BackendKeyData is mandatory?

Tatsuo Ishii <ishii@postgresql.org>

From: Tatsuo Ishii <ishii@postgresql.org>

To: pgsql-hackers@postgresql.org

Date: 2025-06-17T01:10:56Z

Lists: pgsql-hackers

In the Frontend/Backend protocol, it is explained that after
successful authentication following messages can be sent from backend
to frontend[1]:

BackendKeyData
ParameterStatus
ReadyForQuery
ErrorResponse
NoticeResponse

My question is, BackendKeyData is mandatory or not. Currently
Pgpool-II raises a fatal error if BackendKeyData is not sent before
ReadyForQuery arrives. This is because without the message, frontend
cannot send a CancelRequest message later on, as there's no secret
key.

I heard that some "PostgreSQL compatible" servers do not send
BackendKeyData message to frontend. I wonder if this is a protocol
violation.

[1] https://www.postgresql.org/docs/current/protocol-flow.html#PROTOCOL-FLOW-START-UP

Best regards,
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp

Commits

Revert unnecessary check for NULL
- 5a26a3e4eeb2 18.0 landed
- 807ee417e562 19 (unreleased) landed
libpq: Be strict about cancel key lengths
- e7ff96853ea7 18.0 landed
- e411a8d25a4b 19 (unreleased) landed
libpq: Handle OOM by disconnecting instead of hanging or skipping msgs
- 8aa287c9061b 18.0 landed
- f6f0542266f0 19 (unreleased) landed
libpq: Complain about missing BackendKeyData later with PGgetCancel()
- 42b1480eb2c6 18.0 landed
- a4801eb691ed 19 (unreleased) landed