Using MCollective Command Line Applications
MCollective is designed first and foremost for the CLI. You will mostly interact with a single executable called mco which has a number of sub-commands, arguments and flags.
Basic Usage of the mco Command
A simple example of a mco command can be seen below:
In this example the ping sub-command is referred to as an application. Mcollective provides many applications, for a list of them, type mco help. You can also create your own application to plug into the framework. The help sub-command will show you something like this:
You can request help for a specific application using either mco help application or mco application —help. Shown below is part of the help for the rpc application:
The help first shows a basic overview of the command line syntax followed by options specific to this command. Following that you will see some Common Options and Host Filters that generally apply to most applications.
Making RPC Requests
Overview of a Request
The rpc application is the main application used to make requests to your servers. It is capable of interacting with any standard Remote Procedure Call (RPC) agent. Below is an example that shows an attempt to start a webserver on several machines:
The order of events in this process are:
- Perform discovery against the network and discover 10 servers
- Send the request and then show a progress bar of the replies
- Show any results that were out of the ordinary
- Show some statistics
Mcollective client applications aim to only provide the most relevant information. In this case, the application is not showing verbose information about the nine OK results, since the most important issue is the one Failure. Keep this in mind when viewing the results of commands.
Anatomy of a Request
MCollective agents are broken up into actions and each action can take input arguments.
This shows the basic make-up of an RPC command. In this case we are:
- using the rpc application - a generic application that can interact with any agent
- directing our request to machines with the service agent
- sending a request to the stop action of the service agent
- supplying a value, httpd, to the service argument of the stop action
The same command has a longer form as well:
These two commands are functionally identical.
Discovering Available Agents
The above command showed you how to interact with the service agent, but how can you find out that this agent even exists? On a correctly installed MCollective system you can use the plugin application to get a list:
The first part of this list shows all the agents this computer is aware of. In order to show up on this list, an agent must have a DDL file and be installed locally.
To find out the actions, inputs and outputs for a specific agent use the plugin application again:
This shows a truncated example of the auto-generated help for the service agent. First shown is metadata such as version, author and license. This is followed by the list of actions available, in this case the restart, start, status and stop actions.
Further information is shown about each action. For example, you can see that the status action requires an input called service which is a string, has a maximum length of 30, etc. You can also see you will receive one output called status
With this information, you can request the status for a specific service:
Unlike the previous example, in this case specific information is returned on the success of the action. This is because this specific action is meant to retrieve information and so mcollective assumes you would like to see complete, thorough data regardless of success or failure.
Note that this output displays Service Status as shown in the mco plugin doc service help page. Any time you need more information about a display name, the doc for the associated agent will have a Description section for every input and output.
Selecting Request Targets Using Filters
A key capability of mcollective is fast discovery of network resources. Discovery rules are written using filters. For example:
This shows a filter rule that limits the RPC request to being run on machines that are either in the Puppet environment development or belong to the Customer acme.
Filtering can be based on facts, the presence of a Configuration Management Class on the node, the node’s Identity, or installed Agents on the node.
Here are a number of examples of this with short descriptions of each filter:
As you can see, you can filter by Agent, Class and/or Fact, and you can use regular expressions almost anywhere. You can also combine filters additively in a command so that all the criteria have to be matched.
Note: You can use a shortcut to combine Class and Fact filters:
Complex Compound or Select Queries
While the above examples are easy to enter, they are limited in that they can only combine filters additively. If you want to create searches with more complex boolean logic use the -S switch. For example:
The above example shows a scenario where the development environment is usually labeled development but one customer has chosen to use staging. You want to find all machines in those customer’s environments that match the class apache. This search would be impossible using the previously shown methods, but the above command uses -S to allow the use of boolean operators such as and and or so you can easily build the logic of the search.
The -S switch also allows for negative matches using not or !:
Filtering Using Data Plugins
As of version 2.1.0, custom data plugins can also be used to create complex filters:
This will search for the md5 hash of a specific file with matches restricted to the development environment. Note that as before, regular expressions can also be used.
As with agents, you can also discover which plugins are available for use:
For information on the input these plugins take and output they provide use the mco plugin doc fstat command.
Currently, each data function can only accept one input while matches are restricted to a single output field per invocation.
Chaining RPC Requests
The rpc application can chain commands one after the other. The example below uses the package agent to find machines with a specific version of mcollective and then schedules Puppet runs on those machines:
Mcollective results can also be filtered using the opensource gem, jgrep. Mcollective data output is fully compatible with jgrep.
Using with PuppetDB
Recent versions of PuppetDB has a built in query language called
Puppet Query Language that you use via the
puppet query command.
Much like the above example of chaining RPC requests MCollective supports reading results from Puppet Query:
This will run Puppet on all CentOS machines
Seeing the Raw Data
By default the rpc application will try to show human-readable data. To see the actual raw data, add the -v flag to disable the display helpers:
This data can also be returned in JSON format:
When an application encounters an error, it returns an explanatory string:
By default only an abbreviated error string is shown that provides some insight into the nature of the problem. For more details, add the -v flag to show a full stack trace:
When mco finishes, it generates an exit code. The returned exit code depends on the nature of the issue:
- 0: If nodes were discovered and all passed.
- 0: If no discovery was performed, at least 1 response was received, and all responses were OK.
- 1: If no nodes were discovered, or if mco encountered an issue not listed here.
- 2: If nodes were discovered but some RPC requests failed.
- 3: If nodes were discovered, but no responses were received.
- 4: If no discovery was performed, and no responses were received.
The rpc application should suit most needs. However, sometimes the data being returned calls for customization such as custom aggregation, summarising or complete custom display.
In such cases, a custom application may be useful For example, the package application provides concluding summaries and provides some basic safe guards for its use. The agent also provides the commonly required data. Typical package output looks like this:
Notice how this application recognises that you are acting on all possible machines, an action which might have a big impact on your YUM servers. Consequently, package prompts for confirmation and, at the end of processing, displays a brief summary of the network status.
While the behaviors of custom applications are not always consistent with each other, in general they accept the standard discovery flags. For details of which flags are accepted in a given application, use the mco help appname command.
To discover which custom applications are available, run mco or mco help.