Implementing an asynchronous call to a long running process MVC4 - asp.net-mvc-4

Question

I have a large data set that holds up my UI so I thought I would create a background call to fill my local repository and display my other controls in the UI right away and load the results of the async call when I get a response.
I found a helpful tutorial but I am still having to wait until all my results are loaded before I see any controls.
http://www.asp.net/mvc/tutorials/mvc-4/using-asynchronous-methods-in-aspnet-mvc-4
CODE UPDATED
I have created a folder called Services and created FacilitiesService.cs in that folder, see below:
public class FacilitiesService
{
internal async Task<List<Facility>> GetFacilitiesBySourceDbAsync(string sourceDb)
{
var fac = new Facility();
var con = Connect(); // Omitted
try
{
con.Open();
}
catch (Exception ex)
{
Console.WriteLine("Error: GetFacilityBySourceDb " + ex.Message);
}
try
{
OracleDataReader reader = null;
// Requestor
var cmd = new OracleCommand("SELECT FACILITY, FACILITY_ID FROM MyTable where (source_db = '" + sourceDb + "')", con);
reader = cmd.ExecuteReader();
while (reader.Read())
{
fac.Add(new Facility()
{
FacilityName = reader["FACILITY"].ToString(),
FacilityId = reader["FACILITY_ID"].ToString()
});
}
}
catch (Exception ex)
{
throw ex;
}
finally
{
con.Close();
con.Dispose();
}
return fac;
}
}
Then in my HomeController.cs I have the following:
public class HomeController
{
public async Task<List<Facility>> FacilitiesAsync()
{
ViewBag.SyncOrAsync = "Asynchronous";
var service = new FacilitiesService();
this._facilities = new List<Facility>();
var facilities = await service.GetFacilitiesBySourceDbAsync("TEST");
foreach (var item in facilities)
{
Facility fac = new Facility()
{
FacilityName = item.FacilityName,
FacilityId = item.FacilityId
};
_facilities.Add(fac);
}
return _facilities;
}
}
This is my Facility (model) class:
public class Facility : List<Facility>
{
[Required]
[Display(Name = "Facility")]
public string FacilityName { get; set; }
public string FacilityId { get; set; }
public Facility()
{
// Default Constructor
}
public Facility(string facilityName, string facilityId)
{
this.FacilityName = facilityName;
this.FacilityId = facilityId;
}
}
I am using an Ajax call to kick off the FacilitiesAsync method in the code behind from a function call in the About.cshtml page when the user tabs off the tetbox/input control with an id of "tags", I could switch this to something else later but I get the data back when I step through the code-behind and I see both the beforeSend and complete functions fire an alert:
<script type="text/javascript">
$(function () {
var availableTags = [
// Neeed data from function call to populate this list
];
$("#tags").autocomplete({
source: availableTags
});
$("#tags").focusout(function () {
var result = null;
$.ajax({
beforeSend: function() {
alert("Testing");
},
url: "FacilitiesAsync",
success: function(data) {
result = data;
},
complete: function () {
alert(result);
}
});
});
});
</script>
#using (Html.BeginForm()) {
<div class="ui-widget">
<label for="tags">Tags: </label>
<input id="tags" />
</div>
}
This works GREAT! However, I want to take the data from the call made to the code-behind to populate the array availableTags and I'm not sure how to do that. Suggestions?

Answer 1

There's a few things wrong with the implementations, and one problem with the approach.
First, the GetFacilitiesBySourceDbAsync does not contain an await. The compiler will issue a warning in this situation, informing you that it's not really an asynchronous method when you do that; it will run synchronously. That's an important warning. If you want asynchronous code, you'll need to make it asynchronous all the way. This means using the asynchronous database methods (ExecuteReaderAsync, etc).
Secondly, the Task.WhenAll call in Index is meaningless (since you only pass it a single task). Also, since Index is synchronous but calls an asynchronous method, the code not shown is probably calling Result, which is a no-no on ASP.NET. As I explain on my blog, this will actually deadlock once your async code is actually asynchronous.
But even if you fix these, it won't do what you want. There's a problem with the approach, and that is that async doesn't change the HTTP protocol (this is also a link to my blog). ASP.NET MVC understands asynchronous methods and will not complete the request until the async action method completes. You'll need to use something like AJAX to get the web page to do what you want.