initial commit with extraction of retry logic

Author: derwolfe

.gitignore

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+/target
+/classes
+/checkouts
+pom.xml
+pom.xml.asc
+*.jar
+*.class
+/.lein-*
+/.nrepl-port
+.hgignore
+.hg/

.travis.yml

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+language: clojure
+jdk:
+- oraclejdk8
+cache:
+  directories:
+  - "$HOME/.lein"
+  - "$HOME/.m2"
+script:
+  - lein cljfmt check
+  - lein test
+  - lein ancient
+after_success:
+  - CLOVERAGE_VERSION=1.0.7-SNAPSHOT lein cloverage --codecov
+  - bash <(curl -s https://codecov.io/bash) -f target/coverage/codecov.json

CHANGELOG.md

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+# Change Log
+All notable changes to this project will be documented in this file. This change log follows the conventions of [keepachangelog.com](http://keepachangelog.com/).
+
+## 0.1.0 - 2016-06-08
+### Added
+- All retry logic to perform exponential backoff.
+
+[Unreleased]: https://github.com/your-name/banach/compare/0.1.1...HEAD

LICENSE

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README.md

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+# Banach
+
+This contains utilities that can be useful when working with [Manifold](https://github.com/ztellman/manifold).
+
+At the moment the library provides a way to create retry functions for deferred
+producing operations.
+
+
+## License
+
+Copyright © 2016 Rackspace
+
+Distributed under the Eclipse Public License either version 1.0 or (at
+your option) any later version.

project.clj

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+(defproject banach "0.1.0-SNAPSHOT"
+  :description "Utility functions to use with manifold"
+  :url "https://github.com/racksec/banach"
+  :license {:name "Eclipse Public License"
+            :url "http://www.eclipse.org/legal/epl-v10.html"}
+  :dependencies [[org.clojure/clojure "1.8.0"]
+                 [com.taoensso/timbre "4.3.1"]
+                 [manifold "0.1.4"]
+                 [org.clojure/math.numeric-tower "0.0.4"]]
+  :plugins [[lein-auto "0.1.2"]
+            [lein-cljfmt "0.3.0"]
+            [lein-pprint "1.1.1"]
+            [lein-environ "1.0.2"]
+            [lein-ancient "0.6.10"]
+            [lein-cloverage "1.0.7-SNAPSHOT"]])

src/banach/retry.clj

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+(ns banach.retry
+  (:require
+   [clojure.math.numeric-tower :as math]
+   [manifold.deferred :as md]
+   [manifold.time :as mt]))
+
+(defn ^:private exponentially
+  "Returns a function that when evaluated will produce the initial wait
+  raised to the number of failures.
+
+  This is intended to be used in conjunction with 1-arg combinators that take a
+  vector of failures."
+  [wait]
+  (fn [failures]
+    (math/expt wait (count failures))))
+
+(defn ^:private up-to
+  "Returns a function that when evaluated will either
+
+  1) return a number to use to wait until retrying a function again. Or,
+  2) throw an exception because the maximum number of retries, `stop`, has
+  been reached.
+
+  This is intended to be used in conjunction with 1-arg combinators that take a
+  vector of failures."
+  [stop retry?]
+  (fn [failures]
+    (if (< (count failures) stop)
+      (retry? failures)
+      (throw (last failures)))))
+
+(defn ^:private retry
+  "Retry a function multiple times, pausing for a number of seconds between
+  each try.
+
+  f - a function that should be retried; must return a
+      `manifold.deferred/deferred'
+  strategy - a function that will produce a number and takes a vector of
+      exceptions. The number of exceptions is used to record the number
+      of attempts.
+
+  Returns a deferred wrapping the results of `f`."
+  [f strategy]
+  (md/loop [failures []]
+    (md/catch
+     (f)
+     Exception
+      (fn [exc]
+        (let [all-failures (conj failures exc)
+              wait (strategy all-failures)]
+          (mt/in (mt/seconds wait) #(md/recur all-failures)))))))
+
+(defn retry-exp-backoff
+  "Takes a function that returns a `manifold.deferred/deferred`. Retries that
+  function until it succeeds or the number of failures equal the stop value.
+
+  Expects to encounter exceptions when retrying `f`. As such,
+  it will catch all exceptions that `f` might throw and continue retrying.
+
+  f - a function that should be retried; must return a
+      `manifold.deferred/deferred'
+  p - an int representing the initial number of seconds to wait before retrying.
+      This will grow exponentially for each attempt.
+  stop - an int representing the number of tries that the api should make before
+         giving up and returning the last exception encountered.
+
+  Returns a deferred wrapping the results of `f`."
+  [f p stop]
+  (retry f (->> (exponentially p)
+                (up-to stop))))