I am trying to decide if use a java-ee timer in my application or not. The server I am using is Weblogic 10.3.2
The need is: After one hour of a call to an async webservice from an EJB, if the async callback method has not been called it is needed to execute some actions. The information regarding if the callback method has been called and the date of the execution of the call is stored in database.
The two possibilities I see are:
Using a batch process that every half hour looks for all the calls that have been more than one hour without response and execute the needed actions.
Create a timer of one hour after every single call to the ws and in the #Timeout method check if the answer has come and if it has not, execute the required actions.
From a pure programming point of view, it looks easier and cleaner the second one, but I am worry of the performance issues I could have if let's say there are 100.000 Timer created at a single moment.
Any thoughts?
You would be better off having a more specialized process. The real problem is the 100,000 issue. It would depend on how long your actions take.
Because its easy to see that each second, the EJB timer would fire up 30 threads to process all of the current pending jobs, since that's how it works.
Also timers are persistent, so your EJB managed timer table will be saving and deleting 30 rows per second (60 total), this is assuming 100K transactions/hour.
So, that's an lot of work happening very quickly. I can easily see the system simply "falling behind" and never catching up.
A specialized process would be much lighter weight, could perhaps batch the action calls (call 5 actions per thread instead of one per thread), etc. It would be nice if you didn't have to persist the timer events, but that is what it is. You could almost easily simply append the timer events to a file for safety, and keep them in memory. On system restart, you can reload that file, and then roll the file (every hour create a new file, delete the older file after it's all been consumed, etc.). That would save a lot of DB traffic, but you could lose the transactional nature of the DB.
Anyway, I don't think you want to use the EJB Timer for this, I don't think it's really designed for this amount of traffic. But you can always test it and see. Make sure you test restarting your container see how well it works with 100K pending timer jobs in its table.
All depends of what is used by the container. e.g. JBoss uses Quartz Scheduler to implement EJB timer functionality. Quartz is pretty good when you have around 100 000 timer instances.
#Pau: why u need to create a timer for every call made...instead u can have a single timer thread created at start up of application which runs after every half-hour(configurable) period of time and looks in your Database for all web services calls whose response have not been received and whose requested time is past 1 hour. And for selected records, in for loop, it can execute required action.
Well above design may not be useful if you have time critical activity to be performed.
If you have spring framework in your application, you may also look up its timer services.http://static.springsource.org/spring/docs/1.2.9/reference/scheduling.html
Maybe you could use some of these ideas:
Where I'm at, we've built a cron-like scheduler which is powered by a single timer. When the timer fires the system checks which crons need to run using a Quartz CronTrigger. Generally these crons have a lot of work to do, and the way we handle that is each cron spins its individual tasks off as JMS messages, then MDBs handle the messages. Currently this runs on a single Glassfish instance and as our task load increases, we should be able to scale this up with a cluster so multiple nodes are processing the jms messages. We balance the jms message processing load for each type of task by setting the max-pool-size in glassfish-ejb-jar.xml (also known as sun-ejb-jar.xml).
Building a system like this and getting all the details right isn't trivial, but it's proving really effective.
Related
We are currently evaluating quartz-scheduler to use in our project. For our use case, we need only one time trigger to be fired at some point in future, it need not to be a repeatable or cron trigger.
So in my POC, I'm creating a new simple one time trigger when business event occurs. I can see in clustered environment (using JDBC store of quartz), triggers are being balanced/distributed among multiple nodes.
Desired behaviour is observed from POC, but, from performance standpoint, how expensive will it be if we create a new one time trigger each time when we run at scale. From my understanding, one bottleneck would be bloating of database with triggers, possible solution for database cleanup is to add a background task to cleanup old triggers.
I am interested in hearing about experiences and pain points on maintaining scheduler with our design and any inputs for improving design.
You can safely use one-time triggers and they will be automatically removed by Quartz after they have fired. What happens is that Quartz checks all triggers and determines if these triggers are going to fire at some point in the future. If they do not, Quartz simply removes them from the store because it makes no sense to keep them.
A somewhat similar principle applies to jobs. If a job has no associated triggers, Quartz automatically removes it from the store unless the job has the durability flag set to true.
So in your case, you will probably want to register a bunch of durable jobs and then your app will create one-time triggers for these jobs on as needed basis. The jobs will remain in the store and the triggers will be automatically cleaned up when they are done.
