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.
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.
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 for the building of relationships between states with requisite statements. A requisite statement ensures that the named state is evaluated before the state requiring it. There are three types of requisite statements in Salt, require, watch, and prereq.
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, the require requisite is used 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.
Require statements can refer to any state defined in Salt. The basic examples are pkg, service, and file, but any used state can be referenced.
In addition to state declarations such as pkg, file, etc., sls type requisites are also recognized, and essentially allow 'chaining' of states. This provides a mechanism to ensure the proper sequence for complex state formulas, especially when the discrete states are split or groups into separate sls files:
include: - network httpd: pkg.installed:  service.running: - require: - pkg: httpd - sls: network
In this example, the httpd service running state will not be applied (i.e., the httpd service will not be started) unless both the httpd package is installed AND the network state is satisfied.
Requisites match on both the ID Declaration and the name parameter. Therefore, if using the pkgs or sources argument to install a list of packages in a pkg state, it's important to note that it is impossible to match an individual package in the list, since all packages are installed as a single state.
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.
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, so a state with order option should not require or required by other states.
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. Certain circumstances will present a situation where it is desirable to send a state to the end of the line. To do this, set the order to last:
vim: pkg.installed: - order: last
These docs are for Salt's development version: 43857ed.
Docs for previous releases are available on readthedocs.org.
Latest Salt release: 2014.7.0