firstworks SQL Relay FAQ

SQL Relay FAQ
General Questions

- What is SQL Relay?
- What doesn't SQL Relay do?
- What is "persistent database connection pooling"?
- What is "proxying"?
- What is "throttling"?
- What is "load balancing"?
- What is "query routing/filtering"?
- What are "drop-in replacement libraries"?
- What databases does SQL Relay support?
- What platforms does SQL Relay run on?
- What languages does SQL Relay support?
- What database abstraction layers does SQL Relay support?
- What are "bind variables"?
- What are "multi-row fetches"?
- What is "client-side result set caching"?
- What are "suspended transactions"?
- How does SQL Relay work?
- How can SQL Relay speed up or improve the efficiency of my website?
- How does SQL Relay allow me to access a database from a platform not supported by the database vendor?
- How can SQL Relay help migrating my application from one database to another?
- How can SQL Relay be used with replicated or clustered databases?
- Can SQL Relay proxy multiple database users instead of using the same user for every session?
- Can I use SQL Relay to send all insert/update/delete's to a master database and distribute select's over a set of slaves?
- Can I use SQL Relay to make my existing MySQL app send all insert/update/delete's to a master database and distribute select's over a set of slaves?
- Can SQL Relay be configured to filter out certain queries, such as select's without a where clause?

Build Questions

- Can I build SQL Relay against Oracle Instantclient?
- Why won't SQL Relay build properly on Ubuntu?

API Comparison Questions

- Why should I use SQL Relay with CGI's instead of using a native database API?
- How does SQL Relay compare to ODBC, JDBC, Perl::DBI, PHP::ADODB, Ruby::DBI or PythonDB?
- How does SQL Relay compare to DBI::Proxy?
- How does SQL Relay compare to Apache::DBI or PHP's internal persistent database connections?

Database-Specific Questions

- Can I use SQL Relay to connect to XXX database?
- Why should I use SQL Relay with XXX database?
- Does SQL Relay support XXX's specific features?
- Why can't I get SQL Relay to connect to Oracle?
- Why can't I get SQL Relay to connect to Sybase/Microsoft SQL Server?
- I know that Sybase and MS SQL Server support affected row counts, so why does the FreeTDS connection return -1's for affected rows?
- Why are money types crashing my FreeTDS connections?
- How can I keep from getting "Attempt to initiate a new Adaptive Server operation with results pending" errors when using FreeTDS?
- What's the difference between the Sybase and FreeTDS connections?
- Postgresql's native API uses numbers for types but SQL Relay mangles them. How do I get the numbers?
- I'm using a non-ascii character set in my Oracle database, why am I get get garbage back when I do a select?
- How do I get the last insert rowid from SQLite?
- How do I use Oracle's OS-Authentication (users identified externally)?
- Why aren't commit's working in MySQL?
- Why does the first DB2 query causes SQL Relay to reconnect to the DB?
- What's the deal with date columns in MS SQL Server?

Programming Questions

- My PHP application uses up all of the SQL Relay connections, and I keep having to restart SQL Relay. What gives?
- Why does SQL Relay buffer the entire result set?
- How do keep SQL Relay from buffering the entire result set?
- How do I use bind variables?
- Why can't I do vector binds?
- How do I run stored procedures?
- How do I get the result set from a stored procedure that returns a result set?
- How do I get data out of DML with RETURNING clauses?
- Do temporary tables work with SQL Relay?
- Is the SQL Relay API thread-safe?
- How does client-side result-set caching work?
- What about server-side result-set caching?
- What happened to defineOutputBind?
- MySQL Provides mysql_insert_id(). How do I get that from SQL Relay?

Administration Questions

- What system parameters can I tweak to get better performance out of SQL Relay
- If I leave SQL Relay idle for a long time, then come back and try to use it, I get a low level TCP error from the database. What does that mean?
- Why doesn't the last connection die off when I set connections=0 and maxconnections to some larger number and let SQL Relay go idle for a long time?
- Whenever I set maxconnections>connections, it doesn't ever start any new connections and I get "Couldn't parse config file ..." errors on the screen. What's up with that?
- Where'd the configuration GUI go?
- How can I get SQL Relay to only listen on the unix socket and not the inet port?
- If I set up SQL Relay to use dynamic scaling, it eventually spawns off as many connections as it can and they never die off. What's going on?
- If I set up SQL Relay to use dynamic scaling with a maxqueuelength=0, sometimes it spawns off new connections even when there should be enough to handle the clients. What's going on?
- In sqlrelay.conf, I set the cursors parameter to a small number. Why does sqlr-status show Opened Server Cursors growing and growing?
- When using SQL Relay on Cygwin, sqlr-start never returns to a command prompt. How do I fix that?
- When using SQL Relay on Cygwin, I get an error like "child_info_fork::abort: ...xxx.dll: Loaded to different address: parent(xxx)!=child(yyy)" and nothing runs. How do I fix that?
- What happened to the "debug" option? How do I enable the debug logs?
- I don't like storing plaintext passwords in my sqlrelay.conf file. Can I encrypt them somehow?

General Questions
What is SQL Relay?

SQL Relay is powerful database-access middleware consisting of a server-based connection pool and client API's for using it.

SQL Relay sits between your app and the database and provides functionality not typically provided by the database directly.

The main features of SQL Relay are:

Persistent database connection pooling - makes database driven web-based applications faster and more efficient
Solves - "I've analyzed my app's performance and it seems to be spending a lot of time just logging into the database."

Proxing - access dtabases from unsupported platforms
Solves - "I need to access Oracle and MS SQL Server from FreeBSD (or other unsupported platform)."

Throttling - prevents databases from becoming overloaded
Solves - "When I run enough web servers to handle the traffic, it slams the database, even though most of the database connections appear to be idle."

Load Balancing - distributes access over replicated or clustered databases
Solves - "I can't afford a load balancing appliance to distribute access to the nodes in my database cluster."

Query Routing - conditionally sends queries to one database or another
Solves - "My database is getting slammed. I could replicate it but then I'd have to rewrite my app to send DML to the master and somehow distribute selects over the slaves."

Drop-In Replacement Libraries - aims an existing MySQL or PostgreSQL app at SQL Relay without modification
Solves - "I wrote my app using the native MySQL API but the company is migrating to Oracle. I can tweak the queries to work with Oracle but I really don't want to have to modify the app to use OCI."

SQL Relay has client API's for most modern programming languages and drivers for many popular database abstraction API's.

What doesn't SQL Relay do?

SQL Relay is powerful but it does have some limits.

It is a tool in a chain - another tier in a multi-tiered application - designed to solve specific problems. It is not a database-enabled application development environment like Access or Delphi or modern equivalents. It's not a reporting gateway like Crystal Reports. It is not a database itself. The server components currently require a unix-like system (including Linux or Mac OS X) and client support on Windows is limited to C/C++, C# and ADO.NET.

There are some features that SQL Relay currently lacks that are either being worked on, or at least planned though.

