MCollective is a framework for writing feature full agents and clients and provides a rich system to do that. MCollective’s native Client though is very low level, a bit like TCP/IP is to HTTP. Like TCP/IP the native client does not provide any Authentication, Authorization etc.
MCollective Simple RPC is a framework on top of the standard client that abstracts away a lot of the complexity and provides a lot of convention and standards. It’s a bit like using HTTP ontop of TCP/IP to create REST services.
SimpleRPC is a framework that provides the following:
- Provide simple conventions for writing agents and clients, favoring convention over custom design
- Very easy to write agents including input validation and a sensible feedback mechanism in case of error
- Provide audit logging abilities of calls to agents
- Provide the ability to do fine grain Authorization of calls to agents and actions.
- Has a Data Definition Language used to describe agents and assist in giving hints to auto generating user interfaces.
- The provided generic calling tool should be able to speak to most compliant agents
- Should you need to you can still write your own clients, this should be very easy too
- Return data should be easy to print, in most cases the framework should be able to print a sensible output with a single, provided, function. The DDL is used here to improve the standard one-size-fits-all methods.
- The full capabilities of the standard Client classes should still be exposed in case you want to write advanced agents and clients
- A documented standard message format built ontop of the core format.
We’ve provided full tutorials on Writing Simple RPC Agents and Clients. There is also a screencast that will give you a quick look at what is involved in writing agents.
A bit of code probably says more than lots of English, so here’s a simple hello world Agent, it just echo’s back everything you send it in the :msg argument:
1 module MCollective 2 module Agent 3 class Helloworld<RPC::Agent 4 # Basic echo server 5 def echo_action 6 validate :msg, String 7 8 reply.data = request[:msg] 9 end 10 end 11 end 12 end
The nice thing about using a standard abstraction for clients is that you often won’t even need to write a client for it, we ship a standard client that you can use to call the agent above:
% mco rpc helloworld echo msg="Welcome to MCollective Simple RPC" Determining the amount of hosts matching filter for 2 seconds .... 1 devel.your.com : OK "Welcome to MCollective Simple RPC" ---- rpctest#echo call stats ---- Nodes: 1 Start Time: Wed Dec 23 20:49:14 +0000 2009 Discovery Time: 0.00ms Agent Time: 54.35ms Total Time: 54.35ms
You could also use mco rpc like this and achieve the same result:
% mco rpc helloworld echo msg="Welcome to MCollective Simple RPC"
For multiple options just add more key=val pairs at the end
But you can still write your own clients, it’s incredibly simple, full details of a client is out of scope for the introduction - see the SimpleRPCClients page instead for full details - but here is some sample code to do the same call as above including full discovery and help output:
1 #!/usr/bin/ruby 2 3 require 'mcollective' 4 5 include MCollective::RPC 6 7 mc = rpcclient("helloworld") 8 9 printrpc mc.echo(:msg => "Welcome to MCollective Simple RPC") 10 11 printrpcstats
With a standard interface come a lot of possibilities, just like the standard one-size-fits-all CLI client above you can make web interfaces, there’s a simple MCollective <-> REST bridge in the ext directory.
A helper agent called rpcutil is included that helps you gather stats, inventory etc about the running daemon.