Job scheduling is used on many software projects to enable both internal jobs and third-party integration. Clustering can provide a huge boost to reliability by providing fail-over and load-sharing. I believe that clustering should be implemented for reliability on just about all software projects, so I’ve decided to outline how to go about doing that in two popular cluster-enabled Java job schedulers. This post is going to cover how to set up clustering for Quartz and Obsidian. It will explain what work is required to configure each, and help you watch for some common pitfalls. This guide will assume you have both schedulers running in the base configuration already.
Both Quartz and Obsidian have their strong points, and this post won’t debate which is better, but it will provide the information you need to cluster either one.
Quartz is the most popular open-source job scheduling option, and it allows for you to cluster scheduler instances via the JDBCJobStore. Though it’s also possible to do clustering with Terracotta without a persistent job store, I recommend against it for most projects since job execution history is very useful to ongoing operations and troubleshooting, and database-clustering performs adequately in all but extreme cases.
Obsidian is a commercial job scheduler which provides free individual instances, and a single clustering licence free for a year. It provides a similar type of clustering as Quartz, and it also provides a full UI, a REST API, downtime recovery, any many other advanced configuration options.
Configurating Obsidian for Clustering
I’ll cover Obsidian first simply because there’s little to do in comparison to Quartz. Since running Obsidian always uses a database, if you have an instance running, there will be no database configuration to update. If you haven’t set it up yet, check out the Getting Started guide – the installation package comes with an interactive Ant tool to build the properties file for you.
So here’s the thing: to cluster Obsidian, simply start up additional instances. Obsidian doesn’t have a non-clustered mode, and all instances automatically handle adjusting the load-sharing algorithm when new members join the cluster (or drop out). Your system clocks should be roughly in sync, but if they differ by a few seconds, it’s not a problem since Obsidian gracefully handles this. Still, you may synchronize your server times if desired.
Note to those using the free version: right after download, you can start two instances and they will automatically join the same cluster. If you do not have adequate licences, new members will not be able to join the cluster.
There’s no need to deploy different properties files or anything like that. As an example, if you use Amazon’s EC2 service, you could use the same image for multiple nodes in the cluster and everything would work correctly. Each cluster member will automatically assign itself a unique instance name based on its local host name and a unique suffix if required.
However, we recommend you assign each cluster member an explicit name which will help with troubleshooting if there are issues on a specific host. To do so, simply set the Java system property
schedulerDesignation to the host name of your choice. For example, if starting an instance using the standalone scheduler using java directly, simply add the value
-DschedulerDesignation=myHostName to the end of the command.
That’s it! Clustering Obsidian is literally as easy as copying an installation and starting it on another server or virtual machine. As a bonus, unlike other schedulers, in Obsidian you can set a job to only run on a specific host, even with clustering enabled.
In summary, the steps are:
- Grab a copy of your Obsidian installation (WAR, standalone package or embedded application bundle).
- Start it up! At this step you can provide the
schedulerDesignationyou wish to use.
Configurating Quartz for Clustering
For Quartz, we’re only going to cover configuring database clustering since we believe it is the right choice to most projects.
Note before you get started: If you have a non-clustered version of Quartz running, you must shut it down before starting any cluster-enabled instances.
Another note about timing: Since Quartz uses very aggressive timing, you must ensure your different instances have precisely synchronized times. Even a second difference will cause your load-sharing to be effectively disabled and all jobs will run on a single host.
Setting up clustering for Quartz can be a bit overwhelming since it exposes so many properties, and it’s hard to figure out which must be added to your properties file to get up and running, but I will try to simplify the process as much as possible.
Quartz is configured via the
quartz.properties file. Here’s a sample file that outlines the bare minimum properties you will have to configure for clustering. If you want to see full details on any portion of the config, see the Quartz configuration reference. Note that the properties file should be identical on all hosts, with the one exception being the
org.quartz.scheduler.instanceId which is used to identify different hosts.
# Basic Quartz configuration to provide an adequate pool of threads for execution org.quartz.threadPool.class = org.quartz.simpl.SimpleThreadPool org.quartz.threadPool.threadCount = 25 # Datasource for JDBCJobStore org.quartz.dataSource.myDS.driver =com.mysql.jdbc.Driver org.quartz.dataSource.myDS.URL = jdbc:mysql://localhost:3306/scheduler org.quartz.dataSource.myDS.user = myUser org.quartz.dataSource.myDS.password = myPassword # JDBCJobStore org.quartz.jobStore.class = org.quartz.impl.jdbcjobstore.JobStoreTX org.quartz.jobStore.driverDelegateClass = org.quartz.impl.jdbcjobstore.StdJDBCDelegate org.quartz.jobStore.dataSource = myDS # Turn on clustering org.quartz.jobStore.isClustered = true org.quartz.scheduler.instanceName = ClusteredScheduler # If instanceIf is set to AUTO, if will auto-generate an id automatically. # I recommend giving explicit names to each clustered host for easy identification. org.quartz.scheduler.instanceId = Host1
Note about JDBC settings: For whatever reason, you have to configure both the JDBC driver class and the job store’s “driver delegate” class. These will have to be set to the appropriate value for your database platform.
As I mentioned, these are the bare minimum properties you will have to configure to get clustering running. The one big pain with this configuration is that the
instanceId which identifies hosts resides in the same properties file as all the other properties which should all remain the same. Keeping different versions in sync can problematic, and isn’t required for Obsidian. You can use the “AUTO” setting to avoid having to set explicit instance IDs, but as with Obsidian, we recommend you give explicit names as it can help you locate where issues are happening more quickly if you know up front which host is which.
So the main steps to enabling clustering are:
- Prepare properties for each cluster member. Ensure only the
org.quartz.scheduler.instanceIdvaries in the properties file.
- Turn off all running instances by shutting down the application which is running it, or disabling just the Quartz process (see here).
- Start your application, or start the Quartz process (see here).
- Ensure that you never start a non-clustered instance again!
I hope this helps you get off the ground quickly with your new or existing project. Clustering is a great feature in any scheduler, and I feel it provides a lot of value that software and operations teams might be missing out on.