MCollective supports the ability for each node to register with a central inventory. The core functionality of Mcollective doesn’t require registration internally; it’s simply provided as a framework to enable you to build inventory systems or Web UIs.
Registration plugins are easy to write. You can configure your nodes to use your own or the provided one.
The one we provide simply sends a list of agents to the inventory. It’s available in the plugins directory under registration/agentlist.rb and can be seen in its entirety below:
You can see it’s very simple, you just need to subclass MCollective::Registration::Base to ensure they get loaded into the plugin system and provide a body method whose return value will be sent to the registration agent(s).
The registration plugin can decide if a message should be sent or not. If your plugin responds with a nil value the message will not be sent. This can be useful if you wish to only send registration data when some condition has changed. On large collectives, registration messages can be quite high-volume. It’s worthwhile to sample the size of your registration messages and multiply by the number of nodes to determine an appropriate frequency to send them.
To configure it to be used you just need the following in your config:
This will cause the plugin to be called every 300 seconds to the development collective, if you do not configure a target collective explicitely it will target the main collective for the given node.
We do not provide the receiving end of this in the core mcollective. You will need to write an agent called registration and do something useful with the data you receive from all the nodes. You can find a simple monitoring system built using this method as an example. The receiving agent can simply be installed as an extra mcollectived plugin on a node which participates in the collective.
You need to note a few things about the receiving agents:
- They need to be fast. You’ll receive a lot of registration messages so if your agent talks to a database that is slow you’ll run into problems
- They should not return anything other than nil. The MCollective server will interpret nil from an agent as an indication that you do not want to send back any reply. Replying to registration requests is almost always undesired.
There’s nothing preventing you from running more than one type of receiving agent in your collective, you can have one on your monitor server as above and another with completely different code on a web server feeding a local cache for your web interfaces. As long as both agents are called registration you’ll be fine. However this does mean you can’t run more than one registration receiver on the same server.