Chapter 50. Frontend/Backend Protocol

Table of Contents
50.1. Overview
50.1.1. Messaging Overview
50.1.2. Extended Query Overview
50.1.3. Formats and Format Codes
50.2. Message Flow
50.2.1. Start-up
50.2.2. Simple Query
50.2.3. Extended Query
50.2.4. Function Call
50.2.5. COPY Operations
50.2.6. Asynchronous Operations
50.2.7. Canceling Requests in Progress
50.2.8. Termination
50.2.9. SSL Session Encryption
50.3. Streaming Replication Protocol
50.4. Message Data Types
50.5. Message Formats
50.6. Error and Notice Message Fields
50.7. Summary of Changes since Protocol 2.0

PostgreSQL uses a message-based protocol for communication between frontends and backends (clients and servers). The protocol is supported over TCP/IP and also over Unix-domain sockets. Port number 5432 has been registered with IANA as the customary TCP port number for servers supporting this protocol, but in practice any non-privileged port number can be used.

This document describes version 3.0 of the protocol, implemented in PostgreSQL 7.4 and later. For descriptions of the earlier protocol versions, see previous releases of the PostgreSQL documentation. A single server can support multiple protocol versions. The initial startup-request message tells the server which protocol version the client is attempting to use, and then the server follows that protocol if it is able.

In order to serve multiple clients efficiently, the server launches a new "backend" process for each client. In the current implementation, a new child process is created immediately after an incoming connection is detected. This is transparent to the protocol, however. For purposes of the protocol, the terms "backend" and "server" are interchangeable; likewise "frontend" and "client" are interchangeable.