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Κυριακή, 10 Οκτωβρίου 2010 6:41 μμ
από το μέλος
PALLADIN
Πολλές φορες έχουμε σενάρια που θα θέλαμε να εφαρμοσουμε συγκεκριμενα retry policies σε error prone computations.
Τα σενάρια αυτά εμφανίζονται αρκετά συχνά και κυρίως σε cloud computations.
Στο Azure για παράδειγμα, υπάρχουν και συγκεκριμένα APIs που παίζουν με Retry Policies.
Το παρακάτω monad είναι μια έμπνευση που είχα στο να κάνω combine το
reader monad (για το propagation του policy) μαζί με το exception monad έτσι ώστε να δομήσω ένα retry monad.
open System
open System.Threading
type ShouldRetry = ShouldRetry of (RetryCount * LastException -> bool * RetryDelay)
and RetryCount = int
and LastException = exn
and RetryDelay = TimeSpan
type RetryPolicy = RetryPolicy of ShouldRetry
type RetryPolicies() =
static member NoRetry () : RetryPolicy =
RetryPolicy( ShouldRetry (fun (retryCount, _) -> (retryCount < 1, TimeSpan.Zero)) )
static member Retry (currentRetryCount : int , intervalBewteenRetries : RetryDelay) : RetryPolicy =
RetryPolicy( ShouldRetry (fun (retryCount, _) -> (currentRetryCount < retryCount, intervalBewteenRetries)))
static member Retry (currentRetryCount : int) : RetryPolicy =
RetryPolicies.Retry(currentRetryCount, TimeSpan.Zero)
type RetryResult<'a> =
| RetrySuccess of 'a
| RetryFailure of exn
// Reader + Exception Monad DataType
type Retry<'a> = Retry of (RetryPolicy -> RetryResult<'a>)
type RetryBuilder() =
member self.Return (value : 'a) : Retry<'a> = Retry (fun retryPolicy -> RetrySuccess value)
member self.Bind (retry : Retry<'a>, bindFunc : 'a -> Retry<'b>) : Retry<'b> =
Retry (fun retryPolicy ->
let (Retry retryFunc) = retry
match retryFunc retryPolicy with
| RetrySuccess value ->
let (Retry retryFunc') = bindFunc value
retryFunc' retryPolicy
| RetryFailure exn -> RetryFailure exn )
member self.Delay (f : unit -> Retry<'a>) : Retry<'a> =
Retry (fun retryPolicy ->
let mutable resultCell : option<RetryResult<'a>> = None
let mutable lastExceptionCell : exn = null
let (RetryPolicy(ShouldRetry shouldRetry)) = retryPolicy
let mutable canRetryCell : bool = true
let mutable currentRetryCountCell : int = 0
while canRetryCell do
try
let (Retry retryFunc) = f ()
let result = retryFunc retryPolicy
resultCell <- Some result
canRetryCell <- false
with e ->
lastExceptionCell <- e
currentRetryCountCell <- 1 + currentRetryCountCell
match shouldRetry(currentRetryCountCell, lastExceptionCell) with
| (true, retryDelay) ->
canRetryCell <- true
Thread.Sleep(retryDelay)
| (false, _) -> canRetryCell <- false
match resultCell with
| Some result -> result
| None -> RetryFailure lastExceptionCell )
[<AutoOpen>]
module Retry =
let retry = new RetryBuilder()
// override default policy
let retryWithPolicy (retryPolicy : RetryPolicy) (retry : Retry<'a>) =
Retry (fun _ -> let (Retry retryFunc) = retry in retryFunc retryPolicy)
let run (retry : Retry<'a>) (retryPolicy : RetryPolicy) : RetryResult<'a> =
let (Retry retryFunc) = retry
retryFunc retryPolicy
// DivByzero example
let rnd = System.Random()
let example =
retry {
let! a = retry { return 1 / rnd.Next(0, 2) }
let! b = retry { return 2 / rnd.Next(0, 2) }
// override default policy
let! c = retryWithPolicy (RetryPolicies.NoRetry()) (retry { return 3 / rnd.Next(0, 2) })
return a * b * c
}run example (RetryPolicies.Retry 1)