22.26.15. Requisites and Other Global State Arguments

22.26.15.1. Requisites

The Salt requisite system is used to create relationships between states. The core idea being that, when one state is dependent somehow on another, that inter-dependency can be easily defined.

Requisites come in two types: Direct requisites (such as require), and requisite_ins (such as require_in). The relationships are directional: a direct requisite requires something from another state. However, a requisite_in inserts a requisite into the targeted state pointing to the targeting state. The following example demonstrates a direct requisite:

vim:
  pkg.installed

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

In the example above, the file /etc/vimrc depends on the vim package.

Requisite_in statements are the opposite. Instead of saying "I depend on something", requisite_ins say "Someone depends on me":

vim:
  pkg.installed:
    - require_in:
      - file: /etc/vimrc

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

So here, with a requisite_in, the same thing is accomplished as in the first example, but the other way around. The vim package is saying "/etc/vimrc depends on me". This will result in a require being inserted into the /etc/vimrc state which targets the vim state.

In the end, a single dependency map is created and everything is executed in a finite and predictable order.

Note

Requisite matching

Requisites match on both the ID Declaration and the name parameter. This means that, in the example above, the require_in requisite would also have been matched if the /etc/vimrc state was written as follows:

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

22.26.15.1.1. Direct Requisite and Requisite_in types

There are six direct requisite statements that can be used in Salt: require, watch, prereq, use, onchanges, and onfail. Each direct requisite also has a corresponding requisite_in: require_in, watch_in, prereq_in, use_in, onchanges_in, and onfail_in. All of the requisites define specific relationships and always work with the dependency logic defined above.

22.26.15.1.1.1. require

The use of require demands that the dependent state executes before the depending state. The state containing the require requisite is defined as the depending state. The state specified in the require statement is defined as the dependent state. If the dependent state's execution succeeds, the depending state will then execute. If the dependent state's execution fails, the depending state will not execute. In the first example above, the file /etc/vimrc will only execute after the vim package is installed successfully.

22.26.15.1.1.2. Require an entire sls file

As of Salt 0.16.0, it is possible to require an entire sls file. Do this first by including the sls file and then setting a state to require the included sls file:

include:
  - foo

bar:
  pkg.installed:
    - require:
      - sls: foo

22.26.15.1.1.3. watch

watch statements are used to add additional behavior when there are changes in other states.

Note

If a state should only execute when another state has changes, and otherwise do nothing, the new onchanges requisite should be used instead of watch. watch is designed to add additional behavior when there are changes, but otherwise execute normally.

The state containing the watch requisite is defined as the watching state. The state specified in the watch statement is defined as the watched state. When the watched state executes, it will return a dictionary containing a key named "changes". Here are two examples of state return dictionaries, shown in json for clarity:

"local": {
    "file_|-/tmp/foo_|-/tmp/foo_|-directory": {
        "comment": "Directory /tmp/foo updated",
        "__run_num__": 0,
        "changes": {
            "user": "bar"
        },
        "name": "/tmp/foo",
        "result": true
    }
}

"local": {
    "pkgrepo_|-salt-minion_|-salt-minion_|-managed": {
        "comment": "Package repo 'salt-minion' already configured",
        "__run_num__": 0,
        "changes": {},
        "name": "salt-minion",
        "result": true
    }
}

If the "result" of the watched state is True, the watching state will execute normally. This part of watch mirrors the functionality of the require requisite. If the "result" of the watched state is False, the watching state will never run, nor will the watching state's mod_watch function execute.

However, if the "result" of the watched state is True, and the "changes" key contains a populated dictionary (changes occurred in the watched state), then the watch requisite can add additional behavior. This additional behavior is defined by the mod_watch function within the watching state module. If the mod_watch function exists in the watching state module, it will be called in addition to the normal watching state. The return data from the mod_watch function is what will be returned to the master in this case; the return data from the main watching function is discarded.

If the "changes" key contains an empty dictionary, the watch requisite acts exactly like the require requisite (the watching state will execute if "result" is True, and fail if "result" is False in the watched state).

Note

Not all state modules contain mod_watch. If mod_watch is absent from the watching state module, the watch requisite behaves exactly like a require requisite.

