# Mod Aggregate State Runtime Modifications

Added in version 2014.7.0.

The mod_aggregate system was added in the 2014.7.0 release of Salt and allows for runtime modification of the executing state data. Simply put, it allows for the data used by Salt's state system to be changed on the fly at runtime, kind of like a configuration management JIT compiler or a runtime import system. All in all, it makes Salt much more dynamic.

## How it Works

The best example is the `pkg` state. One of the major requests in Salt has long been adding the ability to install all packages defined at the same time. The mod_aggregate system makes this a reality. While executing Salt's state system, when a `pkg` state is reached the `mod_aggregate` function in the state module is called. For `pkg` this function scans all of the other states that are slated to run, and picks up the references to `name` and `pkgs`, then adds them to `pkgs` in the first state. The result is a single call to yum, apt-get, pacman, etc as part of the first package install.

## How to Use it

Note

Since this option changes the basic behavior of the state runtime, after it is enabled states should be executed using test=True to ensure that the desired behavior is preserved.

### In config files

The first way to enable aggregation is with a configuration option in either the master or minion configuration files. Salt will invoke `mod_aggregate` the first time it encounters a state module that has aggregate support.

If this option is set in the master config it will apply to all state runs on all minions, if set in the minion config it will only apply to said minion.

Enable for all states:

    state_aggregate: True

Enable for only specific state modules:

    state_aggregate:
      - pkg

### In states

The second way to enable aggregation is with the state-level `aggregate` keyword. In this configuration, Salt will invoke the `mod_aggregate` function the first time it encounters this keyword. Any additional occurrences of the keyword will be ignored as the aggregation has already taken place.

The following example will trigger `mod_aggregate` when the `lamp_stack` state is processed resulting in a single call to the underlying package manager.

    lamp_stack:
      pkg.installed:
        - pkgs:
          - php
          - mysql-client
        - aggregate: True

    memcached:
      pkg.installed:
        - name: memcached

## Adding mod_aggregate to a State Module

Adding a mod_aggregate routine to an existing state module only requires adding an additional function to the state module called mod_aggregate.

The mod_aggregate function just needs to accept three parameters and return the low data to use. Since mod_aggregate is working on the state runtime level it does need to manipulate low data.

The three parameters are low, chunks, and running. The low option is the low data for the state execution which is about to be called. The chunks is the list of all of the low data dictionaries which are being executed by the runtime and the running dictionary is the return data from all of the state executions which have already be executed.

This example, simplified from the pkg state, shows how to create mod_aggregate functions:

    def mod_aggregate(low, chunks, running):
        """
        The mod_aggregate function which looks up all packages in the available
        low chunks and merges them into a single pkgs ref in the present low data
        """
        pkgs = []
        # What functions should we aggregate?
        agg_enabled = [
            "installed",
            "latest",
            "removed",
            "purged",
        ]
        # The `low` data is just a dict with the state, function (fun) and
        # arguments passed in from the sls
        if low.get("fun") not in agg_enabled:
            return low
        # Now look into what other things are set to execute
        for chunk in chunks:
            # The state runtime uses "tags" to track completed jobs, it may
            # look familiar with the _|-
            tag = __utils__["state.gen_tag"](chunk)
            if tag in running:
                # Already ran the pkg state, skip aggregation
                continue
            if chunk.get("state") == "pkg":
                if "__agg__" in chunk:
                    continue
                # Check for the same function
                if chunk.get("fun") != low.get("fun"):
                    continue
                # Pull out the pkg names!
                if "pkgs" in chunk:
                    pkgs.extend(chunk["pkgs"])
                    chunk["__agg__"] = True
                elif "name" in chunk:
                    pkgs.append(chunk["name"])
                    chunk["__agg__"] = True
        if pkgs:
            if "pkgs" in low:
                low["pkgs"].extend(pkgs)
            else:
                low["pkgs"] = pkgs
        # The low has been modified and needs to be returned to the state
        # runtime for execution
        return low