The ASP.NET runtime is meant for short work loads that can be run in parallel. I need to be able to schedule periodic events and background tasks that may or may not run for much longer periods.
Given the above I have the following problems to deal with:
The AppDomain can shutdown due to changes (Web.config, bin, App_Code, etc.)
IIS recycles the AppPool on a regular basis (daily)
IIS itself might restart, or for that matter the server might crash
I'm not convinced that running this code inside ASP.NET is not the right thing to do, becuase it would allow for a simpler programming model. But doing so would require that an external service periodically makes requests to the app so that the application is keept running and that all background tasks are programmed with utter most care. They will have to be able to pause and resume thier work, in the event of an unexpected error.
My current line of thinking goes something like this:
If all jobs are registered in the database, it should be possible to use the database as a bookkeeping mechanism. In the case of an error, the database would contain all state necessary to resume the operation at the next opportunity given.
I'd really appriecate some feedback/advice, on this matter. I've been considering running a windows service and using some RPC solution as well, but it doesn't have the same appeal to me. And I'd instead have a lot of deployment issues and sycnhronizing tasks and code cross several applications. Due to my business needs this is less than optimial.
This is a shot in the dark since I don't know what database you use, but I'd recommend you to consider dialog timers and activation. Assuming that most of the jobs have to do some data manipulation, and is likely that all have to do only data manipulation, leveraging activation and timers give an extremely reliable job scheduling solution, entirely embedded in the database (no need for an external process/service, not dependencies outside the database bounds like msdb), and is a solution that ensures scheduled jobs can survive restarts, failover events and even disaster recovery restores. Simply put, once a job is scheduled it will run even if the database is restored one week later on a different machine.
Have a look at Asynchronous procedure execution for a related example.
And if this is too radical, at least have a look at Using Tables as Queues since storing the scheduled items in the database often falls under the 'pending queue' case.
I recommend that you have a look at Quartz.Net. It is open source and it will give you some ideas.
Using the database as a state-keeping mechanism is a completely valid idea. How complex it will be depends on how far you want to take it. In many cases you will ended up pairing your database logic with a Windows service to achieve the desired result.
FWIW, it is typically not a good practice to manually use the thread pool inside an ASP.Net application, though (contrary to what you may read) it actually works quite nicely other than the huge caveat that you can't guarantee it will work.
So if you needed a background thread that examined the state of some object every 30 seconds and you didn't care if it fired every 30 seconds or 29 seconds or 2 minutes (such as in a long app pool recycle), an ASP.Net-spawned thread is a quick and very dirty solution.
Asynchronously fired callbacks (such as on the ASP.Net Cache object) can also perform a sort of "behind the scenes" role.
I have faced similar challenges and ultimately opted for a Windows service that uses a combination of building blocks for maximum flexibility. Namely, I use:
1) WCF with implementation-specific types OR
2) Types that are meant to transport and manage objects that wrap a job OR
3) Completely generic, serializable objects contained in a custom wrapper. Since they are just a binary payload, this allows any object to be passed to the service. Once in the service, the wrapper defines what should happen to the object (e.g. invoke a method, gather a result, and optionally make that result available for return).
Ultimately, the web site is responsible for querying the service about its state. This querying can be as simple as polling or can use asynchronous callbacks with WCF (though I believe this also uses some sort of polling behind the scenes).
I tell you what I have do.
I have create a class called Atzenta that have a timer (1-2 second trigger).
I have also create a table on my temporary database that keep the jobs. The table knows the jobID, other parameters, priority, job status, messages.
I can add, or delete a job on this class. When there is no action to be done the timer is stop. When I add a job, then the timer starts again. (the timer is a thread by him self that can do parallel work). I use the System.Timers and not other timers for this.
The jobs can have different priority.
Now let say that I place a job on this table using the Atzenta class. The next time that the timer is trigger is check the query on this table and find the first available job and just run it. No other jobs run until this one is end.
Every synchronize and flags are done from the table. In the table I have flags for every job that show if its |wait to run|request to run|run|pause|finish|killed|
All jobs are all ready known functions or class (eg the creation of statistics).
