How to design a multi-user ajax web application to

Question

I have a web page that shows a large amount of data from the server. The communication is done via ajax.

Every time the user interacts and changes this data (Say user A renames something) it tells the server to do the action and the server returns the new changed data.

If user B accesses the page at the same time and creates a new data object it will again tell the server via ajax and the server will return with the new object for the user.

On A's page we have the data with a renamed object. And on B's page we have the data with a new object. On the server the data has both a renamed object and a new object.

What are my options for keeping the page in sync with the server when multiple users are using it concurrently?

Such options as locking the entire page or dumping the entire state to the user on every change are rather avoided.

If it helps, in this specific example the webpage calls a static webmethod that runs a stored procedure on the database. The stored procedure will return any data it has changed and no more. The static webmethod then forwards the return of the stored procedure to the client.

Bounty Edit:

How do you design a multi-user web application which uses Ajax to communicate with the server but avoids problems with concurrency?

I.e. concurrent access to functionality and to data on a database without any risk of data or state corruption

Answer 1

Server-side push techniques are the way to go here. Comet is (or was?) a buzz word.

The particular direction you take depends heavily on your server stack, and how flexible you/it is. If you can, I would take a look at socket.io, which provides a cross-browser implementation of websockets, which provide a very streamline way to have bidirectional communication with the server, allowing the server to push updates to the clients.

In particular, see this demonstration by the library's author, which demonstrates almost exactly the situation you describe.

Answer 2

I know this is an old question, but I thought I'd just chime in.

OT (operational transforms) seem like a good fit for your requirement for concurrent and consistent multi-user editing. It's a technique used in Google Docs (and was also used in Google Wave):

There's a JS-based library for using Operational Transforms - ShareJS (http://sharejs.org/), written by a member from the Google Wave team.

And if you want, there's a full MVC web-framework - DerbyJS (http://derbyjs.com/) built on ShareJS that does it all for you.

It uses BrowserChannel for communication between the server and clients (and I believe WebSockets support should be in the works - it was in there previously via Socket.IO, but was taken out due to the developer's issues with Socket.io) Beginner docs are a bit sparse at the moment, however.