I'm building a system where users can set a future date(down to hours and minutes) in calendar. At that date a trigger is calling a certain task, unique for every user.
Every user can set a different date. The system will have 10k+ from the start and a user can create more than one trigger.
So assuming I have 10k users each user create on average 3 triggers => 30k triggers with 30k different dates.
All dates are saved in a database.
I'm new to quartz, can this be done in a more optimized way?
I was thinking about making a task run every minute that will get the tasks that will suppose to run in the next hour and remove them from database.
Do you have any better ideas? Did someone used quartz for a large number of triggers.
You have the schedule backed in the database. If I understand the idea - you want the quartz to load all the upcoming tasks to execute them in the future.
This is problematic approach:
Synchronization Issues: I assume that users can edit, remove and add new tasks to the database. You would have to periodically ask the database to refresh the state of the quartz jobs, remove some jobs, edit other jobs etc. This may not be trivial. The state of the program would be a long living cache which needs to be synchronised often.
Performance and scalability issues: Even if proposed solution may be ok for 30K tasks it may not be ok for 70k or 700k tasks. In your approach it's not easy to scale - adding new machine would require additional layer of synchronisation - which machine should actually execute which job (as all of them have all the tasks).
What I would propose:
Add the "stage" to the Tasks table (new, queued, running, finished, failed)
divide your solution into several components. (Initially they can run on a single machine but it will be easy to scale)
Components:
Task Finder: Executed periodically (once every few seconds). Scans the database for tasks that are "new", and due soon. Sends the tasks found to Message Queue and marks the task as "queued" in the db. Marking as "queued" has to be done carefully as there can be multiple "task finders". (As an addition it may find the tasks that have been marked as "queued" or "running" more than N minutes ago and are not "finished" nor "canceled" - probably need to re-run these)
Message Queue: Connector between Taks Finder and Task Executor.
Task Executor: Listens to the Message Queue and process the tasks that it received. Marks the tasks as "running" initially and "finished" or "failed" later on.
With this approach you can have:
multiple Task Executors on multiple machines
multiple Task Schedulers on multiple machines
even if one of the Task Schedulers or Executors will fail it will not be Single Point of Failure. Some of the tasks will be delayed but it will be picked up and run afterwards.
This may not address all the scenarios but would be a good starting point.
I don't see why you need quartz here at all. As far as I remember, quartz is best used to schedule backend internal processes, not user-defined tasks obtained from db.
Just process the trigger as it is created, save a row to your tasks table with start_date based on the trigger and every second select all incomplete tasks with start_date< sysdate. If the job is repeating, calculate next execution time and insert new task row / update previous accordingly.
As Sam pointed out there are some nice topics addressing the same problem:
Quartz Performance
Quartz FAQ
In a system like the mentioned it should not a problem mostly to handle this amount of triggers. But according to my experiance it is a better way to create something like a "JobChecker". If you enable your users to create own triggers it could really break Quartz in some cases. For example if 5000 user creates an event to the exact same time, Quartz will have a hard time to handle them correctly. (It is not likely a situation that will occur often, but it is possible as your specification does not excludes it.) Quartz has difficulties only when a lot of triggers should be fired at the same time.
My recommendation to this problem is to create one job that is running in every hour/minute etc and that should handle every user set events. This way is simmilar to a cron job in bash. With this kind of processing your system will be pretty stable even if the number of "triggers" increases dramatically. Basically your line of thought is correct if you thrive for scalability.
I have two alarms that fire two separate services--one is executed every half-hour, and the other every midnight. At exactly 00:00 of any day, both services will be executed and will need to share a common file. Is there a way for me to execute the two services one after the other (preferably the half-hour one first then the midnight-basis one) when both alarms are fired?
I've tried making the file accessor methods synchronized (that way the services will have to wait for the other to finish) but that doesn't give me any control on which of the services gets executed first.
2 simple options are:
Have only 1 alarm, every 1/2 hour. Each time it fires, check if you are at Midnight. If you are at Midnight, then run both services.
You can have your alarms run in a single IntentService, or #commonsware's WakefulIntentService. These classes are designed to automatically queue, and not to run simultaneously. That way, when the one service is complete, the other will start automatically i.e. the synchronization is automatic.
Personally, I would use the 1st option (and I have seen it recommended by #commonsware, so I think its probably the best route).
It is simple to do, doesn't rely on too many extra classes, and you have full control over which service gets run at which times. You also only need to have a single alarm, rather than 2, which I think will be easier to maintain, and slightly better on power consumption.
