My team and I have been at this for 4 full days now, analyzing every log available to us, Azure Application Insights, you name it, we've analyzed it. And we can not get down to the cause of this issue.
We have a customer who is integrated with our API to make search calls and they are complaining of intermittent but continual 502.3 Bad Gateway errors.
Here is the flow of our architecture:
All resources are in Azure. The endpoint our customers call is a .NET Framework 4.7 Web App Service in Azure that acts as the stateless handler for all the API calls and responses.
This API app sends the calls to an Azure Service Fabric Cluster - that cluster load balances on the way in and distributes the API calls to our Search Service Application. The Search Service Application then generates and ElasticSearch query from the API call, and sends that query to our ElasticSearch cluster.
ElasticSearch then sends the results back to Service Fabric, and the process reverses from there until the results are sent back to the customer from the API endpoint.
What may separate our process from a typical API is that our response payload can be relatively large, based on the search. On average these last several days, the payload of a single response can be anywhere from 6MB to 12MB. Our searches simply return a lot of data from ElasticSearch. In any case, a normal search is typically executed and returned in 15 seconds or less. As of right now, we have already increased our timeout window to 5 minutes just to try to handle what is happening and reduce timeout errors for the fact their searches are taking so long. However, we increased the timeout via the following code in Startup.cs:
services.AddSingleton<HttpClient>(s => {
return new HttpClient() { Timeout = TimeSpan.FromSeconds(300) };
});
I've read in some places that you actually have to do this in the web.config file as opposed to here, or at least in addition to it. Not sure if this is true?
So The customer who is getting the 502.3 errors have significantly increased the volumes they are sending us over the last week, but we believe we are fully scaled to be able to handle it. They are still trying to put the issue on us, but after many days of research, I'm starting to wonder if the problem is actually on their side. Could it be possible that they are not equipped to take the increased payload on their side. Can it be that their integration architecture is not scaled enough to take the return payload from the increased volumes? When we observe our resources usages (CPU/RAM/IO) on all of the above applications, they are all normal - all below 50%. This also makes me wonder if this is on their side.
I know it's a bit of a subjective question, but I'm hoping for some insight from someone who may have experienced this before, but even more importantly, from someone who has experience with a .Net API app in Azure which return large datasets in it's responses.
Any code blocks of our API app, or screenshots from Application Insights are available to post upon request - just not sure what exactly anyone would want to see yet as I type this.
Our website has a long running calculation process which keeps the client waiting for a few minutes until it's finished. We've decided we need a design change, and to farm out the processing to a windows or a WCF service, while the client is presented with another page, while we're doing all the calculations.
What's the best way of implementing the service though?
We've looked at background worker processes, but it looks like these are problematic because if IIS can periodically shut down threads
It seems the best thing to use is either a Windows service or a WCF service. Does anyone have a view on which is better for this purpose?
If we host the service on another machine, would it have to be a WCF service?
It looks like it's difficult to have the service (whatever type it is) to communicate back to the website - maybe instead the service can update its results to a database, and the website polls that for the required results later on.
It's an open ended question I know, but does anyone have any ideas?
thanks
I don't think that the true gain in terms of performance will come from the design change.
If I were to chose between windows service and WCF I would go with the Windows service because I would be able to fix an affinity and prioritize as I want. However I will have to implement the logic for serving multiple clients in the same time (which in a WCF service approach will be handled by IIS).
So in terms of performance if you use .NET framework for both the WCF service and Windows service the performance difference will not be major. Windows service would be more "controllable", WCF would be more straight-forward and with no big performance penalties.
For these types of tasks I would focus on highly optimizing the single thread calculation. If you have a complex calculation, can it be written in native code (C or C++)? You could make a .DLL file that is highly optimized and is used by either the Windows service or the WCF service. Using this approach will allow you to select best compiler option and make best use of your machine resources. Also nothing stops you from creating multiple threads in the .DLL function.
The link between the website and the service can be ensured in both cases: through sockets for Windows service (extra code for creating the protocol) or directly through SOAP for the WCF. If you push the results in a database the difficulty would be letting the website (and knowing to wich particular user session) know that the data is there.
So that's what I would do.
Hope it helps.
Cheers!
