Which mechanism is used for a delayed standby to recover wals. Processes that usually use streaming are not present (wal_sender/wal_receiver). Does the deferred standby fetch the wals one after another based on its offset from the primary or barman if it is configured
Related
I have Debezium in a container, capturing all changes of PostgeSQL database records. But i am unable to understand couple of things regarding how Debezium works. If Debezium starts for the first time it takes a snapshot of the database and then starting streaming based on WAL file accordantly to the Debezium documentation.:
PostgreSQL normally purges write-ahead log (WAL) segments after some period of time. This means that the connector does not have the complete history of all changes that have been made to the database. Therefore, when the PostgreSQL connector first connects to a particular PostgreSQL database, it starts by performing a consistent snapshot of each of the database schemas. After the connector completes the snapshot, it continues streaming changes from the exact point at which the snapshot was made. This way, the connector starts with a consistent view of all of the data, and does not omit any changes that were made while the snapshot was being taken.
The connector is tolerant of failures. As the connector reads changes and produces events, it records the WAL position for each event. If the connector stops for any reason (including communication failures, network problems, or crashes), upon restart the connector continues reading the WAL where it last left off. This includes snapshots. If the connector stops during a snapshot, the connector begins a new snapshot when it restarts.
But there is gap here, or maybe not.
When the snapshot of the database is complete and then it is streaming from WAL file, if the connector goes down and until it goes up the WAL is purged/flushed, how Debezium ensuring data integrity?
Since Debezium is using a replication slot on PG, Debezium constantly sends back to PG the information up to which LSN it has consumed so that PG can flush the WAL files. Now, this information is stored in the table pg_replication_slots. When Debezium starts up again, it reads the restart_lsn from that table and requests changes that happened before that value. This is how Debezium is ensuring data integrity.
Not that if for some reason, that LSN is not available in the WAL files, there is no way to get it back, meaning data loss has happened.
I have Postgresql 14 and I made streaming replication (remote_apply) for 3 nodes.
When the two standby nodes are down, if I tried to do an insert command this will show up:
WARNING: canceling wait for synchronous replication due to user request
DETAIL: The transaction has already committed locally, but might not have been replicated to the standby.
INSERT 0 1
I don't want to insert it locally. I want to reject the transaction and show an error instead.
Is it possible to do that?
No, there is no way to do that with synchronous replication.
I don't think you have thought through the implications of what you want. If it doesn't commit locally first, then what should happen if the master crashes after sending the transaction to the replica, but before getting back word that it was applied there? If it was committed on the replica but rejected on the master, how would they ever get back into sync?
I made a script that checks the number of standby nodes and then make the primary node read-only if the standby nodes are down.
When using HADR do both the primary and standby need to be in same sync mode?
What could be the possible outcomes if both are not same.
The synchronization mode determines when the primary database server considers a transaction complete, based on the state of the logging on the standby database.
So what sync mode you have on standby system does not matter. When this standby becomes new primary after failover, it will use local value of SYNCMODE for synchronisation.
I have a postgres master node which is streaming WAL records to a standby slave node. The slave database runs in read only mode and has a copy of all data on the master node. It can be switched to master by creating a recovery.conf file in /tmp.
On the master node I am also archiving WAL records. I am just wondering if this is necessary if they are already streamed to the slave node? The archived WAL records are 27GB at this point. The disk will fill eventually.
A standby server is no backup; it only protects you from hardware failure on the primary.
Just imagine that somebody by mistakes deletes data or drops a table, then you won't be able to recover from this problem without a backup.
Create a job that regularly cleans up archived WALs if they exceed a certain age.
Once you have a full backup, then you can purge the preceding WAL files associated.
The idea is to preserve the WAL Files for PITR in case if your server crashes.
If your Primary server crashes, then you can certainly use your hot-standby and make it primary, but at this time you have to build another server (as a hot-standby). Typically you don't want to build it using streaming replication.
You will be using full backup+wal backups to build a server and then proceed further instead of relying on streaming replication.
What is the consistency of Postgresql HA cluster with Patroni?
My understanding is that because the fail-over is using a consensus (etc or zookeeper) the system will stay consistent under network partition.
Does this mean that transaction running under the serializable Isolation Level will also provide linearizability.
If not which consistency will I get Sequential Consistency, Causal Consistency .. ?
You shouldn't mix up consistency between the primary and the replicas and consistency within the database.
A PostgreSQL database running in a Patroni cluster is a normal database with streaming replicas, so it provides the eventual consistency of streaming replication (all replicas will eventually show the same values as the primary).
Serializabiliy guarantees that you can establish an order in the database transactions that ran against the primary such that the outcome of a serialized execution in that order is the same as the workload had in reality.
If I read the definition right, that is just the same as “linearizability”.
Since only one of the nodes in the Patroni cluster can be written to (the primary), this stays true, no matter if the database is in a Patroni cluster or not.
In a distributed context, where we have multiple replicas of an object’s state, A schedule is linearizable if it is as if they were all updated at once at a single point in time.
Once a write completes, all later reads (wall-clock time) from any replica should see the value of that write or the value of a later write.
Since PostgreSQL version 9.6 its possible to have multiple synchronous standy node. This mean if we have 3 server and use num_sync = 2, the primary will always wait for write to be on the 2 standby before doing commit.
This should satisfy the constraint of linearizable schedule even with failover.
Since version 1.2 of Patroni, When synchronous mode is enabled, Patroni will automatically fail over only to a standby that was synchronously replicating at the time of the master failure.
This effectively means that no user visible transaction gets lost in such a case.