Chapter 53. Frontend/Backend Protocol

Table of Contents

53.1. Overview
53.1.1. Messaging Overview
53.1.2. Extended Query Overview
53.1.3. Formats and Format Codes
53.2. Message Flow
53.2.1. Start-up
53.2.2. Simple Query
53.2.3. Extended Query
53.2.4. Pipelining
53.2.5. Function Call
53.2.6. COPY Operations
53.2.7. Asynchronous Operations
53.2.8. Canceling Requests in Progress
53.2.9. Termination
53.2.10. SSL Session Encryption
53.2.11. GSSAPI Session Encryption
53.3. SASL Authentication
53.3.1. SCRAM-SHA-256 Authentication
53.4. Streaming Replication Protocol
53.5. Logical Streaming Replication Protocol
53.5.1. Logical Streaming Replication Parameters
53.5.2. Logical Replication Protocol Messages
53.5.3. Logical Replication Protocol Message Flow
53.6. Message Data Types
53.7. Message Formats
53.8. Error and Notice Message Fields
53.9. Logical Replication Message Formats
53.10. 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. If the major version requested by the client is not supported by the server, the connection will be rejected (for example, this would occur if the client requested protocol version 4.0, which does not exist as of this writing). If the minor version requested by the client is not supported by the server (e.g., the client requests version 3.1, but the server supports only 3.0), the server may either reject the connection or may respond with a NegotiateProtocolVersion message containing the highest minor protocol version which it supports. The client may then choose either to continue with the connection using the specified protocol version or to abort the connection.

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.