SimpleMapReduce

Simple MapReduce implementation, written in JavaScript.

Installation

Via npm on Node:

npm install simplemapreduce

Usage

Reference in your program:

var simplemapreduce = require('simplemapreduce');

Synchronous Map Reduce

Synchronous run

simplemapreduce.mapReduceSync(items, mapfn, reducefn);

where

• items: to be processed. In the current version, it's an object with forEach function defined.
• mapfn(key, value, ctx): given a key/value pair, it emits zero, one or more key/value pairs using ctx. See example below.
• reducefn(key, values, ctx): given a key and its associated values, emits zero, one or more key/value pairs using ctx.

Word count example

var result = simplemapreduce.mapReduceSync(
    ["A", "word", "is", "a", "word"], // items to process
    function (key, value, ctx) { ctx.emit(value.toLowerCase(), 1); }, // map
    function (key, values, ctx) { // reduce
        var total = 0;
        values.forEach(function (value) {
            total += value;
        });
        ctx.emit(key, total);
    }
);

Map Reduce

simplemapreduce.mapReduceSync(items, mapfn, reducefn, callbackfn);

where

• items: to be processed. In the current version, it's an object with keys that can be obtained using Object.keys(items).
• mapfn(key, value, ctx, next): given a key/value pair, it emits zero, one or more key/value pairs using ctx. See example below.
• reducefn(key, values, ctx, next): given a key and its associated values, emits zero, one or more key/value pairs using ctx.
• callbackfn(err, results): results is a dictionary with the key/value reduce outcome.

simplemapreduce.mapReduce(
    ["A", "word", "is", "a", "word"], // items to process
    function (key, value, ctx, next) { ctx.emit(value.toLowerCase(), 1); next(); }, // map
    function (key, values, ctx, next) { // reduce
        var total = 0;
        values.forEach(function (value) {
            total += value;
        });
        ctx.emit(key, total);
        next();
    },
    function (err, result) {
		// result.a === 2
		// result.word === 2
		// result.is === 1
    }
);

The above example is simple: it is executed without doing async calls in the map/reduce functions. But you can call next callback at any time.

Run

Synchronous run

simplemapreduce.runSync(items, mapfn, newfn, processfn);

where

• items: to be processed. In the current version, it's an object with forEach function defined.
• mapfn(key, value, ctx): given a key/value pair, it emits zero, one or more key/value pairs using ctx.
• newfn(key): given a new key, it returns the new object to be associated with that key.
• processfn(key, value, result): process an item, usually modifying its associated result object. In addition, it could receive and use the associated key and the map, the dictionary that is being build by the process.

Example

var result = simplemapreduce.runSync(
    ["A", "word", "is", "a", "word"], 
    function (key, value, ctx) { ctx.emit(value.toLowerCase(), 1); },
    function (key) { return { word: key, count: 0 }; },
    function (key, value, result) { result.count += value; }
);
console.dir(result);

Output

{ a: { count: 2 }, word: { count: 2 }, is: { count: 1 } }

There is a run with callback:

simplemapreduce.run(items, mapfn, newfn, processfn);

Example, see the use of next, received as last parameter by mapfn and processfn:

simplemapreduce.run(
    ["A", "word", "is", "a", "word"], 
    function (key, value, context, next) { context.emit(value.toLowerCase(), 1); next(); },
    function (key) { return { word: key, count: 0 }; },
    function (key, value, result, next) { result.count += value; next(); },
    function (err, result) {
		if (err)
			console.log(err);
		else
			console.dir(result);
	}
);

Run Task

Alternatively, you can define a task, an object with functions:

• getItems(): return the items to be processed.
• mapfn(key, value, ctx, next): given a key/value pair, it emits zero, one or more key/value pairs using ctx.
• newResult(key): creates a new object/value to be associated to the key/item. Usually it's used to accumulate results.
• process(key, value, result): function that process a key/value pair, usually updating the associated result object.

Example:

var task = {
    items: ["A", "word", "is", "a", "word"], 
    getItems: function () { return this.items; },
    map: function (key, value, context) { context.emit(value.toLowerCase(), 1); },
    newResult: function (key) { return { count: 0 }; },
    process: function (key, value, result) { result.count += value; }
};

simplemapreduce.runTask(task, function (err, result) { console.dir(result); });

Notice that in this case, getItems returns items defined in the same task. You can provide a more complex function, i.e. reading an stream or file.

Development

git clone git://github.com/ajlopez/SimpleMapReduce.git
cd SimpleMapReduce
npm install
npm test

Samples

Words Word Count sample with callback.

Words Sync Synchronous Word Count sample.

Task Run Task sample with callback.

Task Sync Synchrnous Run Task.

Distributed Workers A server sending tasks to distributed workers.

To do

• Improve async procesing
• Distributed sample

Versions

• 0.0.1 : Published, run, runSync
• 0.0.2 : Published, runTask, runTaskSync, first samples
• 0.0.3 : In master, work in progress. Distributed sample.

Contribution

Feel free to file issues and submit pull requests — contributions are welcome.

If you submit a pull request, please be sure to add or update corresponding test cases, and ensure that npm test continues to pass.