Startup is divided into an authentication phase and a backend startup phase.

Initially, the frontend sends a StartupPacket. The postmaster uses this info and the contents of the pg_hba.conf(5) file to determine what authentication method the frontend must use. The postmaster then responds with one of the following messages:

如果前端不支持 postmaster 请求的认证方法，那么它应当立即关闭连接。

After sending AuthenticationOk, the postmaster attempts to launch a backend process. Since this might fail, or the backend might encounter a failure during startup, the frontend must wait for the backend to acknowledge successful startup. The frontend should send no messages at this point. The possible messages from the backend during this phase are:

ReadyForQuery 消息与后端在每个查询周期之后发出的 消息相同。 Depending on the coding needs of the frontend, it is reasonable to consider ReadyForQuery as starting a query cycle (and then BackendKeyData indicates successful conclusion of the startup phase), or to consider ReadyForQuery as ending the startup phase and each subsequent query cycle.

查询周期由前端向 backend 发送 Query 消息发起。然后后端根据查询命令串的内容发送一个或多个响应消息，最后发送 ReadyForQuery 响应消息。 ReadyForQuery informs the frontend that it may safely send a new query or function call.

The possible response messages from the backend are:

每当期待任何其他类型的消息时，前端必须准备好接受 ErrorResponse 和 NoticeResponse 消息。

Actually, it is possible for NoticeResponse to arrive even when the frontend is not expecting any kind of message, that is, the backend is nominally idle. (In particular, the backend can be commanded to terminate by its postmaster. In that case it will send a NoticeResponse before closing the connection.) It is recommended that the frontend check for such asynchronous notices just before issuing any new command.

此外，如果前端发出任何 listen(l) 命令，那么它必须准备好在任何时间接受 NotificationResponse 消息；见下文。

A Function Call cycle is initiated by the frontend sending a FunctionCall message to the backend. The backend then sends one or more response messages depending on the results of the function call, and finally a ReadyForQuery response message. ReadyForQuery informs the frontend that it may safely send a new query or function call.

The possible response messages from the backend are:

每当期待任何其他类型的消息时，前端必须准备好接受 ErrorResponse 和 NoticeResponse 消息。 Also, if it issues any listen(l) commands then it must be prepared to accept NotificationResponse messages at any time; see below.

If a frontend issues a listen(l) command, then the backend will send a NotificationResponse message (not to be confused with NoticeResponse!) whenever a notify(l) command is executed for the same notification name.

Notification responses are permitted at any point in the protocol (after startup), except within another backend message. Thus, the frontend must be prepared to recognize a NotificationResponse message whenever it is expecting any message. Indeed, it should be able to handle NotificationResponse messages even when it is not engaged in a query.

值得指出的是，listen 和 notify 命令中使用的名字不必与 SQL 数据库中关系（表）的名字 有任何关系。 Notification names are simply arbitrarily chosen condition names.

During the processing of a query, the frontend may request cancellation of the query by sending an appropriate request to the postmaster. The cancel request is not sent directly to the backend for reasons of implementation efficiency: we don't want to have the backend constantly checking for new input from the frontend during query processing. Cancel requests should be relatively infrequent, so we make them slightly cumbersome in order to avoid a penalty in the normal case.

To issue a cancel request, the frontend opens a new connection to the postmaster and sends a CancelRequest message, rather than the StartupPacket message that would ordinarily be sent across a new connection. The postmaster will process this request and then close the connection. For security reasons, no direct reply is made to the cancel request message.

A CancelRequest message will be ignored unless it contains the same key data (PID and secret key) passed to the frontend during connection startup. If the request matches the PID and secret key for a currently executing backend, the postmaster signals the backend to abort processing of the current query.

The cancellation signal may or may not have any effect --- for example, if it arrives after the backend has finished processing the query, then it will have no effect. If the cancellation is effective, it results in the current command being terminated early with an error message.

The upshot of all this is that for reasons of both security and efficiency, the frontend has no direct way to tell whether a cancel request has succeeded. It must continue to wait for the backend to respond to the query. Issuing a cancel simply improves the odds that the current query will finish soon, and improves the odds that it will fail with an error message instead of succeeding.

Since the cancel request is sent to the postmaster and not across the regular frontend/backend communication link, it is possible for the cancel request to be issued by any process, not just the frontend whose query is to be canceled. This may have some benefits of flexibility in building multiple-process applications. It also introduces a security risk, in that unauthorized persons might try to cancel queries. The security risk is addressed by requiring a dynamically generated secret key to be supplied in cancel requests.

The normal, graceful termination procedure is that the frontend sends a Terminate message and immediately closes the connection. On receipt of the message, the backend immediately closes the connection and terminates.

不优雅的终止可能由于任一端的软件故障（即核心转储）而发生。如果前端或后端看到连接意外关闭，应当清理并终止。 The frontend has the option of launching a new backend by recontacting the postmaster, if it doesn't want to terminate itself.