I'm working on an Alexa skill in Node that runs in AWS Lambda, and having trouble getting a callback to execute when I emit an event. The Alexa-skills-kit for Node.JS README demonstrates passing a callback function to an event handler, and recommends using the arrow function to preserve context:
this.emit('JustRight', () => {
this.emit(':ask', guessNum.toString() + 'is correct! Would you like to play a new game?',
'Say yes to start a new game, or no to end the game.');
});
I'm trying to do the same, but find that my callback never appears to be executed. I thought this was because AWS Lambda is limited to Node 4.3.2, and the arrow function is not available, so I tried passing this context back to the callback the old fashioned way:
In New Session handler:
if(!account_id) {
console.log('Access token:' + accessToken);
this.emit('getAccount', accessToken, function (retrieved_id) {
console.log('account id in callback: ' + retrieved_id);
this.emit('welcome');
});
}
In event handler:
accountHandler = {
'getAccount': function (accessToken, cb) {
console.log('fetching account id');
var client = thirdparty.getClient(accessToken);
var r = client.getAccountForToken(client);
r.then(function (data) {
console.log('got it:' + data);
this.attributes['account_id'] = data;
cb.call(this, data);
}).catch(function (err) {
this.emit('handleApiError', err);
});
},
}
I can see that I'm successfully retrieving the account ID, in the logs, but Lambda is executing without error and without invoking my callback function. I'm trying to figure out if there is a problem with invoking the callback within the Promise 'then' function, or if something else is going on.
The exact problem was the lack of context within the promise then function. I fixed this by using the arrow function inside the getAccount handler:
r.then(data => {
console.log('got it:' + data);
this.attributes['account_id'] = data;
this.emit('welcome');
})
Of course this also shows that Lambda Node.JS functions support the arrow function just fine.
Related
Stack: Node.js + Express + TypeScript + Socket.io
Problem: I cannot transfer the callback provided in the library using TypeScript
How do I call the callback correctly? The code below throws an error.
socket.on('method', async (params: any, callback: (res: any) => void) => {
// endpoint's logic
const result = await this._service.ServiceMethodAsync(params);
// acknowledgement
callback(result);
})
Error
TypeError: callback is not a function
If you are performing validation on the event that has triggered on the socket then I would say follow the example provided on their page here.
In the case you have provided above you would have:
socket.on('method', async (params, callback) => {
if (typeof callback !== "function") {
// event is not an acknowledgement, do something i.e. return
return;
}
const result = await this._service.ServiceMethodAsync(params);
// acknowledgement
callback(result);
})
Here is another resource on understanding Acknowledgements in SocketIO.
checking the client-side code and turns out the snippet mentioned above is just fine and there was a problem with emitting the method.
fixed client:
socket.emit('method', msg, (res) => {
console.log(res);
});
I have a function which resolves by taking a callback like function(error, result) { ... } as a parameter. I'm trying to use mocha to test this function, but the problem is that the function returns asynchronously, so there's no good place for me to put the done(). If I put inside my result handler, it takes too long and mocha times out. If I put it outside, the test always passes because the handler hasn't been called yet. Here is my code. What's the best way to get around this?
lbl.createLabels is a function that takes an array of customers, and a directory, and creates a bunch of files in that directory, and then asynchronously calls the callback of type: function(error, callback).
describe('Tests', () => {
it('returns a list of customer objects', (done) => {
lbl.createLabels(customers, __dirname + "/..", (err, result) => {
err.should.equal(undefined)
result.should.be.a('array')
result[0].should.have.property('id')
result[0].should.have.property('tracking')
result[0].should.have.property('pdfPath')
const a = {prop:3}
a.prop.should.be.an('array')
done() // putting done() here results in a timeout
})
done() // putting done here results in the test exiting before the callback gets called
})
})
Mocha's documentation has an entire section describing how to test asynchronous code:
https://mochajs.org/#asynchronous-code
Testing asynchronous code with Mocha could not be simpler! Simply invoke the callback when your test is complete. By adding a callback (usually named done) to it(), Mocha will know that it should wait for this function to be called to complete the test.
describe('User', function() {
describe('#save()', function() {
it('should save without error', function(done) {
var user = new User('Luna');
user.save(function(err) {
if (err) done(err);
else done();
});
});
});
});
How do I get the data back from a lambda invoked with as an event to the calling function?
Essentially the lambda function I have is:
exports.handler = function(event, context, callback) {
var data = {};
data.foo ='hello';
callback(null, data)
}
and the invoking function looks like this:
var AWS = require('aws-sdk');
var lambda = new AWS.Lambda();
var params = {
FunctionName: 'SomeFunction',
InvocationType: 'Event'
};
lambda.invoke(params, function (err, data) {
if (err) {
console.log(err, err.stack); // an error occurred
} else {
console.log(JSON.stringify(data, null, 2));
}
});
However the only thing I get back from the function is
{
"StatusCode": 202,
"Payload": ""
}
I thought the point of the callback parameter was to allow the invoking function to get the data when the function has finished. Am I using it wrong or is what I am asking not possible with Lambdas?
