ASMX Web Service slow first request

The delay that is experienced when a client is calling a webservice for the first time is caused by the fact that by default a XmlSerializers dll for the webservice needs to be compiled. This is causing the 2-4 seconds for the initial call. Of course this is the case when the webservice application is already running, if it's not you would have a recycle. In which case the other answers could help.

To speed up the initial call you can create the XmlSerializers dll at compile time. You can do this by setting your project build 'Generate serialization assembly' to on. This generates an MyApplication.XmlSerializers.dll containing the webservice information. Now the initial call dropped to 300 ms, presumably the loading of the dll. All calls there after take 0 ms.

In Visual Studio right click on your project and choose 'Properties'. Go to the 'Build' Tab. There you have an option 'Generate Serialization assembly' in the 'Output' section. If you change the value to 'On' the serialization assembly will be generated during compile time.

The first time you call the webservice, or the first time after a long delay, the web service needs to start up. This is where you're seeing the delay. After that, it's already started and will respond really quickly to calls. This is standard web service behaviour.

You could configure IIS to have keepalive = true - which may improve the performance.

More information as requested.

It could be that the serialization assemblies are being created at runtime. You can change the settings of the serialization assembly using the dropdown at the bottom of the Build pane of the properties window for the project.

It could be that you have written your web service to perform a lot of operations on application start, which would happen the first time a method on the service is called.

It might be that the operation is very slow, but you are then caching the response, which makes subsequent calls faster.

I recently discovered that in our ASMX-files we only referred to the class name. We got the service implementation in a different assembly for each ASMX-file. This causes .NET framework to scan through the entire bin-folder looking for the assembly containing the implementation. As your webservice application grows this will consume more time. This can be solved by not only including the class name in your ASMX definition but also the assembly name.

Our ASMX looked like this:

<%@ WebService Language=”C#” CodeBehind=”MyService.cs” Class=”MyWebservice” %>

If you change it to include the assembly that contains the implementation it would look like this. This saved us around 10% of our initial load of the webservice application.

<%@ WebService Language=”C#” CodeBehind=”MyService.cs” Class=”MyWebservice, MyWebservice.Implementation.Assembly” %>

That is typical, since ASP.NET apps compile and load the bin\ directory into memory at first request.

Things to do first:

Remove all unnecessary dll's in your bin directory. (I've seen people ship nunit.dll)

Precompile your ASP.NET app so that IIS does not need to. See "VS 2008 Web Deployment Project Support Released"

Not sure if this will solve slow spin up of the WS on the "very first time", as I assume there is a load of compiling and .net DLL's being loaded, but you can almost eliminate any future cold starts by ensuring the application pool the WS is in is configured correctly.

By default, IIS6 has "respawn" on idle, after a number of minutes or "recycle" events that effectively restart the WS each time. If your happy the service is stable then these are not needed.

Ensuring that the WS has it's own dedicated application pool (is not sharing an inappropriate pool) is also a strong recommendation.