In http://docs.meteor.com/#insert there is a statement:
On the server, if you don't provide a callback, then insert blocks until the database acknowledges the write, or throws an exception if something went wrong.
Is it entire node process going blocked there ? Do we always need to provide a callback ?
No, it is not blocking the whole process. It just looks synchronous, in reality the Fiber (current execution context, cooperative thread) yields to other events in the event loop. You can safely use it but be careful: something can execute in the period of time after yielding and regaining the control.
Related
Async i/o operator (1.15.2) is waiting for the future to return (basically let the embedded function complete its processing), till then checkpointing gets halt at this operator. And only proceeds once it gets finished. attaching the screenshot.
Checkpointing in progress
Basically I would like to test the scenario where async io is waiting for the response & job is restarted. So ideally the last checkpoint will get restored & technically the async i/o processing should get restarted again with the same set of data.
But as checkpoint is stuck, & if i restart the job, than previous checkpoint is restored which doesn’t have the correct set of data.
Please guide how should I tackle this scenario.
You should never let Flink end up waiting; all i/o should be done asynchronously. Otherwise, as you've seen, you'll end up interfering with checkpointing.
The async i/o operator is designed to help with this, but you must either use an asynchronous client library for connecting to the external service, or make your synchronous calls in another thread. See https://stackoverflow.com/a/64225825/2000823 for an example of how to do this.
I have a web service that receives requests from users and returns some json. I need to save the json string in the database so for the moment, the write query occurs before the response is sent back.
Is there a way to send the response first and then do the write query, after the response left the web service?
Thanks.
There's a couple of different options here - they all have tradeoffs, though, and would be pretty esoteric. You don't mention why you want to do this, so I'm guessing performance. If that's the case, I think you're barking up the wrong tree - a simple write is almost certainly not your performance problem.
So, off the top of my head:
Queuing, as Ragesh mentions, would be a nice approach. This gets you similar semantics of a transaction, while off loading the write. You still have to write to the queue, though, which may be about the same overhead as writing to the DB.
You could spawn a new thread (using either the ThreadPool or System.Threading.Thread - there's some debates about which is preferable in ASP.NET) to handle the write. This can generally work, but you may have issues with unhandled exceptions, app domain restarts, etc.
You could store the JSON data into a static or Application variable, then use a Timer to periodically write them to the DB. This will be multithreaded code, so you will need to synchronize read/writes to the collection.
Similar to #3, store the JSON data into Cache and use the invalidation callback to write to the DB.
Lots of variations on store somewhere (memory, disk, flat DB table, etc.), process later (ASP.NET, scheduled task, Windows Service, Sql Agent, etc.).
#frenchie says: a response starts by reading the json string from the db and ends with writing it back. In other words, if the user sends a request, the json string that's going to be read must be the one that was written in the previous response.
That complicates things, since inherent in async work is not knowing when something is done. If you require the async portion (writing back to the DB) to be done before handling the next request, you'll have to execute a wait to make sure it actually completed. In order to do that, you'll need to keep server side state on the client - not exactly a best practice as far as services go (though, it sounds like you're already doing that with these JSON request/response pairs).
Given the complications, I would make sure that you've done your profiling and determined it is indeed a performance problem.
You can do schedule a query work like
ThreadPool.QueueUserWorkItem(state =>
this.AsynchronousExecuteReference());
// and run
static void AsynchronousExecuteReference()
{
// run here your sql update
}
One other example using Thread inside an class and you can pass parameters to it.
public class RunThreadProcess
{
// Some parametres
public int cProductID;
// my thread
private Thread t = null;
// start it
public Thread Start()
{
t = new Thread(new ThreadStart(this.work));
t.IsBackground = true;
t.SetApartmentState(ApartmentState.MTA);
t.Start();
return t;
}
// actually work
private void work()
{
// do thread work
all parametres are available here
}
}
And here is how I run it
var OneAction = new RunThreadProcess();
OneAction.cProductID = 100;
OneAction.Start();
Do not worry about memory, CG knows that this process is used until the thread ends, so I have check it and CG not delete it and wait the thread to ends.
You should look at using message queues like MSMQ, ActiveMQ or RabbitMQ to do this. When you receive your request, you'll put the relevant data in to the queue, and send your response to the client. At the other end of the queue, you'll have some process that reads from the queue and inserts data in to your database.
this is missing the point of a request/response. unless you want to get into async commands like a service bus, but that's pub/sub, not request/response. the point of request/response is to do the work on the server after receiving the request and before sending the response. even if the work is sending an async message to a service bus.
You could try moving your web service URL to an ASPX page where the lifecycles come in to play.
In the code-behind, call your routine that does the main portion of the work in Page_Load or Page_Prerender (or whenever is appropriate prior to the response being sent) and then do your DB work in the Page_Unload event which occurs after the response has been sent (http://msdn.microsoft.com/en-us/library/ie/ms178472.aspx).