SQL Relay is not currently a transparent database gateway. Ie. you can't currently aim SQL Relay at MySQL and aim your app at SQL Relay. SQL Relay has it's own client/server protocol and API. If your app uses a database abstraction API like ODBC or Perl DBI, you would need to use the SQL Relay driver. You could also potentially use the SQL Relay Drop-in Replacement Library for MySQL. Otherwise you'd need to modify your app to use the SQL Relay API. Efforts are underway to add support for several native database protocols though.

SQL Relay can provide a single-point of access for replicated databases but it does not provide replication. It cannot keep multiple databases in sync.

Similarly, though SQL Relay can provide a single-point of access for multiple databases, it does not currently provide a mechanism for joining tables across databases or sending the same query to multiple databases and merging the result sets.

SQL Relay cannot currently translate queries from one syntax to another. Though it can be used to aim an app originally written for MySQL at Oracle, for example, it cannot (yet) transparently convert queries in MySQL syntax to Oracle syntax.

What is "persistent database connection pooling"?

See Persistent Database Connection Pooling.

What is "proxying"?

See Proxying.

What is "throttling"?

See Throttling.

What is "load balancing"?

See Load Balancing.

What is "query routing/filtering"?

See Query Routing and Filtering.

What are "drop-in replacement libraries"?

See Using the SQL Relay drop-in replacement library for MySQL and Using the SQL Relay drop-in replacement library for PostgreSQL.

What databases does SQL Relay support?

See Supported Databases.

What platforms does SQL Relay run on?

See the Supported Platforms section of the installation document.

What languages does SQL Relay support?

SQL Relay currently supports C, C++, .NET-capable languages, Perl, Python, PHP, Ruby, Java, TCL and Erlang.

What database abstraction layers does SQL Relay support?

SQL Relay currently supports ODBC, ADO.NET, Perl-DBI, Python-DB, PHP PDO and Ruby-DBI.

The native SQL Relay Client API's are database abstraction layers as well, as the SQL Relay server can be connected to a variety of database and applications written using an SQL Relay Client API can talk to any SQL Relay server.

What are "substitution and bind variables"?

See Substitution and Bind Variables.

What are "multi-row fetches"?

See Multi-Row Fetches.

What is "client-side result set caching"?

See Client-Side Result Set Caching.

What are "suspended transactions"?

See Suspended Transactions.

How does SQL Relay work?

SQL Relay's connection daemons log into and maintain sessions with databases. These connection daemons advertise themselves with a listener daemon which listens on an inet and/or unix port for client connections. When a client connects to the listener, if a connection daemon is available, the listener hands off the client to that connection. If no connection daemon is available, the client must wait in queue until one is. Once a client is handed off to a connection daemon, the client communicates to the database through the session maintained by that daemon.

How can SQL Relay speed up or improve the efficiency of my website?

There are many way that SQL Relay can speed up or improve the efficiency of your website, but here some common examples.

Let's say you're running CGI's againt a transactional database such as PostgreSQL, MS SQL Server or Oracle. CGI's have to log into and out of the database each time they run. If you use SQL Relay to maintain persistent connections to the database and just log into and out of SQL Relay, you can reduce the amount of time spent establishing database connections and handle more CGI requests per-second. This is both because the time-cost of connecting to SQL Relay is smaller than the time-cost of connecting to a transactional database, and because the SQL Relay client library is smaller than most database client libraries, resulting in a more lightweight CGI.

Let's say you're using Apache, PHP and Oracle and you determine by doing all sorts of analysis that you need to keep 30 Apache processes running to provide adequate response. Since most of your site isn't database-driven, on average, no more than 5 PHP's actually access the database simultaneously. Currently, you're using persistent connections to defeat the time-cost of logging into Oracle, but you have to maintain 30 connections (1 per web server process) which takes up a lot of memory on both the web server and database server and you really only need 5 connections. By using SQL Relay you can reduce the number of Oracle connections to the 5 that you need, continue to run 30 Apache processes and reclaim the wasted memory on both machines.

Many websites run a combination of PHP's and Perl modules. Perl modules can use Apache::DBI and PHP's have a persistent database connection system, but a PHP cannot use an Apache::DBI connection and a Perl module cannot use a PHP persistent connection. Thus in order to make sure that there are enough database connections for each platform, many more web-server processes have to be run, perhaps twice as many. If the PHP's and Perl modules used SQL Relay instead, they could share databse connections and reduce the number of web-server processes and database connections.

SQL Relay makes it easy to distribute load over replicated servers. A common scaling solution when using MySQL or PostgreSQL in a read-only web environment is to run several web servers with a dedicated database server for each web server or group of web servers and update all the databases simultaneously at scheduled intervals. This usually works pretty well, but sometimes database or web servers get runs of heavy load while others are idle. In other cases, an uneven number of machines is required. For example, your application may need 3 web servers but only 2 database servers or vice-versa and buying 3 of each would be wasteful. Moreover, in most cases, the servers have to be equivalently powerful machines. You can't usually just add another cheap machine that you have lying around into the pool. SQL Relay can connect to multiple, replicated or clustered database servers, providing web-based applications access to whichever server isn't busy. SQL Relay can also be configured to maintain more connections to more powerful machines and fewer connections to less powerful machines, enabling unevenly matched machines to be used in the same database pool. Collectively, these features allow you to save money by using only the exact number of servers that you need and by enabling you to use spare hardware in your database pools.

How does SQL Relay allow me to access a database from a platform not supported by the database vendor?

See Proxying.

How can SQL Relay help migrating my application from one database to another?

SQL Relay is both a connection pool and database abstraction layer. Lets say that your app currently connects directly to a database which has an insignificant connection delay but you want to migrate it to a database that has features that you want, but also has a significant connection delay. If you first set up instances of SQL Relay to talk to both databases, and then put SQL Relay between your app and the database, you can switch between the databases by changing the connection parameters in your app to point to different instances of SQL Relay, as you would with any other database abstraction layer. However, since SQL Relay is also a connection pool, you don't incur the connection delay associated with the new database.

If your app was written using a database abstraction API supported by SQL Relay such as ODBC, ADO.NET, Perl DBI, etc. then this is straightforward. You can just change the connect string to point to the appropriate intance of SQL Relay.

If your app was written using a database specific API then you might have to port your app to use the SQL Relay API, or a supported DB abstraction layer first.

If your app was written using a MySQL or PostgreSQL API then you might be able to use the SQL Relay Drop-In Replacement Library for MySQL or the SQL Relay Drop-In Replacement Library for PostgreSQL without modifying your app at all.

SQL Relay doesn't currently do query translation though. There might be queries in your app that are written using database-specific syntax that would need to be modified to use the new database.

How can SQL Relay be used with replicated or clustered databases?

See Load Balancing.

Can SQL Relay proxy multiple database users instead of using the same user for every session?

Yes. Set the authtier attribute of the instance tag in the sqlrelay.conf file to "database". See the SQL Relay Configuration Reference for more information.

SQL Relay does it very efficiently when used with Oracle 8i or greater and MySQL version 3.23.3 or greater. For Oracle, the database must be configured properly. See this document for step-by-step instructions.

Oracle 8i or greater and MySQL version 3.23.3 or greater allow a process that is connected to the database to switch users without disconnecting from the database. When used with other databases, SQL Relay logs out and logs back in to the database whenever it needs to switch users.

