Chapter 44. Frontend/Backend Protocol

Table of Contents
44.1. Overview
44.1.1. Messaging Overview
44.1.2. Extended Query Overview
44.1.3. Formats and Format Codes
44.2. Message Flow
44.2.1. Start-Up
44.2.2. Simple Query
44.2.3. Extended Query
44.2.4. Function Call
44.2.5. COPY Operations
44.2.6. Asynchronous Operations
44.2.7. Cancelling Requests in Progress
44.2.8. Termination
44.2.9. SSL Session Encryption
44.3. Message Data Types
44.4. Message Formats
44.5. Error and Notice Message Fields
44.6. 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.