co-log-concurrent- Asynchronous backend for co-log library
Copyright(c) 2018-2020 Kowainik (c) 2020 Alexander Vershilov
MaintainerAlexander Vershilov <>
Safe HaskellNone



For the speed reasons, you may want to dump logs asynchronously. It is especially useful when application threads are CPU bound while logs emitting is I/O bound. This approach allows mitigating bottlenecks from the I/O.

When writing an application user should be aware of the tradeoffs that concurrent log system can provide, in this module, we explain potential tradeoffs and describe if individual building blocks are affected or not.

  1. Unbound memory usage – if there is no backpressure mechanism the user threads, threads may generate more logs that can we can store at the same amount of time. In such cases messages are accumulated in memory. It extends GC times and memory usage.
  2. Persistence requirements – sometimes application may want to ensure that we persisted the logs before it moved to the next statement. It is not a case with concurrent log systems in general; some we lose logs even the thread moves forward. It may happen when the application exits before dumping all logs.
  3. Non-precise logging – sometimes there may be anomalies when storing logs, such as logs reordering or imprecise timestamps.

In case if your application is a subject of those problems you may consider not using concurrent logging system in other cases concurrent logging may be a good default for you.



Concurrent logger consists of the following building blocks (see schema below).

  1. Logger in the application thread. The application runs it in the main thread, and it has access to all the thread state. This logger can work in any m.
  2. Communication channel with backpressure support. In addition to the channel, we have a converter that puts the user message to the communication channel. This converter works in the user thread. Such a logger usually works in IO, but it's possible to make it work in STM as well. At this point, the library provides only IO version, but it can be lifted to any MonadIO by the user.
  3. Logger thread. It's a background thread that performs an actual synchronous write to the log sinks. Loggers there do not have access to the users' thread state.
   +-------------------------+                  +--------------------------------+
   |                         |                  | Logger        |   Sink-1       |
   |   Application Thread    |                  | Thread    +--->                |
   |   -----------------     |  +-----------+   |           |   +----------------+
   |                         |  |           |   +---------+ |   +----------------+
   |           +-------------+  |  channel  |   | Shared  +----->   Sink-2       |
   |           | application||  |          +----> logger  | |   |                |
   |           | logger    +----->          |   +---------+ |   +----------------+
   |           +-------------+  |           |   |           |   +----------------+
   |                         |  +-----------+   |           +--->   Sink3        |
   |                         |                  |               |                |
   |                         |                  |               +----------------+
   |                         |                  |                                |
   +-------------------------+                  +--------------------------------+

So usually user should write the logging system in the way that all 'LogAction.' that populate and filter information should live in the application logger. All loggers that do serialization and formatting should live in the shared logger.

If you need more concurrency it's possible to build multilayer systems:

  +-------------+                         +-------+
  | application |---+                 +---| sink-1|
  +-------------+   |   +---------+   |   +-------+
                    +---| logger  |---+
                        +---------+   |   +-------+
                                      +---| sink-2|

In this approach, the application concurrently writes logs to the logger, then the logger concurrently writing to all sinks.

Simple API.

Simple API provides a handy easy to use API that can be used directly in an application without dealing with internals. Based on users feedback, the internal implementation of the simple API may change, especially in early versions of the library. But the guarantee that we give is that no matter what implementation is, it keeps with reasonable defaults and can be applied to a generic application.

withBackgroundLogger Source #


:: MonadIO m 
=> Capacity

Capacity of messages to handle; bounded channel size

-> LogAction IO msg

Action that will be used in a forked thread

-> IO ()

Action to flush logs

-> (LogAction m msg -> IO a)

Continuation action

-> IO a 

An exception-safe way to create background logger. This method forks a thread that runs 'shared worker', see schema above.

Capacity - provides a backpressure mechanism and tells how many messages in-flight are allowed. In most cases, defCapacity works well. See forkBackgroundLogger for more details.

LogAction - provides a logger action, this action does not have access to the application state or thread info, so you should only pass methods that serialize and dump data there.

IO () - flush provides a function to flush all the logs, it allows flush logs by chunks, so LogAction may not care about flushing.

main :: IO ()
main =
     (pure ())
     (log -> usingLoggerT log $ do
        logMsg @ByteString "Starting application..."
        logMsg @ByteString "Finishing application..."

defCapacity :: Capacity Source #

Default capacity size.

  • 4096 messages that in flight
  • Up to 32 messages can be read in a single chunk

Exceptions in messages.

It worth discussing how the library handles exceptions in messages. User generates a message in the application thread, but it's guaranteed to be evaluated only in the logger thread. It means that on the contrary to the synchronious logger it's possible to store exception in the message and it another thread, so the application thread will never see it.

let message = showt (1/0)
unLogger logger message

In synchronous case exception will be rised in the thread, but in asynchronous thread exception will happen somewhere else.

