Copyright  Ozgun Ataman <ozgun.ataman@soostone.com> 

License  BSD3 
Maintainer  Ozgun Ataman 
Stability  provisional 
Safe Haskell  None 
Language  Haskell2010 
This module exposes combinators that can wrap arbitrary monadic actions. They run the action and potentially retry running it with some configurable delay for a configurable number of times.
The express purpose of this library is to make it easier to work with IO and especially network IO actions that often experience temporary failure that warrant retrying of the original action. For example, a database query may time out for a while, in which case we should delay a bit and retry the query.
Synopsis
 newtype RetryPolicyM m = RetryPolicyM {
 getRetryPolicyM :: RetryStatus > m (Maybe Int)
 type RetryPolicy = forall m. Monad m => RetryPolicyM m
 retryPolicy :: (RetryStatus > Maybe Int) > RetryPolicy
 retryPolicyDefault :: RetryPolicy
 natTransformRetryPolicy :: (forall a. m a > n a) > RetryPolicyM m > RetryPolicyM n
 data RetryStatus = RetryStatus {
 rsIterNumber :: !Int
 rsCumulativeDelay :: !Int
 rsPreviousDelay :: !(Maybe Int)
 defaultRetryStatus :: RetryStatus
 applyPolicy :: Monad m => RetryPolicyM m > RetryStatus > m (Maybe RetryStatus)
 applyAndDelay :: MonadIO m => RetryPolicyM m > RetryStatus > m (Maybe RetryStatus)
 rsIterNumberL :: Lens' RetryStatus Int
 rsCumulativeDelayL :: Lens' RetryStatus Int
 rsPreviousDelayL :: Lens' RetryStatus (Maybe Int)
 retrying :: MonadIO m => RetryPolicyM m > (RetryStatus > b > m Bool) > (RetryStatus > m b) > m b
 recovering :: (MonadIO m, MonadMask m) => RetryPolicyM m > [RetryStatus > Handler m Bool] > (RetryStatus > m a) > m a
 stepping :: (MonadIO m, MonadMask m) => RetryPolicyM m > [RetryStatus > Handler m Bool] > (RetryStatus > m ()) > (RetryStatus > m a) > RetryStatus > m (Maybe a)
 recoverAll :: (MonadIO m, MonadMask m) => RetryPolicyM m > (RetryStatus > m a) > m a
 skipAsyncExceptions :: MonadIO m => [RetryStatus > Handler m Bool]
 logRetries :: (Monad m, Exception e) => (e > m Bool) > (Bool > e > RetryStatus > m ()) > RetryStatus > Handler m Bool
 defaultLogMsg :: Exception e => Bool > e > RetryStatus > String
 constantDelay :: Int > RetryPolicy
 exponentialBackoff :: Int > RetryPolicy
 fullJitterBackoff :: MonadIO m => Int > RetryPolicyM m
 fibonacciBackoff :: Int > RetryPolicy
 limitRetries :: Int > RetryPolicy
 limitRetriesByDelay :: Monad m => Int > RetryPolicyM m > RetryPolicyM m
 limitRetriesByCumulativeDelay :: Monad m => Int > RetryPolicyM m > RetryPolicyM m
 capDelay :: Monad m => Int > RetryPolicyM m > RetryPolicyM m
 simulatePolicy :: Monad m => Int > RetryPolicyM m > m [(Int, Maybe Int)]
 simulatePolicyPP :: Int > RetryPolicyM IO > IO ()
Types and Operations
newtype RetryPolicyM m Source #
A RetryPolicyM
is a function that takes an RetryStatus
and
possibly returns a delay in microseconds. Iteration numbers start
at zero and increase by one on each retry. A *Nothing* return value from
the function implies we have reached the retry limit.
Please note that RetryPolicyM
is a Monoid
. You can collapse
multiple strategies into one using mappend
or <>
. The semantics
of this combination are as follows:
 If either policy returns
Nothing
, the combined policy returnsNothing
. This can be used toinhibit
after a number of retries, for example.  If both policies return a delay, the larger delay will be used. This is quite natural when combining multiple policies to achieve a certain effect.
Example:
One can easily define an exponential backoff policy with a limited number of retries:
> limitedBackoff = exponentialBackoff 50 <> limitRetries 5
Naturally, mempty
will retry immediately (delay 0) for an
unlimited number of retries, forming the identity for the Monoid
.