Can I use SQL Relay to send all insert/update/delete's to a master database and distribute select's over a set of slaves?

Yes, see Routing and Filtering Queries with SQL Relay.

Can I use SQL Relay to make my existing MySQL app send all insert/update/delete's to a master database and distribute select's over a set of slaves?

Probably. You'll need to set up SQL Relay to route queries and use the Drop-in Replacement Library for MySQL to redirect your app to use SQL Relay.

See Routing and Filtering Queries with SQL Relay for details on how to set up SQL Relay as a query router.

See Using the SQL Relay drop-in replacement library for MySQL for details about how to use the drop-in replacement library.

Can SQL Relay be configured to filter out certain queries, such as select's without a where clause?

Yes, see Routing and Filtering Queries with SQL Relay.

Build Questions
Can I build SQL Relay against Oracle Instantclient?

Yes. However, there are two potential issues.

The configure script looks hard for the instantclient installation but if you installed from a non-RPM distribution of instantclient, then you might have to use use the --with-oracle-instantclient-prefix option of the configure script. For example, if you unzipped the instantclient distro in /usr/local and it created a directory called /usr/local/instantclient_11_2, then you should use:

> ./configure --with-oracle-instantclient-prefix=/usr/local/instantclient_11_2

Also older releases of SQL Relay have the following issue...

Some releases of the instantclient software have omitted the libclntsh.so symlink. I noticed it in the zip distribution of the 11.2 version. As a result the -lclntsh argument to the linker fails to find the library and the configure script fails to detect the instantclient installation. Even if you manually tweak the config.mk file, the build will still fail. This can be remedied though, by adding the libclntsh.so symlink manually.

Go to the directory that contains libclntsh.so.11.1 (or whatever version you installed) and, as root, or whatever user you used to install the instantclient software, run the following command:

> ln -s libclntsh.so.11.1 libclntsh.so

Afterwards, the SQL Relay configure script should detect the instantclient installation and the software should build properly.

Why won't SQL Relay build properly on Ubuntu?

This isn't an issue for recent releases of SQL Relay, but it was a problem for lots of software, for a while, not just SQL Relay.

On modern Ubuntu distros, /bin/sh is a symlink to /bin/dash as opposed to /bin/bash. Apparently, older versions of libtool generated code that required more complete support of sh than is provided by dash. The best solution that I have found is to remove the /bin/sh symlink and create a new one between /bin/bash and /bin/sh, as follows (as root).

> cd /bin rm sh ln -s bash sh

This doesn't appear to be an issue these days.

API Comparison Questions
Why should I use SQL Relay with web-based applications instead of using a native database API?

Many web-based apps have to log into and out of the database each time they generate a page. Logging in can take a long time. Also, native database API libraries are often very large. Since SQL Relay maintains persistent database connections, is fast to connect to and has a lightweight client API, using SQL Relay with web-based applications generally results in a faster applications that use less memory.

How does SQL Relay compare to ADO.NET, ODBC, JDBC, Perl::DBI, etc.?

This is sort of an apples-to-oranges comparison. These API's are primarily targeted as abstraction layers and make no attempt to improve application performance. Some have features not supported by the SQL Relay client API's but the SQL Relay client API has features not supported by them too. SQL Relay currently provides drivers for ADO.NET, Perl::DBI, PHP PDO, Ruby::DBI, PythonDB and ODBC and can connect to databases via ODBC.

How does SQL Relay compare to DBI::Proxy?

DBI::Proxy is Perl-specific or at least very challenging to use from other languages. SQL Relay likely outperforms DBI::Proxy since DBI::Proxy is primarily targeted at providing access to databases from unsupported platforms, not at improving application performance, though I have never tested one against the other. SQL Relay can provide access to databases from unsupported platforms as well, even platforms for which there is no unix support using the ODBC connection and an ODBC to ODBC bridge.

How does SQL Relay compare to Apache::DBI or PHP's persistent database connections?

SQL Relay is more lightweight and potentially faster than Apache::DBI and is competitive in speed with PHP's persistent connections. SQL Relay can be used to provide a connection pool to multiple machines and has more backend features than Apache::DBI or PHP. However, the DBI and PHP API's are more full featured than the SQL Relay API's and are generally considered to be simpler to implement.

When using Apache::DBI or PHP's persistent connections, a connection is opened to the database for every web server process. Frequently, web sites need to run large numbers of processes to provide adequate response. As the number of database connections grows, resources get strained and a lot of database connections go unused most of the time.

If a website runs a mixture of Perl modules and PHP scripts, the issue can be doubled.

SQL Relay makes more efficient use of resources by maintaining fewer persistent connections to the database and funnelling all database requests through those connections. When the number of database session requests exceeds the number of persistent connections, the session requests are queued. This ultimately causes delayed response to the client, but keeps the database running smoothly. In most cases, the delay is negligable and the tradeoff is acceptable.

Database-Specific Questions
Can I use SQL Relay to connect to XXX database?

See Supported Databases.

Why should I use SQL Relay with XXX database?

Database | Limit the number of open connections. | Distribute over replicated or clustered databases. | Overcome the connection delay. | Provide remote access.
Oracle | Yes | Yes | Yes | No
MySQL | Yes | Yes | No | No
PostgreSQL | Yes | Yes | Yes | No
Sybase | Yes | Yes | Yes | No
DB2 | Yes | Yes | Yes | No
Firebird | Yes | Yes | Yes | No
SQLite | Yes | Yes | No | Yes
FreeTDS | Yes | Yes | Yes | No
ODBC | Yes | Yes | Yes | No
MDB Tools | Yes | Yes | No | Yes

Does SQL Relay support XXX's specific features?

Database | Queries | Bind Variables | Procedural Language | Auto-Commit
Oracle | Yes | Scalar Input/Output | Yes | Yes
MySQL | Yes | Scalar Input | No | No
PostgreSQL | Yes | Scalar Input | No | No
Sybase | Yes | Scalar Input | No | No
DB2 | Yes | Scalar Input/Output | Unknown | Yes
Firebird | Yes | Scalar Input | Unknown | Yes
SQLite | Yes | Scalar Input | No | No
FreeTDS | Yes | Scalar Input | No | No
ODBC | Yes | Scalar Input/Output if DB supports it. | Yes if DB supports it. | Yes if DB supports it.
MDB Tools | Yes | Scalar Input | No. | No.

Why can't I get SQL Relay to connect to Oracle?

Here's the most common thing that causes this problem...

Following installation, the permissions on the tnsnames.ora file are often set to 640, meaning the read-write for the oracle user, read-only for the oinstall group and no permissions for everyone else.

If the runasuser and/or runasgroup settings in the sqlrelay.conf file are set to a user and/or group that can't read the file, then SQL Relay will fail to connect to the database. The examples generally show runasuser and runasgroup being set to "nobody", which won't work with the default settings for the tnsnames.ora file.

There are multiple solutions. Perhaps the simplest is to change the permissions using:

> chmod o+r tnsnames.ora

Another solution is to run SQL Relay as the oracle user and/or oinstall group, or to run it as a user that is in the oinstall group.

A less common, but not terribly uncommon error is described below.

