An implementation of the Concur algorithm.
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Concur Algorithm

An implementation of the Concur algorithm.

There is a paper describing the algorithm which is (as they say) by no means inaccessible to the non-specialist:


With npm:

npm install concur-algorithm

You can also clone the repository with Git...

git clone

...and then install the necessary modules with npm from within the project's root directory:

npm install

You will need to do this if you want to look at the example.


const concur = require('concur-algorithm'),    ///
      { generateOperations, transformContent } = concur;


Not all of the functionality required to implement the algorithm is exported by the package. If you want to see a full implementation, albeit a basic one, the only place to start is the example.

The example

Once you have cloned the repository, the example server can be run with the following command:

node ./bin/example.js

When the example server is up and running the example client will be available at http://localhost:8888/example.html. To try it out you might want to open several browser tabs or windows with this URL. You can also increase the latency in most browser's developer tools.

For a closer look at the example server and client implementations, see the example.js files in bin and es6 directories, respectively.

The server implementation is straightforward and should present no challenges to anyone familiar with Express.

The client implementation is necessarily more complex. It must handle user interactions, for which it sets up a dedicated textarea, and scheduling. A document is created from the textarea that implements the requisite functionality to get and set its content, and this is passed to an 'agent' that handles scheduling and communication with the server. All of this apparatus obscures the working of the operational transformations and recursive function to transform sequences of operations somewhat, but bear in mind that this layer is also a part of the algorithm.

The example supports only one anonymous session per server instance and only one document within that session. If you want to write a client and a server to support multiple sessions and documents, you will need to abandon the example for the most part and use it for guidance only.

Running the tests

This can be done from the root of the repository:

./node_modules/.bin/mocha --recursive -R spec ./test

As of writing the tests cover the operational transformations and recursive function to transform sequences of operations, but not much else.

Compiling from source

Automation is done with npm scripts, have a look at the package.json file. The pertinent commands are:

npm run build-debug
npm run watch-debug