pg_isready docs doesn't specify what exactly it is checking

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]https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PQPINGPARAMS.

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]https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PQPINGPARAMS 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/

On Fri, 7 Jan 2022 at 03:29, Euler Taveira <euler@eulerto.com> wrote:

On Tue, Jan 4, 2022, at 3:11 PM, PG Doc comments form wrote:

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.

A diagram of the state machine would be very useful.

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.

PostgreSQL docker image can run initialization scripts at container start to
create database, users and maybe load some data. So when pg_isready
reports success, the DB can still be in an inconsistent state for an app to
start
using it.
--
Anatoli Babenia