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@OrKoN

OrKoN/server.js

Last active Feb 25, 2019
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What would you like to do?
Simple server with synchronization of request processing
/**
* WARNING: don't use the code in production, it only works for a single process instance and does not work in cluster mode.
*/
const express = require('express');
const app = express();
let nextRequestId = 1; // request counter
let nextPaymentId = 1; // payment counter
// POST to /payments creates a new empty resource
app.post('/payments', (req, res) => {
const paymentId = nextPaymentId++;
const context = `request(post) #${nextRequestId++}`;
handle(() => createPayment(context, paymentId), res);
});
// PUT to a payment resource charges the user
// app.put('/payments/:id', (req, res) => {
// const context = `request(put) #${nextRequestId++}`;
// const paymentId = req.params.id;
// handle(() => conductPayment(context, paymentId), res);
// });
// PUT to a payment resource charges the user
// with locks
app.put('/payments/:id', (req, res) => {
const context = `request(put) #${nextRequestId++}`;
const paymentId = req.params.id;
handleWithLock(
context,
paymentId,
() => conductPayment(context, paymentId),
res
);
});
app.listen(3000, () => console.log('Example app listening on port 3000!'));
// simple payments database
const _payments = {}; // { id: string, state: EMPTY | PROCESSING | PAID }
async function getPayment(context, paymentId) {
console.log(timestamp(), context, `Payment ${paymentId} retrieved`);
return _payments[paymentId];
}
async function createPayment(context, paymentId) {
console.log(timestamp(), context, `Payment ${paymentId} created`);
_payments[paymentId] = {
id: paymentId,
state: 'EMPTY',
};
return _payments[paymentId];
}
async function processPayment(context, paymentId) {
console.log(timestamp(), context, `Payment ${paymentId} processing started`);
_payments[paymentId].state = 'PROCESSING';
// payment processing is a lengthy operation
await new Promise(resolve => {
setTimeout(() => {
console.log(timestamp(), context, `Procesed payment ${paymentId}`);
_payments[paymentId].state = 'PAID';
resolve(_payments[paymentId]);
}, 3000);
});
}
async function conductPayment(context, paymentId) {
const payment = await getPayment(context, paymentId);
if (!payment) {
throw new Error('Payment does not exist');
}
if (payment.state === 'PROCESSING') {
// TODO improve by waiting for the state change
throw new Error('Payment is in progress. Try again later.');
}
if (payment.state === 'PAID') {
return payment;
}
if (payment.state === 'EMPTY') {
await processPayment(context, paymentId);
}
throw new Error('Payment is in bad state');
}
async function handle(fn, res) {
try {
const result = await fn();
if (result) {
return res.status(200).json(result);
}
res.status(204).end();
} catch (err) {
res.status(409).json({
error: err.message,
});
}
}
async function handleWithLock(context, lockId, fn, res) {
let lockState;
try {
lockState = await lock(context, lockId);
if (lockState === 'locked') throw new Error('Resource is locked.');
const result = await fn();
if (result) {
return res.status(200).json(result);
}
res.status(204).end();
} catch (err) {
res.status(409).json({
error: err.message,
});
} finally {
if (lockState === 'acquired') {
await unlock(context, lockId);
}
}
}
function timestamp() {
return new Date().toUTCString();
}
// simple locking (don't use in production)
const _locks = {};
async function lock(context, lockId) /* : 'locked' | 'acquired' */ {
if (!_locks[lockId]) {
_locks[lockId] = context;
console.log(timestamp(), context, 'lock acquired');
return 'acquired';
} else {
console.log(
timestamp(),
context,
`failed to lock the payment: payment is already locked by ${
_locks[lockId]
}`
);
return 'locked';
}
}
async function unlock(context, lockId) {
console.log(timestamp(), context, `payment is unlocked`);
delete _locks[lockId];
}
@VishwasShashidhar

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@VishwasShashidhar VishwasShashidhar commented Apr 5, 2018

In a cluster setup, I'm assuming you folks use queuing?

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