One way to do this is:
The Client submits the calculation request using a Call to a WCF Service (can be hosted in IIS)
The calculation request is stored in a database With a unique ID
The ID is returned to the Client
A Windows Service (or serveral on several different machines) poll the database for New requests
The Windows service performs the calculation and stores the result to a result table With the ID
The Client polls the result table (using a WCF service) With the ID
When the calculation is finished the result is returned to the client
I have a .NET web service hosted in IIS. The web service has been used by clients over the past few years and there has been occassional timeout events when the client is on a slow connection (e.g. GPRS). On the other hand the clients sometimes have to POST some data to another web page (part of an ASP.NET web app) and usually the size of the data in the POST requests is bigger than the actual payloads in the web service calls. However the POST requests are far quicker as compared to the web service calls.
To establish this further I created a test web service with one method and another single web page with exactly the same operation i.e. receive 100K and send back 100K (random bytes) and I used a test client to call the web service method as well as did a post to the web page and got a response back using the same client. The difference in receiving a reply back from the web service and a response back from the web post request is huge i.e. about 1200 ms. Why is that the case? Is there any such configuration on the web service that would make such a big difference? Is it SOAP call stack? Serialization/Desrialization?
A number of factors could be contributing to this.
The first thing that leaps to mind for me is that SOAP could be considered a verbose protocol. That is, there's a LOT of data in the XML payload going both ways. XML is verbose in and of itself, and it's not exactly the fastest thing in the universe to process. Sure, you can use an optimized library to process it's data, but it'll be parsed out into object trees, then you can walk the nodes to drill down to the data you want. Unless you're using XPath, which will just do the same darned thing.
This is all presuming that you're actually using SOAP. And that your WebService is correctly configured. And that no packet loss is occurring while connecting to the Web Service. And that your firewall isn't creating issues. And that there's no encryption/decryption overhead.
In my own experience, one thing that frequently causes signficant slowdowns server side is one or more thrown exceptions. Try a Fiddler trace.
I'm trying to get a better handle on how threads work in ASP.NET, so I have a test site with a few pages, and I have a test WinForms client that creates 40 roughly concurrent requests to the test site. The requests take about 5-10 seconds to complete--they call a web service on another server. When I run the test client, I can use Fiddler to see that the requests are being made concurrently. However, when I look at Performance Monitor on the web server, with counters "ASP.NET Apps v2.0.xxx/Requests Executing", "ASP.NET/Requests Current", "ASP.NET Requests Queued", these counters never display more than 2. This is the case regardless of whether the test page I'm requesting is set up with Async=True and using the Begin/End pattern of calling the web service, or if it's set up to make the call synchronously. Judging by what I see in Fiddler, I would think I should be seeing a total of 40 requests in one of those states, but I don't. Why is that? Do these counters not mean what I think they mean?
I am building an ASP.NET web application that will be deployed to a 4-node web farm.
My web application's farm is located in California.
Instead of a database for back-end data, I plan to use a set of web services served from a data center in New York.
I have a page /show-web-service-result.aspx that works like this:
1) User requests page /show-web-service-result.aspx?s=foo
2) Page's codebehind queries a web service that is hosted by the third party in New York.
3) When web service returns, the returned data is formatted and displayed to user in page response.
Does this architecture have potential scalability problems? Suppose I am getting hundreds of unique hits per second, e.g.
/show-web-service-result.aspx?s=foo1
/show-web-service-result.aspx?s=foo2
/show-web-service-result.aspx?s=foo3
etc...
Is it typical for web servers in a farm to be using web services for data instead of database? Any personal experience?
What change should I make to the architecture to improve scalability?
You have most definitely a scalability problem: the third-party web service. Unless you have a service-level agreement with that service (agreeing on the number of requests that you can submit per second), chances are real that you overload that service with your anticipated load. That you have four nodes yourself doesn't help you then.
So you should a) come up with an agreement with the third party, and b) test what the actual load is that they can take.
In addition, you need to make sure that your framework can use parallel connections for accessing the remote service. Suppose you have a round-trip time of 20ms from California to New York (which would be fairly good), you can not make more than 50 requests over a single TCP connection. Likewise, starting new TCP connections for every request will also kill performance, so you want pooling on these parallel connections.
I don't see a problem with this approach, we use it quite a bit where I work. However, here are some things to consider:
Is your page rendering going to be blocked while waiting for the web service to respond?
What if the response never comes, i.e. the service is down?
For the first problem I would look into using AJAX to update the page after you get a response back from the web service. You'll also want to consider how to handle the no response or timeout condition.
Finally, you should really think about how you could cache the web service data locally. For example if you are calling a stock quoting service then unless you have a real-time feed, there is no reason to call the web service with every request you get. Store the data locally for a period of time and return that until it becomes stale.
You may have scalability problems but most of these can be carefully engineered around.
