looper

A Clojure library that provides retries around clj-http http calls.

Rationale

Shit's brittle. TODO: expand on this

Usage

If you are using clj-http already, you can just add a dependency on looper:

[com.farmlogs/looper "0.2.0"]

Then turn this:

(ns whatever
  (:require [clj-http.client :as client]))

(defn do-stuff [body]
  (client/post "http://somewhere" {:content-type :json :body body}))

into:

(ns whatever
  (:require [looper.client :as client]))

(defn do-stuff [body]
  (client/post "http://somewhere" {:content-type :json :body body}))

and get retries around that post call.

What it's doing

This uses the retry functionality from the Failsafe library (via diehard) to wrap the clj-http call. By default, it will retry 10 times, with a backoff strategy of: 10ms, 40ms, 160ms, 640ms, 2000ms, 2000ms...

It will only retry if an exception was thrown, and: the exception has no :status in its ex-data or that :status is 5xx.

On each retry, it will log a message of the form:

(log/warn exception "failed-to-<method>" {:url url :attempt num-attempts :elapsed time-elapsed-since-first-call})

So the above example will result in messages like:

WARN: failed-to-post {:url http://somewhere, :attempt 1, :elapsed 10}
WARN: <stacktrace>

It will try to use Timbre, then tools.logging, then fall back to println if neither of those are available. It brings in no logging dependencies.

Overriding defaults

You can override the retry settings by adding a :looper/options map to the options map you pass to the get/post/etc function:

(client/post "http://somewhere" {:content-type :json
                                 :body body
                                 :looper/options {:max-retries 3}})

The following options can be overridden:

• :backoff-ms: should be a vector of [intial-backoff-in-ms max-backoff-in-ms delay-factor]. Default is [10 2000 4]. See the diehard docs for more details.

• :max-retries: number of retry attempts before giving up. Default is 10. Set to nil to retry forever.

• :on-failed-attempt: a fn that will be passed a map of the form:

{:result result-of-call
:exception exception
:method method
:url url
:attempt attempt-number
:elapsed time-elapsed-since-first-attempt}

The default fn log/warns the exception along with the method, url, attempt count, and elapsed time.

• :retry-if: a fn that will be passed the result and exception (if any), and should return truthy if the request should be retried. The default only retries if there is an exception and the exception has no status or has a status >= 500.

TODO

• Add docstrings
• Add metrics(?)

License

Copyright © 2017 FarmLogs

Distributed under the Eclipse Public License either version 2.0 or (at your option) any later version.