async chaining blocks webapi call

Question

I've a flow: WebApi > ServiceFramework > DBLayer > MongoDB.

Since its a new application, I ensured to have async from the ground up in all layers. However, when my DB Layer has async code, webapi never gets a response back.

API CONTROLLER

[HttpGet]
public IHttpActionResult GetAllRecords()
{
   var result = FrameworkApi.GetRecords().Result;
   return Ok(result);
}

above calls > FRAMEWORK API

public async Task<List<Record>> GetRecords()
{
    return await FrameworkDbApi.GetRecords();
}

above calls > DB FRAMEWORK API (which Calls MongoDB)

public async Task<List<Record>> GetRecords()
{
    return await Task.Run(() =>                
       NoSqlDocumentClient.GetDefaultDatabase().Result.
       GetCollection<Record>("record").AsQueryable().ToList());            

      //following Synchronous version works..but defeats the purpose  
      //return NoSqlDocumentClient.GetDefaultDatabase().Result
      //       .GetCollection<Record>("record").AsQueryable().ToList();
}

However, when the operations in either DBLayer or Framework are invoked via test case, I do get result. But when invoked via WebApi controller, the asynchronous version never returns a response while synchronous version works fine.

Yuval Itzchakov · Accepted Answer · 2016-02-14 07:45:31Z

But when invoked via WebApi controller, the asynchronous version never returns a response while synchronous version works fine.

That's because your actual request is deadlocking. When you invoke the method via WebAPI, which has a SynchronizationContext, you're seeing the deadlock, in contrary to your test which hasn't got one, when the test runs fine. This is the reason why you shouldn't block on async code.

Your call-chain should look like this in order to avoid deadlocking (this is what it means to go "async all the way":

[HttpGet]
public async Task<IHttpActionResult> GetAllRecordsAsync()
{
   var result = await FrameworkApi.GetRecordsAsync();
   return Ok(result);
}

public Task<List<Record>> GetRecordsAsync()
{
    return FrameworkDbApi.GetRecordsAsync();
}

public async Task<List<Record>> GetRecordsAsync()
{
    var result = await NoSqlDocumentClient.GetDefaultDatabase();
    return result.GetCollection<Record>("record").AsQueryable().ToList();          
}

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