For stop and start, I use the global.asax and the Application_Start, Application_End to start and pause the object that keep the tasks. For example when I do a job, and I get the Application_End ether I wait to finish and then stop the app, ether I stop the action, notify the table, and start again on application_start.
So I say, Atzenta.RunTheJob(Jobs.StatisticUpdate, ProductID); and then I add this job on table, open the timer, and then on trigger this job is run and I update the statistics for the given product id.
I use a table on a database to synchronize many pools that run the same web app and in fact its work that way. With a common table the synchronize of the jobs is easy and you avoid 2 pools to run the same job at the same time.
On my back office I have a simple table view to see the status of all jobs.
Related
I’m working on a business problem which has to import files which has 1000s of records. Each record has to be registered in a Workflow as individual record which has to go through its own workflow.
WF4 Corporate Purchase Process example has a good solution, as in the first step it create bookmarks for all the required record ids. So the workflow can be resumed with rest of the actions for each individual record/id.
I would like to know how to implement same thing using Workflow services as I could get the benefits of AppFabric for my workflows.
Is there any other solutions to handle batch of records/ids? Otherwise workflow service has to be called 1000s of times just to register every record in a workflow instance which is a not a good solution.
I would like to know how to implement same thing using Workflow services as I could get the benefits of AppFabric for my workflows.
This is pretty straight forward. You're going to have one workflow that reads the file and loops through the results using the looping activities that exist. Then, inside the loop you'll be starting up the workflow that each record needs (the "Service") by calling the endpoint with a Send activity.
Now, as for the workflow that is the Service, you're going to have a Receive activity at the top of the workflow that also has CanCreateInstance set the true. The everything after the Receive is no different than any other workflow. You may consider having a Send activity right after the Receive just to let the caller know that the Service has been started. But that's not a requirement -- the Receive will be required because it forces WF to build the workflow to use the WorkflowServiceHost.
Is there any other solutions to handle batch of records/ids? Otherwise workflow service has to be called 1000s of times just to register every record in a workflow instance which is a not a good solution.
Are you indicating that a for a web server to receive 1000's of requests is not a good solution? Consider the fact that an IIS server can handle roughly 25-50 requests, per instant in time, per core. Now consider the fact that you're loop that's loading the workflows isn't going to average more than maybe 5 in that instant of time but probably more like 1 or 2.
I don't think the web server is going to be your issue. I've started up literally 10,000's of workflows on a server via a loop just like the one you're going to build and it didn't break a sweat.
One way would be to use WCF's MSMQ binding to launch your workflows. Requests can come in normally through HTTP, and WCF would route them to MSMQ and process the load. You can throttle how many workflow instances are used through the MSMQ binding + IIS settings.
Download this word document that describes setting up a workflow application with WCF and MSMQ: http://www.microsoft.com/en-us/download/details.aspx?id=21245
In the spirit of the doing the simplest thing that could work, you can bring the subworkflow in as an activity to the main workflow and use a parallel for each to execute the branch for each input from your file. No extra invoking is required and the tooling supports this out of the box because all workflows are activities. Hosting the main process in a service so you can avoid contention with the rest of your IIS users, real people that they may are, might be a good idea.
I do agree that calling IIS or a WCF service 1000's of times is not a problem though, unless you want to do it in a few seconds!
It is important to remember that one of the good things about workflow is that it has fairly low overhead (compared to other workflow products) so you should be more concerned about what your workflow does than just the idea of launching lots of instances. The idea of batches like your example is very common.
I would like to know the best way to deal with long running processes started on demand from an ASP.NET webpage.
The process may consist of various steps (like upload files to the server, run SSIS packages on them, execute some stored procedures etc.) and sometimes the process could take up to couple of hours to finish.
If I go for asynchronous execution using a WCF service, then what happens if the user closes the browser while the process is running, how the process success or failure result should be displayed to the user? To solve this, I choose one-way WCF service calls, but the problem with this is I need to create a process table and store the result (and error messages if it fails in any of the steps and which steps have completed successfully) in that table which is an additional overhead because there are many such processes with various steps that the user can invoke from the web page and user needs to be made aware of the progress (in simplest case, the status can be "process xyz running") and once it is done, the output needs to be displayed to the user (for example by running a stored procedure).
