Saturday, 22 September 2012

Take your Web API service consumption up to 11 with CacheCow.Client

ASP.NET Web API is here and a lot of teams have already started building software using it. If you have followed this and other blogs on, you probably have seen many various possibilities and avenues this framework brings for designing and building clean and scalable services.

ASP.NET Web API exposes all the goodness of HTTP. Caching is an important feature of HTTP and ASP.NET Web API allows for building services and clients that take advantage of this feature. I, along with a few friends in the Web API community, have been busy building caching extensions for Web API in a project called CacheCow which is hosted on GitHub.

CacheCow has two separate components: server and client. These will be used independently by service providers (server) and their consumers (client).

Server component allows for easy handling of HTTP caching scenarios on server by generating ETag, responding to validation of cache (see earlier posts on this subject, especially this and for full list here), cache invalidation and storage of cache metadata. Storage of cache metadata is possible in various stores, currently in-memory and SQL Server have been implemented and RavenDB and Redis is on the pipeline [UPDATE: RavenDB is implemented and NuGet package is available here]. Since storage has been abstracted away, any storage mechanism can be plugged in without making any server changes.

Client component looks after making cache-aware requests, cache validation and cache storage. Currently in-memory and file-based storage is available but other stores such as Redis, SQL Server, MongoDB and RavenDB are in the pipeline. Since storage has been abstracted away, any storage mechanism can be plugged in without making any client changes. One of important features of storage is total and per-site quota.

It is important to note that while clients can be browsers or native Apps (WPF, Silverlight, iOS, Android, etc), arguably more often than not they will be server components themselves. For example, an ASP.NET web site can call services of an ASP.NET Web API server. Also middleware components could similarly use resources exposed by Web API. As such it is very crucial that cache storage solutions are performant, scalable and configurable.

In this post, I will look into CacheCow.Client a little but more. For more info, you can read previous posts on the topic in this blog.

CacheCow.Client alternatives

The only alternative to CacheCow.Client (that I am aware of) is using WinINET caching. Internet Explorer also uses this so the cache store will be the same. This is basically windows' HTTP request stack which has been exposed in .NET Framework since v 2.0 through WebRequest:

RequestCachePolicy policy = 
        new RequestCachePolicy( RequestCacheLevel.Default);
WebRequest request = WebRequest.Create(uri);
request.CachePolicy = policy;
WebResponse response = request.GetResponse();

As you can see, we can define a cache policy which will be applied to the request and according to the policy, Internet Explorer cache is used. Cache policy has a few possible values that are defined here. Notable values include:

  • CacheOnly: retrieves the request only from cache
  • BypassCache: does not use cache at all and goes straight to the server
  • CacheIfAvailable: retrieves from local or intermediate cache if resource available otherwise retrieve from server
  • Default: Similar to previous but current cache policy takes effect 

This same mechanism is now exposed in HttpClient but basically is built on the top of WebRequest. Henrik fully covers this feature in his blog here.

Basically in order to use WinINET caching with the new Web API stack, you need to create an HttpClient but provide WebRequestHandler as the MessageHandler:

HttpClient client = new HttpClient(new WebRequestHandler()
                               CachePolicy = new RequestCachePolicy( RequestCacheLevel.Default)
// this is a sample. It is not advised to use .Result since can lead to deadlock!
var httpResponseMessage = client.GetAsync("").Result;
var httpResponseMessage2 = client.GetAsync("").Result;

Using this feature, you can enable caching with little coding on the client.

Why I would choose CacheCow.Client rather than WinINET

Because it goes to 11! As we saw, it is very easy to get started with caching in HttpClient. But as we noted, it is very likely that HttpClient could be used in a server context hence having a reliable and scalable solution is very important in production.

Here are a few advantages of CacheCow.Client over WinINET (or rather disadvantages of WinINET):

1. Caching will be shared with Internet Explorer

In a production scenario, you need an implementation which is predictable and reliable. If someone uses Internet Explorer on the machine, storage area for your application's resources will be taken by just simple browsing. This can lead Internet Explorer to flush application resources in order to store resources for the browsing session. 

