Nodejs: Resolving promises with generator function - node.js

Question

I know there are a lot of good examples over the web and I read a lot of them, but currently I'm stucked with resolving promises with the new functionality of generators in nodejs 0.11.x.
For e.g. I have the following function:
SolrBaseDomain.prototype.promisedQuery = function(query, callback) {
var solrClient = solr.createClient(this.configuration);
var defer = Q.defer();
solrClient.search(query, function(err,obj){
if (!err) {
if (obj.response.numFound > 0) {
defer.resolve(obj.response.docs);
} else {
defer.resolve(null);
}
} else {
defer.reject(err);
}
});
var promise = defer.promise;
return Q.async(function* (){
var result = yield promise;
return result;
});
};
I expected that every call to this method will wait until the promise is fullfilled and the return-statement gives back the result of the promise.
But currently it seems that instead the code inside "Q.async..." will not be executed or the async call arrives after the return statement of the method was executed.
It's strange, in every example I know, this is one of the recommended ways in order to wait for async calls in nodejs but currently it does not work for me.
I've tried a lot of different variations of the above example, but the result is everytime the same, I get not back a valid result.
I have nodejs installed in version 0.11.10 and the --harmony-flag is set, when the code is executede.
Can anyone point me to right direction? I'm wondering if I oversee something ... :)
Thanks for your feedback.
Best regards
Udo

Answer 1

I expected that every call to this method will wait until the promise is fullfilled and the return-statement gives back the result of the promise.
No. Generators will not make functions synchronous, you cannot (and don't want to) block while waiting for a result. When calling a generator function and running sequentially through the async steps that it yields, the result you will get back in the end is still asynchronous - and therefore a promise. Only inside of the generator, your code can use synchronous control flow and yield.
This means that the (then-) callback-based code
SolrBaseDomain.prototype.promisedQuery = function(query) {
var promise = Q.ninvoke(solr.createClient(this.configuration), "search", query);
return promise.then(function(obj) {
if (obj.response.numFound > 0) {
return obj.response.docs;
} else {
return null;
}
});
};
becomes
SolrBaseDomain.prototype.promisedQuery = Q.async(function* (query) {
var promise = Q.ninvoke(solr.createClient(this.configuration), "search", query);
var obj = yield promise;
// ^^^^^
if (obj.response.numFound > 0) {
return obj.response.docs;
} else {
return null;
}
});

Answer 2

Try this
SolrBaseDomain.prototype.promisedQuery = Q.async(function*(query) {
var solrClient = solr.createClient(this.configuration);
var obj = yield Q.ninvoke(solrClient, "search", query);
return obj.response.numFound > 0 ? obj.response.docs : null;
});
This does the same thing for promises as this does for callbacks:
SolrBaseDomain.prototype.query = function (query, callback) {
var solrClient = solr.createClient(this.configuration);
solrClient.search(query, function(err, obj) {
if (err) return callback(err);
callback(null, obj.response.numFound > 0 ? obj.response.docs : null);
});
};
Therefore if the first return a promise that resolves to undefined so will the callback version call the callback with undefined.

Answer 3

according to your suggestions, my code looks now like this:
...
SolrBaseDomain.prototype.query = Q.async(function* (query) {
var solrClient = solr.createClient(this.configuration);
var obj = yield Q.ninvoke(solrClient, "search", query);
return obj.response.numFound > 0 ? obj.response.docs : null;
});
...
I share the above query-function over all data access layers in order to have a central method which is querying the different indexes in an asynchronous way.
For e.g. in the domain data access layer, the code which deals with that function looks like this:
SolrHostDomain.prototype.getByName = Q.async(function* (domain) {
var queryObject = {
"domain": domain
};
var query = this.getQuery("byName", queryObject);
var docs = yield this.query(query);
var domain = null;
if (docs != null && docs.length > 0) {
domain = this.dataMapper.merge(docs[0]);
}
return domain;});
Currently I'm not sure if the generator in the "getByName"-function is necessary, but it seems to work. Dealing with promises is some unclear concept for me, since I'm new to nodejs.
So maybe, if you can help me on that topic and point me in the right direction, this would be helpfull.
The main question for me is, how can I ensure, that a synchronous method can call an asynchronous method and get back not a promise but the final result of this promise.
I've searched a long time, but I could not find a good documentation which describes the use of generator functions or promises in conjunction with synchronous calls. Even examples are focusing only of using the mechanism but not working together with synchronous function.
Best regards and many thanks for your help
Udo

Answer 4

Got it!!!
After a few trial and errors, I think I got it now and I have a working solutions:
Query function:
SolrBaseDomain.prototype.query = Q.async(function* (query) {
var solrClient = solr.createClient(this.configuration);
var obj = yield Q.ninvoke(solrClient, "search", query);
return obj.response.numFound > 0 ? obj.response.docs : null;
});
Calling method:
SolrHostDomain.prototype.getByName = function(domain) {
var queryObject = {
"domain": domain
};
var query = this.getQuery("byName", queryObject);
var docsPromise = this.query(query);
var _self = this;
return docsPromise.then(function(docs) {
var domain = null;
if (docs != null && docs.length > 0) {
domain = _self.dataMapper.merge(docs[0]);
}
return domain;
});
};
The solution was to understand, that the "query"-method still returns a promise instead of the concrete result even if yield is used.
So I have to add every code which is working on the result of the promise within the "then"-functions (or "done" if no other caller up in the calling hierarchy of methods will follow).
After the settlement of the promise, each code which is set within the "then"-functions will be processed.
BR
Udo