How many slots do I need to create for 10 replicas with streaming replication?

Question

PostgreSQL

Do I understand correctly that 1 slot = 1 replication? Those. if I need to have 10 replications, then I need to make 10 slots. So?

Answer 1

Depends on the layout of the replicas.
Each replication connection uses one of the allowed max_wal_senders of the host it connects to. If you are configuring replication slots, then you also need the appropriate number of max_replication_slots on that host.
pg_basebackup, in turn, uses up to two max_wal_senders connections and up to one max_replication_slots (depends on launch options and version)
So if you copy all 10 replicas at the same time using basebackup from one master base, you will need 20 max_wal_senders and 10 max_replication_slots. After starting the replicas, 10 max_wal_senders and 0 to 10 max_replication_slots will be used - depending on how you set it up. Each replica has a separate replication slot if you want to use replication slots.
But this is if you configure replication without a cascade.
Replication connections are not counted by cluster, but connected to a specific instance. In the case of a cascade, you can easily have a scheme, for example:
3 replicas read replication from the master, 3 more replicas receive replication from each of them - now 9 replicas, and only 3 walsenders per host were used.
And with such a number of replicas, I highly recommend making a cascade. And save the network on the master, and in case of switching the master, fewer databases will have to be restarted (primary_conninfo still changes only by restarting the entire database, the chance for my patch to be accepted in pg13 is very ghostly)