Writing SimpleRPC Agents

Simple RPC works because it makes a lot of assumptions about how you write agents, we’ll try to capture those assumptions here and show you how to apply them to our Helloworld agent.

We’ve recorded a tutorial that will give you a quick look at what is involved in writing agents.

Conventions regarding Incoming Data

As you’ve seen in SimpleRPCClients our clients will send requests like:

mc.echo(:msg => "Welcome to MCollective Simple RPC")

A more complex example might be:

exim.setsender(:msgid => "1NOTVx-00028U-7G", :sender => "[email protected]")

Effectively this creates a hash with the members :msgid and :sender.

Your data types should be preserved if your Security plugin supports that - the default one does - so you can pass in Arrays, Hashes, OpenStructs, Hashes of Hashes but you should always pass something in and it should be key/value pairs like a Hash expects.

You cannot use the a data item called :process_results as this has special meaning to the agent and client. This will indicate to the agent that the client is’nt going to be waiting to process results. You might choose not to send back a reply based on this.

Sample Agent

Here’s our sample Helloworld agent:

 1 module MCollective
 2   module Agent
 3     class Helloworld<RPC::Agent
 4       # Basic echo server
 5       action "echo" do
 6         validate :msg, String
 7 
 8         reply[:msg] = request[:msg]
 9       end
10     end
11   end
12 end

Strictly speaking this Agent will work but isn’t considered complete - there’s no meta data and no help.

A helper agent called rpcutil is included that helps you gather stats, inventory etc about the running daemon. It’s a full SimpleRPC agent including DDL, you can look at it for an example.

Agent Name

The agent name is derived from the class name, the example code creates MCollective::Agent::Helloworld and the agent name would be helloworld.

Meta Data and Initialization

Simple RPC agents still need meta data like in WritingAgents, without it you’ll just have some defaults assigned, code below adds the meta data to our agent:

NOTE: As of version 2.1.1 the metadata section is deprecated, all agents must have DDL files with this information in them.

 1 module MCollective
 2   module Agent
 3     class Helloworld<RPC::Agent
 4       metadata :name        => "helloworld",
 5                :description => "Echo service for MCollective",
 6                :author      => "R.I.Pienaar",
 7                :license     => "GPLv2",
 8                :version     => "1.1",
 9                :url         => "http://projects.puppetlabs.com/projects/mcollective-plugins/wiki",
10                :timeout     => 60
11 
12       # Basic echo server
13       action "echo" do
14         validate :msg, String
15 
16         reply[:msg] = request[:msg]
17       end
18     end
19   end
20 end

The added code sets our creator info, license and version as well as a timeout. The timeout is how long MCollective will let your agent run for before killing them, this is a very important number and should be given careful consideration. If you set it too low your agents will be terminated before their work is done.

The default timeout for SimpleRPC agents is 10.

Writing Actions

Actions are the individual tasks that your agent can do:

1 action "echo" do
2   validate :msg, String
3 
4   reply[:msg] = request[:msg]
5 end

Creates an action called “echo”. They don’t and can’t take any arguments.

Agent Activation

In the past you had to copy an agent only to machines that they should be running on as all agents were activated regardless of dependencies.

To make deployment simpler agents support the ability to determine if they should run on a particular platform. By default SimpleRPC agents can be configured to activate or not:

plugin.helloworld.activate_agent = false

You can also place the following in /etc/mcollective/plugins.d/helloworld.cfg:

activate_agent = false

This is a simple way to enable or disable an agent on your machine, agents can also declare their own logic that will get called each time an agent gets loaded from disk.

module MCollective
  module Agent
    class Helloworld<RPC::Agent

      activate_when do
        File.executable?("/usr/bin/puppet")
      end
    end
  end
end

If this block returns false or raises an exception then the agent will not be active on this machine and it will not be discovered.

When the agent gets loaded it will test if /usr/bin/puppet exist and only if it does will this agent be enabled.

Help and the Data Description Language

We have a separate file that goes together with an agent and is used to describe the agent in detail, a DDL file for the above echo agent can be seen below:

NOTE: As of version 2.1.1 the DDL files are required to be on the the nodes before an agent will be activated

 1 metadata :name        => "echo",
 2          :description => "Echo service for MCollective",
 3          :author      => "R.I.Pienaar",
 4          :license     => "GPLv2",
 5          :version     => "1.1",
 6          :url         => "http://projects.puppetlabs.com/projects/mcollective-plugins/wiki",
 7          :timeout     => 60
 8 
 9 action "echo", :description => "Echos back any message it receives" do
10    input :msg,
11          :prompt      => "Service Name",
12          :description => "The service to get the status for",
13          :type        => :string,
14          :validation  => '^[a-zA-Z\-_\d]+$',
15          :optional    => false,
16          :maxlength   => 30
17 
18    output :msg,
19           :description => "The message we received",
20           :display_as  => "Message"
21 end

As you can see the DDL file expand on the basic syntax adding a lot of markup, help and other important validation data. This information - when available - helps in making more robust clients and also potentially auto generating user interfaces.

