Promisifying Zookeeper's multiple callback function

Question

Zookeeper provides a getChildren method that takes in the path of the node and returns the children of that node in a callback. It also sets a watch during the process and calls the watcher callback when a watch is triggered

getChildren(path, function(err,event){
        //this is the watcher callback
    },
    function(err,children,stats){
        //children callback
    }
)

So if I use the bluebird's Promise.promisify to promisify this function. How do I know that the promise this function is returning is the watcher or the children ??

Answer 1

If I understand the getChildren() interface correctly, the last callback is designed to be called once with the list of child objects. The first callback is a watcher callback that may be called an indefinite number of times to notify you of various changes occurring.

Given that, the last callback could fit with a promise. The first callback cannot and must remain a callback. In addition, the second callback is returning multiple results (which does not fit perfectly with promises) so you have to make allowances for that too using multiArgs and .spread.

So, you could do something like this:

let getChildrenP = Promise.promisify(getChildren, {multiArgs: true});

getChildrenP(path, function(err, event) {
    // watcher event occurred here
    // this may get called multiple times
}).spread(function(children, stats) {
    // children are available now
}).catch(function(err) {
    // error here
});