2. You have little control over quota

With CacheCow.Client, you can define a global and a per-site quota for storage of resources while such feature is not accessible (although there could be some registry entries for changing these variables) in WinINET caching. Also these variables could be overwritten by installation of a newer version of Internet Explorer.

3. Cache is local to the machine and cannot be shared across servers

In a production scenario, it is desirable to be able to store caches in a central store so network traffic and requests could be limited while with WinINET caching, each server will use its own local cache store.

4. WinINET is file-based

With WinINET, cache is stored in a file location while for a high-throughput production environment, robust caching using solutions such as Redis is required. CacheCow client by abstracting the storage can use any number of storage mechanisms such as Redis, MongoDB, RavenDB, etc.

5. CachePolicy is global for the HttpClient instance

Sometimes you might need to bypass caching. With WinINET, this has to be done with changing policy at the client level which applies across all requests for that HttpClient while CacheCow.Client respects will not use cached resources if you set CacheControl header of the request to no-cache. This basically recommended implementation based on HTTP specification (RFC2616).

6. With WinINET you do not know if request was retrieved from cache

With WinINET, there is no way to tell if response was retrieved from the cache or origin server. CacheCow.Client provides x-cachecow header which provides various information which can be used for debugging and troubleshooting scenarios.

Introducing CacheCow.Client.FileCacheStore

Last week I finished first version of a persistent cache store which is file based. This is available using NuGet and (package name is CacheCow.Client.FileCacheStore) and the code available at GitHub.

Using this persistent store is very easy. After getting the package from NuGet, create an HttpCient while as a delegating handler, pass CachingHandler (covered before here) while setting the store to a new instance of FileStore. While creating a FileStore, you need to specify a folder for storing the cached resources:

var httpClient = new HttpClient(
 new CachingHandler(
 new FileStore("c:\\Cache"))
 InnerHandler = new HttpClientHandler()

That is all you have to do! Now all your requests will store cacheable resources in a file-based persistent store. 

Currently for quota it uses default values but I am in the process of exposing values so you can configure quota.

CacheCow roadmap

After exposing quota settings, I will be working on CacheCow.Client.RedisCacheStore for a high throughput production level cache storage.

Please keep me posted by your comments, feedback and raising bug/issues on the GitHub page. You are awesome!

Monday, 17 September 2012

Server-side Async: Careful with that Axe, Eugene

[Level T3]

In a previous post, I talked about the dangers lurking in doing server-side async operations in .NET 4.0. As you know, .NET 4.5 provides a much better syntax allowing async/await keywords to take your TPL Task-Soups to a much more readable and organised code. But even so, async will make debugging your application more difficult and bugs could take much longer to be reproduced, isolated and fixed.


In .NET 4.0, when we add up continuations to create a chained task, we could end up with a few problems:

  1. We could end up with an unobserved exception problem. This is nicely described by Ayende here
  2. Nested lambda expressions could create unexpected problems with closure of variables
  3. The code becomes hard to read.
On the third note, I will just bring an example from my own code in CacheCow. What is it that we are actually returning here?

return response.Then(r =>
 if (r.Content != null)
  TraceWriter.WriteLine("SerializeAsync - before load",

  return r.Content.LoadIntoBufferAsync()
   .Then(() =>
    TraceWriter.WriteLine("SerializeAsync - after load", TraceLevel.Verbose);
    var httpMessageContent = new HttpMessageContent(r);
    // All in-memory and CPU-bound so no need to async
    return httpMessageContent.ReadAsByteArrayAsync();
   .Then( buffer =>
       TraceWriter.WriteLine("SerializeAsync - after ReadAsByteArrayAsync", TraceLevel.Verbose);
       return Task.Factory.FromAsync(stream.BeginWrite, stream.EndWrite,
        buffer, 0, buffer.Length, null, TaskCreationOptions.AttachedToParent);                                                        


Even looking at brackets gives me headache.

