Quartz is a job scheduling system that can be integrated with, or used along side virtually any other software system. The term "job scheduler" seems to conjure different ideas for different people. As you read this tutorial, you should be able to get a firm idea of what we mean when we use this term, but in short, a job scheduler is a system that is responsible for executing (or notifying) other software components when a pre-determined (scheduled) time arrives.
Quartz is quite flexible, and contains multiple usage paradigms that can be used separately or together, in order to achieve your desired behavior, and enable you to write your code in the manner that seems most 'natural' to your project.
Quartz is very light-weight, and requires very little setup/configuration - it can actually be used 'out-of-the-box' if your needs are relatively basic.
Quartz is fault-tolerant, and can persist ('remember') your scheduled jobs between system restarts.
Although Quartz is extremely useful for simply running certain system processes on given schedules, the full potential of Quartz can be realized when you learn how to use it to drive the flow of your application's business processes.
What is Quartz - From a Software Component View?
Quartz is distributed as a small java library (.jar file) that contains all of the core Quartz functionality. The main interface (API) to this functionality is the Scheduler interface. It provides simple operations such as scheduling/unscheduling jobs, starting/stopping/pausing the scheduler.
If you wish to schedule your own software components for execution they must implement the simple Job interface, which contains the method execute(). If you wish to have components notified when a scheduled fire-time arrives, then the components should implement either the TriggerListener or JobListener interface.
The main Quartz 'process' can be started and ran within your own application, as a stand-alone application (with an RMI interface), or within a J2EE app. server to be used as a resource by your J2EE components.
Why not just use java.util.Timer?
Since JDK 1.3, Java has "built-in" timer capabilities, through the java.util.Timer and java.util.TimerTask classes - why would someone use Quartz rather than these standard features?
There are many reasons! Here are a few:
How do I build the Quartz source?
Although Quartz ships "pre-built" many people like to make their own alterations and/or build the latest 'non-released' version of Quartz from CVS. To do this, follow the instructions in the "README.TXT" file that ships with Quartz.
How many jobs is Quartz capable of running?
This is a tough question to answer... the answer is basically "it depends".
I know you hate that answer, to here's some information about what it depends "on".
First off, the JobStore that you use plays a significant factor. The RAM-based JobStore is MUCH (1000x) faster than the JDBC-based JobStore. The speed of JDBC-JobStore depends almost entirely on the speed of the connection to your database, which data base system that you use, and what hardware the database is running on. Quartz actually does very little processing itself, nearly all of the time is spent in the database. Of course RAMJobStore has a more finite limit on how many Jobs & Triggers can be stored, as you're sure to have less RAM than hard-drive space for a database. You may also look at the FAQ "How do I improve the performance of JDBC-JobStore?"
So, the limitting factor of the number of Triggers and Jobs Quartz can "store" and monitor is really the amount of storage space available to the JobStore (either the amount of RAM or the amount of disk space).
Now, aside from "how many can I store?" is the question of "how many jobs can Quartz be running at the same moment in time?"
One thing that CAN slow down quartz itself is using a lot of listeners (TriggerListeners, JobListeners, and SchedulerListeners). The time spent in each listener obviously adds into the time spent "processing" a job's execution, outside of actual execution of the job. This doesn't mean that you should be terrified of using listeners, it just means that you should use them judiciously - don't create a bunch of "global" listeners if you can really make more specialized ones. Also don't do "expensive" things in the listeners, unless you really need to. Also be mindful that many plug-ins (such as the "history" plugin) are actually listeners.
The actual number of jobs that can be running at any moment in time is limitted by the size of the thread pool. If there are five threads in the pool, no more than five jobs can run at a time. Be careful of making a lot of threads though, as the JVM, Operating System, and CPU all have a hard time juggling lots of threads, and performance degrades just because of all of the management. In most cases performance starts to tank as you get into the hundreds of threads. Be mindful that if you're running within an application server, it probably has created at least a few dozen threads of its own!
