Last Updated: 2024-04-19

Under HTTP 1.0, connections are not considered persistent unless a keep-alive header is included.

Connection: keep-alive

However, in HTTP 1.1, all connections are considered persistent unless declared otherwise.

With keep-alive, the connection is not dropped, but is instead kept open. When the client sends another request, it uses the same connection. This will continue until either the client or the server decides that the conversation is over, and one of them drops the connection.

The default connection timeout of Apache httpd 2.0 is as little as 15 seconds and just 5 seconds for Apache httpd 2.2 and above. This stops too many server processes being used.

Pros and cons

Advantages:

• reduced latency (save on wasteful hand-shaking)
• reduced CPU usage and round-trips (fewer new connections and TLS handshakes)

Disadvantages:

• If the client does not close the connection when all of the data it needs has been received (i.e. once they are through with the website), the resources needed to keep the connection open on the server will be unavailable for other clients. I guess some manual tuning and experimentation is in order.

Resources

• https://en.wikipedia.org/wiki/HTTP_persistent_connection