Is Async worth it at all?

Now we talk a lot about Async operations and its role in improving scalability. But really, is it worth it? How much scalability would it bring? Would it help or hinder?

The answer to these questions is yes, it does help. The more IO you do on your server-side actions, the more you benefit from improvement from scalability. So it is highly advisable to implement your ApiController actions as Async by returning Task or Task<T>

The truth is, it will help even with your non-IO-bound operations although it is not advisable to use Async in such scenarios. You can test it for yourself, create a sync and an async controller to do exactly the same operation and use a benchmarking tool to compare the performance.

I have a CarManager sample on GitHub which I use for testing CacheCow.Server and it contains two simple  controllers: CarController and CarAsyncController. All these do is to use an in-memory repository and their GET only looking up the dictionary by its key:

// sync version
public Car Get(int id)
 return _carRepository.Get(id);

// async version (on another controller)
public Task<Car> GetAsync(int id)
 return Task.Factory.StartNew(() => _carRepository.Get(id));

So if you use a benchmarking tool such as Apache Benchamrk ab.exe, you could see a slight increase in throughput using the async controller. In my case, there was a 10% increase in throughput using async.

My ordeal with a bug

Development of CacheCow has been marred by existent of a problem which as we will see, turns out to be not in my code. I have been battling with this for a few weeks (on and off) and could not progress CacheCow development because of that.

OK, here is how my story begins; I think the Sherlock Holmes nature of this troubleshooting could be amusing for others too. After realising that using simple ContinueWith will not flow the context (see previous post) I was tasked with changing all such cases with Then in the TaskHelpers which checks existence of SynchronizationContext and flows the context if it exists.

On the other hand, lostdev, one of CacheCow's most loyal users, informed me of an occasional null reference exception in CacheCow.Server. Now, I had already fixed a bug related to null reference exception when the a resource was being retrieved for the first time. I attributed the problem to the fix I had made and reported that the problem is fixed in the current version.

So I started developing file-based cache storage for CacheCow.Client (which will have its own post very soon) and replaced all ContinueWith cases with Then.

And then I started to experience deadlocks in CacheCow.Client when I was using file-based caching and sending concurrent GET requests to the server. As soon as I would remove FileStore, and replace with InMemoryCacheStore, it would work. So I started searching through the client code, debug, look at the threads, debug again, change code, debug... to no avail. As soon as I was using file-based caching it would start to appear so it had to be on the client.

Then I noticed a strange thing: I could only run 4 concurrent calls and rest would be blocked. Why? Then I started playing with the maxconnection property of the configuration:

   <add address = "*" maxconnection = "N" />

and interestingly, by setting the N to a high number, I would get more concurrent connections - but only up to the number defined. Hmmm... so the requests do not quite finish. OK, I fired up Sysinternals' TcpView but unfortunately these connections did not show up (and I do not know why).

I was getting nowhere until I accidentally loaded an earlier version of the server code. To my surprise, I did not get the deadlock but this error which @Tugberk separately reported earlier but attributed to order of handlers:

[NullReferenceException: Object reference not set to an instance of an object.]
System.Web.Http.WebHost.HttpControllerHandler.EndProcessRequest(IAsyncResult result) +112
System.Web.Http.WebHost.HttpControllerHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +10
System.Web.CallHandlerExecutionStep.OnAsyncHandlerCompletion(IAsyncResult ar) +129

OK, so it is probably happening on the server but the continuation code gets deadlocked on unhandled exception. I am close! So it was time to go to bed and I was positive that I would nail it the day after.

It was funny that I woke up the day after and with my in-bed reading on tweets, stumbled on @Tugberk's tweet on issue he had just created. That sounds exceedingly similar, so we just doubled checked our scenarios and it turned out that an HttpResponseMessage with empty RequestMessage property is not handled in Web API and a null reference exception is thrown at the end of the response clean-up code. And the reason I was seeing it only with file-based cache store was that the part of server-side code to return such responses was being triggered only using file-based store (since it was capable of persisting caches and was trying to validate the cache).

