Dear Alvaro,
Thanks for your answers. Unfortunately, I was unaware of a shutdown
record, that makes a difference then. So, I definitely must stop the
primary first, then use pg_controldata to obtain checkpoint info. Then,
can I query the replicas while they are up and running if they've
received the shutdown record or not? So, after shutting down the
primary, how will I know if a replica has received the mentioned record,
and is safe to shutdown?
Thanks for the clarifications.
Best regards,
Richard
2025-02-19 16:54 időpontban Álvaro Herrera ezt írta:
On 2025-Feb-19, richard@xxxxxxxxx wrote:
With this, I have the question, that after the shutdown of primary,
what is
the guarantee for replicas having the same checkpoint location? Why
does the
order of shutting down the servers matter? What would be the really
exact
and reliable way to ensure that replicas will have the same checkpoint
location as the primary?
The replicas can't write WAL by themselves, but they will replay
whatever the primary has sent; by shutting down the primary first and
letting the replicas catch up, you ensure that the replicas will
actually receive the shutdown record and replay it. If you shut down
the replicas first, they can obviously never catch up with the shutdown
checkpoint of the primary.
As I recall, if you do shut down the primary first, one potential
danger
is that the primary fails to send the checkpoint record before shutting
down, so the replicas won't receive it and obviously will not replay
it;
or simply that they are behind enough that they receive it but don't
replay it.
You could use pg_controldata to read the last checkpoint info from all
nodes. You can run it on the primary after shutting it down, and then
on each replica while it's still running to ensure that the correct
restartpoint has been created.