The DDL is a complex topic, read all about it in DDL.

Validating Input

If you’ve followed the conventions and put the incoming data in a Hash structure then you can use a few of the provided validators to make sure your data that you received is what you expected.

If you didn’t use Hashes for input the validators would not be usable to you. In future validation will happen automatically based on the DDL so I strongly suggest you follow the agent design pattern shown here using hashes.

In the sample action above we validate the :msg input to be of type String, here are a few more examples:

1    validate :msg, /[a-zA-Z]+/
2    validate :ipaddr, :ipv4address
3    validate :ipaddr, :ipv6address
4    validate :commmand, :shellsafe
5    validate :mode, ["all", "packages"]

The table below shows the validators we support currently

All of these checks will raise an InvalidRPCData exception, you shouldn’t catch this exception as the Simple RPC framework catches those and handles them appropriately.

We’ll make input validators plugins so you can provide your own types of validation easily.

Additionally if can escape strings being passed to a shell, escaping is done in line with the Shellwords#shellescape method that is in newer version of Ruby:

1    safe = shellescape(request[:foo])

As of version 2.2.0 you can add your own types of validation using Validator Plugins.

Agent Configuration

You can save configuration for your agents in the main server config file:

 plugin.helloworld.setting = foo

In your code you can retrieve the config setting like this:

 setting = config.pluginconf["helloworld.setting"] || ""

This will set the setting to whatever is the config file of “” if unset.

Accessing the Input

As you see from the echo example our input is easy to get to by just looking in request, this would be a Hash of exactly what was sent in by the client in the original request.

The request object is in instance of MCollective::RPC::Request, you can also gain access to the following:

Since data is the actual Hash you can gain access to your input like:

 request.data[:msg]

OR

request[:msg]

Accessing it via the first will give you full access to all the normal Hash methods where the 2nd will only give you access to include?.

Running Shell Commands

A helper function exist that makes it easier to run shell commands and gain access to their STDOUT and STDERR.

We recommend everyone use this method for calling to shell commands as it forces LC_ALL to C as well as wait on all the children and avoids zombies, you can set unique working directories and shell environments that would be impossible using simple system that is provided with Ruby.

The simplest case is just to run a command and send output back to the client:

reply[:status] = run("echo 'hello world'", :stdout => :out, :stderr => :err)

Here you will have set reply[:out], reply[:err] and reply[:status] based on the output from the command

You can append the output of the command to any string:

out = []
err = ""
status = run("echo 'hello world'", :stdout => out, :stderr => err)

Here the STDOUT of the command will be saved in the variable out and not sent back to the caller. The only caveat is that the variables out and err should have the « method, so if you supplied an array each line of output will be a single member of the array. In the example out would be an array of lines while err would just be a big multi line string.

By default any trailing new lines will be included in the output and error:

reply[:status] = run("echo 'hello world'", :stdout => :out, :stderr => :err)
reply[:stdout].chomp!
reply[:stderr].chomp!

You can shorten this to:

reply[:status] = run("echo 'hello world'", :stdout => :out, :stderr => :err, :chomp => true)

This will remove a trailing new line from the reply[:out] and reply[:err].

If you wanted this command to run from the /tmp directory:

reply[:status] = run("echo 'hello world'", :stdout => :out, :stderr => :err, :cwd => "/tmp")

Or if you wanted to include a shell Environment variable:

reply[:status] = run("echo 'hello world'", :stdout => :out, :stderr => :err, :environment => {"FOO" => "BAR"})

The status returned will be the exit code from the program you ran, if the program completely failed to run in the case where the file doesn’t exist, resources were not available etc the exit code will be -1

You have to set the cwd and environment through these options, do not simply call chdir or adjust the ENV hash in an agent as that will not be safe in the context of a multi threaded Ruby application.

Constructing Replies

Reply Data

The reply data is in the reply variable and is an instance of MCollective::RPC::Reply.

reply[:msg] = request[:msg]

Reply Status

As pointed out in the ResultsandExceptions page results all include status messages and the reply object has a helper to create those.

def rmmsg_action
  validate :msg, String
  validate :msg, /[a-zA-Z]+-[a-zA-Z]+-[a-zA-Z]+-[a-zA-Z]+/
  reply.fail "No such message #{request[:msg]}", 1 unless have_msg?(request[:msg])

  # check all the validation passed before doing any work
  return unless reply.statuscode == 0

  # now remove the message from the queue
end

The number in reply.fail corresponds to the codes in ResultsandExceptions it would default to 1 so you could just say:

reply.fail "No such message #{request[:msg]}" unless have_msg?(request[:msg])

This is hypothetical action that is supposed to remove a message from some queue, if we do have a String as input that matches our message id’s we then check that we do have such a message and if we don’t we fail with a helpful message.