I have an application that checks a resource on the internet for new mails. If there is are new mails it does some processing on them. This means that depending on the amount of mails it might take just a few seconds to hours of processing.
Now the object/program that does the processing is already a singleton. So right now I already took care of there really only being 1 instance that's handling the checking and processing.
However I only have it running once now and I'd like to have it continuously running, checking for new mails more or less every 10 minutes or so to handle them in a timely manner.
I understand I can take care of this with Timer/Timertask or even better I found a resource here: http://www.ibm.com/developerworks/java/library/j-schedule/index.html that uses Scheduler/SchedulerTask. But what I am afraid of.. is if I set it to run every 10 minutes and a previous session is already processing data it will put the new task in a stack waiting to be executed once the previous one is done. So what I'm afraid of is for instance the first run running for 5 hours and then, because it was busy all the time, after that it will launch 5*6-1=29 runs immediately after each other checking for mails and/do some processing without giving the server a break.
Does anyone know how I can solve this?
P.S. the way I have my application set up right now is I'm using a Java Servlet on my tomcat server that's launched upon server start where it creates a Singleton instance of my main program, then calls some method to do the fetching/processing. And what I want is to repeat that fetching/processing every "x" amount of time (10 minutes or so), making sure that really only 1 instance is doing this and that really after each run 10 minutes or so are given to rest.
Actually, Timer + TimerTask can deal with this pretty cleanly. If you schedule something with Timer.scheduleAtFixedRate() You will notice that the docs say that it will attempt to "make up" late events to maintain the long-term period of execution. However, this can be overcome by using TimerTask.scheduledExecutionTime(). The example therein lets you figure out if the task is too tardy to run, and you can just return instead of doing anything. This will, in effect, "clear the queue" of TimerTask.
Of note: TimerTask uses a single thread to execute, so it won't spawn two copies of your task side-by-side.
On the side note part, you don't have to process all 10k emails in the queue in a single run. I would suggest processing for a fixed amount of time using TimerTask.scheduledExecutionTime() to figure out how long you have, then returning. That keeps your process more limber, cleans up the stack between runs, and if you are doing aggregates, ensures that you don't have to rebuild too much data if, for example, the server is restarted in the middle of the task. But this recommendation is based on generalities, since I don't know what you're doing in the task :)
I have an app that runs over several instances and all requests come through one servlet.
I need to run a cron job which executes once a week for about 3 minutes. During that cron call some kind of flag/boolean will be modified somewhere so that the servlet can pick up and send an "server temporarily unavailable" type message back instead of processing the request. Once the cron job is complete it will flag it back to true.
I cannot use a singleton or a static boolean as the app will be in multiple instances. Nor do I want the servlet to have to fetch a value from the datastore on every request, as it will mean hundreds of thousands of extra datastore reads.
What can I do? Any ideas?
I think you may be able to store boolean in memcached. GAE has a Cache API for Memcached. However note that cache values are not persistent and may not be survived for even 3 minutes. I think you should have a firm time to start cron task hardcoded in one of your Java classes or .properties file and then when your task finishes, it should look at that hard-coded time and schedule itself for next round according to that time.
And by this way your servlet can also look at that time and do not serve requests in the interval you are going to specify. Yeah, that will be very fast but your jobs will be scheduled to a fixed time periodically and you won't be able to change this unless you re-deploy application.
I think the better solution is you should keep the boolean in the datastore and make use of cache. See the following algorithm:
is my boolean in the cache?
yes:
[alright, then choose to serve or not to serve request using it.]
no:
[fetch variable from datastore and put it on the cache.] (cache miss)
Again, cache will be fast, but not as much as hard-coding the schedule in the program.
EDIT: Another solution. (however not possible to implement)
If you want to serving pages during the task execution, you should use a task api
First of all you should be familiar with using countdown for your task (in this case next week) http://code.google.com/appengine/docs/java/javadoc/com/google/appengine/api/taskqueue/TaskOptions.html#countdownMillis(long)
Then you can use size() method of Queue – which I was expecting it to be there but apparently Google didn't implement it– to see if task queue size is 0, then it means it is processed right now because when the task finishes, it submits itself again to 1 week later.
One approach would be to have the cron job publish a message to a JMS topic to which all the servlet instances were listening. The messages could inform the servlet instances to set a value in the static boolean you mentioned to true or false.