On Tue, Jan 4, 2022, at 3:11 PM, PG Doc comments form wrote: > `pg_isready` docs don't specify how its different from a port probe - what > does on top of checking that a port is open? Does it send some handshake > message that a server responds to when initialization is finished? pg_isready uses a PQping function that reports the current Postgres status. The main advantage of pg_isready is that it uses the Postgres protocol to report the server status. A port probe is not a sufficient step to determine if the server is running and accepting connections. Why? A Postgres server can be running but in a state that disallows connections such as during a startup or crash recovery phase. pg_isready is capable of reporting such state (PQPING_REJECT -- 1) [1].
We could include a sentence explaining that a port probe is not a sufficient condition to successfully connect to a Postgres server because the state machine has more than 2 states. PQping [1] already states "reports the status of the server" and describe each state it provides. Feel free to suggest an improvement to documentation. > People here are telling that `pg_isready` is not reliable and that > `pg_isready` may return when database is not actually ready > https://github.com/docker-library/postgres/issues/880#issuecomment-1004251102 > Without further info about `pg_isready` it is not clear if it should be > filled as a bug. Or if it should be filled as a bug only when an > initialization script is involved. There is no known issues with pg_isready and that report isn't clear. He says "database and/or user wasn't available". What's the exact error message? If there is a reproducible test case, share it here. [1] https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PQPINGPARAMS -- Euler Taveira EDB https://www.enterprisedb.com/
