Q的承诺：如何顺序尝试多个注册表

Question

I could need some help with Q promises for a node.js application. My use case is as follows:

• we need to find (by ID) some information (say an order)
• we have multiple repositories which may contain the information (directory, database, nosql, REST service ...)
• We have a search sequence. That is we do not want to search them all in parallel and the fastest wins, but we need to search sequentially one repository after the other
• We can stop searching and ignore the remaining repositories once we found the information
• we already have promises for each repository

What I'm struggling with is to create/return a promise that finishes, when the information has been found.

Something like (pseudo code):

var deferred = Q.defer(id)
foreach repo in repositories {
    var p = repo.findById(id)
               .then deferred.resolve(order)
    p.waitForThePromiseToFinish
    if (p.success) break 
}

An idea how to implement this? Any help is appreciated.

Answer 1

Pretty much like your pseudocode except we use thenable chaining since the actions are async:

var p = Promise.resolve(); // start empty promise;
repositories.forEach(function(repo){
    p = p.then(function(result){ // chain
        if(result.success) return result; // assumes that result.success is true if good
        return repo.findById(id); // call find by Id
    });
});

This will resolve to a promise p that will be the first repository to be successfull, or a rejection if all failed - note that this assumes your findById API is such that it returns a value with result.success === true on a successful find and result.success === false otherwise. Let me know if you have issues adapting this to your actual API or if you have issues. Have fun :)