A good example of using watch is with a service.running state. When a service watches a state, then the service is reloaded/restarted when the watched state changes, in addition to Salt ensuring that the service is running.

ntpd:
  service.running:
    - watch:
      - file: /etc/ntp.conf
  file.managed:
    - name: /etc/ntp.conf
    - source: salt://ntp/files/ntp.conf

22.26.15.1.1.4. prereq

New in version 0.16.0.

prereq allows for actions to be taken based on the expected results of a state that has not yet been executed. The state containing the prereq requisite is defined as the pre-requiring state. The state specified in the prereq statement is defined as the pre-required state.

When prereq is called, the pre-required state reports if it expects to have any changes. It does this by running the pre-required single state as a test-run by enabling test=True. This test-run will return a dictionary containing a key named "changes". (See the watch section above for examples of "changes" dictionaries.)

If the "changes" key contains a populated dictionary, it means that the pre-required state expects changes to occur when the state is actually executed, as opposed to the test-run. The pre-required state will now actually run. If the pre-required state executes successfully, the pre-requiring state will then execute. If the pre-required state fails, the pre-requiring state will not execute.

If the "changes" key contains an empty dictionary, this means that changes are not expected by the pre-required state. Neither the pre-required state nor the pre-requiring state will run.

The best way to define how prereq operates is displayed in the following practical example: When a service should be shut down because underlying code is going to change, the service should be off-line while the update occurs. In this example, graceful-down is the pre-requiring state and site-code is the pre-required state.

graceful-down:
  cmd.run:
    - name: service apache graceful
    - prereq:
      - file: site-code

site-code:
  file.recurse:
    - name: /opt/site_code
    - source: salt://site/code

In this case the apache server will only be shutdown if the site-code state expects to deploy fresh code via the file.recurse call. The site-code deployment will only be executed if the graceful-down run completes successfully.

22.26.15.1.1.5. onfail

New in version 2014.7.0.

The onfail requisite allows for reactions to happen strictly as a response to the failure of another state. This can be used in a number of ways, such as executing a second attempt to set up a service or begin to execute a separate thread of states because of a failure.

The onfail requisite is applied in the same way as require as watch:

primary_mount:
  mount:
    - mounted
    - name: /mnt/share
    - device: 10.0.0.45:/share
    - fstype: nfs

backup_mount:
  mount:
    - mounted
    - name: /mnt/share
    - device: 192.168.40.34:/share
    - fstype: nfs
    - onfail:
      - mount: primary_mount

22.26.15.1.1.6. onchanges

New in version 2014.7.0.

The onchanges requisite makes a state only apply if the required states generate changes, and if the watched state's "result" is True. This can be a useful way to execute a post hook after changing aspects of a system.

22.26.15.1.1.7. use

The use requisite is used to inherit the arguments passed in another id declaration. This is useful when many files need to have the same defaults.

/etc/foo.conf:
  file.managed:
    - source: salt://foo.conf
    - template: jinja
    - mkdirs: True
    - user: apache
    - group: apache
    - mode: 755

/etc/bar.conf
  file.managed:
    - source: salt://bar.conf
    - use:
      - file: /etc/foo.conf

The use statement was developed primarily for the networking states but can be used on any states in Salt. This makes sense for the networking state because it can define a long list of options that need to be applied to multiple network interfaces.

The use statement does not inherit the requisites arguments of the targeted state. This means also a chain of use requisites would not inherit inherited options.

22.26.15.1.1.8. The _in versions of requisites

All of the requisites also have corresponding requisite_in versions, which do the reverse of their normal counterparts. The examples below all use require_in as the example, but note that all of the _in requisites work the same way: They result in a normal requisite in the targeted state, which targets the state which has defines the requisite_in. Thus, a require_in causes the target state to require the targeting state. Similarly, a watch_in causes the target state to watch the targeting state. This pattern continues for the rest of the requisites.

If a state declaration needs to be required by another state declaration then require_in can accommodate it. Therefore, these two sls files would be the same in the end:

Using require

httpd:
  pkg:
    - installed
  service:
    - running
    - require:
      - pkg: httpd

Using require_in

httpd:
  pkg:
    - installed
    - require_in:
      - service: httpd
  service:
    - running

The require_in statement is particularly useful when assigning a require in a separate sls file. For instance it may be common for httpd to require components used to set up PHP or mod_python, but the HTTP state does not need to be aware of the additional components that require it when it is set up:

http.sls

httpd:
  pkg:
    - installed
  service:
    - running
    - require:
      - pkg: httpd

php.sls

include:
  - http

php:
  pkg:
    - installed
    - require_in:
      - service: httpd

mod_python.sls

include:
  - http

mod_python:
  pkg:
    - installed
    - require_in:
      - service: httpd

Now the httpd server will only start if php or mod_python are first verified to be installed. Thus allowing for a requisite to be defined "after the fact".