Aside from those factors, it really comes down to what your jobs DO. If your jobs take a long time to complete their work, and/or their work is very CPU-intensive, then you're obviously not going to be able to run very many jobs at once, nor very many in a given spanse of time.
Finally, if you just can't get enough horse-power out of one Quartz instance, you can always load-balance many Quartz instances (on separate machines). Each will run the jobs out of the shared database on a first-come first-serve basis, as quickly as the triggers need fired.
So here you are this far into the answer of "how many", and I still haven't given you a number ;-) And I really hate to, because of all of the variables mentioned above. So let me just say, there are installments of Quartz out there that are managing hundreds-of-thousands of Jobs and Triggers, and that at any given moment in time are executing dozens of jobs -- and this excludes using load-balancing. With this in mind, most people should feel confident that they can get the performance out of Quartz that they need.
I'm having issues with using Quartz via RMI -- HELP!?!
RMI can be a bit problematic, especially if you don't have an understanding of how class loading via RMI works. I highly recommend reading all of the JavaDOC available about RMI, and strongly suggest you read the following references, dug up by a kind Quartz user (Mike C):
An awesome description of RMI and codebase: http://www.kedwards.com/jini/codebase.html. One of the important points is to realize that "codebase" is used by the client!
Quick info about security managers: http://gethelp.devx.com/techtips/java_pro/10MinuteSolutions/10min0500.asp.
And finally from the java API docs read the docs for the RMISecurityManager... http://java.sun.com/j2se/1.3/docs/api/java/rmi/RMISecurityManager.html. One of the important points of this document is: "RMI's class loader will not download any classes from remote locations if no security manager has been set."
How do I keep a Job from being removed after it completes?
Set the property JobDetail.setDurability(true) - which instructs Quartz not to delete the Job when it becomes an "orphan" (when the Job not longer has a Trigger referencing it).
How do I keep a Job from firing concurrently?
Make the job class implement StatefulJob rather than Job. Read the JavaDOC for StatefulJob for more information.
How do I chain Job execution? Or, how do I create a workflow?
There currently is no "easy" or "free" way to chain triggers with Quartz. However there are several ways you can accomplish it, below is an outline of a couple approaches:
One way is to use a listener (i.e. a TriggerListener, JobListener or SchedulerListener) that can notice the completion of a job/trigger and then immediately schedule a new trigger to fire. This approach can get a bit involved, since you'll have to inform the listener which job follows which - and you may need to worry about persistence of this information.
Another way is to build a Job that contains within its JobDataMap the name of the next job to fire, and as the job completes (the last step in its execute() method) have the job schedule the next job. Several people are doing this and have had good luck. Most have made a base (abstract) class that is a Job that knows how to get the job name and group out of the JobDataMap using special keys (constants) and contains code to schedule the identified job. Then they simply make extensions of this class that included the additional work the job should do.
In the future, Quartz will provide a much cleaner way to do this, but until then, you'll have to use one of the above approaches, or think of yet another that works better for you.
The most common reason for this is not having called Scheduler.start(), which tells the scheduler to start firing triggers.
How do I improve the performance of JDBC-JobStore?
There are a few known ways to speed up JDBC-JobStore, only one of which is very practical.
First, the obvious, but not-so-practicle: (1) Buy a better (faster) network between the machine that runs Quartz, and the machine that runs your RDBMS. (2) Buy a better (more powerful) machine to run your database on (3) Buy a better RDBMS.
Now for something simple, but effective: Build indexes on the Quartz tables.
Most database systems will automatically put indexes on the primary-key fields, many will also automatically do it for the foreign-key field. Make sure yours does this, or make the indexes on all key fields of every table manually.
Next, manually add some additional indexes: most important to index are the TRIGGER table's "next_fire_time" and "state" fields. Last (but not as important), add indexes to every column on the FIRED_TRIGGERS table.
My DB Connections don't recover properly if the database server is restarted.
If you're having Quartz create the connection data source (by specifying the connection parameters in the quartz properties file) make sure you have a connection validation query specified, such as:
If you're datasource is managed by your application server, make sure the datasource is configured in such a way that it can detect failed connections.