SQL Relay fails to start against Oracle with error: OCIEnvCreate() failed. How do I fix this?

This error means that the Oracle OCI library, which SQL Relay uses, couldn't find something that it needed to set up the environment.

The most common cause is an incomplete installation. Try running sqlplus as the oracle user. If it works, then the installation probably isn't incomplete. If it doesn't, then you probably need to reinstall Oracle, or at least the client components, or Instant Client components if you're using that.

The next most common cause of this is a permissions issue. Something under the ORACLE_HOME (or Instant Client installation directory, if you're using Instant Client) isn't readable or executable by the user that SQL Relay is trying to run as. Try running sqlplus as the same user and see if it works. If it does then there probably isn't a permissions issue. If it doesn't, then there could be a permissions issue.

The next most common cause is an invalid ORACLE_HOME environment variable, or invalid oracle_home setting in the sqlrelay.conf file. The ORACLE_SID environment variable, or oracle_sid setting in the sqlrelay.conf file, can either refer to an entry in the tnsnames.ora file or it can just be an fully qualified entry itself. In the case of the former, the ORACLE_HOME must be set to a valid directory, under which network/admin/tnsnames.ora can be found. In the case of the latter, when using Oracle 8 and 8i or any version of Oracle Instant Client, ORACLE_HOME can be set to anything at all or left empty, but when using Oracle 9i and up, ORACLE_HOME must be set to a valid location.

Why can't I get SQL Relay to connect to Sybase/Microsoft SQL Server?

The server parameter in the string attribute of the connection tag does not refer to the DNS name of the server. Rather it refers to an entry in the "interfaces" file. The Sybase and FreeTDS libraries look for that file in default places, but if the file is installed somewhere else and the library can't find it, it will not be able to figure out what host/port the server is running on. One way to tell SQL Relay where the file is located is to set the SYBASE environment variable to the directory containing the file before starting SQL Relay. Starting with version 0.32, the string attribute of the connection tag takes a sybase parameter which sets the environment variable.

Another problem that people have connecting to Sybase/Microsoft SQL Server is related to database selection. Until version 0.32, the Sybase and FreeTDS connection daemons ignored the "db" connectstring parameter. This was an oversight. The connection daemons would connect to the correct server but would not use the correct database. Instead, the connection daemons would be connected to the master database on that server. This is fixed in 0.32. To work around this problem in an older release it is necessary to fully qualify table names if the table is not in the master database. A fully qualified table name is dbname.username.tablename.

I know that Sybase and MS SQL Server support affected row counts, so why does the FreeTDS connection return -1's for affected rows?

Before version 0.53, calling the FreeTDS function to get the number of affected rows would cause a segmentation fault. As of version 0.32, SQL Relay figures out what version of FreeTDS is installed at compile time and only enables affected rows if the FreeTDS version is greater than 0.52. If you compile against an earlier version of FreeTDS, a -1 is returned for affected rows as if the database didn't support the feature.

Why are money types crashing my FreeTDS connections?

Before version 0.53, calling the FreeTDS function ct_fetch when a result set had a MONEY or SMALLMONEY column in it would cause a segmentation fault. As of version 0.32, SQL Relay figures out what version of FreeTDS is installed at compile time and only enables queries selecting MONEY or SMALLMONEY columns if the FreeTDS version is greater than 0.52. If you compile against an earlier version of FreeTDS, any attempt to run a query selecting MONEY or SMALLMONEY column will fail with an error indicating that you should recompile SQL Relay against a newer version of FreeTDS.

How can I keep from getting "Attempt to initiate a new Adaptive Server operation with results pending" errors when using FreeTDS?

The short answer... If you're doing nested selects, don't use setResultSetBufferSize() with a non-0 argument on the client when using FreeTDS on the server.

The longer answer is that the TDS protocol doesn't support fetching mulitple result sets simultaneously through the same connection. Somehow the Sybase client API works around this but FreeTDS does not.

SQL Relay can work around it by fetching the entire result set and caching it on the client, which is SQL Relay's default behavior, but using setResultSetBufferSize() with a non-0 argument defeats this behavior, causing the result set to be fetched in chunks rather than all-at-once. If you do this and then run nested selects then the result set from the outer select will still be pending when the inner select is run and FreeTDS will return this error.

So, when using FreeTDS on the server and nested selects on the client, don't use setResultSetBufferSize() with a non-0 argument and everything will work (though it might work slowly and consume inordinate amounts of memory with very large result sets), otherwise you'll get that error.

See the FreeTDS FAQ for more info.

What's the difference between the Sybase and FreeTDS connections?

The libsqlrelay_sqlrconnection_sybase.so plugin is compiled against Sybase ctlib; the libraries that come with Sybase Adaptive Server Enterprise. They use a protocol called TDS (Tabular Data Stream) to talk to the database.

The libsqlrelay_sqlrconnection_freetds.so program is compiled against FreeTDS, an open-source implementation of the TDS protocol and ctlib.

Older versions of Microsoft SQL Server are compatible with Sybase ctlib, but newer versions are not. FreeTDS is compatible with all versions of Sybase Adaptive Server Enterprise and Microsoft SQL Server.

Unfortunately, FreeTDS is an incomplete implementation of TDS. Depending on the version of FreeTDS you're using, features may be buggy, inconsistent or non-existent. For example...

- Programs compiled against FreeTDS<0.53 crash when fetching MONEY and SMALLMONEY datatypes. SQL Relay handles this by preventing queries which fetch MONEY or SMALLMONEY datatypes from running if it's compiled against FreeTDS<0.53.
- Programs compiled against FreeTDS<0.53 crash when attempting to find out how many rows were affected by an insert/update/delete query. SQL Relay handles this by not requesting the affected row count and just returning -1 if it's compiled against FreeTDS<0.53.
- Columns of type CHAR have trailing spaces truncated. For a CHAR(20) column, inserting a value

      'hello'

  will be returned as

      'hello'

  instead of

      'hello               '

  To add to the problem CHAR and VARCHAR datatypes are both represented as CHAR in FreeTDS and Sybase ctlib, so there's no good way to know whether to append trailing spaces or not.
- Under Sybase ctlib, MONEY datatypes have 2 decmial places by default but can have up to 4. Under FreeTDS, they have 4 decimal places.
- Date formats are slightly different. For example, Sybase ctlib returns 1:00 AM on 01/01/2001 as

      Jan  1 2001  1:00AM'

  while FreeTDS returns it as

      Jan  1 2001 01:00:00:000AM'

- Sybase ctlib returns REAL and FLOAT datatypes with arbitrary decimal places. Inserting 1.1 as a REAL or FLOAT type then selecting it might return 1.09888874310 or something similar. With FreeTDS, inserting 1.1 then selecting it returns 1.1. Sybase behaves like FreeTDS for DECIMAL and NUMBER types.
- FreeTDS only supports one cursor per connection. You can write code that uses multiple cursors, but it will eventually throw errors like: "Error: Attempt to initiate a new SQL Server operation with results pending." Prior to SQL Relay 0.37, this was a common error to see under certain circumstances. As of SQL Relay 0.37 the connection daemon funnels everything through a single cursor and this error should no longer appear. From the client-side as long as you don't use setResultSetBufferSize() to specify a buffer size other than 0, it will appear like more than 1 cursor is available, but in fact only 1 is being used.

