Chap Ambrose

The Performance Penalty of Repeated Connections

Before the latest improvements to the Heroku Router, every connection between the router and your application dyno risked incurring the latency penalty of a TCP slow start . To understand why this is a performance bottleneck for modern web applications, we must look at the fundamentals of the Transmission Control Protocol (TCP) and its history with HTTP.

Maybe you’ve heard of a keep-alive connection, but haven’t thought much about what it is or why they exist. In this post, we’re going to peel away some networking abstractions in order to explain what a keep-alive…