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目录
由 Phil Thompson 编写。协议 2.0 的更新由 Tom Lane 完成。
Postgres 使用一种基于消息的协议在frontend和backend 之间通信。 The protocol is implemented over TCP/IP and also on Unix sockets. Postgres v6.3 introduced version numbers into the protocol. This was done in such a way as to still allow connections from earlier versions of frontends, but this document does not cover the protocol used by those earlier versions.
This document describes version 2.0 of the protocol, implemented in Postgres v6.4 and later.
Higher level features built on this protocol (for example, how libpq passes certain environment variables after the connection is established) are covered elsewhere.
The three major components are the frontend (running on the client) and the postmaster and backend (running on the server). The postmaster and backend have different roles but may be implemented by the same executable.
A frontend sends a startup packet to the postmaster. This includes the names of the user and the database the user wants to connect to. The postmaster then uses this, and the information in the pg_hba.conf(5) file to determine what further authentication information it requires the frontend to send (if any) and responds to the frontend accordingly.
The frontend then sends any required authentication information. Once the postmaster validates this it responds to the frontend that it is authenticated and hands over the connection to a backend. The backend then sends a message indicating successful startup (normal case) or failure (for example, an invalid database name).
随后的通信是前端与后端之间交换的查询和结果包。postmaster 不再参与普通的查询/结果通信。 (However, the postmaster is involved when the frontend wishes to cancel a query currently being executed by its backend. Further details about that appear below.)
When the frontend wishes to disconnect it sends an appropriate packet and closes the connection without waiting for a response for the backend.
Packets are sent as a data stream. The first byte determines what should be expected in the rest of the packet. The exception is packets sent from a frontend to the postmaster, which comprise a packet length then the packet itself. The difference is historical.
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