I want to invoke an HTTP POST with a request body and wait until it has reached the receiver, but NOT wait for any full response if the receiving server is slow to send the response.
Is this possible at all to do reliably? It's been years since I studied the internals of TCP/IP so I don't really remember the entire state machine here.
I guess that if I simply incur a timeout of say 1 seconds and then close the socket, there's no guarantee that the request has reached the remote server. Is there any signalling at all happening when the receiving server has received the entire request, but before it starts sending its response?
In practical terms I want to call a webhook URL without having to wait for a potentially slow server implementation of that webhook - I want to make the webhook request as "fire and forget" and simply ignore the responses (even if they are intermediate errors in gateways etc and the request actually didn't reach its final destination), but I'm hesitant to simply setting a low timeout (if so, how low would be "sufficient", etc)?
Is there a time out for a http request which is kept in the IIS request queue?
If there is a time out, what will happens if a request stayed longer time in the IIS request queue ?
a - Does it discards or execute by the server when threads available?
Good question, I'm surprised it's infinite by default, as a surge would overload IIS with requests (up to the limit, which is 3000 by default).
If you have a well tuned application, I would say 1-3 seconds is a good range. Users typically don't wait longer than a second anyway, they'll hit refresh. In my case I have a dinosaur with all kinds of clunky reports so have set to 30 seconds.
I think I know what is happening here, but would appreciate a confirmation and/or reading material that can turn that "think" into just "know", actual questions at the end of post in Tl,DR section:
Scenario:
I am in the middle of testing my MVC application for a case where one of the internal components is stalling (timeouts on connections to our database).
On one of my web pages there is a Jquery datatable which queries for an update via ajax every half a second - my current task is to display correct error if that data requests times out. So to test, I made a stored procedure that asks DB server to wait 3 seconds before responding, which is longer than the configured timeout settings - so this guarantees a time out exception for me to trap.
I am testing in Chrome browser, one client. Application is being debugged in VS2013 IIS Express
Problem:
Did not expect the following symptoms to show up when my purposeful slow down is activated:
1) After launching the page with the rigged datatable, application slowed down in handling of all requests from the client browser - there are 3 other components that send ajax update requests parallel to the one I purposefully broke, and this same slow down also applied to any actions I made in the web application that would generate a request (like navigating to other pages). The browser's debugger showed the requests were being sent on time, but the corresponding break points on the server side were getting hit much later (delays of over 10 seconds to even a several minutes)
2) My server kept processing requests even after I close the tab with the application. I closed the browser, I made sure that the chrome.exe process is terminated, but breakpoints on various Controller actions were still getting hit for 20 minutes afterward - mostly on the actions that were "triggered" by automatically looping ajax requests from several pages I was trying to visit during my tests. Also breakpoints were hit on main pages I was trying to navigate to. On second test I used RawCap monitor the loopback interface to make sure that there was nothing actually making requests still running in the background.
Theory I would like confirmed or denied with an alternate explanation:
So the above scenario was making looped requests at a frequency that the server couldn't handle - the client datatable loop was sending them every .5 seconds, and each one would take at least 3 seconds to generate the timeout. And obviously somewhere in IIS express there has to be a limit of how many concurrent requests it is able to handle...
What was a surprise for me was that I sort of assumed that if that limit (which I also assumed to exist) was reached, then requests would be denied - instead it appears they were queued for an absolutely useless amount of time to be processed later - I mean, under what scenario would it be useful to process a queued web request half an hour later?
So my questions so far are these:
Tl,DR questions:
Does IIS Express (that comes with Visual Studio 2013) have a concurrent connection limit?
If yes :
{
Is this limit configurable somewhere, and if yes, where?
How does IIS express handle situations where that limit is reached - is that handling also configurable somewhere? ( i mean like queueing vs. immediate error like server is busy)
}
If no:
{
How does the server handle scenarios when requests are coming faster than they can be processed and can that handling be configured anywhere?
}
Here - http://www.iis.net/learn/install/installing-iis-7/iis-features-and-vista-editions
I found that IIS7 at least allowed unlimited number of silmulatneous connections, but how does that actually work if the server is just not fast enough to process all requests? Can a limit be configured anywhere, as well as handling of that limit being reached?
Would appreciate any links to online reading material on the above.
First, here's a brief web server 101. Production-class web servers are multithreaded, and roughly one thread = one request. You'll typically see some sort of setting for your web server called its "max requests", and this, again, roughly corresponds to how many threads it can spawn. Each thread has overhead in terms of CPU and RAM, so there's a very real upward limit to how many a web server can spawn given the resources the machine it's running on has.
When a web server reaches this limit, it does not start denying requests, but rather queues requests to handled once threads free up. For example, if a web server has a max requests of 1000 (typical) and it suddenly gets bombarded with 1500 requests. The first 1000 will be handled immediately and the further 500 will be queued until some of the initial requests have been responded to, freeing up threads and allowing some of the queued requests to be processed.
A related topic area here is async, which in the context of a web application, allows threads to be returned to the "pool" when they're in a wait-state. For example, if you were talking to an API, there's a period of waiting, usually due to network latency, between sending the request and getting a response from the API. If you handled this asynchronously, then during that period, the thread could be returned to the pool to handle other requests (like those 500 queued up requests from the previous example). When the API finally responded, a thread would be returned to finish processing the request. Async allows the server to handle resources more efficiently by using threads that otherwise would be idle to handle new requests.
