fcm - FCM configuration management

fcm is a configuration management system that aims to manage configuration, and nothing more.

Design goals

• Current state live on-hosts is never trusted.
• All data about what configuration a host should have is derived from group membership.
• fcm shall provide a trailer hitch on the front and on the back; no deep integration with other software is required and fcm will not interfere with other software.
• fcm shall not deal with dependency management in any way. All operations are expected to happen at any time.

How to run it

This command will read configs from ../testdata and dump all configs in /tmp/test:

$ ./fcm-builder -d ../testdata/ -o /tmp/test

This command will read configs from ../testdata and show you what host1's passwd file will look like:

$ ./fcm-builder -d ../testdata/ -H host1 -f passwd
root:x:0:0:root:/root:/bin/bash

How it works

A YAML file specifies a group-to-host map. A series of file on disks maps groups to configuration file transformation operations. When invoked, fcm-builder reads the group-to-host map, builds a set of configuration files based on group configuration, and produces a complete set of configuration files for each host.

On-host, a lightweight agent - such as rsync in a loop - downloads host-specific configurations and dumps them into a directory. A set of lightweight asynchronous agents are then free to read these files and do what they will with them. For example, an httpd.conf agent could atomically copy fcm's httpd.conf into /etc/httpd and then reload apache if it was changed.

An agent for a file shall be a standalone executable program that will be passed the name of its input file as its first argument. These agents are expected to do the right thing with regard to file atomicity, fsync, etc.

An example agent that installs a file into /tmp/hello:

#!/usr/bin/ruby
require 'tempfile'
filename = ARGV[0]
data = File.open(filename).read()
file = Tempfile.new('/tmp/test')
file.write(data)
File.rename(file.path, "/tmp/hello")
file.fsync
file.close

Trailer hitches?

Any good vehicle is designed to tow or be towed. fcm does not assume that you're using any particular CMDB, node classifier, or distribution mechanism.

The trailer hitch on the front is the group-to-host map YAML file; you're free to edit it directly or to generate it from whatever source of truth you prefer.

The trailer hitch on the back is that fcm produces raw configuration files. fcm supplies a lightweight agent to run on end hosts, but you are similarly free to, for example, copy them into a files/ directory in Puppet and use them from there.

File formats

Group transformations are applied in the order in which they appear in hosts.yaml.

DATADIR/hosts.yaml:

- GROUPNAME:
    - host1
    - host2
    - host3
- GROUP2:
    - host3
    - host4
    - hostZ

DATADIR/GROUPNAME/named.conf:

- INCLUDE: "/var/fcm/files/named.conf.base"
- APPEND: "controls {"
- APPEND: "  inet 127.0.0.1 port 54 allow {any;}"
- APPEND: "  keys { "rndc-key"; };"
- APPEND: "};"

This will cause all hosts in GROUPNAME to start with a base named.conf configuration, and then append some extra lines to it. In this case, host1,2,3 will each get this named.conf.

Supported transformation commands

- INCLUDE: string

Includes the whole file under DATADIR/raw/string.

- APPEND: string

Appends the line "string" to the file.

- REPLACERE:
   regex: foo
   sub: bar

Replaces all instances of "foo" with "bar" on all lines of the file.

- TRUNCATE

Truncates the file, start from scratch.

- DELETERE: regex

Deletes all lines matching regex.

- INCLUDELINE:
    file: filename
    regex: some_regex

Includes all lines matching some_regex from DATADIR/raw/filename. Example: when you want to include just one line out of a passwd file.