Hang In There, Query
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And so I return from my break, with a fresh reenergized hatred for terrible and unexpected technical issues.
Speaking of which…
Jacked Up And Good To Go
I’ve been sitting on a data export project for a while now. Its a simple C# command line application that connects to one of our databases, correlates some entities together, pulls some data out and dumps it into an S3 bucket as CSV files. It has a wide variety of automated unit tests proving that the various components of the system function as expected, and a few integration and end-to-end tests that show that the entire export process works (i.e. given a set of command line arguments + an S3 bucket, does running the app result in data in the bucket).
From a purely technical point of view, the project has been “complete” for a while now, but I couldn’t turn it on until some other factors cleared up.
Well, while I was on holidays those other factors cleared up, so I thought to myself “I’ll just turn on the schedule in TeamCity and all will be well”.
Honestly, you’d think having written as much software as I have that I would know better.
That first run was something of a crapshow, and while some of the expected files ended up in the S3 bucket, a bunch of others were missing completely.
Even worse, the TeamCity task that we use for job execution thought everything completed successfully. The only indicator that it had failed was that the task description in TeamCity was not updated to show the summary of what it had done (i.e. it simply showed the generic “Success” message instead of the custom one we were supplying), which is suspicious, because I’ve seen TeamCity fail miserably like that before.
Not Enough Time In The World
Breaking it down, there were two issues afoot:
- Something failed, but TeamCity thought everything was cool
- Something failed
The first problem was easy to deal with; there was a bug in the way the process was reporting back to TeamCity when there was an unhandled exception in the data extraction process. With that sorted, the next run at least indicated a failure had occurred.
With the process indicating correctly that a bad thing had happened, the cause of the second problem became obvious.
Which explained perfectly why the tests had not picked up any issues, as while they run the full application (from as close to the command line as possible), they don’t run a full export, instead leveraging a limiter parameter to avoid the test taking too long.
Entirely my fault really, as I should have at least done a full export at some stage.
Normally I would look at a timeout with intense suspicion, as they typically indicate a inefficient query or operation of some sort. Simply raising the time allowed when timeouts start occurring is often a route to a terrible experience for the end-user, as operations take longer and longer to do what they want them to.
In this case though, it was reasonable that the data export would actually take a chunk of time greater than the paltry 60 seconds applied to command execution in Npgsql by default. Also, the exports that were failing were for the larger data sets (one of which had some joins onto other data sets) and being full exports, could not really make effective use of any indexes for optimisation.
So I upped the timeouts via the command line parameters and off it went.
Three hours later though, and I was pretty sure something else was wrong.
Socket To Me
Running the same process with the same inputs from my development environment showed the same problem. The process just kept on chugging along, never finishing. Pausing the execution in the development environment showed that at least one thread was stuck waiting eternally for some database related thing to conclude.
The code makes use of parallelisation (each export is its own isolated operation), so my first instinct was that there was some sort of deadlock involving one or more exports.
With the hang appearing to be related to database connectivity, I thought that maybe it was happening in the usage of Npgsql connections, but each export creates its own connection, so that seemed unlikely. There was always the possibility that the problem was related to connection pooling though, which is built into the library and is pretty much static state, but I had disabled that via the connection string, so it shouldn’t have been a factor.
I ripped out all of the parallelisation and ran the process again and it still hung. On closer investigation, it was only one specific export that was hanging, which was weird, because they all use exactly the same code.
Turning towards the PostgreSQL end of the system, I ran the process again, except this time started paying attention to the active connections to the database, the queries they were running, state transitions (i.e. active –> idle) and execution timestamps. This is pretty easy to do using the query:
SELECT * FROM pg_stat_activity
I could clearly see the export that was failing execute its query on a connection, stay active for 15 minutes and then transition back to idle, indicating that the database was essentially done with that operation. On the process side though, it kept chugging along, waiting eternally for some data to come through the Socket that it was listening on that would apparently never come.
The 15 minute query time was oddly consistent too.
It turns out the query was actually being terminated server side because of replica latency (the export process queries a replica DB), which was set to max out at, wait for it, 15 minutes.
For some reason the stream returned by the NpgsqlConnection.BeginTextExport(sql) just never ends when the underlying query is terminated on the server side.
My plan is to put together some information and log an issue in the Npgsql Github Repository, because I can’t imagine that the behaviour is intended.
Solve For X
With the problem identified, the only question remaining was what to do about it.
I don’t even like that our maximum replica latency is set to 15 minutes, so raising it was pretty much out of the question (and this process is intended to be automated and ongoing, so I would have to raise it permanently).
The only real remaining option is to break down the bigger query into a bunch of smaller queries and then aggregate myself.
So that’s exactly what I did.
Luckily for me, the data set had a field that made segmentation easy, though running a bunch of queries and streaming the results into a single CSV file meant that they had to be run sequentially, so no parallelization bonus for me.
This was one of those issues where I really should have had the foresight to see the first problem (timeouts when dealing with large data sets), but the existence of what looks to be a real bug made everything far more difficult than it could have been.
Still, it just goes to show that no matter how confident you are in a process, there is always the risk that when you execute it in reality that it all might go belly up.
Which just reinforces the idea that you should always be running it in reality as soon as you possibly can, and paying attention to what the results are.