Locking on an object?

Question

I'm very new to Node.js and I'm sure there's an easy answer to this, I just can't find it :(

I'm using the filesystem to hold 'packages' (folders with a status extensions 'mypackage.idle') Users can perform actions on these which would cause the status to go to something like 'qa', or 'deploying' etc... If the server is accepting lots of requests and multiple requests come in for the same package how would I check the status and then perform an action, which would change the status, guaranteeing that another request didn't alter it before/during the action took place?

so in c# something like this

lock (someLock) { checkStatus(); performAction(); }

Thanks :)

Answer 1

If you are making asynchronous calls with callbacks, this means multiple clients could potentially make the same, or related requests, and receive responses in different orders. This is definitely a case where locking is useful. You won't be 'locking a thread' in the traditional sense, but merely ensuring asynchronous calls, and their callbacks are made in a predictable order. The async-lock package looks like it handles this scenario.

https://www.npmjs.com/package/async-lock

Answer 2

If checkStatus() and performAction() are synchronous functions called one after another, then as others mentioned earlier: their exectution will run uninterupted till completion. However, I suspect that in reality both of these functions are asynchoronous, and the realistic case of composing them is something like:

function checkStatus(callback){
  doSomeIOStuff(function(something){
    callback(something == ok);
  });
}

checkStatus(function(status){
  if(status == true){
    performAction();
  }
});

The above code is subject to race conditions, as when doSomeIOStuff is being perfomed instead of waiting for it new request can be served.

You may want to check https://www.npmjs.com/package/rwlock library.