Zero-downtime deployment concept in Express - Step-by-Step Execution

Concept Flow - Zero-downtime deployment concept

Start Current Server

↓

Deploy New Server Version

↓

New Server Listens on Same Port

↓

Redirect Incoming Requests to New Server

↓

Old Server Finishes Active Requests

↓

Old Server Shuts Down

↓

Deployment Complete - No Downtime

This flow shows how a new server version starts alongside the old one, takes over requests, and the old server closes only after finishing active work, ensuring no downtime.

Execution Sample

Express

const http = require('http');

// Old server
const oldServer = http.createServer((req, res) => {
  res.end('Old Server Response');
});
oldServer.listen(3000);

// New server starts
const newServer = http.createServer((req, res) => {
  res.end('New Server Response');
});
// New server listens on same port after old server closes
oldServer.close(() => {
  newServer.listen(3000);
});

This code shows two servers: old and new. The new server waits to listen until the old server finishes, to avoid downtime.

Execution Table

Step	Action	Old Server State	New Server State	Request Handling
1	Start old server listening on port 3000	Listening	Not started	Handled by old server
2	Deploy new server code	Listening	Not started	Handled by old server
3	New server attempts to listen on port 3000	Listening	Error (port busy)	Handled by old server
4	Send signal to old server to stop accepting new connections	Closing to new requests	Not started	Handled by old server
5	Old server finishes active requests	Closed	Not started	No requests handled
6	New server starts listening on port 3000	Closed	Listening	Handled by new server
7	All new requests handled by new server	Closed	Listening	Handled by new server
8	Old server fully shut down	Stopped	Listening	Handled by new server

💡 Old server stops and releases port; new server listens on port 3000; requests handled without downtime.

Variable Tracker

Variable	Start	After Step 3	After Step 5	After Step 6	Final
oldServerState	Not started	Listening	Closed	Closed	Stopped
newServerState	Not started	Error (port busy)	Not started	Listening	Listening
requestHandling	None	Old server	None	New server	New server

Key Moments - 3 Insights

Why can't the new server listen on the same port while the old server is still running?

Because the port is already in use by the old server (see execution_table step 3), the new server gets an error until the old server releases it.

How does the old server avoid dropping active requests during deployment?

The old server stops accepting new requests but finishes active ones before closing (see execution_table steps 4 and 5), ensuring no requests are dropped.

When does the new server start handling requests?

Only after the old server fully closes and releases the port, the new server starts listening and handles all new requests (see execution_table step 6).

Visual Quiz - 3 Questions

Look at the execution table, at which step does the old server stop accepting new requests?

A Step 4

B Step 2

C Step 6

D Step 8

💡 Hint

Check the 'Old Server State' column for when it changes to 'Closing to new requests'.

According to the variable tracker, what is the new server state after step 3?

A Not started

B Error (port busy)

C Listening

D Closed

💡 Hint

Look at the 'newServerState' row under 'After Step 3' in the variable tracker.

If the old server did not finish active requests before closing, what would happen?

A New server would start earlier

B Old server would keep running forever

C Requests might be dropped causing downtime

D Port would be free immediately

💡 Hint

Refer to key moment about finishing active requests to avoid dropped requests.

Concept Snapshot

Zero-downtime deployment means running new server version alongside old one.
Old server stops accepting new requests but finishes active ones.
New server starts listening only after old server releases port.
This avoids downtime by smoothly switching servers.
Use signals or process managers to coordinate shutdown and startup.

Full Transcript

Zero-downtime deployment in express means updating your server without stopping service. First, the old server runs and handles requests. When deploying a new version, it tries to listen on the same port but can't because the old server is still using it. So, the old server stops accepting new requests but finishes any active ones. Once done, it closes and frees the port. Then the new server starts listening on that port and handles all new requests. This way, users never experience downtime during deployment.