There are possibly other inconsistencies, but these are the only ones that I've run into so far.

FreeTDS is great software despite its inconsistencies. Sybase ctlib is impossibly complex. Anyone attempting to re-engineer it is braver than I am. I'm impressed that FreeTDS works as well as it does, and it's getting better with each release. In fact, it's possible that any or all of those inconsistencies have been fixed in the current release.

Postgresql's native API uses numbers for types but SQL Relay mangles them. How do I get the numbers?

Prior to version 0.28, SQL Relay mangled Postgresql numeric types into pseudo-standard datatype names. I read a thread in a discussion group indicating that someone was specifically unhappy with SQL Relay because of this behavior though, and decided to change it. So, as of version 0.28, by default, SQL Relay returns numeric types when run against Postgresql. If you prefer getting type names, you can set the mangletypes connect string value to "yes" in your sqlrelay.conf file.

For example, in version 0.28 or higher the following connectstring will instruct SQL Relay to return type names instead of numbers:

> user=myuser;password=mypass;db=testdb;mangletypes=yes

In version 0.28 or higher the following connectstring will instruct SQL Relay to return type numbers:

> user=myuser;password=mypass;db=testdb;mangletypes=no

Leaving the mangletypes parameter out altogether is the same as setting it to "no".

Why am I getting question marks and other garbage back when I do a select? It worked fine when I was using the native database command line tool.

The most common cause of this has to do with character set translation.

Data is transferred from the SQL Relay server process to the SQL Relay client process unmodified, but if you're using one character set in your database (or in a specific table or column of your database) and the SQL Relay server process is using a different character set, the database client library will translate the result set when it is transferred from the database to the SQL Relay process and will then be passed on to the SQL Relay client in its translated state.

If the character set used by the SQL Relay server process is missing characters from the character set used by the database, then ?'s are often substituted. Other translations can occur too such as the removal of accents or creation of total garbage.

Configuring the character set used by the SQL Relay process is different for each database.

Since SQL Relay 0.43, when using MySQL, PostgreSQL, FreeTDS, Sybase and Firebird databases, the character set can be set by setting the charset variable in the connect string of the sqlrelay.conf file.

Since SQL Relay 0.37, when using Oracle, the character set can be set by setting the nls_lang variable in the connect string.

Since SQL Relay 0.43, when using DB2, the character set can be set by setting the lang variable in the connect string.

Other databases don't support any character set translation at all. Data is just returned from the database as-is, byte-by-byte.

In the case of Oracle and DB2, the nls_lang and lang connect string variables just set the NLS_LANG and LANG environment variables. The NLS_LANG or LANG environment variables may be set directly to set the character set when using versions of SQL Relay older than listed above.

How do I get the last insert rowid from SQLite?

You can run a special query: "SELECT LAST INSERT ROWID". When SQL Relay sees this query, it returns a result set with a single field containing the last insert rowid.

How do I use Oracle's OS-Authentication (users identified externally)?

To set up Oracle to authenticate a user against the OS, first create a user in linux/unix, then log into oracle as the sys user and create a corresponding user as follows. In this example, we'll assume that you created an OS-level user named mused:

>     CREATE USER ops$mused IDENTIFIED EXTERNALLY
>     GRANT CONNECT TO ops$mused

Now you can log in as mused and connect to the database using sqlplus / and you will not be prompted for a username and password.

In the sqlrelay.conf file, set the runasuser attribute of the instance tag to the user that you want to connect as and leave out the user, password and oracle_sid parameters of the string attribute of the connection tag.

For example, here's a normal (non-OS-authentication) configuration which connects to the ora1 instance of oracle using testuser/testpassword:

>     <instance ... runasuser="oracle" ...>
>             ...
>             <connections>
>                     <connection ... string="user=testuser;password=testpassword;oracle_sid=ora1" .../>
>             </connections>
>     </instance>

Here's one that uses the oracle OS-user to connect. This is analagous to logging in as oracle and running sqlplus /:

>     <instance ... runasuser="oracle" ...>
>             ...
>             <connections>
>                     <connection ... string="" .../>
>             </connections>
>     </instance>

It's possible to set up an externally authenticated user to have access to other SID's and use sqlplus /@someotherschema to connect to them. To configure SQL Relay to connect to another schema, just add the oracle_sid parameter to the string attribute of the connection tag. For example, use the following to connect to ora1.

>             <connection ... string="oracle_sid=ora1" .../>

Why aren't commit's working in MySQL?

Older versions of MySQL don't support transactions at all but modern versions do, if you create tables using a table type that supports transactions. The InnoDB table type is the most commonly used transaction-supporting table type. To use the InnoDB table type, use a create statement like the following:

>     create table testtable (col1 int) type=innodb

However, you may be surprised at MySQL's behavior even when you're using InnoDB tables or another table type that supports transactions.

With most databases, if a client has a connection open and commits a set of inserts, updates or deletes in, then those changes are immediately visible to other clients using separate connections.

MySQL doesn't support this behavior, at least not with InnoDB tables. Instead, with MySQL, after beginning a transaction, the database will appear to be unaffected by queries run in other transactions until the transaction you are running is committed. So, if another client commits a set of inserts, you won't see them until you commit your transaction.

If you're in autocommit mode though and haven't begun a transaction, then you will immediately see all changes committed by other clients. But as soon as you begin a transaction, you will no longer see changes to the database until you commit your transaction.

It's really a bit more complicated than that. MySQL supports several different isolation levels, each with slightly different behavior. See the official MySQL documentation for more information on this.

It's possible (probable?) that non-MySQL databases have different isolation levels as well, but I don't have any experience with different isolation levels in other databases. It's also possible that there is some way to get the "normal" behavior out of MySQL, perhaps by using a different table type, but again I don't have any experience with that. If anyone has insight into this, please email me about it.

In SQL Relay 0.39, when using MySQL, whenver a client connects to the server, the MySQL connection daemon executes a commit before running the first query to sync things up. This emulates the behavior that a client would expect if it was making a new connection directly to the database.

Why does the first DB2 query causes SQL Relay to reconnect to the DB?

This started in SQL Relay 0.39, see SQL Relay - Getting Started With IBM DB2 for information about this.

What's the deal with date columns in MS SQL Server?

SQL Relay uses FreeTDS to connect to MS SQL Server. SQL Relay can also use FreeTDS to connect to Sybase.

FreeTDS recognizes Sybase DATE/DATETIME columns and MS SQL Server DATETIME columns as DATETIME types and formats them according to the rules defined in FreeTDS's locales.conf file for the locale of the SQL Relay server.

However, FreeTDS doesn't recognizes MS SQL Server DATE columns as DATETIME types. Rather it recognizes them as CHAR types and formats them in YYYY-MM-DD format universally.

So when using MS SQL Server, if you select a DATE and DATETIME column you might get results like:

> DATE: 2013-08-05
> DATETIME: Aug 5 2013 09:25:25:917PM

instead of what you'd get with Sybase:

> DATE: Aug 5 2013 12:00:00:000AM
> DATETIME: Aug 5 2013 09:25:25:917PM

The dateformat parameter in the sqlrelay.conf file can be used to normalize these DATE fields but there is an interesting potential issue.

