Ordering States¶

The way in which configuration management systems are executed is a hotly debated topic in the configuration management world. Two major philosophies exist on the subject, to either execute in an imperative fashion where things are executed in the order in which they are defined, or in a declarative fashion where dependencies need to be mapped between objects.

Imperative ordering is finite and generally considered easier to write, but declarative ordering is much more powerful and flexible but generally considered more difficult to create.

Salt has been created to get the best of both worlds. States are evaluated in a finite order, which guarantees that states are always executed in the same order, and the states runtime is declarative, making Salt fully aware of dependencies via the requisite system.

State Auto Ordering¶

Salt always executes states in a finite manner, meaning that they will always execute in the same order regardless of the system that is executing them. But in Salt 0.17.0, the state_auto_order option was added. This option makes states get evaluated in the order in which they are defined in sls files.

The evaluation order makes it easy to know what order the states will be executed in, but it is important to note that the requisite system will override the ordering defined in the files, and the order option described below will also override the order in which states are defined in sls files.

If the classic ordering is preferred (lexicographic), then set state_auto_order to False in the master configuration file.

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.

Note

Requisite matching

Requisites match on both the ID Declaration and the name parameter. Therefore, if you are using the pkgs or sources argument to install a list of packages in a pkg state, it's important to note that you cannot have a requisite that matches on an individual package in the list.

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. When mod_watch is called depends on the execution of the watched state, which:

If no changes then just run the watching state itself as usual. mod_watch is not called. This behavior is same as using a require.

If changes then run the watching state AND if that changes nothing then react by calling mod_watch.

When reacting, 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, reload=False, full_restart=False):
    '''
    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):
        if 'service.reload' in __salt__ and reload:
            restart_func = __salt__['service.reload']
        elif 'service.full_restart' in __salt__ and full_restart:
            restart_func = __salt__['service.full_restart']
        else:
            restart_func = __salt__['service.restart']
    else:
        restart_func = __salt__['service.start']

    result = restart_func(name)
    return {'name': name,
            'changes': {name: result},
            'result': result,
            'comment': 'Service restarted' if result else \
                       'Failed to restart the service'
           }

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.

Also notice that a mod_watch may accept additional keyword arguments, which, in the sls file, will be taken from the same set of arguments specified for the state that includes the watch requisite. This means, for the earlier service.running example above, you can tell the service to reload instead of restart like this:

redis:

  # ... other state declarations omitted ...

    service.running:
      - enable: True
      - reload: True
      - watch:
        - file: /etc/redis.conf
        - pkg: redis

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