I have a service that does image processing on an image supplied by the client.
Each processing takes CPU (3min aprox runtime/image), so I will not allow more than 1 image to be processed at a time.
what I did is that when the service is called, the image is saved on the server and an entry is added into the database, with the status queued.
Now I would like to create a background task or something that takes every entry from the database that has a status Queued, processes that image,updates the entry status to Done, and than takes the new entry with the status Queued and so on.
There may be the case that no image is queued at some time.
How do you suggest me implementing this?
It sounds like what you want is a queued service.
http://msdn.microsoft.com/en-us/library/ms731089.aspx
It allows you to focus on your core algorithm and not worry much about the mechanics of queuing the messages (e.g. making custom DB tables for queues etc.). Queuing sounds easy, but to get it to work reliably is harder than it sounds - better to leave it to the experts at MS :o)
It also provides some good features like durability, poison message handling etc.
You could use Windows Server AppFabric to host a workflow-backed WCF service. Instead of service.svc, the extension is service.xamlx. AppFabric is designed to run long running processes like this and will scale to your needs.
Perhaps you can develop a windows service which polls the database every minute for any images to process.
Related
I am getting my head around messaging. Currently we are spiking a few scenarios using Rebus. We are also considering NServiceBus.
The scenario we are trying to build is a proof of concept for a background task processing system. Today we have a handful of backend services hosted in different ways. (web, windows services, console apps) I am looking to hook them up to rebus and start consuming messages using competing consumer, some mesages will have one listener and some will share the load of messages. Elegant :)
I got a pretty good start from this other question How should I set rebus up for one producer and many consumers and it is working nicely in the proof of concept.
Now I want to start reporting progress. My intital approach is to set up pub/sub as well and spin up a service that listens to progress events from all the services. And if a service is interrested in a specific progress in the future it is easy to subscripe of interrest to the messages and start listening.
But how shall I approach setting up both competing consumer and pub/sub? it is dimply two separate things? (In the rebus case one adapter using UseSqlServerInOneWayClientMode / UseSqlServer and another adapter that is set up for the pub/sub using whatever protocol we want?)
Or is there a better solution then having two "buses" here?
I've built something like that myself a couple of times, and I've had pretty good results with using SignalR to report progress from this kind of backend worker processes.
Our setup had a bunch of WPF clients, one single SignalR hub, and a bunch of backend worker processes. All WPF clients and all backend workers would then establish a connection to the hub, allowing workers to send progress reports while doing their work.
SignalR has some nice properties that makes it very suitable for this exact kind of problem:
The published messages "escape" the Rebus unit of work, allowing progress report messages to be sent several times from within one single message handler even though it could take a long time to complete
It was easy to get the messages all the way to the clients because they subscribe directly
We could use the hub groups functionality to group users so we could target progress/status messages from the backend at either all users or a single user (could also be used for departments, etc.)
The most important point, I guess, is that this progress reporting thing (at least in our case) was not as important as our Rebus messages, i.e. it didn't require the same reliability etc, which we could use to our advantage and then pick a technology with some other nice properties that turned out to be cool.
I'm working on a system which amongst other things, runs payroll, a heavy load process. It is likely that soon, there may be so many requests to run payroll at peak times that the batch servers will be overwhelmed.
I'm looking to put together a proof of concept to cope with this by using MSMQ (probably replacing this with a commercial solution like nservicebus later). I using this this example as a basis. I can see how to set up the bindings and stick it together, but I still need a way to tell the subscribers hosted by WAS to only process the 'run heavy payroll process' message if they are not busy. Otherwise the messages on the queue will get picked up straightaway and we have the same problem as before.
Can I set up the subscribing service to say, "I'm busy, I can't take the message, leave it on the queue"? Does the queue need to be transactional?
If you're using WCF then there's no way to conditionally activate the channel thereby leaving the messages on the queue for later.
A better solution is to host the message receiver in a completely different process, for example as a windows service. These can then be enabled/disabled according to your service window requirement.
You also get the additional benefit of being able to very easily scale out the message receivers to handle greater loads (by hosting more instances of your receiver).
One way to do this is to have 2 queues, your polling always checks the high priority queue first, only if there are no items in that queue does it take an item from the other
I'm thinking of adding a queue function in a product based on a bunch of WCF services. I've read some about MSMQ, first I thought that was what I needed but I'm not sure and are considering to just put the queue in a database table. I wonder if somone here got some feedback on which way to go.
Basicly I'm planning to have a facade WCF service called over http. The facade service should only write all incoming messages to a queue to give a fast response to the calling system. The messages in the queue should then be processed by another component, either a WCF service or a Windows service depending om my choice of queue.
The product is running in a load balanced enviroment with 2 to n web servers.
The options I'm considering and the questions I got are:
To let the facade WCF write to a MSMQ and then have anothther WCF service reading from this queue to do the processing of the messages. What I don't feel confident about for this alternative from what I've read is how this will work in a load balanced enviroment.
