Extending External SLS Data

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# Extending External SLS Data

Sometimes a state defined in one SLS file will need to be modified from a separate SLS file. A good example of this is when an argument needs to be overwritten or when a service needs to watch an additional state.

## The Extend Declaration

The standard way to extend is via the extend declaration. The extend declaration is a top level declaration like `include` and encapsulates ID declaration data included from other SLS files. A standard extend looks like this:

    include:
      - http
      - ssh

    extend:
      apache:
        file:
          - name: /etc/httpd/conf/httpd.conf
          - source: salt://http/httpd2.conf
      ssh-server:
        service:
          - watch:
            - file: /etc/ssh/banner

    /etc/ssh/banner:
      file.managed:
        - source: salt://ssh/banner

A few critical things happened here, first off the SLS files that are going to be extended are included, then the extend dec is defined. Under the extend dec 2 IDs are extended, the apache ID's file state is overwritten with a new name and source. Than the ssh server is extended to watch the banner file in addition to anything it is already watching.

## Extend is a Top Level Declaration

This means that `extend` can only be called once in an sls, if if is used twice then only one of the extend blocks will be read. So this is WRONG:

    include:
      - http
      - ssh

    extend:
      apache:
        file:
          - name: /etc/httpd/conf/httpd.conf
          - source: salt://http/httpd2.conf
    # Second extend will overwrite the first!! Only make one
    extend:
      ssh-server:
        service:
          - watch:
            - file: /etc/ssh/banner

## The Requisite "in" Statement

Since one of the most common things to do when extending another SLS is to add states for a service to watch, or anything for a watcher to watch, the requisite in statement was added to 0.9.8 to make extending the watch and require lists easier. The ssh-server extend statement above could be more cleanly defined like so:

    include:
      - ssh

    /etc/ssh/banner:
      file.managed:
        - source: salt://ssh/banner
        - watch_in:
          - service: ssh-server

## Rules to Extend By

There are a few rules to remember when extending states:

1. Always include the SLS being extended with an include declaration
2. Requisites (watch and require) are appended to, everything else is overwritten
3. extend is a top level declaration, like an ID declaration, cannot be declared twice in a single SLS
4. Many IDs can be extended under the extend declaration

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