What is SPDY?
SPDY (pronounced "speedy") is a communication protocol developed by Google, released in 2009, and based on HTTP. Its primary purpose was to enhance web page load speeds to accommodate increasingly rich web content. Rather than altering the methods or headers defined by HTTP/1.1, SPDY redefined the session protocol to achieve its performance goals.
The most notable feature of SPDY is its ability to handle multiple communications simultaneously over a single TCP connection, which optimizes connection resources and improves response times. In SPDY, a sequence of request-and-response exchanges is referred to as a stream. By adding a session layer atop a TLS connection, multiple streams can remain open within a single SPDY connection, enabling simultaneous communications.
Streams can be initiated not only by the client but also by the server, allowing server push functionality. This lets the server proactively send content to the client without waiting for a request, reducing response times through content preloading. Additionally, SPDY uses binary encoding and header compression to reduce the size of transmitted data, further enhancing speeds.
Another important feature of SPDY is its integration with TLS, which enables automatic protocol selection. For example, if a SPDY-enabled server communicates with a non-SPDY-compatible browser, the server automatically falls back to HTTP. Since the application-layer semantics (such as methods and headers) remain unchanged, there is no need to modify web applications, ensuring seamless compatibility during the transition to SPDY. However, since SPDY assumes integration with TLS, HTTPS is a mandatory requirement for its use.
The features of SPDY were incorporated into the specifications of HTTP/2, which was standardized and released by the IETF in May 2015. Today, many web browsers support HTTP/2. Google, the developer of SPDY, announced on February 11, 2016, that Chrome would end support for SPDY on May 15, 2016. While SPDY gained widespread support among web browsers and was implemented on major websites like Google's, the transition to HTTP/2 is now the focus for modern web technologies.