The date/time conversion routines take any fields in the result set that appear to be a date, time or date/time combination and translate them to the format specified by the dateformat, timeformat and datetimeformat parameters. However the routines must also use the dateddmm parameter to determine whether the month or day comes first. For example, the date 03/04/2005 could be recognized as either March 4, 2005 or April 3, 2005. If dateddmm="yes" is set then it will be recognized as the latter.

Without a special exception though, setting dateddmm="yes" would cause problems when fetching DATE columns from MS SQL Server because they are always in YYYY-MM-DD format. To resolve this, for MS SQL Server, when a date in the result set appears to be in the YYYY-XX-XX format, the dateddmm parameter is ignored and it is presumed to be in YYYY-MM-DD format.

Ideally we'd only ignore dateddmm for MS SQL Server, on DATE type columns that appeared to be in the YYYY-XX-XX format. But, since DATE columns are returned as CHAR type and we can't tell whether a column was fetched from the database or just a literal in the original queriy then we can't be that selective. We just have to ignore dateddmm for everything in YYYY-XX-XX format, whether fetched from a DATE column or from a CHAR column or from a string literal in the original query.

What all of this means is that when using date/time conversion with MS SQL Server and dateddmm="yes" is set, dates stored in CHAR fields in YYYY-XX-XX format must be stored in YYYY-MM-DD format and that string literals in the original query that are in YYYY-XX-XX format must also be in YYYY-MM-DD format.

Of course, if you're not using MS SQL Server and date/time conversion and setting dateddmm="yes" then you don't have to worry about any of this.

Programming Questions
My PHP application uses up all of the SQL Relay connections, and I keep having to restart SQL Relay. What gives?

Prior to version 0.36, PHP scripts needed to set up a shutdown handler to clean up connections and cursors and call register_shutdown_function() or call ignore_user_abort(). Otherwise, if someone hit stop in their browser, the page would stop executing immediately and wouldn't close its connections to the SQL Relay server. In version 0.36, cleanup code was added to the PHP module, making it unnecessary to set up a shutdown handler.

The cleanup code works with PHP 4.1 or greater. If you're using PHP 4.0, then the old issue still exists. Below is a description of the issue and how to deal with it.

PHP pages that use SQL Relay are usually set up like this:

>     <?
>     dl("sql_relay.so");

>     var $con=sqlrcon_alloc(...);
>     var $cur=sqlrcur_alloc($con);

>     ... do some stuff ...

>     sqlrcur_free($cur)
>     sqlrcon_free($con)
>     ?>

If someone clicks stop in their browser before sqlrcur_free is called, execution of the page stops there and sqlrcur_free never gets called. As a result, the socket connection between the web server and SQL Relay is never severed and the SQL Relay connection remains occupied even though the PHP page that opened the connection isn't running anymore.

The best solution to this problem is to create a shutdown function and register it with a call to register_shutdown_function().

For example:

>     <?
>     dl("sql_relay.so");

>     var $con;
>     var $cur;

>     function shutdown() {
>             sqlrcur_free($cur);
>             sqlrcon_free($con);
>     }

>     $con=sqlrcon_alloc(...);
>     $cur=sqlrcur_alloc($con);

>     register_shutdown_function(shutdown);

>     ... do some stuff ...
>     ?>

Note that in this example, sqlrcur_free() and sqlrcon_free() aren't called at the end of the page. This is because the shutdown function gets called whether the page was aborted or completed successfully.

If you want to create a function who's contents are executed only on abort, you can use the connection_aborted() function. In the following code, the shutdown() function free's the cursor and connection if the script was aborted but lets the script free them on its own if it completed successfully. This code is redundant but it illustrates the functionality of connection_aborted().

>     <?
>     dl("sql_relay.so");

>     var $con;
>     var $cur;

>     function shutdown() {
>             if (connection_aborted()) {
>                     sqlrcur_free($cur);
>                     sqlrcon_free($con);
>             }
>     }

>     $con=sqlrcon_alloc(...);
>     $cur=sqlrcur_alloc($con);

>     register_shutdown_function(shutdown);

>     ... do some stuff ...

>     sqlrcur_free($cur);
>     sqlrcon_free($con);
>     ?>

Another option is to call ignore_user_abort(TRUE) early on in the script. If ignore_user_abort() is used, then if a user clicks stop, the PHP script is not aborted. Rather, the remainder of its output is just never sent to the browser. This solution is less efficient than using register_shutdown_function() but also works. For example:

>     <?
>     dl("sql_relay.so");

>     var $con;
>     var $cur;

>     $con=sqlrcon_alloc(...);
>     $cur=sqlrcur_alloc($con);

>     ignore_user_abort(TRUE);

>     ... do some stuff ...

>     sqlrcur_free($cur);
>     sqlrcon_free($con);
>     ?>

SQL Relay version 0.37 features an idle client timeout. If a client remains connected to SQL Relay for a configurable amount of time but does nothing, then SQL Relay will close the connection to that client. This should make the results less catastrophic when people forget to call register_shutdown_function() or ignore_user_abort() but is still less efficient.

Why does SQL Relay buffer the entire result set?

SQL Relay is targeted for web-based applications. For the most part, queries with relatively small result sets are used to build web pages. For small result sets, it more efficient to buffer the entire result set than to step through it, building the page. It's usually faster because it reduces network round-trips and allows one program to drop the connection to SQL Relay, freeing it up for more programs to use while the first program builds its page.

How do keep SQL Relay from buffering the entire result set?

For large result sets it can be impractical to buffer the entire result set. Use the setResultSetBufferSize() method in the C++, Perl, Python, Ruby and Java API's or the sqlrcur_setResultSetBufferSize() function in the C or PHP API's to specify how many rows of the result set to buffer at once.

How do I use bind variables?

That depends on the database you're using. Oracle supports named bind variables while other databases only support binds by position. Below are pseudocode examples of both.

Oracle example:

    sqlrconnection	*con=new sqlrconnection(...);
    sqlrcursor	*cur=new sqlrcursor(cur);
    cur->prepareQuery("select * from table where charcol=:charval and intcol=:intval and floatcol=:floatval");
    cur->inputBind("charval","hello");
    cur->inputBind("intval",10);
    cur->inputBind("floatval",5.5,1,1);
    cur->executeQuery();
    delete cur;
    delete con;

Sybase/MS SQL Server example:

    sqlrconnection	*con=new sqlrconnection(...);
    sqlrcursor	*cur=new sqlrcursor(cur);
    cur->prepareQuery("select * from table where charcol=@charval and intcol=@intval and floatcol=@floatval");
    cur->inputBind("charval","hello");
    cur->inputBind("intval",10);
    cur->inputBind("floatval",5.5,1,1);
    cur->executeQuery();
    delete cur;
    delete con;

Other DB example:

    sqlrconnection	*con=new sqlrconnection(...);
    sqlrcursor	*cur=new sqlrcursor(cur);
    cur->prepareQuery("select * from table where charcol=? and intcol=? and floatcol=?");
    cur->inputBind("0","hello");
    cur->inputBind("1",10);
    cur->inputBind("2",5.5,1,1);
    cur->executeQuery();
    delete cur;
    delete con;

Output bind variables work similarly but are only supported on Oracle.

    sqlrconnection	*con=new sqlrconnection(...);
    sqlrcursor	*cur=new sqlrcursor(con);
    cur->prepareQuery("insert into table values ('hello') returning :charval");
    cur->defineOutputBind("charval","hello",10);
    cur->executeQuery();
    cout << "charval is: " << cur->getOutputBind("charval") << endl;
    delete con;
    delete cur;

ODBC appears to only support binding by position, so when using ODBC to connect SQL Relay to a database, you must bind by position, even if the underlying database supports binding by name.

