One of the hardest practices to learn as a developer is to actually sit down and read the information that you have. Ever since we became a product company, we have to deal with partial information, because some data wasn’t captured, or it isn’t possible to capture more information because it is running in a production environment.

That means that we have to do the most with the information we have available. Here are a few recent cases of having information, and ignoring it. A customer complained that a query was timing out. He provided the following error.

{"Url":"/databases/Events/streams/query/
EventsByTimeStampAndSequenceIndex?&start=9134999&sort=EventTimeStamp&sort=EventSequence_Range&SortHint-
EventTimeStamp=String&SortHint-EventSequence_Range=Long","Error":"System.OperationCanceledException: The operation 
was canceled.\r\n   at System.Threading.CancellationToken.ThrowIfCancellationRequested()\r\n   at Raven.Database.
Indexing.Index.IndexQueryOperation.<Query>d__5d.MoveNext()\r\n   at Raven.Database.Util.ActiveEnumerable`1..ctor(
IEnumerable`1 enumerable)\r\n   at Raven.Database.Actions.QueryActions.DatabaseQueryOperation.Init()\r\n   at Raven
.Database.Server.Controllers.StreamsController.SteamQueryGet(String id)\r\n   at lambda_method(Closure , Object , 
Object[] )\r\n   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass10.<
GetExecutor>b__9(Object instance, Object[] methodParameters)\r\n   at System.Web.Http.Controllers.
ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, 
CancellationToken cancellationToken)\r\n--- End of stack trace from previous location where exception was thrown 
---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.
CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.
CompilerServices.TaskAwaiter`1.GetResult()\r\n   at System.Web.Http.Controllers.ApiControllerActionInvoker.<
InvokeActionAsyncCore>d__0.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown 
---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.
CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.
HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter`1.
GetResult()\r\n   at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext()\r\n--- End of 
stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.
ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task 
task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task 
task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()\r\n   at System.Web.Http.Controllers.
ExceptionFilterResult.<ExecuteAsync>d__0.MoveNext()"}

As you can see, that is a big chunk of text, and it require careful parsing. I’m actually quite proud of this, even though the error is obtuse, it is giving us all the information that we need to figure out what the problem is.

In particular, there are two pieces of information that we need to see here:

• OperationCancelledException is thrown when we have a timeout processing the query on the server side.
• start=9134999 is a request to do deep paging of about 10 million records, and then start reading.

Those two combine tell us that the issue is that we are doing deep paging, which cause us to timeout internally before we start sending any data.

But why are we timing out on deep paging? What is expensive about that?

Turns out that RavenDB does de-dup during queries. So if you got events/1 in page #1, you won’t see it in page #2 even if there are results from the index for that document in page #2. If you care to know more, the issue is related to fanout and one document having many index entries pointing to it. But the major issue here is that we need to scan 10 million docs to avoid duplicates before we can return any value to the user. There is a flag you can send that would avoid this, but that wasn’t used in this case.

Another issue that was solved by reading the message from the customer was:

Message
Version store out of memory (cleanup already attempted)
 
Exception
Microsoft.Isam.Esent.Interop.EsentVersionStoreOutOfMemoryException: Version store out of memory (cleanup already attempted) 
at Raven.Database.Storage.Esent.StorageActions.DocumentStorageActions.RemoveAllBefore(String name, Etag etag) in c:\Builds\RavenDB-Stable-3.0\Raven.Database\Storage\Esent\StorageActions\Lists.cs:line 74 
at Raven.Database.Smuggler.SmugglerEmbeddedDatabaseOperations.<>c__DisplayClass12.<PurgeTombstones>b__11(IStorageActionsAccessor accessor) in c:\Builds\RavenDB-Stable-3.0\Raven.Database\Smuggler\SmugglerEmbeddedDatabaseOperations.cs:line 219 
at Raven.Storage.Esent.TransactionalStorage.ExecuteBatch(Action`1 action, EsentTransactionContext transactionContext) in c:\Builds\RavenDB-Stable-3.0\Raven.Database\Storage\Esent\TransactionalStorage.cs:line 799 at Raven.Storage.Esent.TransactionalStorage.Batch(Action`1 action) in c:\Builds\RavenDB-Stable-3.0\Raven.Database\Storage\Esent\TransactionalStorage.cs:line 778 
at Raven.Abstractions.Smuggler.SmugglerDatabaseApiBase.<ExportData>d__2.MoveNext() in c:\Builds\RavenDB-Stable-3.0\Raven.Abstractions\Smuggler\SmugglerDatabaseApiBase.cs:line 187 --- End of stack trace from previous location where exception was thrown --- 
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

Reported
25 minutes ago (08/02/15, 9:43am)
Level
Error

Version store out of memory happens when the size of the uncommitted transaction is too large, and we error. Usually that indicate some unbounded operation that wasn’t handled well.

Those are almost always batch operations of some kind that do various typed of cleanups. In order to handle them, we are pulsing the transaction. Effectively committing the transaction and starting a new one. That frees the uncommitted values.

The problem was that this method had this behavior:

So the investigation was focused on why MaybePulseTransaction didn’t work properly. There is some heuristics there to avoid being too costly, so it was very strange.

Can you see the error? We were looking at the wrong overload, this is the one that takes a time, and the one that was called takes an etag. Indeed, in the version the customer had, that call wasn’t there. In the current version, it was added.

Knowing how to read errors and logs, and actually paying attention to what they are saying is crucial when the time to fix a problem is at hand.