Currently the library does not take any action to prevent that. The problem is that there is no safe strategy that will work in general case. There are some approaches though:

  1. Use deepseq to fully evaluate message before passing it to the logger. Doesn't work for all types, leads to additional computations.
  2. Require message to be WHNF-strict and call seq before sending message to the logger.
  3. Serialize message in the user thread.

Extended API.

Extended API explains how asynchronous logging is working and provides basic building blocks for writing your combinators. It is the part of the public API and does not change without prior notice.

The main abstraction for the concurrent worker is BackgroundWorker. It is a wrapper of the thread, that has a communication channel to talk to and threadId.

Background worker may provide a backpressure mechanism, but does not provide notification of completeness unless it's included in the message itself.

data BackgroundWorker msg Source #

Wrapper for the background thread that may receive messages to process.

backgroundWorkerWrite :: BackgroundWorker msg -> msg -> STM () Source #

Method for communication with the thread.

killBackgroundLogger :: BackgroundWorker msg -> IO () Source #

Stop background logger thread.

The thread is blocked until background thread will finish processing all messages that were written in the channel.

mkCapacity Source #


:: Natural

Size of the queue. Number of the messages in flight

-> Maybe Natural

Maximum number of messages that logger can read in a chunk.

-> Capacity 

Creates new capacity.


Background logger

forkBackgroundLogger :: Capacity -> LogAction IO msg -> IO () -> IO (BackgroundWorker msg) Source #

Creates background logger with given Capacity, takes a LogAction that should describe how to write logs.

capacity - parameter tells how many in-flight messages are allowed, if that value is reached then user's thread that emits logs is blocked until any message is written. Usually, if the value is chosen reasonably high and if this value is reached it means that the application environment experiences severe problems.

N.B. The LogAction is run in the background thread so that logger should not add any thread-specific context to the message.

N.B. On exit, even in case of exception thread will dump all values that are in the queue. But it will stop doing that in case if another exception will happen.

  • Exception handing*. forkBackoundLogger forks a private that and does not expect that someone sends it an asynchronous exception. It means that any asynchronous exception is treated as a command to stop the worker. Logger is not aware of synchronous exceptions as well, so handing of the application specific exceptions should be a concern of the logging action.

convertToLogAction :: MonadIO m => BackgroundWorker msg -> LogAction m msg Source #

Convert a given 'BackgroundWorker msg' into a 'LogAction msg' that will send log message to the background thread, without blocking the thread.

If logger dies for any reason then thread that emits logs will receive BlockedIndefinitelyOnSTM exception.

You can extend result worker with all functionality available with co-log. This logger will have an access to the thread state.

Worker thread

While generic background logger is enough for the most of the usecases, sometimes you may want even more.

There are at least two cases where that may happen:

  1. You need to modify logger, for example different threads wants to write to different sources. Or you want to change lgo mechanism in runtime.
  2. You may want to implement some notification machinery that allows you to guarantee that your logs were written before processing further.

In order to solve those problems worker thread abstraction was introduced. This is a worker that accepts any action and performs that.

mkBackgroundThread :: Capacity -> IO (BackgroundWorker (IO ())) Source #

Create a background worker with a given capacity. If capacity is reached, then the thread that tries to write logs will be blocked.

This method is more generic than forkBackgroundLogger but it's less effective, as you have to pass entire closure to be run and that leads to extra memory usage and indirect calls happening.

When closed it will dump all pending messages, unless another asynchronous exception will arrive, or synchronous exception will happen during the logging.

Note. Limit parameter of capacity is ignored here as the function performs IO actions and seems that doesn't benefit from the chunking. However it may change in the future versions if proved to be wrong.

runInBackgroundThread :: BackgroundWorker (IO ()) -> LogAction IO msg -> LogAction IO msg Source #

Run logger action asynchronously in the worker thread. Logger is executed in the other thread entirely, so if logger takes any thread related context it will be read from the other thread.

Usage example

Consider following example. (Leaving resource control aside).

data M msg = M (MVar ()) msg

notificationLogger :: MonadIO m => LoggerAction m msg -> LoggerAction m (M msg)
notificationLogger logger = LogAction $ (M lock msg) ->
   (unLogger logger msg) finally (putMVar lock ())

example = do
   worker <- mkBackgroundThread defCapacity
   lock <- newEmptyMVar
   -- Log message with default logger.
      (runInBackgroundThread worker
      (notificationLogger $ withLogByteStringFile "/var/log/myapp/log")
      (M lock "my message")
   -- Log message with a different logger.
      (runInBackgroundThread worker
      (withLogByteStringFile "varlogmyapplog")
      ("another message")
   -- Block until first message is logged.
   _ <- takeMVar lock