httpbackoff

Automatic http retries for intermittent failures, with exponential backoff, based on https://github.com/cenkalti/backoff.

The reason for a separate library, is that this library handles http status codes to know whether to retry or not. HTTP codes in range 500-599, and code 429 are retried. Connection failures are also retried. Status codes 400-499 (except for 429) are considered permanent errors and are not retried.

The Retry function performs the http request and retries if temporary errors occur. It takes a single parameter as its input - a function to perform the http request. This function must return (resp *http.Response, tempError error, permError error) where tempError must be non-nil if a temporary error occurs (e.g. dropped connection), and permError must be non-nil if an error occurs that does not warrant retrying the request (e.g. badly formed url).

For example, the following code that is not using retries:

res, err := http.Get("http://www.google.com/robots.txt")

can be rewritten as:

res, attempts, err := httpbackoff.Retry(func() (*http.Response, error, error) {
	resp, err := http.Get("http://www.google.com/robots.txt")
	// assume all errors are temporary
    return resp, err, nil
})

Please note the additional return value attempts is an int specifying how many http calls were made (i.e. = 1 if no retries, otherwise > 1).

The go http package has 9 functions that return (*http.Reponse, error) that can potentially be retried:

• http://golang.org/pkg/net/http/#Client.Do
• http://golang.org/pkg/net/http/#Client.Get
• http://golang.org/pkg/net/http/#Client.Head
• http://golang.org/pkg/net/http/#Client.Post
• http://golang.org/pkg/net/http/#Client.PostForm
• http://golang.org/pkg/net/http/#Get
• http://golang.org/pkg/net/http/#Head
• http://golang.org/pkg/net/http/#Post
• http://golang.org/pkg/net/http/#PostForm

To simplify using these functions, 9 utility functions have been written that wrap these. Therefore you can simplify this example above further with:

res, _, err := httpbackoff.Get("http://www.google.com/robots.txt")

Configuring back off settings

To use cusom back off settings (for example, in testing, you might want to fail quickly), instead of calling the package functions, you can call methods of HTTPRetryClient with the same name:

package main

import (
	"log"
	"net/http/httputil"
	"time"

	"github.com/cenkalti/backoff"
	"github.com/taskcluster/httpbackoff"
)

func main() {
	// Note, you only need to create a client if you want to customise
	// the back off settings. In this example, we want to, but if you
	// wish to use the reasonable default settings, no need to do this.
	retryClient := httpbackoff.Client{
		BackOffSettings: &backoff.ExponentialBackOff{
			InitialInterval:     1 * time.Millisecond,
			RandomizationFactor: 0.2,
			Multiplier:          1.2,
			MaxInterval:         5 * time.Millisecond,
			MaxElapsedTime:      20 * time.Millisecond,
			Clock:               backoff.SystemClock,
		},
	}

	res, _, err := retryClient.Get("http://www.google.com/robots.txt")
	if err != nil {
		log.Fatalf("%s", err)
	}
	data, err := httputil.DumpResponse(res, true)
	if err != nil {
		log.Fatalf("%s", err)
	}
	log.Print(string(data))
}

Testing

The package has tests, which run in travis. See http://travis-ci.org/taskcluster/httpbackoff.

Concurrency

As far as I am aware, there is nothing in this library that prevents it from being used concurrently. Please let me know if you find any problems!

Contributing

Contributions are welcome. Please fork, and issue a Pull Request back with an explanation of your changes. Also please include tests for any functional changes.