This plugin uses Gearman to support multiple Jenkins masters. 

{jenkins-plugin-info:pluginId=gearman-plugin|jiraComponent=gearman|sourceDir=gearman-plugin}

** This project is hosted on git.openstack.org **

Summary

We on Openstack infrastructure team use Jenkins extensively. Our jenkins servers, at peak load, runs 20,000+ jobs per day.   At that load we require many jenkins slaves (900+) to process all of those build jobs.  We have found that our requirement was pushing Jenkins beyond it's limits therefore we've decided to create the Gearman Plugin to support multiple Jenkins masters.  The gearman plugin was designed to support extra slaves, allow load balancing of build jobs, and provide redundancy.  

Jenkins core does not support multiple masters.  You can setup multiple Jenkins masters but there is no coordination between them.  One problem with scheduling builds on Jenkins master (“MasterA”) server is that MasterA only knows about its connected slaves.  If all slaves on MasterA are busy then MasterA will just put the next scheduled build on the jenkin server queue.  Now MasterA needs to wait for an available slave to run the build.  This will be very in-efficient if your builds take a long time to run.  So.. what if there is another Jenkins master (“MasterB”) that has free slaves to service the next scheduled build on the server's queue?  Your probably saying.. “Then slaves on MasterB should run the build instead of waiting for slaves on MasterA to run them”, then I would say "good thought!".  However MasterB will never service the builds on MasterA's queue.  The client that schedules the builds must know about MasterB and then schedule builds on MasterB. This is what we mean by lack of coordination between masters. This  gearman-plugin attempts to fill the gap.

This plugin integrates Gearman with Jenkins and will make it so that any Jenkins slave on any Jenkins master can service a job in the queue.   This plugin will essentially replace the Jenkins (master) build queue with the Gearman job queue.  The job will stay in the Gearman queue until there is a Jenkins node (master or slave) that can run that job.  The gearman job queue is shared by multiple jenkins masters therefore gearman can hand out jobs to the next available slave on any jenkins master.

Features

Known Issues

Getting Started

This assumes some familiarity with Jenkins and Gearman

Install

Configuration

Workflow

Starting the Gearman workers:

  1. When the gearman plugin is enabled a gearman worker threads are spawned for each executor on the master and slave nodes.  We'll call these "executor worker threads". Each executor worker thread is associated 1:1 with an executor on a jenkins node.
  2. We spawn one more  Gearman worker thread to handle job management (i.e. abort job/update description/etc..).  We'll call it the "management worker thread" and it will register a "stop:$hostname" and "set_description:$hostname" function with the gearman server.  We use these functions to manage jenkins builds.  
  3. The gearman plugin will register gearman a function for each Gearman executor depending on the projects, labels and nodes that have been setup on the Jenkins master. You can check the registered gearman functions using the administration protocol.  It should look something like this..

Notes:

  1. Red text denotes gearman admin commands
  2. Blue text denotes gearman workers.  There is a default manager worker for the master and an executor worker for a jenkins executor on master.  There are two gearman executor workers for oneiric-668599 slave (exec-0 & exec-1).  These executor workers map to two jenkins executors on the oneiric-668599 slave.  
  3. Functions like "build:guava:ubuntu" map to build:$projectName:$nodeLabel"

Here's the corresponding Jenkins master UI:

Sample Clients

A gearman client can be written in any language.  Here are a few sample clients that work with this plugin

Running a Jenkins build

To  execute a Jenkins job the gearman client just needs to provide the Gearman hostname, port, function, and UUID to start a jenkins build.   

Example:  

python gear_client.py -s MyGearmanServer --function=build:myProject \
    --params='{"OFFLINE_NODE_WHEN_COMPLETE":"false","param1":"moon","param1":"sun"}'

Stopping/aborting a jenkins build

A Gearman request can stop/abort a jenkins build.  

Example: 

python gear_client.py -s MyGearmanServer --function=stop:MyGearmanSever \
    --params='{"name":"myProject","number":"130"}'

The job is stopped differently depending on the current state of the job.  The table below explains the state, transitions and when cancellations happen.

State

Transitions

Cancellation

Gearman queue

Sending a job request to gearman puts it on the gearman queue

the job is removed from the gearman queue

Jenkins queue

jobs on the gearman queue will transition to the jenkins queue

the job is removed from the Jenkins queue

Jenkins executor

job on the jenkins queue transition to the jenkins executor to run

the build is aborted while on the jenkins executor

Updating a build description

You can send a gearman request to update a build's description.  To do this you pass in the following parameters: name of project, build number, description.  

Example:  

python gear_client.py -s MyGearmanSever --function=set_description:MyGearmanSever \
    --params='{"name":"myProject","number":"105","html_description":"<h1>My New Description</h1>"}'

Set slave to offline after a build completes

Our infrastructure employees many 'single use slaves' so what we like to do is run a job and then immediately set the slave offline.  You can do this by passing in the parameter OFFLINE_NODE_WHEN_COMPLETE.

Example:  

python gear_client.py -s MyGearmanSever --function=build:myProject \
    --params='{"OFFLINE_NODE_WHEN_COMPLETE":"true"}'

Configuring Multiple Jenkins Masters

To configure the gearman plugin to work with multiple Jenkins masters you will need to do following:

Now multiple Jenkins masters will be able to service the same jobs.

The typical workflow for this configuration is something like this:

  1. The Gearman workers, running on the Jenkins Masters, are waiting to service the configured jenkins jobs 
  2. A Gearman client submits a request to the Gearman server to run a job.
  3. The Gearman server tells the Gearman worker(s) (on a Jenkins Master) to execute the job(s).  
  4. The first Gearman worker that can service that request will execute the job.  If all workers are busy then the request is placed on the gearman queue to be processed when a gearman worker is available.
  5. The Gearman worker(s) continuously pull jobs off of the Gearman queue and execute each job.  
  6. The Gearman worker reports the job result to the Gearman server when complete.
  7. The Gearman server reports the job result back to the Gearman client.
  8. Loop back to step 1.

Configure logging

Instructions to make the gearman plugin send log messages to the Jenkins logger:

  1. goto http://host:8080/log/levels
  2. add "org.gearman.session.logger" with level "WARNING"
  3. goto http://host:8080/log/all

Now you should see logs from gearman plugin.

Plugin In Action

Plugin In Production

The above images just show how the plugin might work in a simple case.  To see the plugin used in production check out openstack jenkins servers, yes that's servers with an s:

All of the above masters use this plugin which means all of them can run any jobs that are sent to gearman server. We have lots of documentation on how we run the system in production.  

References

Versions