What is the best way to design the solution for this?
As I see it, you have three options
Have a long running page where the user waits for the response. If this is several hours, you're going to have many usability problems, so I wouldn't even consider it.
Create a process table to store the results of operations. Run service functions asynchronously and delegate logging the results to the service. There can be a page that the user refreshes which gets the latest results of this table.
If you really don't want to create a table, then store all the current process details in the users' session state, and have a current processes page as above. You have the possible issue that the session might timeout, or the web app might restart and you'll lose all this.
I can't see that number 2 is such a great hardship. You could make the table fairly generic to encompass all types of processes: process details could just be encoded as binary or xml and interpreted by the web application. You then have the most robust solution.
I cant say what the best way would be but using Windows Workflow Foundation for such long running processes is definitely one way to go about it.
You can do tracking of the process to see what stage it is at, even persist it if you have steps where it is awaiting user input etc.
WF provides a lot of features out of the box (especially if your storage medium is SQL Server) and may be a good option to consider.
http://www.codeproject.com/KB/WF/WF4Extensions.aspx might help give you some more insight into the same.
I think you are in the right track. You should run the process asynchronously, store the execution somewhere (a table), and keep status of the running process in there.
Your user should see a pending display label while the process is executing, and a finished label with the result when the process finished. If the user closed the browser, she will see the result of her running process next time she logs in.
A part of the application I'm working on is an swf that shows a test with some 80 questions. Each question is saved in SQL Server through WebORB and ASP.NET.
If a candidate finishes the test, the session needs to be validated. The problem is that sometimes 350 candidates finish their test at the same moment, and the CPU on the web server and SQL Server explodes (350 validations concurrently).
Now, how should I implement queuing here? In the database, there's a table that has a record for each session. One column holds the status. 1 is finished, 2 is validated.
I could implement queuing in two ways (as I see it, maybe you have other propositions):
A process that checks the table for records with status 1. If it finds one, it validates the session. So, sessions are validated one after one.
If a candidate finishes its session, a message is sent to a MSMQ queue. Another process listens to the queue and validates sessions one after one.
Now:
What would be the best approach?
Where do you start the process that will validate sessions? In your global.asax (application_start)? As a windows service? As an exe on the root of the website that is started in application_start?
To me, using the table and looking for records with status 1 seems the easiest way.
The MSMQ approach decouples your web-facing application from the validation logic service and the database.
This brings many advantages, a few of which:
It would be easier to handle situations where the validation logic can handle 5 sessions per second, and it receives 300 all at once. Otherwise you would have to handle copmlicated timeouts, re-attempts, etc.
It would be easier to do maintanance on the validation service, without having to interrupt the rest of the application. When the validation service is brought down, messages would queue up in MSMQ, and would get processed again as soon as it is brought up.
The same as above applies for database maintanance.
If you don't have experience using MSMQ and no infrastructrure set up, I would advice against it. Sure, it might be the "proper" way of doing queueing on the Microsoft platform, but it is not very straight-forward and has quite a learning curve.
The same goes for creating a Windows Service; don't do it unless you are familiar with it. For simple cases such as this I would argue that the pain is greater than the rewards.
The simplest solution would probably be to use the table and run the process on a background thread that you start up in global.asax. You probably also want to create an admin page that can report some status information about the process (number of pending jobs etc) and maybe a button to restart the process if it for some reason fails.
What is validating? Before working on your queuing strategy, I would try to make validating as fast as possible, including making it set based if it isn't already so.
I have recently been investigating this myself so wanted to mention my findings. The location of the Database in comparison to your application is a big factor on deciding which option is faster.
I tested inserting the time it took to insert 100 database entries versus logging the exact same data into a local MSMQ message. I then took the average of the results of performing this test several times.
What I found was that when the database is on the local network, inserting a row was up to 4 times faster than logging to an MSMQ.
