Again

A Clojure library for retrying an operation based on a retry strategy.

Clojars:

[listora/again "0.1.0"]

Usage

Require the library:

(require '[again.core :as again])

Again provides a very simple (too simple?) API for retrying an operation: given a retry strategy and an operation, the operation will be retried based on the provided strategy if it throws an exception.

A retry strategy is just a sequence of integers that represent a delay in milliseconds before retrying the operation. Once the sequence runs out, with-retries will rethrow the last exception.

Basic usecase:

(again/with-retries
  [100 1000 10000]
  (my-operation arg-1 arg-2))

The above will retry my-operation three times, for a total of four tries, with 100ms, 1000ms and 10000ms delays between the retries.

The library provides a numbers of functions for generating and manipulating retry strategies. Most of the provided strategies are inifinite sequences. The strategies can be restricted with the manipulator functions.

Generators:

• constant-strategy - constant delays between retries
• immediate-strategy - 0ms delays between retries
• additive-strategy - incrementally increasing delays between retries
• stop-strategy - no retries
• multiplicative-strategy - exponentially inccreasing delays between retries

Manipulators:

• randomize-strategy - scale each delay with a new random number
• max-retries - limit the number of retries to a given number
• clamp-delay - limit the delay to a given number
• max-delay - stop retrying when the delay crosses a given number
• max-duration - stop retrying when the combined delay crosses a given number

Exponential backoff example:

The generators and manipulators can be combined to create a desired retry strategy. Eg an exponential backoff retry strategy with an initial delay of 500ms and a multiplier of 1.5, limited to either 10 retries or a maximum duration of 10s can be generated as follows:

(def exponential-backoff-strategy
  (again/max-duration
    10000
    (again/max-retries
      10
      (again/randomize-strategy
        0.5
        (again/multiplicative-strategy 500 1.5)))))

We can also prepend a 0 to the strategy in order to execute the first retry immediately:

(def exponential-backoff-strategy-with-immediate-retry
  (cons 0 exponential-backoff-strategy))

License

Copyright © 2014 Listora

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.