feature-change

Module to run current and new versions of a feature simultaneously. It helps find differences between results without breaking user applications.

Once you are sure that there are no more differences you can just remove it.

This blog post explains how it is used at Auth0.

Install

npm i -S feature-change

Usage

Let's assume you currently perform searches against mongodb and to improve upon this you want searches to be done in Elastic Search.

var feature_change = require('feature-change');

var current_implementation = mongo_search;
var new_implementation = es_search;

var options = {
  expected: function(cb){
      current_implementation(current_opts, cb);
  },
  actual: function(cb){
      new_implementation(es_opts, cb);
  },
  logAction: function(mongo_result, es_result){
      // invoked when there is a difference in the results (useful for logging)
  }
};

feature_change(options, function(err, result){
    // this is the original callback you were using for mongo
    // err and result always come from mongo_search
});

API

The feature_change function takes the following arguments:

• options - This parameter should contain the following properties:
  • expected - A function that takes a cb that performs the operation that results in the expected outcome. Commonly, the current implementation of your feature.
    • cb(err, result) - Takes err as the first parameter if an error occurred, otherwise result should be passed.
  • actual - A function that takes a cb that performs the operation that results in the actual outcome. Commonly, the new implementation of your feature.
    • cb(err, result) - Takes err as the first parameter if an error occurred, otherwise result should be passed.
  • logAction - A function that takes expected_result and actual_result. Only invoked if there is an error in expected or the results are different.
    • expected_result - An object that has an err property if an error occurred or a value if everything was successful.
    • actual_result - An object that has an err property if an error occurred or a value if everything was successful.
  • areEqual - (optional) A function that takes both results and must return a boolean indicating if both results are equal or not. By default the function from module deep-equal (https://www.npmjs.com/package/deep-equal) will be used.
• done - A function to be invoked with the result of the expected operation. This is commonly the callback you were using for the original implementation of the feature.

Implementation details

The result or err from the expected operation is always the one that is provided in done. The goal of this module is not to provide one or the other, but to provide a way to find differences in results.

As soon as the expected callback completes done will be invoked, even if actual has not completed. That is because there is no need to delay the result of the operation.

Contributing

Feel free to open issues with questions/bugs/features. PRs are also welcome.

License

MIT