In modern web applications, it is common to have multiple instances of an Express.js server running in a clustered or load-balanced environment. However, maintaining data consistency across these instances can be challenging. Without a proper synchronization mechanism, each instance may have its own version of data, leading to conflicts and discrepancies.

In this blog post, we will look at how to implement a data synchronization mechanism between multiple Express.js instances using a pub-sub (publish-subscribe) pattern and a shared cache.

Pub-Sub Pattern

The pub-sub pattern is a messaging pattern where senders of messages, called publishers, do not program the messages to be sent directly to specific receivers, called subscribers. Instead, the publishers broadcast messages without any knowledge of who may be listening.

Setting Up a Pub-Sub System

To implement the pub-sub pattern in our Express.js application, we can leverage existing libraries like Redis or RabbitMQ. These libraries provide robust pub-sub systems that can handle high volumes of messages and distribute them to multiple subscribers.

Here’s an example of setting up a publisher and subscriber using Redis as the pub-sub system:

const redis = require("redis");
const publisher = redis.createClient();
const subscriber = redis.createClient();

subscriber.on("message", (channel, message) => {
  // Handle incoming message
  console.log(`Received message: ${message}`);
});

subscriber.subscribe("data-updates");

// Publishing a message
const message = "New data added";
publisher.publish("data-updates", message);

Shared Cache

A shared cache is a centralized store that allows different Express.js instances to share data. By utilizing a shared cache, we can ensure that all instances have access to the same version of data.

Setting Up a Shared Cache

One popular caching solution is Memcached, which provides a distributed in-memory caching system. To use Memcached with Express.js, we can utilize packages like memcached-promisify to simplify the API usage.

Here’s an example of setting up a shared cache using Memcached:

const Memcached = require("memcached-promisify");

const memcached = new Memcached("localhost:11211");

// Storing data in the cache
const key = "my-data";
const value = "Some important data";
const expireTime = 300; // 5 minutes
memcached.set(key, value, expireTime, (err) => {
  if (err) {
    throw new Error("Failed to store data in cache");
  }
  console.log("Data stored in cache");
});

// Retrieving data from the cache
memcached.get(key)
  .then((data) => {
    console.log(`Data retrieved from cache: ${data}`);
  })
  .catch((err) => {
    throw new Error("Failed to retrieve data from cache");
  });

Putting it Together

To implement data synchronization between multiple Express.js instances, we can combine the pub-sub pattern and a shared cache. Whenever data changes on one instance, it can publish a message indicating the change. Other instances subscribed to the relevant channel can then update their local cached data accordingly.

Here’s an example of how the synchronization can be achieved:

// On data update
const updatedData = { ... }; // New data
publisher.publish("data-updates", JSON.stringify(updatedData));

// On message received
subscriber.on("message", (channel, message) => {
  const updatedData = JSON.parse(message);
  // Update the shared cache with the new data
  memcached.set("my-data", updatedData, expireTime, (err) => {
    if (err) {
      throw new Error("Failed to update shared cache");
    }
    console.log("Shared cache updated");
  });
});

By combining the pub-sub pattern and a shared cache, we can ensure that data changes are propagated and synchronized across multiple Express.js instances effectively.

#Express.js #data-synchronization