The default under def
implements a constant 50ms delay, up to 5 times:
> def = constantDelay 50000 <> limitRetries 5
For anything more complex, just define your own RetryPolicyM
:
> myPolicy = retryPolicy $ \ rs > if rsIterNumber n > 10 then Just 1000 else Just 10000
Since 0.7.
RetryPolicyM  

Instances
Monad m => Semigroup (RetryPolicyM m) Source #  
Defined in Control.Retry (<>) :: RetryPolicyM m > RetryPolicyM m > RetryPolicyM m # sconcat :: NonEmpty (RetryPolicyM m) > RetryPolicyM m # stimes :: Integral b => b > RetryPolicyM m > RetryPolicyM m #  
Monad m => Monoid (RetryPolicyM m) Source #  
Defined in Control.Retry mempty :: RetryPolicyM m # mappend :: RetryPolicyM m > RetryPolicyM m > RetryPolicyM m # mconcat :: [RetryPolicyM m] > RetryPolicyM m # 
type RetryPolicy = forall m. Monad m => RetryPolicyM m Source #
Simplified RetryPolicyM
without any use of the monadic context in
determining policy. Mostly maintains backwards compatitibility with
type signatures pre0.7.
retryPolicy :: (RetryStatus > Maybe Int) > RetryPolicy Source #
Helper for making simplified policies that don't use the monadic context.
retryPolicyDefault :: RetryPolicy Source #
Default retry policy
natTransformRetryPolicy :: (forall a. m a > n a) > RetryPolicyM m > RetryPolicyM n Source #
Applies a natural transformation to a policy to run a RetryPolicy
meant for the monad m
in the monad n
provided a transformation
from m
to n
is available. A common case is if you have a pure
policy, RetryPolicyM Identity
and want to use it to govern an
IO
computation you could write:
purePolicyInIO :: RetryPolicyM Identity > RetryPolicyM IO purePolicyInIO = natTransformRetryPolicy (pure . runIdentity)
data RetryStatus Source #
Datatype with stats about retries made thus far. The constructor is deliberately not exported to make additional fields easier to add in a backwardcompatible manner. To read or modify fields in RetryStatus, use the accessors or lenses below. Note that if you don't want to use lenses, the exported field names can be used for updates:
> retryStatus { rsIterNumber = newIterNumber } > retryStatus & rsIterNumberL .~ newIterNumber
RetryStatus  

Instances
defaultRetryStatus :: RetryStatus Source #
Initial, default retry status. Exported mostly to allow user code to test their handlers and retry policies. Use fields or lenses to update.
applyPolicy :: Monad m => RetryPolicyM m > RetryStatus > m (Maybe RetryStatus) Source #
applyAndDelay :: MonadIO m => RetryPolicyM m > RetryStatus > m (Maybe RetryStatus) Source #
Apply policy and delay by its amount if it results in a retry. Return updated status.
Lenses for
RetryStatus
rsIterNumberL :: Lens' RetryStatus Int Source #
rsCumulativeDelayL :: Lens' RetryStatus Int Source #
rsPreviousDelayL :: Lens' RetryStatus (Maybe Int) Source #
Applying Retry Policies
:: MonadIO m  
=> RetryPolicyM m  
> (RetryStatus > b > m Bool)  An action to check whether the result should be retried. If True, we delay and retry the operation. 
> (RetryStatus > m b)  Action to run 
> m b 
Retry combinator for actions that don't raise exceptions, but
signal in their type the outcome has failed. Examples are the
Maybe
, Either
and EitherT
monads.
Let's write a function that always fails and watch this combinator retry it 5 additional times following the initial run:
>>>
import Data.Maybe
>>>
let f _ = putStrLn "Running action" >> return Nothing
>>>
retrying def (const $ return . isNothing) f
Running action Running action Running action Running action Running action Running action Nothing
Note how the latest failing result is returned after all retries have been exhausted.
:: (MonadIO m, MonadMask m)  
=> RetryPolicyM m  Just use 
> [RetryStatus > Handler m Bool]  Should a given exception be retried? Action will be retried if this returns True *and* the policy allows it. This action will be consulted first even if the policy later blocks it. 
> (RetryStatus > m a)  Action to perform 
> m a 
Run an action and recover from a raised exception by potentially
retrying the action a number of times. Note that if you're going to
use a handler for SomeException
, you should add explicit cases
*earlier* in the list of handlers to reject AsyncException
and
SomeAsyncException
, as catching these can cause thread and
program hangs. recoverAll
already does this for you so if you
just plan on catching SomeException
, you may as well ues
recoverAll
:: (MonadIO m, MonadMask m)  
=> RetryPolicyM m  Just use 
> [RetryStatus > Handler m Bool]  Should a given exception be retried? Action will be retried if this returns True *and* the policy allows it. This action will be consulted first even if the policy later blocks it. 
