Chapter 43. Frontend/Backend Protocol

Table of Contents
43.1. Overview
43.1.1. Messaging Overview
43.1.2. Extended Query Overview
43.1.3. Formats and Format Codes
43.2. Message Flow
43.2.1. Start-Up
43.2.2. Simple Query
43.2.3. Extended Query
43.2.4. Function Call
43.2.5. COPY Operations
43.2.6. Asynchronous Operations
43.2.7. Cancelling Requests in Progress
43.2.8. Termination
43.2.9. SSL Session Encryption
43.3. Message Data Types
43.4. Message Formats
43.5. Error and Notice Message Fields
43.6. 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 may 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.