Control.Retry

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.

Types and Operations

newtype RetryPolicyM m

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:

1. If either policy returns Nothing, the combined policy returns Nothing. This can be used to inhibit after a number of retries, for example.
2. 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 50000 <> limitRetries 5

Naturally, mempty will retry immediately (delay 0) for an unlimited number of retries, forming the identity for the Monoid.

The default retry policy retryPolicyDefault implements a constant 50ms delay, up to 5 times:

> retryPolicyDefault = constantDelay 50000 <> limitRetries 5

For anything more complex, just define your own RetryPolicyM:

> myPolicy = retryPolicy $ \ rs -> if rsIterNumber rs > 10 then Just 1000 else Just 10000

Since 0.7.

Constructors

RetryPolicyM

Fields

• getRetryPolicyM :: RetryStatus -> m (Maybe Int)

Instances

Monad m => Semigroup (RetryPolicyM m)

Defined in Control.Retry

Methods

(<>) :: 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)

Defined in Control.Retry

Methods

mempty :: RetryPolicyM m

mappend :: RetryPolicyM m -> RetryPolicyM m -> RetryPolicyM m

mconcat :: [RetryPolicyM m] -> RetryPolicyM m

type RetryPolicy = forall m. Monad m => RetryPolicyM m

Simplified RetryPolicyM without any use of the monadic context in determining policy. Mostly maintains backwards compatitibility with type signatures pre-0.7.

retryPolicy :: Monad m => (RetryStatus -> Maybe Int) -> RetryPolicyM m

Helper for making simplified policies that don't use the monadic context.

retryPolicyDefault :: Monad m => RetryPolicyM m

Default retry policy

natTransformRetryPolicy :: (forall a. m a -> n a) -> RetryPolicyM m -> RetryPolicyM n

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 RetryAction

How to handle a failed action.

Constructors

DontRetry

Don't retry (regardless of what the RetryPolicy says).

ConsultPolicy

Retry if the RetryPolicy says so, with the delay specified by the policy.

ConsultPolicyOverrideDelay Int

Retry if the RetryPolicy says so, but override the policy's delay (number of microseconds).

Instances

Eq RetryAction

Defined in Control.Retry

Methods

(==) :: RetryAction -> RetryAction -> Bool

(/=) :: RetryAction -> RetryAction -> Bool

Read RetryAction

Defined in Control.Retry

Methods

readsPrec :: Int -> ReadS RetryAction

readList :: ReadS [RetryAction]

readPrec :: ReadPrec RetryAction

readListPrec :: ReadPrec [RetryAction]

Show RetryAction

Defined in Control.Retry

Methods

showsPrec :: Int -> RetryAction -> ShowS

show :: RetryAction -> String

showList :: [RetryAction] -> ShowS

Generic RetryAction

Defined in Control.Retry

Associated Types

type Rep RetryAction :: Type -> Type

Methods

from :: RetryAction -> Rep RetryAction x

to :: Rep RetryAction x -> RetryAction

type Rep RetryAction

Defined in Control.Retry

type Rep RetryAction = D1 ('MetaData "RetryAction" "Control.Retry" "retry-0.9.0.0-1kxQXwrMn4iC0TToNFtCE5" 'False) (C1 ('MetaCons "DontRetry" 'PrefixI 'False) (U1 :: Type -> Type) :+: (C1 ('MetaCons "ConsultPolicy" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons "ConsultPolicyOverrideDelay" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 Int))))

toRetryAction :: Bool -> RetryAction

Convert a boolean answer to the question "Should we retry?" into a RetryAction.

data RetryStatus

Datatype with stats about retries made thus far. The constructor is deliberately not exported to make additional fields easier to add in a backward-compatible 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

Constructors

RetryStatus

Fields

• rsIterNumber :: !Int

  Iteration number, where 0 is the first try

• rsCumulativeDelay :: !Int

  Delay incurred so far from retries in microseconds

• rsPreviousDelay :: !(Maybe Int)

  Latest attempt's delay. Will always be Nothing on first run.

Instances

Eq RetryStatus

Defined in Control.Retry

Methods

(==) :: RetryStatus -> RetryStatus -> Bool

(/=) :: RetryStatus -> RetryStatus -> Bool

Read RetryStatus

Defined in Control.Retry

Methods

readsPrec :: Int -> ReadS RetryStatus

readList :: ReadS [RetryStatus]

readPrec :: ReadPrec RetryStatus

readListPrec :: ReadPrec [RetryStatus]