I recommend you use ASP.NET's asynchronous tasks so that the web service is queued up, the thread is released while the request waits for the web service to respond, and then another thread picks up when the web service is done to finish off the request.
MSDN Magazine - Wicked Code - Asynchronous Pages in ASP.NET 2.0
Local caching is an absolute must. The fewer times you have to go from California to New York, the better. You might want to look into Microsoft's Velocity (although that's still in CTP) or NCache, or another distributed cache, so that each of your 4 web servers don't all have to make and cache the same data from the web service - once one server gets it, it should be available to all.
Microsoft Project Code Named "Velocity"
NCache
Other things that can go wrong that you should engineer around:
The web service is down (obviously) and data falls out of cache, and you can't get it back. Try to make it so that the data is not actually dropped from cache until you're sure you have an update available. Then the only risk is if the service is down and your application pool is reset, so don't reset it as a first-line troubleshooting maneuver!
There are two different timeouts on web requests, a connect and an overall timeout. Make sure both are set extremely low and you handle both of them timing out. If the service's DNS goes down, this can look like quite a different failure.
Watch perfmon for ASP.NET Queued Requests. This number will rise rapidly if the service goes down and you're not covering it properly.
Research and adjust ASP.NET performance registry settings so you have a highly optimized ASP.NET thread pool. I don't remember the specifics, but I seem to remember that there's a limit on IO Completion Ports and something else of that nature that are absurdly low for the powerful hardware I'm assuming you have on hand.
the trendy answer is REST. Any GET request can be HTTP Response cached (with lots of options on how that is configured) and it will be cached by the internet itself (your ISP, essentially).
Your project has an architecture that reflects they direction that Microsoft and many others in the SOA world want to take us. That said, many people try to avoid this type of real-time risk introduced by the web service.
Your system will have a huge dependency on the web service working in an efficient manner. If it doesn't work, or is slow, people will just see that your page isn't working properly.
At the very least, I would get a web stress tool and performance test your web service to at least the traffic levels you expect to get at peaks, and likely beyond this. When does it break (if ever?), when does it start to slow down? These are good metrics to know.
Other options to look at: perhaps you can get daily batches of data from the web service to a local database and hit the database for your web site. Then, if for some reason the web service is down or slow, you could use the most recently obtained data (if this is feasible for your data).
Overall, it should be doable, but you want to understand and measure the risks, and explore any potential options to minimize those risks.
It's fine. There are some scalability issues. Primarily, with the number of calls you are allowed to make to the external web service per second. Some web services (Yahoo shopping for example) limit how often you can call their service and will lock out your account if you call too often. If you have a large farm and lots of traffic, you might have to throttle your requests.
Also, it's typical in these situations to use an interstitial page that forks off a worker thread to go and do the web service call and redirects to the results page when the call returns. (Think a travel site when you do search, you get an interstitial page while they call out to an external source for the flight data and then you get redirected to a results page when the call completes). This may be unnecessary if your web service call returns quickly.
I recommend you be certain to use WCF, and not the legacy ASMX web services technology as the client. Use "Add Service Reference" instead of "Add Web Reference".
One other issue you need to consider, depending on the type of application and/or data you're pulling down: security.
Specifically, I'm referring to authentication and authorization, both of your end users, and the web application itself. Where are these things handled? All in the web app? by the WS? Or maybe the front-end app is authenticating the users, and flowing the user's identity to the back end WS, allowing that to verify that the user is allowed? How do you verify this? Since many other responders here mention a local data cache on the front end app (an EXCELLENT idea, BTW), this gets even MORE complicated: do you cache data that is allowed to userA, but not for userB? if so, how do you verify that userB cannot access data from the cache? What if the authorization is checked by the WS, how do you cache the permissions then?
On the other hand, how are you verifying that only your web app is allowed to access the WS (and an attacker doesn't directly access your WS data over the Internet, for instance)? For that matter, how do you ensure that your web app contacts the CORRECT WS server, and not a bogus one? And of course I assume that all the connection to the WS is only over TLS/SSL... (but of course also programmatically verify the cert applies to the accessed server...)
In short, its complicated, and many elements to consider here.... but it is NOT insurmountable.
(as far as input validation goes, that's actually NOT an issue, since this should be done by BOTH the front end app AND the back end WS...)
Another aspect here, as mentioned by #Martin, is the need for an SLA on whatever provider/hosting service you have for the NY WS, not just for performance, but also to cover availability. I.e. what happens if the server is inaccessible how quickly they commit to getting it back up, what happens if its down for extended periods of time, etc. That's the only way to legitimately transfer the risk of your availability being controlled by an externality.