Worklight JSON Store, can we get race conditions?

Question

Worklight 6.1 on both Windows (colleague) and Mac (me), building an a Hybrid app destined for Android device but to speed up development we do initial testing as Mobile Web App in Chrome browser on desktop.

We get a weird symptom that I'm trying to fine-down to a reproducible test case. I think I see different behaviours when stepping in debugger and just letting it run. Want to check whether a certain coding pattern could be the cause of the symptom before I go any further.

Fundamental question: should we wait for the resolution of a promise returned by a JSONSTore request for an action on a collection before issuing another request? more explanation below.

The overall intent is to load some data into the JSONStore, with some intelligent replace/merge action if a record is already present. Pseudo code:

  for each record retrieved from back-end

        if ( record already present in Store )
              do some data merging 
              replace record
        else
              add record

The application code actually works like this, just considering the add() case, the problem manifests when the store is empty, all records need to be added

  for each record to add 
        addPromise = store.get().add(record);
        listOfPromises.insert(addPromise);

  examine the list of promises recording any errors

That is there is no "wait" for add to finish before issuing the next add request. Hence in effect we've initiated a set of adds "in parallel" whatever that might mean in JavaScript in Chrome.

The code appears to run just fine, no errors reported. On android device it works reliably. In Chrome under normal running (no stepping in debugger) we end up with no reported errors but only one record inserted - indeed as though a snapshot of the initial "empty" store had been taken and each add is working on that "empty" copy.

After writing this I'm now pretty convinced that the coding pattern described above is vulnerable to a kind of race and that the better approach is build a list of documents to be added and insert them in a single operation.

djna djna · Accepted Answer · 2014-03-30T06:21:55

A more detailed answer will be coming later, but I now know that this

the coding pattern described above is vulnerable to a kind of race and that the better approach is build a list of documents to be added and insert them in a single operation.

is true. In the browser the JSONStore does require that we wait for the result of one request before issuing another one. The recommended approach is

var dataToAdd = buildArrayOfDataToAdd(responseFromServer);
var dataToReplace = buildArrayOfDataToReplace(responseFromServer);

jsonstore.add( dataToAdd ).then( function() { jsonstore.replace( dataToReplace); })

Worklight JSON Store, can we get race conditions?

1 Answers