Chapter 46. Frontend/Backend Protocol

Table of Contents
46.1. Overview
46.1.1. Messaging Overview
46.1.2. Extended Query Overview
46.1.3. Formats and Format Codes
46.2. Message Flow
46.2.1. Start-up
46.2.2. Simple Query
46.2.3. Extended Query
46.2.4. Function Call
46.2.5. COPY Operations
46.2.6. Asynchronous Operations
46.2.7. Canceling Requests in Progress
46.2.8. Termination
46.2.9. SSL Session Encryption
46.3. Streaming Replication Protocol
46.4. Message Data Types
46.5. Message Formats
46.6. Error and Notice Message Fields
46.7. 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.

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