Schedule (Retry & Repeat)

The Schedule module provides a powerful DSL for describing policies that govern how and when a workflow should be executed again. You can use schedules to implement sophisticated retry strategies (like exponential backoff with jitter) or recurring tasks.

Note: Schedule is currently available on .NET only.

Basic Schedules

A schedule decides two things:

1. Whether to continue (recur).
2. How long to wait before the next attempt.

Fixed Number of Recursions

// Recur 5 times (6 attempts total)
let fiveTimes = Schedule.recurs 5

Fixed Spacing

// Recur indefinitely with 1 second between attempts
let everySecond = Schedule.spaced (TimeSpan.FromSeconds 1.0)

Exponential Backoff

// Delays: 100ms, 200ms, 400ms, 800ms...
let backoff = Schedule.exponential (TimeSpan.FromMilliseconds 100.0)

Adding Jitter

Jitter adds randomness to delays to prevent “thundering herd” problems in distributed systems. Schedule.jittered adds a random factor between 0.5x and 1.5x to the current delay.

let policy = 
    Schedule.exponential (TimeSpan.FromMilliseconds 100.0)
    |> Schedule.jittered

Retrying Failures

Use Flow.Retry to apply a schedule to a flow that might fail with an expected domain error (Cause.Fail).

Important: If the flow fails with a defect (Cause.Die) or is interrupted (Cause.Interrupt), it will not be retried. This ensures that logic errors and cancellation signals are respected immediately.

let unstableCall = 
    flow {
        printfn "Attempting call..."
        return! Flow.fail "temporary-error"
    }

// This will attempt the call up to 4 times (initial + 3 retries)
let resilientCall = 
    Flow.Retry(unstableCall, Schedule.recurs 3)

Repeating Successes

Use Flow.Repeat to execute a successful flow again. This is useful for polling, heartbeats, or recurring background tasks.

let pollStatus = 
    flow {
        printfn "Polling status..."
        return "Still working"
    }

// This will poll every 5 seconds until it fails or is cancelled
let recurringPoll = 
    Flow.Repeat(pollStatus, Schedule.spaced (TimeSpan.FromSeconds 5.0))

API Reference: Module Schedule

API Reference: Flow Extensions