Show RetryStatus

Defined in Control.Retry

Methods

showsPrec :: Int -> RetryStatus -> ShowS

show :: RetryStatus -> String

showList :: [RetryStatus] -> ShowS

Generic RetryStatus

Defined in Control.Retry

Associated Types

type Rep RetryStatus :: Type -> Type

Methods

from :: RetryStatus -> Rep RetryStatus x

to :: Rep RetryStatus x -> RetryStatus

type Rep RetryStatus

Defined in Control.Retry

type Rep RetryStatus = D1 ('MetaData "RetryStatus" "Control.Retry" "retry-0.9.0.0-1kxQXwrMn4iC0TToNFtCE5" 'False) (C1 ('MetaCons "RetryStatus" 'PrefixI 'True) (S1 ('MetaSel ('Just "rsIterNumber") 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Int) :*: (S1 ('MetaSel ('Just "rsCumulativeDelay") 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Int) :*: S1 ('MetaSel ('Just "rsPreviousDelay") 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 (Maybe Int)))))

defaultRetryStatus :: RetryStatus

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)

Apply policy on status to see what the decision would be. Nothing implies no retry, Just returns updated status.

applyAndDelay :: MonadIO m => RetryPolicyM m -> RetryStatus -> m (Maybe RetryStatus)

Apply policy and delay by its amount if it results in a retry. Return updated status.

Lenses for RetryStatus

rsIterNumberL :: Lens' RetryStatus Int

rsCumulativeDelayL :: Lens' RetryStatus Int

rsPreviousDelayL :: Lens' RetryStatus (Maybe Int)

Applying Retry Policies

retrying

Arguments

:: 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 retryPolicyDefault (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.

retryingDynamic

Arguments

:: MonadIO m

=> RetryPolicyM m

-> (RetryStatus -> b -> m RetryAction)

An action to check whether the result should be retried. The returned RetryAction determines how/if a retry is performed. See documentation on RetryAction.

-> (RetryStatus -> m b)

Action to run

-> m b

Same as retrying, but with the ability to override the delay of the retry policy based on information obtained after initiation.

For example, if the action to run is a HTTP request that turns out to fail with a status code 429 ("too many requests"), the response may contain a "Retry-After" HTTP header which specifies the number of seconds the client should wait until performing the next request. This function allows overriding the delay calculated by the given retry policy with the delay extracted from this header value.

In other words, given an arbitrary RetryPolicyM rp, the following invocation will always delay by 1000 microseconds:

retryingDynamic rp (\_ _ -> return $ ConsultPolicyOverrideDelay 1000) f

Note that a RetryPolicys decision to not perform a retry cannot be overridden. Ie. when to stop retrying is always decided by the retry policy, regardless of the returned RetryAction value.

recovering

Arguments

:: (MonadIO m, MonadMask m)

=> RetryPolicyM m

Just use retryPolicyDefault for default settings

-> [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

recoveringDynamic

Arguments

:: (MonadIO m, MonadMask m)

=> RetryPolicyM m

Just use retryPolicyDefault for default settings

-> [RetryStatus -> Handler m RetryAction]

Should a given exception be retried? Action will be retried if this returns either ConsultPolicy or ConsultPolicyOverrideDelay *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

The difference between this and recovering is the same as the difference between retryingDynamic and retrying.

stepping

Arguments

:: (MonadIO m, MonadMask m)

=> RetryPolicyM m

Just use retryPolicyDefault for default settings

-> [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 external-interfacing systems.

recoverAll :: (MonadIO m, MonadMask m) => RetryPolicyM m -> (RetryStatus -> m a) -> m a

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 retryPolicyDefault 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]

List of pre-made 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.

logRetries

Arguments

:: (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

For use with logRetries.

Retry Policies

constantDelay

Arguments

:: Monad m

=> Int

Base delay in microseconds

-> RetryPolicyM m

Implement a constant delay with unlimited retries.

exponentialBackoff

Arguments

:: Monad m

=> Int

Base delay in microseconds

-> RetryPolicyM m

Grow delay exponentially each iteration. Each delay will increase by a factor of two.

fullJitterBackoff

Arguments

:: 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)

fibonacciBackoff

Arguments

:: Monad m

=> Int

Base delay in microseconds

-> RetryPolicyM m

Implement Fibonacci backoff.

limitRetries

Arguments

:: Int

Maximum number of retries.

-> RetryPolicy

Retry immediately, but only up to n times.

Policy Transformers

limitRetriesByDelay

Arguments

:: Monad m

=> Int

Time-delay limit in microseconds.

-> RetryPolicyM m

-> RetryPolicyM m

Add an upperbound to a policy such that once the given time-delay 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 time-delay amount, use limitRetriesByCumulativeDelay

limitRetriesByCumulativeDelay

Arguments

:: Monad m

=> Int

Time-delay 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.

capDelay

Arguments

:: Monad m

=> Int

A maximum delay in microseconds

-> RetryPolicyM m

-> RetryPolicyM m

Set a time-upperbound 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)]

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 ()

Run given policy up to N iterations and pretty print results on the console.