This FAQ was adapted from Quartz Java
What is Quartz
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 dynamically linked library (.dll 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, or a stand-alone application (with an remote interface).
Why not just use System.Timers.Timer?
.NET Framework has "built-in" timer capabilities, through the System.Timers.Timer class - why would someone use Quartz rather than these standard features?
There are many reasons! Here are a few:
- Timers have no persistence mechanism.
- Timers have inflexible scheduling (only able to set start-time & repeat interval, nothing based on dates, time of day, etc.
- Timers don't utilize a thread-pool (one thread per timer)
- Timers have no real management schemes - you'd have to write your own mechanism for being able to remember, organize and retreive your tasks by name, etc.
...of course to some simple applications these features may not be important, in which case it may then be the right decision not to use Quartz.NET.
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, so 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 ADO.NET-based JobStore. The speed of AdoJobStore 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 AdoJobStore?"
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 VM, 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 Java 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.
Questions About Jobs
How can I control the instantiation of Jobs?
See Quartz.Spi.IJobFactory and the Quartz.IScheduler.JobFactory property.
How do I keep a Job from being removed after it completes?
Set the property JobDetail.Durable = 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?
Implement IJob and also decorate your job class with
[DisallowConcurrentExecution] attribute. Read the API documentation for
DisallowConcurrentExecutionAttribute for more information.
Make the job class implement
IStatefulJob rather than
IJob. Read the API documentation for
IStatefulJob for more information.
How do I stop a Job that is currently executing?
Quartz 1.x and 2x: See the
Quartz.IInterruptableJob interface, and the
IScheduler.Interrupt(string, string) method.
Quartz 3.x: See
Questions About Triggers
How do I chain Job execution? Or, how do I create a workflow?
There currently is no "direct" or "free" way to chain triggers with Quartz. However there are several ways you can accomplish it without much effort. 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.
Why isn't my trigger firing?
The most common reason for this is not having called
Scheduler.Start(), which tells the scheduler to start firing triggers.
The second most common reason is that the trigger or trigger group has been paused.
Daylight Saving Time and Triggers
CronTrigger and SimpleTrigger each handle daylight savings time in their own way - each in the way that is intuitive to the trigger type.
First, as a review of what daylight savings time is, please read this resource: http://webexhibits.org/daylightsaving/g.html . Some readers may be unaware that the rules are different for different nations/contents. For example, the 2005 daylight savings time starts in the United States on April 3, but in Egypt on April 29. It is also important to know that not only the dates are different for different locals, but the time of the shift is different as well. Many places shift at 2:00 am, but others shift time at 1:00 am, others at 3:00 am, and still others right at midnight.
SimpleTrigger allows you to schedule jobs to fire every N milliseconds. As such, it has to do nothing in particular with respect to daylight savings time in order to "stay on schedule" - it simply keeps firing every N milliseconds. Regardless your SimpleTrigger is firing every 10 seconds, or every 15 minutes, or every hour or every 24 hours it will continue to do so. However the implication of this which confuses some users is that if your SimpleTrigger is firing say every 12 hours, before daylight savings switches it may be firing at what appears to be 3:00 am and 3:00 pm, but after daylight savings 4:00 am and 4:00 pm. This is not a bug
- the trigger has kept firing exacly every N milliseconds, it just that the "name" of that time that humans impose on that moment has changed.
CronTrigger allows you to schedule jobs to fire at certain moments with respect to a "gregorian calendar". Hence, if you create a trigger to fire every day at 10:00 am, before and after daylight savings time switches it will continue to do so. However, depending on whether it was the Spring or Autumn daylight savings event, for that particular Sunday, the actual time interval between the firing of the trigger on Sundary morning at 10:00 am since its firing on Saturday morning at 10:00 am will not be 24 hours, but will instead be 23 or 25 hours respectively.
There is one additional point users must understand about CronTrigger with respect to daylight savings. This is that you should take careful thought about creating schedules that fire between midnight and 3:00 am (the critical window of time depends on your trigger's locale, as explained above). The reason is that depending on your trigger's schedule, and the particular daylight event, the trigger may be skipped or may appear to not fire for an hour or two. As examples, say you are in the United States, where daylight savings events occur at 2:00 am. If you have a CronTrigger that fires every day at 2:15 am, then on the day of the beginning of daylight savings time the trigger will be skipped, since, 2:15 am never occurs that day. If you have a CronTrigger that fires every 15 minutes of every hour of every day, then on the day daylight savings time ends you will have an hour of time for which no triggerings occur, because when 2:00 am arrives, it will become 1:00 am again, however all of the firings during the one o'clock hour have already occurred, and the trigger's next fire time was set to 2:00 am
- hence for the next hour no triggerings will occur.
In summary, all of this makes perfect sense, and should be easy to remember if you keep these two rules in mind:
- SimpleTrigger ALWAYS fires exacly every N seconds, with no relation to the time of day.
- CronTrigger ALWAYS fires at a given time of day and then computes its next time to fire. If that time does not occur on a given day, the trigger will be skipped. If the time occurs twice in a given day, it only fires once, because after firing on that time the first time, it computes the next time of day to fire on.
Questions About AdoJobStore
How do I improve the performance of AdoJobStore?
There are a few known ways to speed up AdoJobStore, only one of which is very practical.
First, the obvious, but not-so-practical:
- Buy a better (faster) network between the machine that runs Quartz, and the machine that runs your RDBMS.
- Buy a better (more powerful) machine to run your database on.
- Buy a better RDBMS.
Secondly, use driver delegate implementation that is specific to your database, like
SQLServerDelegate, for best performance.
You should also always prefer the latest version of the library. Quartz.NET 2.0 is much more efficient than 1.x series and 2.2.x line again has AdoJobStore related performance improvements over earlier 2.x releases.
Quartz in web environment
Scheduler keeps stopping when application pool gets recycled
By default IIS recycles and stops app pools from time to time. This means that even if you have Application_Start event to start Quartz when web app is being first accessed, the scheduler might get disposed later on due to site inactivity.
If you have a IIS 8 available, you can configure your site to be preloaded and kept running. See this blog post for details.