MongoDB replication, it has 3 servers(Server1, Server2, Server3). Due to any reason, Server1 goes down and Server2 acts as Primary and Server3 as Secondary mode.
Query: As Server1 is down and after 2-3 hours we made it up(running). The 3 hrs gap between Server1 data and Server2 data, how it will be sync up?
The primary maintains an oplog detailing all of the writes that have been done to the data. The oplog is capped by size, the oldest entries are automatically removed to keep it below the configured size.
When a secondary node replicates from the primary, it reads the oplog and creates a local copy. If a secondary is offline for a period of time, when it comes back online, it will ask the primary for all oplog entries since the last one that it successfully copied.
If the primary still has the entry that the secondary most recently saw, the secondary will begin applying the events it missed.
If the primary no longer has that entry, the secondary will log a message that it is too stale to catch up, and manual intervention will be required. This would usually require a manual resync
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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.
Under my understanding, in ClickHouse ReplicatedMergeTree, insert operation will write log in ZK "/log", other replica pull log, execute task and sync date.
My question is when one replica is unavailable(machine is down or clickhouse instance is down), this replica cannot pull log and sync data. If other replica still insert data and push log to ZK. How long the operation log will keep in ZK? Is there valid period? Maybe ZK will not keep these log forever, is there exact keep time?
And if insert log in ZK is removed and the prior unavailable replica is normal again, how this replica sync data with other replica?
Appreciate for any answer or discussion, thank you.
SELECT *
FROM system.merge_tree_settings
WHERE name LIKE '%replicated_logs%'
FORMAT Vertical
Query id: 534466cf-1624-4ca0-b559-bc8c381ff547
Row 1:
──────
name: max_replicated_logs_to_keep
value: 1000
changed: 0
description: How many records may be in log, if there is inactive replica. Inactive replica becomes lost when when this number exceed.
type: UInt64
Row 2:
──────
name: min_replicated_logs_to_keep
value: 10
changed: 0
description: Keep about this number of last records in ZooKeeper log, even if they are obsolete. It doesn't affect work of tables: used only to diagnose ZooKeeper log before cleaning.
type: UInt64
max_replicated_logs_to_keep now is 1000.
During the past this default value were changing, it was 10000, 100, 1000 :) .
If a replication log is "rotated" (a replica delay is >1000), it's not a problem at all, the stale replica starts a special bootstrap procedure, it does not use log at all, but it syncs it's metadata and a list of parts with other replicas, this procedure is slightly longer than rolling the log.
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.
I have two mongod instances without replication each having same collection name but different data.Now initialized replication between them.Secondary machine copies all data from primary machine and looses it's original data.Can I recover original data present in secondary machine ?
This is the expected behaviour with MongoDB replica sets: data from the primary is replicated to the secondaries. When you add a server as a new secondary, it does an "initial sync" which copies data from the primary. The replica sets are designed for failover and redundancy; your secondary nodes should have data consistent with the primary and their current replication lag.
If you have overwritten your previous database, your only option is to restore from a backup.
See also:
Backup and Restoration Strategies
Replica Set Internals Part V: Initial Sync