A more complete explanation of bind variables is given in here and the programming guides for each language explain how to use them with queries and stored procedures.

Why can't I do vector binds?

Vector binds just aren't implemented yet.

How do I run stored procedures?

Stored procedures work in most databases that support them, with some caveats. Stored procedures are not supported when using FreeTDS against Sybase/MS SQL Server. Binding clob/blob values are only supported in Oracle 8i or greater. When using stored procedures with Sybase, output parameters must be varchar types.

The syntax for creating and executing a stored procedure is different for each database. Each of the "Programming with SQL Relay using the XXX API" documents explain how to use stored procedures in detail.

How do I get the result set from a stored procedure that returns a result set?

This depends entirely on the database you're using.

For Firebird you just run a query like select * from myprocedure. For DB2 and MySQL you just run a query like call myprocedure. For Sybase/FreeTDS you just run a query like exec myprocedure. For PostgreSQL you call select * from myprocedure() ... where ... is replaced with a description of the columns that the procedure returns. In all of these cases, the result set is available as if you were selecting from a table.

Oracle is a bit different. A stored procedure may declare a cursor variable, assign the results of a query to it and return it either as a return value or as an output bind variable. You have to use defineOutputBindCursor(), getOutputBindCursor() and fetchFromBindCursor(). The use of these are documented in the various language guides for SQL Relay such as the C++ guide. After calling fetchFromBindCursor(), the result set is available as if you were selecting from a table.

Some databases allow a stored procedure to return multiple result sets, one after the other. SQL Relay does not currently support fetching more than one result set from any query, including one that calls a stored procedure.

There is an exception to this is with Oracle. Since Oracle exposes cursor variables directly, and each cursor variable can be bound to a client-side cursor, one result set can be fetched from each cursor variable exposed by a stored procedure.

How do I get data out of DML with RETURNING clauses?

DML with RETURNING clauses are only known to work with Oracle. You can use output bind variables to get data out of DML with RETURNING clauses if the values returned are scalar. Vector values are not supported. For example:

    insert into testtable values ("one",2) returning :first, :second

Do temporary tables work with SQL Relay?

Yes. Support for PostgreSQL temporary tables was added in version 0.34 and support for all other databases was added in 0.35.

The reason temporary tables are an issue is because normally temporary tables are either dropped or truncated when an application closes its connection to the database or commits/rolls-back a transaction. Transaction-scope tables work naturally with SQL Relay. But, since SQL Relay never logs out of the database, the database doesn't know to drop or truncate tables when an application closes its connection to SQL Relay. To remedy this, SQL Relay parses each query according to the rules for creating a temporary table for the database that it is connected to and keeps a list of temporary tables. Then, when the application closes its connection, SQL Relay drops or truncates the tables as appropriate.

In Oracle, temporary tables are never dropped when the session or transaction ends, only truncated. Transaction-scope tables are handled naturally by the database, but session-scope tables must be handled by SQL Relay. In other databases, a create table query would have to have been issued but not with Oracle. SQL Relay only knows to truncate the table if a create table query was issued. So, when using session-scope tables in Oracle, you should issue a create table query, even if you know the table already exists. The query will fail, but SQL Relay will know to truncate the table when the session ends.

Is the SQL Relay API thread-safe?

Yes. However, you cannot share an instance of an sqlrconnection or sqlrcursor between threads without protecting the calls with mutexes.

How does client-side result-set caching work?

When the cacheToFile() and setCacheTtl() methods are called prior to running a query, the client caches the result set from the query in a file on the local file system and attaches a time-to-live tag to the file. The full pathname of this file can be retrieved using the getCacheFileName() method.

The file sits in the cache directory (usually /usr/local/firstworks/var/sqlrelay/cache) until it is removed by the sqlr-cachemanager program. sqlr-cachemanager scans the files in the cache directory every so often and removes the ones who's time to live has expired.

Until the file is removed, other applications can open the file by name using the openCachedResultSet() methods. At this point, the API acts as if it had run a query that generated that result set.

What about server-side result-set caching?

Server-side result set caching has not been implemented. This feature is on the TODO list though.

What happened to defineOutputBind?

defineOutputBind defined a string output bind. Version 0.37 added support for integer and double output binds and defineOutputBind was replaced with defineOutputBindString, defineOutputBindInteger and defineOutputBindDouble and getOutputBind was replaced with getOutputBindString, getOutputBindInteger and getOutputBindDouble.

Prior to 0.37, stored procedures had to convert integers and doubles to strings before returning them. As of 0.37, this is no longer necessary.

Apps which used defineOutputBind/getOutputBind must replace those calls with defineOutputBindString/getOutputBindString to get the exact same behavior.

MySQL Provides mysql_insert_id(). How do I get that from SQL Relay?

SQL Relay doesn't provide an API call like mysql_insert_id(), however, when using MySQL as a backend database, you can run the query "select last_insert_id()" to get the last insert_id.

Administration Questions
What system parameters can I tweak to get better performance out of SQL Relay?

See Tuning SQL Relay...

If I leave SQL Relay idle for a long time, then come back and try to use it, my program gets hung or I get a low level TCP error from the database. What does that mean?

If you are running SQL Relay on one machine and the database on a seperate machine, and there is a firewall between the two machines, and no queries have been run in a while, the firewall may "close" the connection between SQL Relay and the database by discarding any packets that it receives from either. The firewall does not send "close packets" to either SQL Relay or the database, so SQL Relay thinks it's still connected to the database, but when it sends a query, the firewall discards it.

Some database client API's catch this condition and return an obscure error but many do not, they just hang silently forever.

Cisco PIX firewalls are known to cause this problem. Others may as well.

Allegedly, an IOS upgrade will solve the problem for Cisco PIX firewalls, but I don't know what version you have to upgrade to.

Another solution is to turn off the time out.

Yet another solution is to set up a cron job that runs a query periodically and keeps the connection from timing out.

You could also just set up the timeout to be so long that it's really unlikely that it will ever occur. The PIX firewall's default is 1 hour. You can change it from the PIX's command line using a command such as the following:

> timeout conn 04:00:00

You can also set it using the graphical web interface as follows:

Why doesn't the last connection die off when I set connections=0 and maxconnections to some larger number and let SQL Relay go idle for a long time?

When SQL Relay first starts up, since no connections are started, no connections are registered with the listener. Clients connect, connections fire up to handle them, clients disconnect, the connections time out and die. However, one of the connections will still remain registered with the listener as an "available connection". This connection cannot die off.