Then, there's the concept of client-server. In protocols like HTTP, the client makes a request and the server responds to that request. However, there's no persistent connection between the two. (This is somewhat untrue as of HTTP 1.1. Connections between the client and server are sometimes persisted, but this is only to allow faster future requests/responses, as the time it takes to initiate the connection is not a factor. However, there's no real persistent communication about the status of the client/server still in this scenario). The main point here is that if a client, like a web browser, sends a request to the server, and then the client is closed (such as closing the tab in the browser), that fact is not communicated to the server. All the server knows is that it received a request and must respond, and respond it will, even though there's technically nothing on the other end to receive it, any more. In other words, just because the browser tab has been closed, doesn't mean that the server will just stop processing the request and move on.
Then there's timeouts. Both clients and servers will have some timeout value they'll abide by. The distributed nature of the Internet (enabled by protocols like TCP/IP and HTTP), means that nodes in the network are assumed to be transient. There's no persistent connection (aside from the same note above) and network interruptions could occur between the client making a request and the server responding to the request. If the client/server did not plan for this, they could simply sit there forever waiting. However, these timeouts are can vary widely. A server will usually timeout in responding to a request within 30 seconds (though it could potentially be set indefinitely). Clients like web browsers tend to be a bit more forgiving, having timeouts of 2 minutes or longer in some cases. When the server hits its timeout, the request will be aborted. Depending on why the timeout occurred the client may receive various error responses. When the client times out, however, there's usually no notification to the server. That means that if the server's timeout is higher than the client's, the server will continue trying to respond, even though the client has already moved on. Closing a browser tab could be considered an immediate client timeout, but again, the server is none the wiser and keeps trying to do its job.
So, what all this boils down is this. First, when doing long-polling (which is what you're doing by submitting an AJAX request repeatedly per some interval of time), you need to build in a cancellation scheme. For example, if the last 5 requests have timed out, you should stop polling at least for some period of time. Even better would be to have the response of one AJAX request initiate the next. So, instead of using something like setInterval, you could use setTimeout and have the AJAX callback initiate it. That way, the requests only continue if the chain is unbroken. If one AJAX request fails, the polling stops immediately. However, in that scenario, you may need some fallback to re-initiate the request chain after some period of time. This prevents bombarding your already failing server endlessly with new requests. Also, there should always be some upward limit of the time polling should continue. If the user leaves the tab open for days, not using it, should you really keep polling the server for all that time?
On the server-side, you can use async with cancellation tokens. This does two things: 1) it gives your server a little more breathing room to handle more requests and 2) it provides a way to unwind the request if some portion of it should time out. More information about that can be found at: http://www.asp.net/mvc/overview/performance/using-asynchronous-methods-in-aspnet-mvc-4#CancelToken
I have a message driven bean which serves messages in a following way:
1. It takes data from incoming message.
2. Calls external service via HTTP (literally, sends GET requests using HttpURLConnection), using the data from step 1. No matter how long the call takes - the message MUST NOT be dropped.
3. Uses the outcome from step 2 to persist data (using entity beans).
Rate of incoming messages is:
I. Low most of the time: an order of units / tens in a day.
II. Sometimes high: order of hundreds in a few minutes.
QUESTION:
Having that service in step (2) is relatively slow (20 seconds per request and degrades upon increasing workload), what is the best way to deal with situation II?
WHAT I TRIED:
1. Letting MDB to wait until service is executed, no matter how long it takes. This tends to rollback MDB transactions by timeout and to re-deliver message, increasing workload and making things even worse.
2. Setting timeout for HttpURLConnection gives some guarantees in terms of completion time of MDB onMessage() method, but leaves an open question: how to proceed with 'timed out' messages.
Any ideas are very much appreciated.
Thank you!
In that case you can just increase a transaction timeout for your message driven beans.
This is what I ended up with (mostly, this is application server configuration):
Relatively short (comparing to transaction timeout) timeout for HTTP call. The
rationale: long-running transactions from my experience tend to
have adverse side effects such as threads which are "hung" from app.
server point of view, or extra attention to database configuration,
etc.I chose 80 seconds as timeout value.
Increased up to several minutes re-delivery interval for failed
messages.
Careful adjustment of the number of threads which handle messages
simultaneously. I balanced this value with throughput of HTTP service.
I am submitting POST requests to an external server running IIS6. This is a time critical request where I want to ensure that my request is processed at a specific time (say 10:00:00 AM). No earlier. And I want to ensure that at that specific time, my request is assigned the highest priority over other requests. Would any of this help:
Sending most of the message a few seconds early and sending the last byte or so a few milliseconds prior to 10:00:00. Not sure if this will help as I will be competing with other requests that come in around that time. Will IIS assign a higher priority to my request based on how long I am connected?
Anything that I can add to the message header to tell the server to queue my request and process only at a specific time?
Any known hacks that I can leverage?
No - HTTP is not a real time protocol. It usually runs on top of TCP/IP which is not a real time protocol. While you can get near real-time behaviour out of such an architecture its far from simple - don't take my word for it - go read the source code for xntpd.
Having said that you give no details of the actual level of precision you require - but your post implies that it could be up to a second - which is a very long time for submitting a request to a webserver. On the other hand, scheduling such an event to fire client side with this level of accuracy is very difficult - I've not tried measuring the accuracy of the scheduler on MSWindowsNT but elsewhere I'd only expect it to be accurate to about 5 minutes. So you'd need to schedule the job to start 5 minutes early then sleep for 10 milliseconds at a time until the target time rolls around.
But then again, thinking about why you need to run any job with any sort of timing accuracy makes me think that you're trying to solve the problem the wrong way.
C.
It sounds like you need more of a scheduler system then trying to use http. HTTP is a stateless protocol, you send a request to IIS, you get a response.
What you might want to consider is taking that request, and then storing the information you require somewhere (database). Then using some sort of scheduler (cronjobs, scheduled tasks) you action that information at the desired time.
What you want, you probably can't achieve with IIS, it's not what it is designed to do.