When you invoke the Lambda function you need to set the InvocationType to 'RequestResponse' instead of 'Event'.
When using the Event type your callback is invoked when the payload has been received by AWS's servers. When using the RequestResponse type your callback is invoked only after the Lambda function has completed and you will receive the data it provided to its callback. It is not possible to do what you want with the Event type.
As #MatthewPope commented in this answer, if you do need the results from an asynchronous Lambda execution, you should consider having the Lambda function write its results to S3 (or something to that effect) at a known location where clients of the Lambda function can periodically check for the existence of the result.
I am using the test function from AWS console:
console.log('Loading event');
exports.handler = function(event, context) {
console.log('value1 = ' + event.key1);
console.log('value2 = ' + event.key2);
console.log('value3 = ' + event.key3);
context.done(null, 'Hello World'); // SUCCESS with message
};
And calling it in nodejs as follows:
var params = {
FunctionName: 'MY_FUNCTION_NAME', /* required */
InvokeArgs: JSON.stringify({
"key1": "value1",
"key2": "value2",
"key3": "value3"
})
};
lambda.invokeAsync(params, function(err, data) {
if (err) {
// an error occurred
console.log(err, err.stack);
return cb(err);
}
// successful response
console.log(data);
});
and everything works fine:
//Console Output
{ Status: 202 }
But I was expecting to receive the message from context.done(null, 'Message') as well...
Any idea how to get the message?
As Eric mentioned, currently Lambda doesn't offer a REST endpoint to run the function and return its result, but may in the future.
Right now, your best bet would be to use a library like lambdaws, which wraps the function deployment and execution for you and handles returning results via an SQS queue. If you'd like more control by rolling your own solution, the process is straightforward:
Create an SQS queue
Have your Lambda function write its result to this queue
In your client, poll the queue for a result
You are calling invokeAsync, so your Lambda function is run asynchronously. This mean you get the success means back at the point your function is successfully started, and not after it completes.
As of this writing, AWS Lambda does not yet offer a way to invoke a function synchronously, returning information from the function directly back to the caller. However, this appears to be a common request and Amazon has publicly stated they are considering the feature.
I have an ExpressJS app where I have api.js in routes that manages connecting to Couchbase and then emits event couchbaseConnected that is awaited by init() function inside api.js.
Inside init() I want to push those exports.someFunction(req, res){return something;}. But when I just put these exports inside init() function, I get an error .get() requires callback functions but got a [object Undefined] so it seems like I am doing it wrong.
The question is how I can export functions from another function in NodeJS?
Here is the code:
//connecting to couchbase and emitting event on connection
couchbase.connect(dbConfiguration, function (err, bucket) {
if (err) {
console.log(err);
}
cb = bucket;
eventEmitter.emit('couchbaseConnected');
});
//listening for the event and fire init() when it's there
eventEmitter.on('couchbaseConnected', function (e) {
console.log('Connected to Couchbase.'.green);
init();
});
function init() {
exports.getUserData = function (req, res) {
if (req.user != undefined && req.user.meta != undefined) {
res.json(200, {result: 'ok'})
}
else {
res.json(401, {error: 'Unauthorized request.'})
}
};
}
Here is the ExpressJS .get() that is located in app.js:
app.get('/api/user/data/:type', api.getUserData);
Here is the ExpressJS .get() that is located in app.js:
app.get('/api/user/data/:type', api.getUserData);
The error message .get() requires callback functions but got a [object Undefined] makes quite clear what happens: You require the API module, it starts to connect to the db, you are defining your express app by passing a non-existent property to .get - which fails, since init has not yet been called and getUserData has not yet been assigned. What you need to do is
var api = require('api');
eventEmitter.on('couchbaseConnected', function () {
app.get('/api/user/data/:type', api.getUserData); // now it is available
});
However, this does not look like good code. Instead of loosely coupling them via that couchbaseConnected event you better should use explicit callbacks that are invoked with the requested values (i.e. the cb bucket, or the getUserData method). At least pass them as parameters to the emitted event.
Also, your setup is unconventional. I don't see why getUserData would need to be asynchronously defined - it should always be available. If the couchbase connection failed, I would not expect the /api/user/data/ service to not exist, but to respond with some 500 internal server error message.
This is an answer that I have made some assumptions as the data provided by you is not sufficient to know what you are doing when you start your app.
I would suggest some change in code:
module.exports.connect = function(callback){
couchbase.connect(dbConfiguration, function (err, bucket) {
if (err) {
console.log(err);
callback(err);
}
cb = bucket;
module.exports.getUserData = getUserData();
callback(err); //just callback with no error as you don't require to send the database
});
}
function getUserData(req, res){
if (req.user != undefined && req.user.meta != undefined) {
res.json(200, {result: 'ok'})
}
else {
res.json(401, {error: 'Unauthorized request.'})
}
};
and in your app.js file where you are starting the app just do this
var api = require('./api');
api.connect(function(error){
if (error) throw error;
app.listen(3000);
console.log('Express started on port 3000');
});
And then you can continue doing exactly what you were doing. It should work.
I have assumed that you are not explicitly calling the connect or its equivalent when you are starting the app.....So your getting that error.