Chapter 45. Frontend/Backend Protocol

Table of Contents
45.1. Overview
45.1.1. Messaging Overview
45.1.2. Extended Query Overview
45.1.3. Formats and Format Codes
45.2. Message Flow
45.2.1. Start-Up
45.2.2. Simple Query
45.2.3. Extended Query
45.2.4. Function Call
45.2.5. COPY Operations
45.2.6. Asynchronous Operations
45.2.7. Cancelling Requests in Progress
45.2.8. Termination
45.2.9. SSL Session Encryption
45.3. Message Data Types
45.4. Message Formats
45.5. Error and Notice Message Fields
45.6. 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.

Higher level features built on this protocol (for example, how libpq passes certain environment variables when the connection is established) are covered elsewhere.

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.