1A. Where should the MSMQ(s) be placed? One on each web server? One on a separate server? Mulitple on a separate server? (not considering need of redundance and that data in rare cases could be lost and re-sent)
1B. How it the design affected if I want the system redundant? I'd like to be alble to lose a server (it never comes up online again) holding the MSMQ without losing the data in that queue. From what I've read about MSMQ that leaves me to the only option of placing the MSMQ on a windows cluster. Is that correct? (I'd like to avoid using a windows cluster fo this).
The second design alternative is to let the facade WCF service write the queue to a database. Then have two or more Windows services to do the processing of the queue. I don't have any questions on this alternative. If you wonder why I don't pick this one as it seems simpler to me then it is because I'd like to build this not introducing any windows services to the solution, that I beleive the MSMQ got functionality I don't want to code myself and I'm also curious about using MSMQ as I've never used it before.
Best Regards
Håkan
OK, so you're not using WCF with MSMQ integration, you're using WCF to create MSMQ messages as an end-product. That simplifies things to "how do I load balance MSMQ?"
The arrangement you use is based on what works best for you.
You could have multiple webservers sending messages to a remote queue on a central machine.
Instead you could have a webservers putting messages in local queues with a central machine polling the queues for new arrivals.
You don't need to cluster MSMQ to make it resilient. You can instead make your code resilient so that it copes with lost messages using dead letter queues, transactional queues, journaling, and so on. Hardware clustering is the easy option :-)
Load-balancing MSMQ - a brief
discussion
Oil and water - MSMQ transactional
messages and load balancing
After reading some more on the subjet I haver decided to not use MSMQ. It seems like I really got no reason to go down this road. I need this to be non-transactional and as I understand it none of the journaling or dead letter techniques will help me with my redundancy requirement.
All my components will be online most of the time (maybe a couple of hours per year when they got access problems).
The MSQM will only add complexity to the exciting solution, another technique and maybe another server to keep track of.
To get full redundance to prevent data loss in MSMQ I will need a windows cluster or implement send/recieve to multiple identical queues. I don't want to do either of those.
All this lead me to front my recieving application with a WCF facade accepting http calls writing to a database queue. This database is already protected from data loss. The queue will be polled by muliple active instances of a Windows Servce containing all the heavy business logic. With low process priority these services could be hosted on the already existing nodes used by the load balaced web application. If I got time to use MSMQ or if I needed it for another reason in my application I might change my decision.
I have a Windows service that's running all the time, and takes some action every 15 minutes. I also have a client WinForms app that displays some information about what the service is doing. I'd like the forms application to keep itself updated with a recent status, but I'm not sure if polling every second is a good move performance-wise.
When it starts, my Windows Service opens a WCF named pipe to receive queries (from my client form)
Every second, a timer on the winform sends a query to the pipe, and then displays the results. If the pipe isn't there, the form displays that the service isn't running.
Is that the best way to do this? If my service opens the pipe when it starts, will it always stay open (until I close it or my service stops)? In addition to polling the service, maybe there's some way for the service to notify any watching applications of certain events, like starting and stopping processing? That way, I could poll less, since I'd presumably know about big events already, and would only be polling for progress.
Anything that I'm missing?
I would steer clear of polling if possible. You're much better off adopting an event-based mechanism, which reduces overhead and provides just-in-time status updates as necessary.
You're already ahead of the curve. WCF is definitely the way to go here for .NET-based applications. And since your form and service are on the same system, the named pipe binding is the one to use as well. Now all you need is to have your service publish events, including periodic status updates, that your form can listen to when it's open.
The easiest way I've found to do this is to use Juval Lowy's Publish-Subscribe Framework. The neat thing about this framework is that it decouples the publisher from the subscriber. The publisher publishes events, not caring if a subscriber is there or not. If a subscriber is present, the event is forwarded for its consumption. Based on your description, I think this would work very well for you.
I have a web service, implemented with WCF and hosted in IIS7, with a submit-poll communication pattern. An initial request is made, which returns quickly and kicks off a background process. The client polls for the status of the background process. This interface is set and can't be changed (it's a simulation of an external service we depend on).
I implemented the background processing by adding another service contract to the existing service with a one-way message contract that starts the long-running process. The "background" service keeps a database updated with the status in order to communicate with the main service. This avoids creating any new web services or items to deploy.
The problem is that the background process is very CPU intensive, and it seems to be starving the other service calls out. It will take up an entire processor, and while a single instance of the background process is running, status polling calls to the main service can take over a minute. I don't care how long the background process takes.
Is there any way to throttle the resource usage of the background method? Or an obvious way to do long running async processes in WCF without changing my submit/poll service contract? Would separating them into different web services help if the two services were still running on the same server?
The first thing I would try would be to lower the priority.
If you're actually spinning off a separate process for the background work, then you can do it like this:
Process.GetCurrentProcess().PriorityClass = ProcessPriorityClass.BelowNormal;
If it's really just a background thread, use this instead (from within the thread):
Thread.CurrentThread.Priority = ThreadPriority.BelowNormal;
(Actually, it's better to start the thread suspended and change the priority at the caller before running it, but it's generally OK to lower your own priority.)
At the very least it should help determine whether or not it's really a CPU issue. If you still have problems after lowering the priority then it might be something else that's getting starved, like file or network I/O.