> (RetryStatus > m ())  Action to run with updated status upon failure. 
> (RetryStatus > m a)  Main action to perform with current status. 
> RetryStatus  Current status of this step 
> m (Maybe a) 
A version of recovering
that tries to run the action only a
single time. The control will return immediately upon both success
and failure. Useful for implementing retry logic in distributed
queues and similar externalinterfacing systems.
recoverAll :: (MonadIO m, MonadMask m) => RetryPolicyM m > (RetryStatus > m a) > m a Source #
Retry ALL exceptions that may be raised. To be used with caution;
this matches the exception on SomeException
. Note that this
handler explicitly does not handle AsyncException
nor
SomeAsyncException
(for versions of base >= 4.7). It is not a
good idea to catch async exceptions as it can result in hanging
threads and programs. Note that if you just throw an exception to
this thread that does not descend from SomeException, recoverAll
will not catch it.
See how the action below is run once and retried 5 more times before finally failing for good:
>>>
let f _ = putStrLn "Running action" >> error "this is an error"
>>>
recoverAll def f
Running action Running action Running action Running action Running action Running action *** Exception: this is an error
skipAsyncExceptions :: MonadIO m => [RetryStatus > Handler m Bool] Source #
List of premade handlers that will skip retries on
AsyncException
and SomeAsyncException
. Append your handlers to
this list as a convenient way to make sure you're not catching
async exceptions like user interrupt.
:: (Monad m, Exception e)  
=> (e > m Bool)  Test for whether action is to be retried 
> (Bool > e > RetryStatus > m ())  How to report the generated warning message. Boolean is whether it's being retried or crashed. 
> RetryStatus  Retry number 
> Handler m Bool 
Helper function for constructing handler functions of the form required
by recovering
.
defaultLogMsg :: Exception e => Bool > e > RetryStatus > String Source #
For use with logRetries
.
Retry Policies
:: Int  Base delay in microseconds 
> RetryPolicy 
Implement a constant delay with unlimited retries.
:: Int  Base delay in microseconds 
> RetryPolicy 
Grow delay exponentially each iteration. Each delay will increase by a factor of two.
:: MonadIO m  
=> Int  Base delay in microseconds 
> RetryPolicyM m 
FullJitter exponential backoff as explained in AWS Architecture Blog article.
http://www.awsarchitectureblog.com/2015/03/backoff.html
temp = min(cap, base * 2 ** attempt)
sleep = temp / 2 + random_between(0, temp / 2)
:: Int  Base delay in microseconds 
> RetryPolicy 
Implement Fibonacci backoff.
:: Int  Maximum number of retries. 
> RetryPolicy 
Retry immediately, but only up to n
times.
Policy Transformers
:: Monad m  
=> Int  Timedelay limit in microseconds. 
> RetryPolicyM m  
> RetryPolicyM m 
Add an upperbound to a policy such that once the given timedelay
amount *per try* has been reached or exceeded, the policy will stop
retrying and fail. If you need to stop retrying once *cumulative*
delay reaches a timedelay amount, use
limitRetriesByCumulativeDelay
limitRetriesByCumulativeDelay Source #
:: Monad m  
=> Int  Timedelay limit in microseconds. 
> RetryPolicyM m  
> RetryPolicyM m 
Add an upperbound to a policy such that once the cumulative delay over all retries has reached or exceeded the given limit, the policy will stop retrying and fail.
:: Monad m  
=> Int  A maximum delay in microseconds 
> RetryPolicyM m  
> RetryPolicyM m 
Set a timeupperbound for any delays that may be directed by the
given policy. This function does not terminate the retrying. The policy
`capDelay maxDelay (exponentialBackoff n)` will never stop retrying. It
will reach a state where it retries forever with a delay of maxDelay
between each one. To get termination you need to use one of the
limitRetries
function variants.
Development Helpers
simulatePolicy :: Monad m => Int > RetryPolicyM m > m [(Int, Maybe Int)] Source #
Run given policy up to N iterations and gather results. In the
pair, the Int
is the iteration number and the Maybe Int
is the
delay in microseconds.
simulatePolicyPP :: Int > RetryPolicyM IO > IO () Source #
Run given policy up to N iterations and pretty print results on the console.