Ordering States

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# Ordering States

When creating Salt SLS files, it is often important to ensure that they run in a specific order. While states will always execute in the same order, that order is not necessarily defined the way you want it.

A few tools exist in Salt to set up the correct state ordering. These tools consist of requisite declarations and order options.

Note

Salt does not execute state declarations in the order they appear in the source.

## Requisite Statements

Note

This document represents behavior exhibited by Salt requisites as of version 0.9.7 of Salt.

Often when setting up states any single action will require or depend on another action. Salt allows you to build relationships between states with requisite statements. A requisite statement ensure that the named state is evaluated before the state requiring it. There are two types of requisite statements in Salt, require and watch.

These requisite statements are applied to a specific state declaration:

    httpd:
      pkg:
        - installed
      file.managed:
        - name: /etc/httpd/conf/httpd.conf
        - source: salt://httpd/httpd.conf
        - require:
          - pkg: httpd

In this example we use the require requisite to declare that the file /etc/httpd/conf/httpd.conf should only be set up if the pkg state executes successfully.

The requisite system works by finding the states that are required and executing them before the state that requires them. Then the required states can be evaluated to see if they have executed correctly.

### Multiple Requisites

The requisite statement is passed as a list, allowing for the easy addition of more requisites. Both requisite types can also be separately declared:

    httpd:
      pkg:
        - installed
      service.running:
        - enable: True
        - watch:
          - file: /etc/httpd/conf/httpd.conf
        - require:
          - pkg: httpd
          - user: httpd
          - group: httpd
      file.managed:
        - name: /etc/httpd/conf/httpd.conf
        - source: salt://httpd/httpd.conf
        - require:
          - pkg: httpd
      user:
        - present
      group:
        - present

In this example the httpd service is only going to be started if the package, user, group and file are executed successfully.

### The Require Requisite

The foundation of the requisite system is the `require` requisite. The require requisite ensures that the required state(s) are executed before the requiring state. So, if a state is declared that sets down a vimrc, then it would be pertinent to make sure that the vimrc file would only be set down if the vim package has been installed:

    vim:
      pkg:
        - installed
      file.managed:
        - source: salt://vim/vimrc
        - require:
          - pkg: vim

In this case, the vimrc file will only be applied by Salt if and after the vim package is installed.

### The Watch Requisite

The `watch` requisite is more advanced than the `require` requisite. The watch requisite executes the same logic as require (therefore if something is watched it does not need to also be required) with the addition of executing logic if the required states have changed in some way.

The watch requisite checks to see if the watched states have returned any changes. If the watched state returns changes, and the watched states execute successfully, then the watching state will execute a function that reacts to the changes in the watched states.

Perhaps an example can better explain the behavior:

    redis:
      pkg:
        - latest
      file.managed:
        - source: salt://redis/redis.conf
        - name: /etc/redis.conf
        - require:
          - pkg: redis
      service.running:
        - enable: True
        - watch:
          - file: /etc/redis.conf
          - pkg: redis

In this example the redis service will only be started if the file /etc/redis.conf is applied, and the file is only applied if the package is installed. This is normal require behavior, but if the watched file changes, or the watched package is installed or upgraded, then the redis service is restarted.

### Watch and the mod_watch Function

The watch requisite is based on the `mod_watch` function. Python state modules can include a function called `mod_watch` which is then called if the watch call is invoked. In the case of the service module the underlying service is restarted. In the case of the cmd state the command is executed.

The `mod_watch` function for the service state looks like this:

    def mod_watch(name, sig=None):
        '''
        The service watcher, called to invoke the watch command.

        name
            The name of the init or rc script used to manage the service

        sig
            The string to search for when looking for the service process with ps
        '''
        if __salt__['service.status'](name, sig):
            changes = {name: __salt__['service.restart'](name)}
            return {'name': name,
                    'changes': changes,
                    'result': True,
                    'comment': 'Service restarted'}

        return {'name': name,
                'changes': {},
                'result': True,
                'comment': 'Service {0} started'.format(name)}

The watch requisite only works if the state that is watching has a `mod_watch` function written. If watch is set on a state that does not have a `mod_watch` function (like pkg), then the listed states will behave only as if they were under a `require` statement.

## The Order Option

Before using the order option, remember that the majority of state ordering should be done with a requisite declaration, and that a requisite declaration will override an order option.

The order option is used by adding an order number to a state declaration with the option order:

    vim:
      pkg.installed:
        - order: 1

By adding the order option to 1 this ensures that the vim package will be installed in tandem with any other state declaration set to the order 1.

Any state declared without an order option will be executed after all states with order options are executed.

But this construct can only handle ordering states from the beginning. Sometimes you may want to send a state to the end of the line. To do this, set the order to `last`:

    vim:
      pkg.installed:
        - order: last

Remember that requisite statements override the order option. So the order option should be applied to the highest component of the requisite chain:

    vim:
      pkg.installed:
        - order: last
        - require:
          - file: /etc/vimrc

    /etc/vimrc:
      file.managed:
        - source: salt://edit/vimrc

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