I have tried to implement an optimistic concurrency 'worker'.
Goal is to read a batch of data from the same database table (single table with no relations) with multiple parallel 'worker'. This did seem to work so far. I get optimistic concurrency exceptions here and there, catch them and retry.
So far so good, and the function to get the data works stable on my local setup. When moving the application to a test environment however, I get a strange timeout exception, which even if caught, will end the async function (breaks the while loop). Does someone see a flaw in the implementation? What could cause the timeout? What could cause the end of the async function?
public async IAsyncEnumerable<List<WorkItem>> LoadBatchedWorkload([EnumeratorCancellation] CancellationToken token, int batchSize, int runID)
{
DataContext context = null;
try
{
context = GetNewContext(); // create a new dbContext
List<WorkItem> workItems;
bool loadSuccessInner;
while (true)
{
if (token.IsCancellationRequested) break;
loadSuccessInner = false;
context.Dispose();
context = GetNewContext(); // create a new dbContext
RunState currentRunState = context.Runs.Where(a => a.Id == runID).First().Status;
try
{
// Error happens on the following line: Microsoft.Data.SqlClient.SqlException: Timeout
workItems = context.WorkItems.Where(a => a.State == ProcessState.ToProcess).Take(batchSize).ToList();
loadSuccessInner = true;
}
catch (Exception ex)
{
workItems = new List<WorkItem>();
}
if (workItems.Count == 0 && loadSuccessInner)
{
break;
}
//... update to a different RunState
//... if set successful yield the result
//... else cleanup and retry
}
}
finally
{
if (context != null) context.Dispose();
}
}
I verified that EntityFramework (here with MS SQL Server adapter) does full server side query, which translates to a simple query like this:
SELECT TOP 10 field_1, field_2 FROM WorkItems WHERE field_2 = 0
The query usually takes <1ms and the timeout is left on default of 30s
I verified that there are no cancelation requests fired
This happens also when there is only a single worker and no one else is accessing the database. I'm aware that a timeout can happen when the resource is busy or blocked. But until now, I never saw a timeout on any other query yet.