The Apache JServ Protocol (AJP) is a method for a web server to communicate with an associated application server. The web server is a "reverse proxy," meaning, its purpose is to handle incoming traffic from the Internet on behalf of the application server.
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Web traffic is carried out in HTTP (Hypertext Transfer Protocol), which also carries through to communication between servers working together to fulfill a request. HTTP is a plain-text format where actual words make up the messages running between computers. This create an overhead of traffic. AJP converts these messages into a binary code, reducing the amount of space taken by each message.
Speed between collaborating servers is a distinctive feature of the protocol. The process maintains permanent connections between servers to reduce the time taken to establish contact. The link is dedicated to a particular request and then maintained for the next request to be tunneled down, rather than each session causing a session to be created and broken.
AJP is particularly used for "Load Balancing." This is where one server distributes incoming traffic among several application servers, sharing the load equally.