I'd like to start using asynchronous processing in IIS. Edit: I'm talking about using the task parallel library.
For example, on certain page loads I want to log a bunch of crap, send an email, update some tables, etc. But I don't want to make the user wait for me to log all that crap.
So normally what I do is I have a static Queue that I push the log info onto, and then I have a cron job that calls a special page every 10 minutes whose OnLoad flushes out the queue. This works, but it's kind of clunky to setup, especially when you want to log 50 things. I'd rather do this:
Task.CreateNew(() => Log(theStuff));
However I'm terrified of running tasks in IIS because one slip up and your entire website goes down.
So now I have
SafeTask.FireAndForget(() => Log(theStuff));
This wraps the delegate in some try/catch and passes it into Task.CreateNew. So if someone changes something that affects something else that generates an exception somewhere else that accidentally gets thrown on the task thread, we get a notification instead of a crashed website. Also, the error notification inside the catch is also inside its own try/catch, and the catch for that also has a try/catch that tries to log in a different way.
Now that I can safely run stuff asynchronously in IIS, what other things do I need to worry about before I can start using my SafeTask class?
Every request in IIS and .net is processed in one thread by default. This thread comes from a thread pool called the "Application Pool". Existing threads are reused so you can't really use them for thread state unless you clear or set it every time. You define the size of this thread pool using a formula from MSDN in the machine.config or even your web.config.
Now, every async function call is put on a different thread. This includes async web service calls, async page functions, async delegates, etc. This thread comes from the "application pool" thus reducing the number of thread available for IIS to service new requests.
Most likely, your application will work just fine while using async function calls. In case you are worried or you have a lot of async tasks then you may want to create your own thread pool or look at SmartThreadPool on codeplex.
Hope this helps.
Consider using the page's OnUnload event. Read about it here: http://msdn.microsoft.com/en-us/library/ms178472.aspx
This event fires after the content is sent to the user (so the user isn't blocked while you do work), and should completely satisfy your requirement without introducing additional threads.
Specific to your question, you should be concerned about thread pool exhaustion only if your load and performance testing suggests you're running up against thread limits. If you're not then what you propose is certainly reasonable.
I'm not talking about asynchronous pages or asynchronous handlers, I just want to know if I should be afraid of any side effect when I invoke an asynchronous method that will end after the page is finished rendering.
Example given: Each time that a user do login, I have to launch a heavy and time consuming SQL operation, but the user doesn't need to know the result of that operation, so I can execute the query using BeginExecuteNonQuery without pass any callback, and finish rendering the page.
My concern is, what happen if the HTTP call ends (because the page is served) and whatever I've executed asynchronously is already running? is ASP.NET or IIS going to cut, destroy, void anything?
Cheers.
That operation will run, even when the request has finished. However, please note that the ASP.NET host aggressively kills threads. When IIS has any reason for unloading or recycling the AppDomain, your background thread will be killed. Unloading happens in several situations. For instance when no new requests have come in for a certain period of time. Or when too many exceptions are fired from the application within a certain period of time. Or when the memory pressure gets too high.
If you need the guarantee, that the operation will finish, I think there are three things you can do:
Speed up the operation so that it can run synchronously, or
Move that that heavy operation to a Windows Service and let that execute it, or
You can hook onto the HostingEnvironment.RegisterObject method (as Phill Haack explains here) (demands full trust) to prevent the AppDomain to go down while that thread is running.
If you have a callback registered, the process will comeback to notify the callback otherwise it will still complete the job. AFAIK - neither ASP.NET or IIS will cut/destroy or void anything as the execution was already ordered and it has to complete.
I have an ASP.Net application that needs needs to have some work performed by another machine. To do this I am leaving a message on queue visible to both machines. When the work is done a message is left on second queue.
I need the ASP.Net application to check the second queue periodically to see if any of the tasks are complete.
Where is the best place to but such a loop? Global.asax?
I remember reading somewhere that you can get a function called after an interval. Would that be suitable?
To achieve periodical tasks on asp.net, I've found two acceptable approaches:
Spawn a thread during Application_Start at global.asax, in a while loop (1) Do the work (2) Sleep the thread for an interval.
Again in Application_Start, insert a dummy item into asp.net cache, expires in a certain interval and give that cache item a callback to be called when it's expired. In that callback, you can do the work and insert the cache item back the same way.
In both ways, you need to make sure that your thread keeps working even if there happens an error. You may place a restore code in SessionStart and BeginRequest to check your thread or cache item is there, and renew it if something has happened to it.
I assume that this is done on a regular basis, and that some other process puts the items on the queue?
If that is the case, you might put something in Global.asax that on application start creates a separate thread that simply monitors the queue, you could use a timer to have that thread sleep for X seconds, then check for results.