Race conditions with other tabs

[Flimm]

Open this URL which uses Backbone.localStorage twice in two separate tabs in the same browser:

http://backbonejs.org/examples/todos/index.html

In the first tab, add a todo item labelled "a", and open the second tab, and add a todo item labelled "b". Then refresh both tabs. Both tabs will now have a list of one item, it being "b". What should have happened is that both tabs have a list of two items, "a" and "b".

This bug happens because of this piece of code in the function create:

this.records.push(model.id.toString());
this.save();

this.records may be out-of-date at this point, and the subsequent line this.save() does not consider this, and simply overwrites what other tabs may have written.

I do not know if there is an easy way to overcome this race condition, but if there isn't, there ought to be a warning in the documentation.

[mateuszwrobel]

You should do sync between tabs by yourself. Library like this should not do it for you (it just provide interface to use LocalStorage).
Storage feature provides event that you can listen to, and update your models. Example: http://html5demos.com/storage-events

[scott-w]

Hi @Flimm and @mateuszwrobel, thanks for contributing this.

I agree that we don't need to document this in this case - a similar issue can happen with server-side APIs so developers should generally be aware of these possibilities.

I would definitely support adding support for events firing from localStorage, however, as this could greatly enhance the usability of this library.

[Flimm]

What would it hurt to warn developers about this?

Intelligent and well-educated developers who already know about this race condition get a friendly reminder. Not so intelligent or well-educated developers like me who make bad assumptions about being able to use a library in multiple tabs get their faulty assumptions corrected.

I don't see a downside.

In general, server-side libraries in many languages frequently point out if there are thread-safe or not (although that isn't relevant in Node), so I don't think it would be at all unusual for a library's documentation to point out that the code is not "tab-safe".

[scott-w]

Hi @Flimm - it's common in other libraries because being thread-safe or not is a well understood statement that is usually important to know in the context of the library. People generally need to understand the effect on memory during execution of a program as the standard assumption is that it's not affected by other parts of the program.

The localStorage is more like writing to disk (and like synchronising over a network) - you don't assume that it'll remain totally in sync without doing something. If anything, I'd be more inclined to merge #174 and put efforts into improving the ability of the system to remain in sync with your local disk.