How do I achieve a synchronous requirement using asynchronous NodeJS - node.js

Question

I am adding user validation an data modification page on a node.js application.
In a synchronous universe, in a single function I would:
Lookup the original record in the database
Lookup the user in LDAP to see if they are the owner or admin
Do the logic and write the record.
In an asynchronous universe that won't work. To solve it I've built a series of hand-off functions:
router.post('/writeRecord', jsonParser, function(req, res) {
post = req.post;
var smdb = new AWS.DynamoDB.DocumentClient();
var params = { ... }
smdb.query(params, function(err,data){
if( err == null ) writeRecordStep2(post,data);
}
});
function writeRecord2( ru, post, data ){
var conn = new LDAP();
conn.search(
'ou=groups,o=amazon.com',
{ ... },
function(err,resp){
if( err == null ){
writeRecordStep3( ru, post, data, ldap1 )
}
}
}
function writeRecord3( ru, post, data ){
var conn = new LDAP();
conn.search(
'ou=groups,o=amazon.com',
{ ... },
function(err,resp){
if( err == null ){
writeRecordStep4( ru, post, data, ldap1, ldap2 )
}
}
}
function writeRecordStep4( ru, post, data, ldap1, ldap2 ){
// Do stuff with collected data
}
Additionally, because the LDAP and Dynamo logic are in their own source documents, these functions are scattered tragically around the code.
This strikes me as inefficient, as well as inelegant. I'm eager to find a more natural asynchronous pattern to achieve the same result.

Answer 1

Any promise library should sort your issue out. My preferred choice is bluebird. In summary they help you in performing blocking operations.

Answer 2

If you haven't heard about bluebird then just use it. It converts all function of a module and return promise which is then-able. Simply put, it promisifies all functions.
Here is the mechanism:
Module1.someFunction() \\do your job and finally pass the return object to next call
.then() \\Use that object which is return from the first call, do your job and return the updated value
.then() \\same goes on
.catch() \\do your job when any error occurs.
Hope you understand. Here is an example:
var readFile = Promise.promisify(require("fs").readFile);
readFile("myfile.js",
"utf8").then(function(contents) {
return eval(contents);
}).then(function(result) {
console.log("The result of evaluating
myfile.js", result);
}).catch(SyntaxError, function(e) {
console.log("File had syntax error", e);
//Catch any other error
}).catch(function(e) {
console.log("Error reading file", e);
});

Answer 3

I could not tell from your pseudo-code exactly which async operations depend upon results from with other ones and knowing that is key to the most efficient way to code a series of asynchronous operations. If two operations do not depend upon one another, they can run in parallel which generally gets to an end result faster. I also can't tell exactly what data needs to be passed on to later parts of the async requests (too much pseudo-code and not enough real code to show us what you're really attempting to do).
So, without that level of detail, I'll show you two ways to approach this. The first runs each operation sequentially. Run the first async operation, when it's done, run the next one and accumulates all the results into an object that is passed along to the next link in the chain. This is general purpose since all async operations have access to all the prior results.
This makes use of promises built into the AWS.DynamboDB interface and makes our own promise for conn.search() (though if I knew more about that interface, it may already have a promise interface).
Here's the sequential version:
// promisify the search method
const util = require('util');
LDAP.prototype.searchAsync = util.promisify(LDAP.prototype.search);
// utility function that does a search and adds the result to the object passed in
// returns a promise that resolves to the object
function ldapSearch(data, key) {
var conn = new LDAP();
return conn.searchAsync('ou=groups,o=amazon.com', { ... }).then(results => {
// put our results onto the passed in object
data[key] = results;
// resolve with the original object (so we can collect data here in a promise chain)
return data;
});
}
router.post('/writeRecord', jsonParser, function(req, res) {
let post = req.post;
let smdb = new AWS.DynamoDB.DocumentClient();
let params = { ... }
// The latest AWS interface gets a promise with the .promise() method
smdb.query(params).promise().then(dbresult => {
return ldapSearch({post, dbresult}, "ldap1");
}).then(result => {
// result.dbresult
// result.ldap1
return ldapSearch(result, "ldap2")
}).then(result => {
// result.dbresult
// result.ldap1
// result.ldap2
// doSomething with all the collected data here
}).catch(err => {
console.log(err);
res.status(500).send("Internal Error");
});
});
And, here's a parallel version that runs all three async operations at once and then waits for all three of the to be done and then has all the results at once:
// if the three async operations you show can be done in parallel
// first promisify things
const util = require('util');
LDAP.prototype.searchAsync = util.promisify(LDAP.prototype.search);
function ldapSearch(params) {
var conn = new LDAP();
return conn.searchAsync('ou=groups,o=amazon.com', { ... });
}
router.post('/writeRecord', jsonParser, function(req, res) {
let post = req.post;
let smdb = new AWS.DynamoDB.DocumentClient();
let params = { ... }
Promise.all([
ldapSearch(...),
ldapSearch(...),
smdb.query(params).promise()
]).then(([ldap1Result, ldap2Result, queryResult]) => {
// process ldap1Result, ldap2Result and queryResult here
}).catch(err => {
console.log(err);
res.status(500).send("Internal Error");
});
});
Keep in mind that due to the pseudo-code nature of the code in your question, this is also pseudo-code where implementation details (exactly what parameters you're searching for, what response you're sending, etc...) have to be filled in. This should be illustrative of promise chaining to serialize operations and the use of Promise.all() for parallelizing operations and promisifying a method that didn't have promises built in.