22.26.15.2. Altering States

The state altering system is used to make sure that states are evaluated exactly as the user expects. It can be used to double check that a state preformed exactly how it was expected to, or to make 100% sure that a state only runs under certain conditions. The use of unless or onlyif options help make states even more stateful. The check_cmds option helps ensure that the result of a state is evaluated correctly.

22.26.15.2.1. Unless

New in version 2014.7.0.

The unless requisite specifies that a state should only run when any of the specified commands return False. The unless requisite operates as NOR and is useful in giving more granular control over when a state should execute.

vim:
  pkg.installed:
    - unless:
      - rpm -q vim-enhanced
      - ls /usr/bin/vim

In the example above, the state will only run if either the vim-enhanced package is not installed (returns False) or if /usr/bin/vim does not exist (returns False). The state will run if both commands return False.

However, the state will not run if both commands return True.

22.26.15.2.2. Onlyif

New in version 2014.7.0.

onlyif is the opposite of unless. If all of the commands in onlyif return True, then the state is run. If any of the specified commands return False, the state will not run.

stop-volume:
  module.run:
    - name: glusterfs.stop_volume
    - m_name: work
    - onlyif:
      - gluster volume status work
    - order: 1

remove-volume:
  module.run:
    - name: glusterfs.delete
    - m_name: work
    - onlyif:
      - gluster volume info work
    - watch:
      - cmd: stop-volume

The above example ensures that the stop_volume and delete modules only run if the gluster commands return a 0 ret value.

22.26.15.2.3. Listen/Listen_in

New in version 2014.7.0.

listen and its counterpart listen_in trigger mod_wait functions for states, when those states succeed and result in changes, similar to how watch its counterpart watch_in. Unlike watch and watch_in, listen and listen_in will not modify the order of states and can be used to ensure your states are executed in the order they are defined. All listen/listen_in actions will occur at the end of a state run, after all states have completed.

restart-apache2:
  service.running:
    - name: apache2
    - listen:
      - file: /etc/apache2/apache2.conf

configure-apache2:
  file.managed:
    - path: /etc/apache2/apache2.conf
    - source: salt://apache2/apache2.conf

This example will cause apache2 to be restarted when the apache2.conf file is changed, but the apache2 restart will happen at the end of the state run.

restart-apache2:
  service.running:
    - name: apache2

configure-apache2:
  file.managed:
    - path: /etc/apache2/apache2.conf
    - source: salt://apache2/apache2.conf
    - listen_in:
      - service: apache2

This example does the same as the above example, but puts the state argument on the file resource, rather than the service resource.

22.26.15.2.4. check_cmd

New in version 2014.7.0.

Check Command is used for determining that a state did or did not run as expected.

comment-repo:
  file.replace:
    - path: /etc/yum.repos.d/fedora.repo
    - pattern: ^enabled=0
    - repl: enabled=1
    - check_cmd:
      - grep 'enabled=0' /etc/yum.repos.d/fedora.repo && return 1 || return 0

This will attempt to do a replace on all enabled=0 in the .repo file, and replace them with enabled=1. The check_cmd is just a bash command. It will do a grep for enabled=0 in the file, and if it finds any, it will return a 0, which will prompt the && portion of the command to return a 1, causing check_cmd to set the state as failed. If it returns a 1, meaning it didn't find any 'enabled=0' it will hit the || portion of the command, returning a 0, and declaring the function succeeded.

22.26.15.2.5. Overriding Checks

There are two commands used for the above checks.

mod_run_check is used to check for onlyif and unless. If the goal is to override the global check for these to variables, include a mod_run_check in the salt/states/ file.

mod_run_check_cmd is used to check for the check_cmd options. To override this one, include a mod_run_check_cmd in the states file for the state.


These docs are for Salt's development version: 7c3a137.

Docs for previous releases are available on readthedocs.org.

Latest Salt release: 2014.1.10

Try the shiny new release candidate of Salt, v2014.7.0rc2! More info here.

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