node async call return data in response

Question

I am new to nodejs so I have a basic question and this is my scanrio

1. I have a javascript client which is making a http request to a node server to read a value from the database.

2. Once the node server receives the request it makes a simple db call and returns the data to the client in the response, and this is where the problem is.

   router.get('/state', function(req, res){        
      var result = dbServer.makeDBCall();//Before this line executes and returns the result the next line executes
      res.send(result); 
   }
   

The database call from the node server is asynchronous, therefore before the result is returned the node server has already sent a blank response to the client. What is the standard/acceptable way of getting this achieved, I know I can block the node thread using async, but then the whole purpose of node is gone right?

Answer 1

It depends on what kind of database node module you are using.

Other than the standard callback approach, there are also the promise way. The pg-promise library is 1 of those kind.

See sample code:

this.databaseConnection.makeDBCall('your query...')
    .then(function(dbResponse) {
        // Parse the response to the format you want then...
        res.send(result);    
    })
    .catch(function(error) {
        // Handle error
        res.send(error.message);
    });

@spdev : I saw 1 of your comments about you being worried about how Node actually knows who to reply the response to, especially when there are multiple requests.

This is a very good question, and to be honest with you - I don't know much about it as well.

In short the answer is yes, Node somehow handles this by creating a corresponding ServerResponse object when a HTTP request comes through. This object seems to have some smartness to tell the Nodejs network stack how to route itself back to the caller when it gets parsed as data packets.

I tried Googling a bit for an answer but didn't got too far. I hope the ServerResponseObject documentation can provide more insight for you. Share with me if you got an answer thanks!

https://nodejs.org/api/all.html#http_class_http_serverresponse

Answer 2

Try below code.

router.get('/state', function(req, res){        
   var result = dbServer.makeDBCall(function(err,result){
    if(!err) {
      res.send(result); 
     }
  });
}

Hope this Help.

Answer 3

The dbServer.makeDBCall(); must have a callback that runs when the statement completes executing. Something like -

dbServer.makeDBCall({query: 'args'}, function(err, result){
    if (err) // handle error
    res.send(result); 
})

You return the response from db from that callback function.

Learn more about callback from here-

nodeJs callbacks simple example

https://docs.nodejitsu.com/articles/getting-started/control-flow/what-are-callbacks/