Safe Haskell | None |
---|---|
Language | Haskell2010 |
This module is where you should start if you are interested in adding tracing to an
application. It provides backend-agnostic utilities to generate traces. Trace publication and
other backend-specific features are available in the modules below Monitor.Tracing
(e.g.
Monitor.Tracing.Zipkin). The additional functionality exposed under Control.Monad
in this
package is useful if you wish to implement a new tracing backend.
Synopsis
- class Monad m => MonadTrace m
- data Sampling
- alwaysSampled :: Sampling
- neverSampled :: Sampling
- sampledEvery :: Int -> Sampling
- sampledWhen :: Bool -> Sampling
- debugEnabled :: Sampling
- rootSpan :: MonadTrace m => Sampling -> Name -> m a -> m a
- childSpan :: MonadTrace m => Name -> m a -> m a
- data Zipkin
Overview
Let's assume we are interested in tracing the two following functions:
listTaskIDs' :: MonadIO m => m [Int] -- Returns a list of all task IDs. fetchTasks' :: MonadIO m => [Int] -> m [Task] -- Resolves IDs into tasks.
We can do so simply by wrapping them inside childSpan
calls and adding a MonadTrace
constraint:
import Monitor.Tracing listTaskIDs :: (MonadIO m, MonadTrace m) => m [Int] listTaskIDs = childSpan "list-task-ids" listTaskIDs' fetchTasks :: (MonadIO m, MonadTrace m) => [Int] -> m [Task] fetchTasks = childSpan "fetch-tasks" . fetchTasks'
Spans will now automatically get generated any time these actions are run! Each span will be
associated with various useful pieces of metadata, including lineage. For example, if we wrap
the two above functions in a rootSpan
, the spans will correctly be nested:
printTasks :: (MonadIO m, MonadTrace m) => m () printTasks = rootSpan alwaysSampled "list-tasks" $ listTaskIDs >>= fetchTasks >>= print
Spans can then be published to various backends. For example, to run the above action and publish its spans using Zipkin:
import qualified Monitor.Tracing.Zipkin as ZPK main :: IO () main = ZPK.with ZPK.defaultSettings $ ZPK.run printTasks
Generic trace creation
class Monad m => MonadTrace m Source #
A monad capable of generating traces.
There are currently two instances of this monad:
Instances
MonadTrace Identity Source # | |
MonadUnliftIO m => MonadTrace (TraceT m) Source # | |
(Monad m, MonadTrace m) => MonadTrace (ExceptT e m) Source # | |
(Monad m, MonadTrace m) => MonadTrace (StateT s m) Source # | |
(Monad m, MonadTrace m) => MonadTrace (StateT s m) Source # | |
(Monad m, MonadTrace m, Monoid w) => MonadTrace (WriterT w m) Source # | |
(Monad m, MonadTrace m, Monoid w) => MonadTrace (WriterT w m) Source # | |
(Monad m, MonadTrace m) => MonadTrace (ReaderT r m) Source # | |
(Monad m, MonadTrace m, Monoid w) => MonadTrace (RWST r w s m) Source # | |
(Monad m, MonadTrace m, Monoid w) => MonadTrace (RWST r w s m) Source # | |
Controlling the sampling rate
alwaysSampled :: Sampling Source #
Returns a Sampling
which always samples.
neverSampled :: Sampling Source #
Returns a Sampling
which never samples.
sampledEvery :: Int -> Sampling Source #
Returns a Sampling
which randomly samples one in every n
spans.
sampledWhen :: Bool -> Sampling Source #
debugEnabled :: Sampling Source #
Returns a debug Sampling
. Debug spans are always sampled.
Building hierarchical traces
By default, traces created by trace
are independent from each other. However, we can get a
lot more value out of tracing by organizing a trace's spans. The simplest and most common
approach is to build a tree of spans, with a single root span and zero or more children for
each span. rootSpan
and childSpan
below set up spans such that the lineage of spans is
automatically propagated.
childSpan :: MonadTrace m => Name -> m a -> m a Source #
Extends a trace if it is active, otherwise do nothing.
Backends
As a convenience, the top-level type for each backend is exported here.