Distributing files from a central location

CFEngine can manage many machines simply by distributing policies to all its hosts. This tutorial describes how to distribute files to hosts from a central policy server location. For this example, we will distribute software patches.

Files are centrally stored on the policy server (hub). In our example, they are stored in /storage/patches. These patch files must also exist on the agent host (client) in /storage/deploy/patches. To do this, perform the following instructions:

Check out masterfiles from your central repository

CFEngine stores the master copy of all policy in the /var/cfengine/masterfiles directory. Ensure that you are working with the latest version of your masterfiles.

git clone url

or

git pull origin master

Make policy changes

Define locations

Before files can be copied we must know where files should be copied from and where files should be copied to. If these locations are used by multiple components, then defining them in a common bundle can reduce repetition. Variables and classes that are defined in common bundles are accessible by all CFEngine components. This is especially useful in the case of file copies because the same variable definition can be used both by the policy server when granting access and by the agent host when performing the copy.

The policy framework includes a common bundle called def. In this example, we will add two variables--dir_patch_store and dir_patch_deploy--to this existing bundle. These variables provide path definitions for storing and deploying patches.

Add the following variable information to the masterfiles/def.cf file:

"dir_patch_store"
  string => "/storage/patches",
  comment => "Define patch files source location",
  handle => "common_def_vars_dir_patch_store";

"dir_patch_deploy"
  string => "/storage/deploy/patches",
  comment => "Define patch files deploy location",
  handle => "common_def_vars_dir_patch_deploy";

}

These common variables can be referenced from the rest of the policy by using their fully qualified names, $(def.dir_patch_store) and $(def.dir_patch_deploy)

Grant file access

Access must be granted before files can be copied. The right to access a file is provided by cf-serverd, the server component of CFEngine. Enter access information using the access promise type in a server bundle. The default access rules defined by the MPF (Masterfiles Policy Framework) can be found in controls/cf_serverd.cf.

There is no need to modify the vendored policy, instead define your own server bundle. For our example, add the following to services/main.cf:

bundle server my_access_rules
{
  access:
    "$(def.dir_patch_store)"
      handle => "server_access_grant_locations_files_patch_store_for_hosts",
      admit => { ".*$(def.domain)", @(def.acl) },
      comment => "Hosts need to download patch files from the central location";
}

Create a custom library for reusable synchronization policy

You might need to frequently synchronize or copy a directory structure from the policy server to an agent host. Thus, identifying reusable parts of policy and abstracting them for later use is a good idea. This information is stored in a custom library.

Create a custom library called lib/custom/files.cf. Add the following content:

bundle agent sync_from_policyserver(source_path, dest_path)
# @brief Sync files from the policy server to the agent
#
# @param source_path  Location on policy server to copy files from
# @param dest_path Location on agent host to copy files to
{
  files:
    "$(dest_path)/."
      handle       => "sync_from_policy_server_files_dest_path_copy_from_source_path_sys_policy_hub",
      copy_from    => sync_cp("$(source_path)", "$(sys.policy_hub)"),
      depth_search => recurse("inf"),
      comment      => "Ensure files from $(sys.policy_hub):$(source_path) exist in $(dest_path)";
}

This reusable policy will be used to synchronize a directory on the policy server to a directory on the agent host.

Create a patch policy

Organize in a way that makes the most sense to you and your team. We recommend organizing policy by services.

Create services/patching.cf with the following content:

# Patching Policy

bundle agent patching
# @brief Ensure various aspects of patching are handeled

# We can break down the various parts of patching into separate bundles. This
# allows us to become less overwhelmed by details if numerous specifics
# exist in one or more aspect for different host classifications.
{
  methods:

    "Patch Distribution"
      handle    => "patching_methods_patch_distribution",
      usebundle => "patch_distribution",
      comment   => "Ensure patches are properly distributed";
}

bundle agent patch_distribution
# @brief Ensures that our patches are distributed to the proper locations
{
  files:
    "$(def.dir_patch_deploy)/."
      handle  => "patch_distribution_files_def_dir_patch_deploy_exists",
      create  => "true",
      comment => "If the destination directory does not exist, we have no place
                  to which to copy the patches.";

  methods:

    "Patches"
      handle    => "patch_distribution_methods_patches_from_policyserver_def_dir_patch_store_to_def_dir_patch_deploy",
      usebundle => sync_from_policyserver("$(def.dir_patch_store)", "$(def.dir_patch_deploy)"),
      comment   => "Patches need to be present on host systems so that we can use
                   them. By convention we use the policy server as the central
                   distribution point.";
}

The above policy contains two bundles. We have separated a top-level patching bundle from a more specific patch_distribution bundle. This is an illustration of how to use bundles in order to abstract details. You might, for example, have some hosts that you don't want to fully synchronize so you might use a different method or copy from a different path. Creating numerous bundles allows you to move those details away from the top level of what is involved in patching. If people are interested in what is involved in patch distribution, they can view that bundle for specifics.

Integrate the policy

Now that all the pieces of the policy are in place, they must be integrated into the policy so they can be activated. Add each policy file to the inputs section which is found under body common control. Once the policy file is included in inputs, the bundle can be activated. Bundles can be activated by adding them to either the bundlesequence or they can be called as a methods type promise.

Add the following entries to promises.cf under body common control -> inputs:

"lib/custom/files.cf",
"services/patching.cf",

and the following to promises.cf under body common control -> bundlesequence:

"patching",

Now that all of the policy has been edited and is in place, check for syntax errors by running cf-promises -f ./promises.cf. This promise is activated from the service_catalogue bundle.

Commit Changes

Set up trackers in the Mission Portal (Enterprise Users Only)

Before committing the changes to your repository, log in to the Mission Portal and set up a Tracker so that you can see the policy as it goes out. To do this, perform the following:

Navigate to the Hosts section. Select All hosts. Select the Events tab, located in the right-hand panel. Click Add new tracker.

Name it Patch Failure. Set the Report Type to Promise not Kept. Under Watch, enter .patch. Set the Start Time to Now and then click Done to close the Start Time window. Click Start to save the new tracker. This tracker watches for any promise handle that includes the string patch where a promise is not kept.

Add another tracker called Patch Repaired. Set the Report Type to Promise Repaired. Enter the same values as above for Watch and Start Time. Click Start to save the new tracker. This tracker allows you to see how the policy reacts as it is activated on your infrastructure.

Deploy changes (Enterprise and Community Users)

Always inspect what you expect. git status shows the status of your current branch.

git status

Inspect the changes contained in each file.

git diff file

Once satisfied, add them to Git's commit staging area.

git add file

Iterate over using git diff, add, and status until all of the changes that you expected are listed as Changes to be committed. Check the status once more before you commit the changes.

git status

Commit the changes to your local repository.

git commit

Push the changes to the central repository so they can be pulled down to your policy server for distribution.

git push origin master