So as you can see, a seemingly unrelated problem can really confuse the nature of the bugs in async scenarios.


First of all, always use request.CreateResponse() instead of using new HttpResponseMessage. I googled for cases of new HttpResponseMessage and found +3000 entries. This is really dangerous and I think this is a bug in Web API and needs to be fixed. If you are using new, make sure you set the RequestMessage property.

And in general, be careful with doing server-side async operations. It is really a powerful axe but with it you are not quite sure what a slightly off swing could bring. Careful with that axe Eugene.

Monday, 10 September 2012

Going away

There are feelings that are so complex they don't have a name - unless we make one. They can be described, but not easily. Now "Going away" is one of those feelings.

Have you ever been to a place, on a holiday, and enjoying the places you visit and yet you cannot stop thinking about the fact that this is probably the last time you see this place? That is why you start taking pictures, videos, you want to register everything since you know your memory won't. It is likely that you would not go back to those pictures but having them is like you "own" those memories.

* * *

Beaumaris (map) is a tiny town in the remotest corner of the north west of Wales - in the Anglesey island. Once visiting Wales (we love Wales), on our hunt for nice places to eat, we found ourselves in there. Passing sailing boats on a green road with the Irish Sea on the right and stony walls on the left, we approached a town boasting a rich history - later found out about its Castle. We had no idea what we will find there.

Memory of that night was not captured on any picture. It did not need to. It felt like a surreal experience of two invisible tourists/observers entering an almost dream-like banquet. It was almost fictional. It reminds me of the opening (and closing) scene of the Russian Ark.

We parked in the corner of the square overlooking the Irish Sea and found our way through a small entrance to the main street. Ye Olde Bull's Head Inn was easy to find: a very old traditional pub with a classy hotel on the top. We ordered drinks and sat down in very old wooden chair and benches. While walking we were careful of our head: like all old pubs ceiling is low and wooden bars are visible.

And there was a banquet: a reunion of army forces of some sort - many of them American. Now out of  all places, why Beaumaris? Had they travelled all the way from the ocean to meet at such a remote place? I had no answer. I did chat to a few officers walking around there but my inquiry did not produce much more information. I have not been able to find any reference to this reunion on internet.

After 40 minutes, we were ushered to the brasserie where food was served. Suddenly atmosphere changed. It was a modern extension to the old building and an ethereal ambient light had made the chic contemporary design even more magical. Starter, food and dessert were excellent. Probably among best foods we ever had.

On leaving, I felt like "going away". But it did not matter. I had absorbed all those beautiful yet surreal moments.
* * *

I am visiting my family in Iran now. I do go back almost every year although now most my friends and my brother and sister are abroad.

My mother-in-law is ill. Medically speaking she is terminal. But everything is possible. Isn't it?

This is a chance to spend time with her and my parents. Things are not easy. Seeing suffering and not being able to cure is not easy - although there are things we can do. My wife is doing most of them so really there is not much for me to do. I occasionally help them out and I am just there, in case. My mother-in-law is a brave woman. She has been fighting and keeps fighting. This is so important for someone battling with a fierce illness. My wife is also very brave. She is fighting her emotions trying to help medically as much as she can - she is a doctor, a real one not like me who chickened out to do what he likes to do.

My holiday is running out - and the feeling of "going away" is coming back.

* * *

We visited Beaumaris again only after two years. This time there were no banquets but the place had no less magic: it was exactly as we had seen in that evening two years ago. Ye Olde pub with its crisp beers and the food excellent like before. 

I felt really special. It felt like an honour to visit the place again. I was fortunate enough to visit a "gone away" place.

And with my mother-in-law, I think it is possible - and perhaps definite. In this world. Or in the next.