re

2019-09-11

re是什么

什么是re,Do it again, after a bit.

re使用教程帮助文档

Re

Do it again, if it doesn't work the first time. Supports various configurable retry strategies, including: constant, exponential backoff and linear backoff.

Functions are styled to match the simplicity and ease of use found in the asynclibrary.

Install

npm install re

Quick Example

var Re = require('re'),
    re = new Re();

re.try(repeatMe, doMeAtTheEnd);

var repeatMe = function(retryCount, done){
    if(retryCount < 2) done(new Error("Not there yet!"));
    else done(null, retryCount);
};

var doMeAtTheEnd = function(err, retryCount){
    console.log("It took this many tries: " + retryCount);
};

In the Browser

Tested in recent versions of Internet Explorer, Firefox and Chrome. Usage:

<script type="text/javascript" src="re.js"></script>
<script type="text/javascript">
  var re = new Re();

  re.try(repeatMe, doMeAtTheEnd);

  // repeatMe and doMeAtTheEnd are exactly as above

</script>

Try it in your browser with this test: test/test.htmlor play with the test in this fiddle: re-fiddle(these pages don't work in IE, because it's recently gone from lax to pedantic).

Usage

If you like the defaults, call it like this:

var Re = require('re'),
    re = new Re();

re.try(function(retryCount, done){
    if(retryCount < 2) done(new Error("Not there yet!"));
    else done(null, retryCount);
  },
  function(err, retryCount){
    console.log("It took this many retries: " + retryCount);
});

The re.tryfunction takes two arguments, a function to call until it works (or we run out of retries) and a function to call when it finally succeeds (or we fail too many times). As the name suggests we automatically wrap your function in a standard tryblock and, if an exception occurs, call it again according to the retry schedule.

This first function passed to re.tryshould take 2 arguments like this:

function operation(retryCount, done)

The retryCountargument is the number of the current retry. It'll be zero the first time and get bigger every time.

The doneargument is a function to call when you've completed your operation. If you encounter an error condition, pass in the errobject as the first argument. If you don't encounter an error, pass in a falsy first argument (null works). If you give us a falsy error and no exception happens, we call your callback with all the arguments passed into this function.

The second function passed to re.trycan take as many arguments as you like but should always start with an error parameter. This will be falsy, if no error happens.

The re.dofunction is like re.tryexcept it doesn't wrap your operation in a try...catch.

Options

The default options look like this:

var options = {
    retries : 10,
    strategy : {
      "type": Re.STRATEGIES.EXPONENTIAL,
      "initial":100,
      "base":2
    }
}

You pass this options object into the Reconstructor.

var Re = require('re'),
    re = new Re(options);

This gives you 10 retries and an exponential backoff strategy with the following progression (in milliseconds): 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600, 51200

Retry Strategy Examples

The following will retry every 400 milliseconds:

{"type": Re.STRATEGIES.CONSTANT, "initial": 400}

The following will give a linear backoff strategy that has the following progression (when paired with retries: 10) : 200, 400, 600, 800, 1000, 1200, 1400, 1600, 1800, 1800

{"type": Re.STRATEGIES.LINEAR, "initial": 200, "max":1800}

Both progressive strategies accept the maxoption. All strategies also accept a randoption. This is a Booleanthat adds a random multiplier between 1 and 2. This makes them act like the tradition backoff function. This option is set to falseby default.

Stability

Test coverage is good and expanding. We use mocha.

Technical Details

The traditional exponential backoff function is described here: Exponential Backoff in Distributed Systems. This is equivalent to our exponential backoff function with the randoption set to true.

Our formula for exponential backoff looks something like this, when using all the options:

return Math.min(random * initial * Math.pow(base, retry), max);

Where randomis a random number in the half-open interval [1, 2). When randomness is turned off, the value of this variable is always 1.

If you don't specify the maxoption, the formula looks like this:

return random * initial * Math.pow(base, retry);

I'm shamelessly stealing the following link from node-retryjust because it's fun for nerdy math people to play with. You can use it to calculate the exact value you need for the baseoption so that all retry intervals sum to a desired amount: Wolfram Alpha.


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