convert a NodeJS lambda function (AWS) to use "async" (promises, etc.) instead of callbacks - mysql

Question

I have a lambda function that connects to mysql and runs a set of queries, but I actually have a sequence of mysql queries that need to run one after another. I.e., the value of one query is used in the next query, etc.
Currently, I have a bunch of callbacks to achieve this, but this is leading to "callback hell". How would I rewrite this to use async / await?
My code is actually split into 2 files. The first file does an initial query, and then the value is passed into a function of the second file. Please note that the mysql node_module is included but not shown here. The AWS API gateway calls index.js
// index.js
var mysql = require('mysql'); // from node_modules
var config = require('./config.json');
var dashboard = require('./dashboard.js');
var pool = mysql.createPool({
host : config.dbhost,
user : config.dbuser,
password : config.dbpassword,
database : config.dbname
});
exports.handler = (event, context, callback) => {
context.callbackWaitsForEmptyEventLoop = false;
pool.getConnection(function(err, connection) {
// check for mysql connection error first
if ( err ) {
throw err;
}
let qry = "select id from some_table where some_field = ?";
let someval = event.queryStringParameters.someval;
connection.query(qry, [someval], function(error, result) {
if ( error ) {
throw err;
}
else {
dashboard.processRequest(connection, callback, event, res[0].id);
}
});
});
}
// dashboard.js
module.exports = {
jsonResponse: function(results) {
return {
"statusCode": 200,
"body": JSON.stringify({ results }),
"isBase64Encoded": false,
"headers": {
"Access-Control-Allow-Origin": "*"
}
};
},
processRequest: function(connection, callback, event, val) {
let qry = "update first_table set some_field = ?";
connection.query(qry, [val], function(error, results) {
// return to client if error
if (error) {
callback(null, this.jsonResponse(error));
}
else {
// assume that this table must be update AFTER the previous statement
qry = "select id from second_table where some_field = ?";
connection.query(qry, [val], function(error1, results1) {
// return to client if error
if ( error1 ) {
callback(null, this.jsonResponse(error1));
}
qry = "update third_table set some_field = ? where id = ?";
connection.query(qry, [results1[0].id], function(error2, results2) {
// release connection when all queries are completed
connection.release();
if ( error2 ) {
callback(null, this.jsonResponse(error2));
}
else {
callback(null, this.jsonResponse(results2));
}
});
});
}
});
}
};
It was suggested to me that something like the below code might work. Unfortunately, it does not. I was curious to know why using try...catch blocks in the way shown below is not working, and is it the same thing as what you've shown, but just written differently?
// index.js
var mysql = require('mysql'); // from node_modules
var config = require('./config.json');
var dashboard = require('./dashboard.js');
var pool = mysql.createPool({
host : config.dbhost,
user : config.dbuser,
password : config.dbpassword,
database : config.dbname
});
exports.handler = (event, context, callback) => {
context.callbackWaitsForEmptyEventLoop = false;
pool.getConnection(function(err, connection) {
// check for mysql connection error first
if ( err ) {
throw err;
}
let qry = "select id from users where username = ? limit 1;";
let username = event.queryStringParameters.username;
try {
let res = await connection.query(qry, [event.queryStringParameters.username]);
dashboard.processRequest(connection, callback, event, res[0].id);
} catch (err) {
console.log(err);
}
});
}
// dashboard.js
module.exports = {
jsonResponse: function (results) {
return {
"statusCode": 200,
"body": JSON.stringify({results}),
"isBase64Encoded": false,
"headers": {
"Access-Control-Allow-Origin": "*"
}
};
},
processRequest: async function (connection, callback, event, val) {
let qry = "update first_table set some_field = ?";
try {
let results = await connection.query(qry, [val]);
qry = "select id from second_table where some_field = ?";
try {
let results1 = await connection.query(qry, [val]);
qry = "update third_table set some_field = ? where id = ?";
try {
let results2 = await connection.query(qry, [results1[0].id]);
connection.release();
callback(null, this.jsonResponse(results2));
} catch (error2) {
callback(null, this.jsonResponse(error2));
}
} catch (error1) {
callback(null, this.jsonResponse(error1));
}
} catch (error) {
callback(null, this.jsonResponse(error));
}
}
};

Answer 1

We need use promises.
Typically I follow this approach:
Create one async method mainProcess and have bunch of methods step by step called with in that method. one after the other with await or all at once.
Each async method getConnection and runQuery in this case, called within mainProcess must a Promise.
If any errors from these methods i.e promise rejects from individual methods, goes in catch block of mainProcess().
If no errors, all methods within mainProcess gets executed and goes to then block of mainProcess()
Your code can be refactored like this (just wrote in an editor untested)
var pool = mysql.createPool({
host: config.dbhost,
user: config.dbuser,
password: config.dbpassword,
database: config.dbname,
});
exports.handler = (event, context, callback) => {
context.callbackWaitsForEmptyEventLoop = false;
/**
* Main Lambda Process
*/
const mainProcess = async () => {
// Get Connection
let connection = await getConnection();
// Run Step 1
let qry1 = "select id from some_table1 where some_field = ?";
const response1 = await runQuery(connection, qry1, { someFiledValue: 1222})
// Run Step 2
let qry2 = "select id from some_table2 where some_field = ?";
const resonse2 = await runQuery(connection, qry2, { someFiledValue: 1222})
return 'All Good';
});
}
mainProcess()
.then(result => {
// All lambda success messages are returned from here
callback(null, result);
})
.catch(error => {
// All lambda errors thrown from here
callback(error);
});
};
function getConnection(qry, parms) {
return new Promise((resolve, reject) => {
pool.getConnection(function (error, connection) {
if (error) {
// return to client if error
reject(error);
} else {
// Return response
resolve(connection);
}
});
});
}
/**
* Runs a query, either resolves or rejects
*/
function runQuery(connection, qry, parms) {
return new Promise((resolve, reject) => {
connection.query(qry, [val], function (error, results) {
if (error) {
// return to client if error
reject(error);
} else {
// Return response
resolve(result);
}
});
});
}

Answer 2

When you're dealing with a lambda function which performs an async task you have two solutions:
you can use non async handlers, in which case you need to invoke "callback" on promises as you did in your example
you can use async handlers, which does not requires the "callback" input and that allows you to write async/await code, like the following example:
const mysql = require('mysql2/promise');
exports.handler = async(event, context) => {
//get path variable
const { pathVar } = event.pathParameters;
// get connection
const connection = await mysql.createConnection({
host : process.env.RDS_HOSTNAME,
user : process.env.RDS_USERNAME,
password : process.env.RDS_PASSWORD,
database : process.env.RDS_DB_NAME
});
// get text query
const textQuery = `SELECT field FROM entity WHERE attribute = ${pathVar}`;
// get res
const results = await connection.execute(textQuery);
return {
"statusCode": 200,
"body": results,
"isBase64Encoded": false
}
}
You can have a look at the AWS docs: https://docs.aws.amazon.com/lambda/latest/dg/nodejs-handler.html