nag

Retry/poll mechanism with backoff support and some handy defaults.

npm install nag
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nag.js

Nag is a small utility to handle automatic retries of an asynchronous operation.

It can:

  • Wait a specified amount of time between each attempt.
  • Retry a maximum number of times before giving up.
  • Wait longer between each attempt (exponential backoff).

There are multiple similar modules out there, but this one strives to be less verbose to invoke for the most common use cases. Also, I really wanted to write one.

Module is a callable function.

Example

var nag = require('nag');
var net = require('net');

function checkRedisUp(next) {
  net.connect(6379); // will throw exception if not responding
  next();
};

child_process.spawn('redis-server');

nag(checkRedisUp, function() {
  console.log("redis operational");
});

nag([config], poll, next)

The poll function must accept a standard callback(err, data) function as its only parameter.

Will repeatedly call poll until it gives a successful callback, in which it will call next(null, data) where data is whatever data poll yielded in its callback.

If a maximum number of attempts has been exceeded, it will call next(err) where err is the error object of the last failed poll call.

If poll throws an exception, it is treated the same as an error callback. E.g. if poll throws the exception ex, it is treated exactly as if poll called callback(ex).

Config format:

{
  wait: <default: 1000, number of miliseconds to wait between each attempt>,
  attempts: <default: infinite, number of attempts before giving up>,
  backoff: undefined <default - wait time is kept constant>
        || { multiplier: <default: 2, multiply wait time by this after each attempt>,
             maximum: <default: 60000, wait time will not exceed this> }
        || <multiplier - shorthand for above format>
        || <function taking current wait value and returning next wait value>,
  canRetry: undefined <default - all errors are retryable>
         || <function taking poll() error and returning whether it makes sense to retry>
}

Preconfigured variants:

Some variants of nag() with different default behavior have been provided for convenience. Arguments and config format are same as for nag().

  • nag.backoff - Waits 1 second after first attempt, 2 seconds after second, then 4 seconds, 8 seconds, and so on until a maximum of 60 seconds.
  • nag.minute - Retries every minute.
  • nag.rabid - Retries immediately after a failure (0 miliseconds between each attempt).
  • nag.burst - 10 miliseconds between each attempt. Gives up after 1000 failures.
  • nag.patient - Waits 1 minute after first failure, and adds 50% to the wait time after each failure. Max wait time is 1 hour.

License

MIT. See file LICENSE for details.

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