Technically this will just set statuscode and statusmsg fields in the reply to appropriate values.

It won’t actually raise exceptions or exit your action though you should do that yourself as in the example here.

There is also a fail! instead of just fail it does the same basic function but also raises exceptions. This lets you abort processing of the agent immediately without performing your own checks on statuscode as above later on.

Actions in external scripts

Actions can be implemented using other programming languages as long as they support JSON.

action "test" do
  implemented_by "/some/external/script"
end

The script /some/external/script will be called with 2 arguments:

• The path to a file with the request in JSON format
• The path to a file where you should write your response as a JSON hash

You can also access these 2 file paths in the MCOLLECTIVE_REPLY_FILE and MCOLLECTIVE_REQUEST_FILE environment variables

Simply write your reply as a JSON hash into the reply file.

The exit code of your script should correspond to the ones in ResultsandExceptions. Any text in STDERR will be logged on the server at error level and used in the text for the fail text.

Any text to STDOUT will be logged on the server at level info.

These scripts can be placed in a standard location:

action "test" do
  implemented_by "script.py"
end

This will search each configured libdir for libdir/agent/agent_name/script.py. If you specified a full path it will not try to find the file in libdirs.

Sharing code between agents

Sometimes you have code that is needed by multiple agents or shared between the agent and client. MCollective has name space called MCollective::Util for this kind of code and the packagers and so forth supports it.

Create a class with your shared code given a name like MCollective::Util::Yourco and save this file in the libdir in util/yourco.rb

A sample class can be seen here:

module MCollective
  module Util
    class Yourco
      def dosomething
      end
    end
  end
end

You can now use it in your agent or clients by first loading it from the MCollective lib directories:

MCollective::Util.loadclass("MCollective::Util::Yourco")

helpers = MCollective::Util::Yourco.new
helpers.dosomething

Authorization

You can write a fine grained Authorization system to control access to actions and agents, please see SimpleRPCAuthorization for full details.

Auditing

The actions that agents perform can be Audited by code you provide, potentially creating a centralized audit log of all actions. See SimpleRPCAuditing for full details.

Logging

You can write to the server log file using the normal logger class:

Log.debug ("Hello from your agent")

You can log at levels info, warn, debug, fatal or error.

Data Caching

As of version 2.2.0 there is a system wide Cache you can use to store data that might be costly to create on each request.

The Cache is thread safe and can be used even with multiple concurrent requests for the same agent.

Imagine your agent interacts with a customer database on the node that is slow to read data from but this data does not change often. Using the cache you can arrange for this be read only every 10 minutes:

action "get_customer_data" do
  # Create a new cache called 'customer' with a 600 second TTL,
  # noop if it already exist
  Cache.setup(:customer, 600)

  begin
    customer = Cache.read(:customer, request[:customerid])
  rescue
    customer = Cache.write(:customer, request[:customerid], get_customer(request[:customerid])
  end

  # do something with the customer data
end

Here we setup a new cache table called :customer if it does not already exist, the cache has a 10 minute validity. We then try to read a cached customer record for request[:customerid] and if it’s not been put in the cache before or if it expired I create a new customer record using a method called get_customer and then save it into the cache.

If you have critical code in an agent that can only ever be run once you can use the Mutex from the same cache to synchronize the code:

action "get_customer_data" do
  # Create a new cache called 'customer' with a 600 second TTL,
  # noop if it already exist
  Cache.setup(:customer, 600)

  Cache(:customer).synchronize do
     # Update customer record
  end
end

Here we are using the same Cache that was previously setup and just gaining access to the Mutex protecting the cache data. The code inside the synchronize block will only be run once so you won’t get competing updates to your customer data.

If the lock is held too long by anyone the mcollectived will kill the threads in line with the Agent timeout.

Processing Hooks

We provide a few hooks into the processing of a message, you’ve already used this earlier to set meta data.

You’d use these hooks to add some functionality into the processing chain of agents, maybe you want to add extra logging for audit purposes of the raw incoming message and replies, these hooks will let you do that.

startup_hook

Called at the end of the RPC::Agent standard initialize method use this to adjust meta parameters, timeouts and any setup you need to do.

This will not be called right when the daemon starts up, we use lazy loading and initialization so it will only be called the first time a request for this agent arrives.

before_processing_hook

Called just after a message was received from the middleware before it gets passed to the handlers. request and reply will already be set, the msg passed is the message as received from the normal mcollective runner and the connection is the actual connector.

You can in theory send off new messages over the connector maybe for auditing or something, probably limited use case in simple agents.

after_processing_hook

Called at the end of processing just before the response gets sent to the middleware.

This gets run outside of the main exception handling block of the agent so you should handle any exceptions you could raise yourself. The reason it is outside of the block is so you’ll have access to even status codes set by the exception handlers. If you do raise an exception it will just be passed onto the runner and processing will fail.

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