The Save Protocol Between the File Daemon and the Storage Daemon
Keywords: developers

Subsections

- Command and Control Information
- Data Information

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The Save Protocol Between the File Daemon and the Storage Daemon

Once the Director has send a save command to the File daemon, the File daemon will contact the Storage daemon to begin the save.

In what follows: FD: refers to information set via the network from the File daemon to the Storage daemon, and SD: refers to information set from the Storage daemon to the File daemon.

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Command and Control Information

Command and control information is exchanged in human readable ASCII commands.

    FD: listens
    SD: makes connection
    FD: append open session = <JobId> [<password>]
    SD: 3000 OK ticket = <number>
    FD: append data <ticket-number>
    SD: 3000 OK data address = <IPaddress> port = <port>

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Data Information

The Data information consists of the file attributes and data to the Storage daemon. For the most part, the data information is sent one way: from the File daemon to the Storage daemon. This allows the File daemon to transfer information as fast as possible without a lot of handshaking and network overhead.

However, from time to time, the File daemon needs to do a sort of checkpoint of the situation to ensure that everything is going well with the Storage daemon. To do so, the File daemon sends a packet with a negative length indicating that he wishes the Storage daemon to respond by sending a packet of information to the File daemon. The File daemon then waits to receive a packet from the Storage daemon before continuing.

All data sent are in binary format except for the header packet, which is in ASCII. There are two packet types used data transfer mode: a header packet, the contents of which are known to the Storage daemon, and a data packet, the contents of which are never examined by the Storage daemon.

The first data packet to the Storage daemon will be an ASCII header packet consisting of the following data.

File-Index Stream-Id Info where File-Index is a sequential number beginning from one that increments with each file (or directory) sent.

where Stream-Id will be 1 for the Attributes record and 2 for uncompressed File data. 3 is reserved for the MD5 signature for the file.

where Info transmit information about the Stream to the Storage Daemon. It is a character string field where each character has a meaning. The only character currently defined is 0 (zero), which is simply a place holder (a no op). In the future, there may be codes indicating compressed data, encrypted data, etc.

Immediately following the header packet, the Storage daemon will expect any number of data packets. The series of data packets is terminated by a zero length packet, which indicates to the Storage daemon that the next packet will be another header packet. As previously mentioned, a negative length packet is a request for the Storage daemon to temporarily enter command mode and send a reply to the File daemon. Thus an actual conversation might contain the following exchanges:

    FD: <1 1 0> (header packet)
    FD: <data packet containing file-attributes>
    FD: Null packet
    FD: <1 2 0>
    FD: <multiple data packets containing the file data>
    FD: Packet length = -1
    SD: 3000 OK
    FD: <2 1 0>
    FD: <data packet containing file-attributes>
    FD: Null packet
    FD: <2 2 0>
    FD: <multiple data packets containing the file data>
    FD: Null packet
    FD: Null packet
    FD: append end session <ticket-number>
    SD: 3000 OK end
    FD: append close session <ticket-number>
    SD: 3000 OK Volumes = <number of volumes>
    SD: 3001 Volume = <volumeid> <start file> <start block>
         <end file> <end block> <volume session-id>
    SD: 3002 Volume data = <date/time of last write> <Number bytes written>
         <number errors>
    SD: ... additional Volume / Volume data pairs for
         volumes 2 .. n
    FD: close socket

The information returned to the File daemon by the Storage daemon in response to the append close session is transmit in turn to the Director.