When the database was being accessed over a decent internet connection, inserting a row into the database was up to 6 times slower than logging to an MSMQ.
So:
Local database - DB is faster, otherwise MSMQ is.
I know there's a bunch of APIs out there that do this, but I also know that the hosting environment (being ASP.NET) puts restrictions on what you can reliably do in a separate thread.
I could be completely wrong, so please correct me if I am, this is however what I think I know.
A request typically timeouts after 120 seconds (this is configurable) but eventually the ASP.NET runtime will kill a request that's taking too long to complete.
The hosting environment, typically IIS, employs process recycling and can at any point decide to recycle your app. When this happens all threads are aborted and the app restarts. I'm however not sure how aggressive it is, it would be kind of stupid to assume that it would abort a normal ongoing HTTP request but I would expect it to abort a thread because it doesn't know anything about the unit of work of a thread.
If you had to create a programming model that easily and reliably and theoretically put a long running task, that would have to run for days, how would you accomplish this from within an ASP.NET application?
The following are my thoughts on the issue:
I've been thinking a long the line of hosting a WCF service in a win32 service. And talk to the service through WCF. This is however not very practical, because the only reason I would choose to do so, is to send tasks (units of work) from several different web apps. I'd then eventually ask the service for status updates and act accordingly. My biggest concern with this is that it would NOT be a particular great experience if I had to deploy every task to the service for it to be able to execute some instructions. There's also this issue of input, how would I feed this service with data if I had a large data set and needed to chew through it?
What I typically do right now is this
SELECT TOP 10 *
FROM WorkItem WITH (ROWLOCK, UPDLOCK, READPAST)
WHERE WorkCompleted IS NULL
It allows me to use a SQL Server database as a work queue and periodically poll the database with this query for work. If the work item completed with success, I mark it as done and proceed until there's nothing more to do. What I don't like is that I could theoretically be interrupted at any point and if I'm in-between success and marking it as done, I could end up processing the same work item twice. I might be a bit paranoid and this might be all fine but as I understand it there's no guarantee that that won't happen...
I know there's been similar questions on SO before but non really answers with a definitive answer. This is a really common thing, yet the ASP.NET hosting environment is ill equipped to handle long-running work.
Please share your thoughts.
Have a look at NServiceBus
NServiceBus is an open source
communications framework for .NET with
build in support for publish/subscribe
and long-running processes.
It is a technology build upon MSMQ, which means that your messages don't get lost since they are persisted to disk. Nevertheless the Framework has an impressive performance and an intuitive API.
John,
I agree that ASP.NET is not suitable for Async tasks as you have described them, nor should it be. It is designed as a web hosting platform, not a back of house processor.
We have had similar situations in the past and we have used a solution similar to what you have described. In summary, keep your WCF service under ASP.NET, use a "Queue" table with a Windows Service as the "QueueProcessor". The client should poll to see if work is done (or use messaging to notify the client).
We used a table that contained the process and it's information (eg InvoicingRun). On that table was a status (Pending, Running, Completed, Failed). The client would submit a new InvoicingRun with a status of Pending. A Windows service (the processor) would poll the database to get any runs that in the pending stage (you could also use SQL Notification so you don't need to poll. If a pending run was found, it would move it to running, do the processing and then move it to completed/failed.
In the case where the process failed fatally (eg DB down, process killed), the run would be left in a running state, and human intervention was required. If the process failed in an non-fatal state (exception, error), the process would be moved to failed, and you can choose to retry or have human intervantion.
