Node.js Promise concept during if else condition in then block - mysql

Question

I have a scenario where the webservice needs to check for existense of key in redis if present give it as a response else get it from mysql, store in redis and then give it as response.
So i am using promises concept where first time when i call return new Set_Data(); it doesn't go to next then block it just stays idle. But next time since data already exists the return new Set_Data();
is not executed which is correct.
But why is that i am getting problem for first time when i call return new Set_Data(); which is not going for next then block.
Below is my code
constants.js file
var Promise = require('bluebird');
module.exports =
{
getRedisConnection: function ()
{
return require("redis").createClient(6379, 'path', { auth_pass: 'key' });
},
getMySqlConnection: function ()
{
var conObj = { host: "localhost", user: "root", password: "", database: "deccan" };
var connection = require("mysql").createConnection(conObj);
return new Promise(function (resolve, reject)
{
connection.connect(function (error)
{
if (error)
reject(error);
else
resolve(connection);
});
});
}
};
webservicefile.js
var Promise = require('bluebird');
var express = require('express');
var router = express.Router();
var constants = require("../constants");
function getSettings(request, response)
{
var client = constants.getRedisConnection();
get_Data();
function get_Data()
{
return new Promise(function (resolve, reject)
{
client.get("url", function (error, reply)
{
if (error)
reject(error);
else
{
if (reply == null)
resolve(); // Key not present so create
else
resolve(reply);
}
});
}).
catch(function (e)
{
console.log("Error at : " + new Date().toString() + ", => " + e);
}).
then(function (urlResult)
{
return new Promise(function (resolve, reject)
{
if (urlResult == undefined || urlResult == null)
{
return new Set_Data();
}
else
{
client.quit();
return resolve(urlResult);
}
});
}).
then(function (urlResult)
{
if (urlResult)
response.status(200).send({ url : urlResult });
else
response.status(500).send();
})
}
function Set_Data()
{
constants.getMySqlConnection().then(function (connection)
{
return new Promise(function (resolve, reject)
{
connection.query("select url from table where id = 1", function (error, results)
{
connection.end();
if (error)
reject(error);
else
resolve(results);
});
});
}).
then(function (url)
{
return new Promise(function (resolve, reject)
{
client.set('url', url, function (err, reply)
{
if (err)
reject(err);
else
resolve(url);
});
});
});
}
}

Answer 1

A couple of changes should do the trick, first Set_Data() doesn't return a promise like you think it does, add a return:
function Set_Data() {
return constants.getMySqlConnection().then(function (connection).then()
// ...
}
Inside this callback, you don't have a resolve() in the if so the promise is never resolved, returning something doesn't resolve:
// your code
then(function (urlResult) {
return new Promise(function (resolve, reject) {
if (urlResult == undefined || urlResult == null) {
return new Set_Data();
} else {
client.quit();
return resolve(urlResult);
}
});
}).
Return Set_Data() which is now a promise or the url:
then(function (urlResult) {
if (urlResult == undefined || urlResult == null) {
return new Set_Data();
} else {
client.quit();
return urlResult;
}
}).
On a side note, don't format your js code like C#, { shouldn't be on a new line.

Answer 2

You can return new Set_Data() directly if no urlResult, or return urlResult otherwise - inside then any (non Promise) value returned is a resolved Promise - so the .then chain will continue as required
function get_Data() {
// ...
.then(function (urlResult) {
// you don't need a "new Promise" here
if (urlResult == undefined || urlResult == null) {
return new Set_Data();
} else {
client.quit();
return urlResult;
}
})
// ...
}
One thing I noticed in your code is a .catch in the middle of your .then chain which will effectively turn an error into a fulfilled promise - not sure if that's the behaviour you are looking for. Something else to look out for