Keepalive connections in Nginx - Step-by-Step Execution

Process Flow - Keepalive connections

Client sends first request

↓

Server accepts connection

↓

Process request and send response

↓

Check if keepalive enabled?

No→Close connection

Yes↓

Keep connection open for next request

↓

Client sends next request on same connection

↓

Repeat processing without new TCP handshake

↓

Connection closed after timeout or client closes

The server keeps the TCP connection open after sending a response, allowing the client to send multiple requests without reconnecting.

Execution Sample

Nginx

keepalive_timeout 15;
keepalive_requests 100;

server {
    listen 80;
    location / {
        proxy_pass http://backend;
    }
}

This config sets the keepalive timeout to 15 seconds and allows up to 100 requests per connection before closing.

Process Table

Step	Action	Connection State	Request Number	Response Sent	Next Step
1	Client connects and sends request 1	Open	1	Response 1 sent	Keep connection open (keepalive)
2	Client sends request 2 on same connection	Open	2	Response 2 sent	Keep connection open (keepalive)
3	Client sends request 3 on same connection	Open	3	Response 3 sent	Keep connection open (keepalive)
4	No new requests within 15 seconds	Closed	3	No response	Server closes connection (timeout)
5	Client reconnects and sends request 4	Open	4	Response 4 sent	Keep connection open (keepalive)
6	Client sends request 5 on same connection	Open	5	Response 5 sent	Keep connection open (keepalive)
7	Client closes connection	Closed	5	No response	Connection closed by client

💡 Connection closes after timeout or client closes it, ending keepalive session.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	Final
Connection State	Closed	Open	Open	Open	Closed	Open	Open	Closed
Request Number	0	1	2	3	3	4	5	5

Key Moments - 3 Insights

Why does the connection stay open after the first response?

Because keepalive is enabled, the server keeps the connection open to allow multiple requests without reconnecting, as shown in execution_table rows 1 to 3.

What causes the server to close the connection after some time?

If no new requests arrive within the keepalive_timeout period (15 seconds here), the server closes the connection to free resources, as shown in execution_table row 4.

Can the client close the connection before the timeout?

Yes, the client can close the connection anytime, which ends the keepalive session immediately, as shown in execution_table row 7.

Visual Quiz - 3 Questions

Look at the execution table, what is the connection state after the client sends the third request?

A Timed out

B Open

C Closed

D Reset

💡 Hint

Check the 'Connection State' column at step 3 in the execution_table.

At which step does the server close the connection due to timeout?

A Step 4

B Step 2

C Step 6

D Step 7

💡 Hint

Look for the step where 'Server closes connection (timeout)' is mentioned in the 'Next Step' column.

If keepalive_timeout was set to 5 seconds instead of 15, how would the execution_table change?

A Number of requests per connection would increase

B Connection would stay open longer

C Connection would close earlier, possibly after step 3

D Client would have to reconnect less often

💡 Hint

Consider the 'No new requests within 15 seconds' event in step 4 and how timeout affects connection closing.

Concept Snapshot

Keepalive connections in nginx keep TCP connections open after a response.
This allows multiple requests on one connection, reducing overhead.
Key directives: keepalive_timeout (time to wait before closing), keepalive_requests (max requests per connection).
If no new requests arrive before timeout, server closes connection.
Client can also close connection anytime.
Improves performance by avoiding repeated TCP handshakes.

Full Transcript

Keepalive connections in nginx allow the server to keep a TCP connection open after sending a response. This means the client can send multiple requests over the same connection without reconnecting each time. The server uses the keepalive_timeout setting to decide how long to wait for new requests before closing the connection. The keepalive_requests setting limits how many requests can be sent on one connection. If the client does not send a new request within the timeout, the server closes the connection. The client can also close the connection at any time. This mechanism improves performance by reducing the overhead of opening and closing connections repeatedly.