If there were multiple processors, the first one to move it to a running state got that job. You can use this method to prevent the job being run twice. Alternate is to do the select then update to running under a transaction. Make sure either of these outside a transaction larger transaction. Sample (rough) SQL:
UPDATE InvoicingRun
SET Status = 2 -- Running
WHERE ID = 1
AND Status = 1 -- Pending
IF ##RowCount = 0
SELECT Cast(0 as bit)
ELSE
SELECT Cast(1 as bit)
Rob
Use a simple background tasks / jobs framework like Hangfire and apply these best practice principals to the design of the rest of your solution:
Keep all actions as small as possible; to achieve this, you should-
Divide long running jobs into batches and queue them (in a Hangfire queue or on a bus of another sort)
Make sure your small jobs (batched parts of long jobs) are idempotent (have all the context they need to run in any order). This way you don't have to use a quete which maintains a sequence; because then you can
Parallelise the execution of jobs in your queue depending on how many nodes you have in your web server farm. You can even control how much load this subjects your farm to (as a trade off to servicing web requests). This ensures that you complete the whole job (all batches) as fast and as efficiently as possible, while not compromising your cluster from servicing web clients.
Have thought about the use the Workflow Foundation instead of your custom implementation? It also allows you to persist states. Tasks could be defined as workflows in this case.
Just some thoughts...
Michael
We have an ASP.Net application that provides administrators to work with and perform operations on large sets of records. For example, we have a "Polish Data" task that an administrator can perform to clean up data for a record (e.g. reformat phone numbers, social security numbers, etc.) When performed on a small number of records, the task completes relatively quickly. However, when a user performs the task on a larger set of records, the task may take several minutes or longer to complete. So, we want to implement these kinds of tasks using some kind of asynchronous pattern. For example, we want to be able to launch the task, and then use AJAX polling to provide a progress bar and status information.
I have been looking into using the BackgroundWorker class, but I have read some things online that make me pause. I would love to get some additional advice on this.
For example, I understand that the BackgroundWorker will actually use the thread pool from the current application. In my case, the application is an ASP.Net web site. I have read that this can be a problem because when the application recycles, the background workers will be terminated. Some of the jobs I mentioned above may take 3 minutes, but others may take a few hours.
Also, we may have several hundred administrators all performing similar operations during the day. Will the ASP.Net application thread pool be able to handle all of these background jobs efficiently while still performing it's normal request processing?
So, I am trying to determine if using the BackgroundWorker class and approach is right for our needs. Should I be looking at an alternative approach?
Thanks and sorry for such a long post!
Kevin
In your case it actually sounds like the solution you will be looking for is multifaceted (and not a simple in and done project).
Since you said that some processes can last for hours that is absolutely not something for ASP.NET to own. This should be ran inside a windows service and managed with native windows threading.
You will need to implement some type of work queue in your service and a way to communicate with the queue. One way is to expose a WCF service for all actions your service will govern. Another would be to have service poll a database table and pick up work from the table.
To be able express the status of the process you will want the ASP.NET application to be able to have some reference to the processID for example the WCF service returns a guid identifier. Then you have a method that when you give it the processID it will return the status of the process. You can then implement the polling of that service call using AJAX and display any type of modal you wish.
Another thing to remember is that you need to design your processes to have knowledge of where it is and where it will be when it is finished so it can track the state it's in. For example, BatchJobA is run and will have 1000 records to process. The service needs to know what record it's on or what the current % of competition is for it to be able to return information to the UI. For sql queries that take a very long time to execute this can be very problematic to accurately gauge where it is unless you do alot of pre and post processing of temp tables that you can in the middle of it read the status of the temp tables to understand where it is.
Based on what you are saying I think that BackgroundWorker is not a good choice.
Furthermore keeping this functionality as a part of your main app can be problematic, specifically because you do not want the submitted processing to be interrupted if the main app recycles. You can play with asynch processing but it still will be a part of the main app AppDomain - all of it will die if the app recycles.
I would suggest buidling a separate app implementing this functionality. In a similar situation I separated background processing to a Windows service and hosted a web service in it as a means of communication
You might consider a slightly different approach.
For example, have a command and control table in which you send commands like "REFORMAT PHONE NUMBERS" or whatever.
Then have a windows service monitoring that table. Whenever a record shows up, run the command.
This eliminates any sort of worry about a background thread. Further you have a bit more flexibility with regards to what's in the queue, order of operations including priority, etc. Finally, you would have a definitive list of what is running or needs to run.
As an option, instead of a windows service you might just use a SQL job to execute every so often to watch your control table and perform the requested action.