Exponential backoff retry logic
retry: libraries should not log at warn/err
All retry-attempt notifications are dropped to debug level. A retry
library emits nothing at or above info by default: the caller already
receives the outcome back and decides what to surface.
Users who want visibility into retry behaviour still can -- the [retry]
log source is kept so it can be enabled explicitly at debug.
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lib/retry.ml
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lib/retry.ml
···
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let config ?(max_retries = 3) ?(backoff_factor = 0.3) ?(backoff_max = 120.0)
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?(jitter = true) () =
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Log.debug (fun m ->
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m "Creating retry config: max_retries=%d backoff_factor=%.2f jitter=%b"
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max_retries backoff_factor jitter);
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{ max_retries; backoff_factor; backoff_max; jitter }
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let calculate_backoff ~config ~attempt =
···
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let delay =
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if config.jitter then base_delay +. Random.float base_delay else base_delay
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in
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let final_delay = min delay config.backoff_max in
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Log.debug (fun m ->
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m "Backoff: attempt=%d base=%.2f jitter=%b -> %.2fs" attempt base_delay
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config.jitter final_delay);
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final_delay
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min delay config.backoff_max
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+
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(* All retry events are logged at [debug]: a library should not inject
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warnings or errors into its caller's output. Callers wanting visibility
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into retry behaviour enable the [retry] log source at debug. *)
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let with_retry ~clock ~config ~should_retry f =
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let rec attempt n =
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Log.info (fun m -> m "Attempt %d/%d" n (config.max_retries + 1));
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Log.debug (fun m -> m "attempt %d/%d" n (config.max_retries + 1));
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match f () with
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| result ->
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if n > 1 then Log.info (fun m -> m "Succeeded after %d attempts" n);
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if n > 1 then Log.debug (fun m -> m "succeeded after %d attempts" n);
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result
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| exception exn when n <= config.max_retries && should_retry exn ->
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let delay = calculate_backoff ~config ~attempt:n in
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Log.warn (fun m ->
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m "Attempt %d/%d failed: %s. Retrying in %.2fs..." n
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Log.debug (fun m ->
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m "attempt %d/%d failed: %s; sleeping %.2fs" n
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(config.max_retries + 1) (Printexc.to_string exn) delay);
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Eio.Time.sleep clock delay;
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attempt (n + 1)
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| exception exn ->
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if n > config.max_retries then
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Log.err (fun m ->
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m "Failed after %d attempts: %s" n (Printexc.to_string exn))
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else
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Log.err (fun m ->
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m "Failed and won't retry: %s" (Printexc.to_string exn));
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raise exn
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| exception exn -> raise exn
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in
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attempt 1
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let with_retry_result ~clock ~config ~should_retry f =
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let rec attempt n =
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Log.info (fun m -> m "Attempt %d/%d" n (config.max_retries + 1));
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Log.debug (fun m -> m "attempt %d/%d" n (config.max_retries + 1));
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match f () with
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| Ok _ as result ->
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if n > 1 then Log.info (fun m -> m "Succeeded after %d attempts" n);
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if n > 1 then Log.debug (fun m -> m "succeeded after %d attempts" n);
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result
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| Error e when n <= config.max_retries && should_retry e ->
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let delay = calculate_backoff ~config ~attempt:n in
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Log.warn (fun m ->
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m "Attempt %d/%d returned error. Retrying in %.2fs..." n
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Log.debug (fun m ->
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m "attempt %d/%d returned error; sleeping %.2fs" n
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(config.max_retries + 1) delay);
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Eio.Time.sleep clock delay;
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attempt (n + 1)
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| Error _ as result ->
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if n > config.max_retries then
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Log.err (fun m -> m "Failed after %d attempts" n)
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else Log.err (fun m -> m "Failed and won't retry");
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result
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| Error _ as result -> result
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in
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attempt 1
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