I'm using Meteor (1.0.3) in general, but for one particular case I'm using a raw server side route to render a file -- so I'm outside a Meteor method.
I'm using node fs.writeFile/fs.readFile and exec commands to call out to Linux command-line utilities too.
My only point in brining this up is that the node calls are async of course. And so I'm opted to use the node Q library in order to manage async callbacks.
This all worked until I added a line to call out to the MongoDB database.
A call like so:
var record_name = Mongo_Collection_Name.findOne({_personId: userId}, {fields: {'_id': 0}});
Produces the following error:
[Error: Can't wait without a fiber]
The error only occurs when I wrap the function in a Promise.
For example, something like this will throw:
getRecordExample = function () {
var deferred = Q.defer();
var record_name = Mongo_Collection_Name.findOne({_personId: userId}, {fields: {'_id': 0}});
// do something
// if no error
deferred.resolve(record_name);
return deferred.promise;
}
If I use the Meteor Fibers library I don't get the error:
getRecordExample = function () {
var deferred = Q.defer();
Fiber = Npm.require('fibers');
var record_name
Fiber(function () {
record_name = Mongo_Collection_Name.findOne({_personId: userId});
}).run()
// do something
// if no error
deferred.resolve(record_name);
return deferred.promise;
}
but, the record_name variable is undefined outside the fiber, so I don't have a way to pass the variable outside of the Fiber scope as far as I can tell.
A More Precise Example
This is a little long, so you have to scroll down to see it all. I'm basically building a workflow here so there are processes and subprocesses.
// both/routes.js
Router.route('/get-route', function(req, res) {
// get the userId then start the workflow below
// using Promises here because these were firing concurrently
Q(userId)
.then(process_1)
.then(process_2)
.done();
}, { name: 'server-side-ir-route', where: 'server' }
// server.js
process_1 = function (userId) {
sub_process_1(userId);
sub_process_2(userId);
return userId;
}
process_2 = function (userId) {
sub_process_3(userId);
sub_process_4(userId);
return userId;
}
sub_process_1 = function (userId) {
var result = get_record_1(userId);
// do stuff with result
// using Q library to call out to async fs.writeFile, return Promise
fs_writeFile_promise(result)
.catch(function (error) {
console.log('error in sub_process_1_write', error);
})
.done(function () {
console.log('done with sub_process_1');
}
return userId;
}.future() // <-- if no future() here, the exception is thrown.
sub_process_2 = function (userId) {
var result = get_record_2(userId);
// do stuff with result
// using Q library to call out to async fs.writeFile, return Promise
fs_writeFile_promise(result)
.catch(function (error) {
console.log('error in sub_process_1_write', error);
})
.done(function () {
console.log('done with sub_process_1');
}
return userId;
}.future()
// async because of I/O operation (I think)
get_record_1 = function (userId) {
var record_1 = Mongo_Collection_Name.findOne({'userId': userId});
// do stuff
return record_1;
}
get_record_2 = function (userId) {
var record_2 = Mongo_Collection_Name.findOne({'userId': userId});
// do stuff
return record_2;
}
// async operation using Q library to return a Promise
fs_writeFile_promise = function (obj) {
var deferred = Q.defer();
fs.writeFile(obj.file, obj.datas, function (err, result) {
if (err) deferred.reject(err);
else deferred.resolve('write data completed');
});
return deferred.promise;
}
For now, lets assume that the process_2 function is exactly like process_1
Also, we should assume I have console.log('step_start') and console.log('step_end') in each function. This is what it would look like on the command line:
- start processes
- end processes
- start processes 1
- end processes 1
- start processes 2
- start sub processes 1
- getting record 1
- start sub processes 2
- getting record 2
- returning record 1
- end sub processes 1
- called writeData in sub process 1
- returning record 2
- called writeData in sub process 2
- end processes 2
- ending sub processes 1
The reason I had to place a Fiber (future) on the sub_process_1() function was because when I placed the function process_1() in the Q chain at the top I got the Error: Can't wait without a fiber].
If I remove the process_1() in the Q chain at the top and remove the .future() from sub_process_1() no exception is thrown.
Questions
- Why does calling out to a Mongo collection within a Promise cause a fiber error within a Meteor application?
- Does calling a async function within a sync function in general cause the sync function to become a async function?
- How do I solve this problem?