It's really, really complicated why. I tried lots of approaches and couldn't find one that didn't have a fundamental problem.

Basically, here's the code:

>     listener:
>     	acquireExclusiveAccessToSharedMemoryAmongListenerProcesses();
>     	waitForConnectionToSignalUsToRead();
>     	readRegistration();
>     	signalConnectionThatWeHaveRead();
>     	releaseEclusiveAccessToSharedMemoryAmongListenerProcesses();

>     connection:
>     	acquireExclusiveAccessToSharedMemoryAmongConnectionProcesses();
>     	writeRegistration();
>     	signalListenerToRead();
>     	waitForListenerToSignalUsThatItHasRead();
>     	releaseEclusiveAccessToSharedMemoryAmongConnectionProcesses();

If the connection dies between signalListenerToRead() and waitForListenerToSignalUsThatItHasRead() or while waiting then obscure problems occur. Most notably, the next connection with a that starts up will not die off. Also, it is certain that the currently registered connection will die at this point because that's where it sits and waits for the listener to hand it a client.

Fundamentally, the listener needs to know that the connection has died off and needs not to signalConnectionThatWeHaveRead(). For the time being, I'm not certain how to do that. So for now, I avoid the problem by making it impossible for a connection to die off there.

Of course, it's always possible to kill the connection manually. In that case though, you would cause the problem that I'm trying to avoid.

Whenever I set maxconnections>connections, it doesn't ever start any new connections and I get "Couldn't parse config file ..." errors on the screen. What's going on?

SQL Relay is typically started as root. The sqlr-start command runs as root and starts up the sqlr-listener, sqlr-connection and sqlr-scaler processes as root. They then read the config file and switch to run as whatever user/group are specified in the runasuser and runasgroup parameters. The sqlr-scaler is most likely now running as a non-root user (such as nobody). When it spawns new connections, they start out running as that same non-root user. The root user can always read the config file, but the config file may not have permissions set so that the non-root user can read it. In that case, the sqlr-connection daemon will start up, fail to read the config file, display a message and exit.

To remedy the situation, make sure that the sqlrelay.conf file is readable by each of the users and groups specified in the runasuser/runasgroup attributes inside the file.

Where'd the configuration GUI go?

The GTK-based configuration GUI was removed in version 0.40. It was buggy, used legacy GTK+ version 1.2 and virtually nobody used it.

How can I get SQL Relay to only listen on the unix socket and not the inet port?

This was broken prior to version 0.40. But in 0.40 or newer, you can just set port="" in the instance tag of the sqlrelay.conf file or leave the attribute out altogether. However, in either case, you must supply a value for the socket attribute. If neither are specified then SQL Relay will fall back to listening on the default port 9000 and not listening on the unix socket at all.

If I set up SQL Relay to use dynamic scaling, it eventually spawns off as many connections as it can and they never die off. What's up with that?

This can happen if a spike of client traffic comes in. If configured to do so, SQL Relay will fork off more connection daemons to handle the traffic. Later when the traffic dies down, the connections that were forked off will still end up handling clients. Each connection daemon will experience longer idle periods than before the spike, but unless those idle periods are shorter than the ttl parameter, the connections will not die off. Setting the ttl shorter should take care of this, but it may take a lot of fine tuning to find the correct ttl.

As of version 0.41, SQL Relay supports a new parameter: maxsessioncount. If it is set greater than 0, then connections that were forked off will only handle that many client sessions before dying off, even if it doesn't go idle. Using this new parameter will ensure that all forked-off connections will eventually die off. Like the ttl, this parameter still requires some tuning but setting it too low will result in poorer performance rather than additional resource consumption.

In version 0.41, the default maxsessioncount value was 10. This caused performance problems as most people just didn't set the value at all. As of 0.42, the default has been 0, effectively disabling this feature by default.

If I set up SQL Relay to use dynamic scaling with a maxqueuelength=0, sometimes it spawns off new connections even when there should be enough to handle the clients. What's going on?

This can happen if maxqueuelength=0, all connection daemons are occupied and a client calls endSession and immediately runs another query. The connection daemon that the client was using has to loop back and tell the listener that it's ready for another client. The client will attempt to reconnect to SQL Relay to run the next query, but depending on the exact timing, if the connetion daemon hasn't finished looping back, to the scaler daemon, it will appear that all connections are busy, but there's a new client attempting to connect and it will fork off a new connection daemon to service it.

There is no easy fix for this. Anything that might be done to guarantee that a new connection daemon won't be spawned would reduce overall performance. So, if you're using maxqueuelength=0, then expect for this to happen. It should not happen if maxqueuelength is set to some value greater than 0.

In sqlrelay.conf, I set the cursors parameter to a small number. Why does sqlr-status show Opened Server Cursors growing and growing?

Some database API's provide functions for doing commits and rollbacks. Others do not. For those that do not, SQL Relay allocates a new cursor and uses it to execute the commit or rollback as a query. Each time this happens, the Opened Server Cursors count increases. Aside from being confusing, this shouldn't cause any problems.

When using SQL Relay on Cygwin, sqlr-start never returns to a command prompt. How do I fix that?

For some reason, only under Cygwin, the exit() call at the end of sqlr-start waits for the various processes that were started to finish. I'm not sure why exactly and I don't remember it happening prior to Cygwin 1.7. It's possible that it doesn't happen on all platforms or all versions of Cygwin.

You can run sqlr-start into the background:

> sqlr-start -id instance> &

...to get the command prompt back, but doing that will return you to the command prompt before everything is started, so it's not a great solution.

Ultimately I'll get to the bottom of the problem, but for now, that's the only solution.

When using SQL Relay on Cygwin, I get an error like "child_info_fork::abort: ...xxx.dll: Loaded to different address: parent(xxx)!=child(yyy)" and nothing runs. How do I fix that?

The various SQL Relay server processes "fork themselves into the background" on startup. Windows doesn't have the same concept of fork() that unix has though. Cygwin provides a reasonable facimile but the implementation is highy complex and can get fouled up in a number of ways.

I encountered this problem when building with g++ 4.x with Cygwin 1.7 on Windows 7 and switching to g++ 3.x solved it.

However, there are more common causes and the Cygwin FAQ provides a good explanation of them and provides potential solutions.

What happened to the debug option? How do I enable the debug logs?

Prior to version 0.49, the instance tag of the sqlrelay.conf file had a debug attribute that made it possible to enable debug logs for the listeners and connections.

As of 0.49, this has been replaced by an modular logging framework and the classic debug has been reimplemented as a module.

The modern equivalent configuration to the classic debug="listener and connection" configuration is:

> <instance ...>
> ...
> <loggers>
> <logger module="debug" listener="yes" connection="yes"/>
> </logger>
> ...
> </intstance>

You can disable logging of either listener or connection daemons by setting "no" to "yes".

See Logger Modules for more information.

I don't like storing plaintext passwords in my sqlrelay.conf file. Can I encrypt them somehow?

Yes. Both database passwords and passwords for accessing SQL Relay itself can be encrypted via extension modules. You can use one of the provided modules or develop your own.

See Password Encryption Modules for more information.