stm-stats: Retry statistics for STM transaction

Two days ago, Stefan Wehr talked at the Haskell in Leipzig workshop about the experiences that factis research had with Haskell when creating real-world applications. When he mentioned that we have code that keeps count of retries of STM transaction, someone from the audience asked us to publish the code. So, after some refactoring to make the interface generally usable, here it is. The stm-stats package provides a few functions (in Control.Concurrent.STM.Stats) that can replace atomically: trackSTM and variants that allow you to name the transaction or have more control about the warnings that will be emitted when the number of retries exceeds a certain value. The following code demonstrates how the module works:

import Control.Concurrent
import Control.Concurrent.STM
import Control.Monad

import Control.Concurrent.STM.Stats

main = do
var <- trackSTM $ newTVar 0
forkIO $ forM_ [1..23] $ \i -> do
    threadDelay (100*1000)
    trackNamedSTM "writer" $ writeTVar var i
putStrLn "Starting reader..."
trackNamedSTM "reader" $ do
    i <- readTVar var
    when (i < 23) retry
putStrLn "Reader finished."
dumpSTMStats

When run, you will see this output:

Starting reader...
STM transaction reader finished after 23 retries
Reader finished.
STM transaction statistics (2011-10-09 16:26:27.226675 UTC):
Transaction     Commits    Retries      Ratio
_anonymous_           1          0       0.00
reader                1         23      23.00
writer               23          0       0.00

PS: As I usually post on my own blog, I should explain why I from now on will also post on the factis research company blog. Factis research relies heavily on Free Software and wants to contribute back to the community where possible. But in the course of every-day work, this sometimes falls by the wayside. Therefore, I was hired to help out as the community interface: My task is identifying, packaging and uploading components of internal code that are of wider interest, following up on user requests and bug reports, and talk about it. This module is the first brought to you by this new strategy, but expect more to come.