Question

Please could I ask for some advice on a control flow issue with node and redis? (aka Python coder trying to get used to JavaScript)

I don’t understand why client.smembers and client.get (Redis lookups) need to be callbacks rather than simply being statements – it makes life very complicated.

Basically I’d like to query a set, and then when I have the results for the set, I need to carry out a get for each result. When I’ve got all the data, I need to broadcast it back to the client.

Currently I do this inside two callbacks, using a global object, which seems messy. I’m not even sure if it’s safe (will the code wait for one client.get to complete before starting another?).

The current code looks like this:

var all_users = [];
// Get all the users for this page.
client.smembers("page:" + current_page_id, function (err, user_ids ) {
  // Now get the name of each of those users.
  for (var i = 0; i < user_ids.length; i++) {
     client.get('user:' + user_ids[i] + ':name', function(err, name) {
       var myobj = {};
       myobj[user_ids[i]] = name;
       all_users.push(myobj);  
       // Broadcast when we have got to the end of the loop, 
       // so all users have been added to the list - 
       // is this the best way? It seems messy.  
       if (i === (user_ids.length - 1)) {
           socket.broadcast('all_users', all_users); 
       }
     });       
   }
 });

But this seems very messy. Is it really the best way to do this? How can I be sure that all lookups have been performed before calling socket.broadcast?

scratches head Thanks in advance for any advice.

Answer 1

I don’t understand why client.smembers and client.get (Redis lookups) need to be callbacks rather than simply being statements – it makes life very complicated.

That’s what Node is. (I’m pretty sure that this topic was discussed more than enough times here, look through other questions, it’s definitely there)

How can I be sure that all lookups have been performed before calling socket.broadcast?

That’s what is err for in callback function. This is kinda Node’s standard – first parameter in callback is error object (null if everything fine). So just use something like this to be sure no errors occurred:

if (err) {
  ...    // handle errors.
  return // or not, it depends.
}

... // process results

But this seems very messy.

You’ll get used to it. I’m actually finding it nice, when code is well formatted and project is cleverly structured.

Other ways are:

• Using libraries to control async code-flow (Async.js, Step.js, etc.)
• If spaghetti-style code is what you think mess is, define some functions to process results and pass them as parameters instead of anonymous ones.