Re: Improve handling of parameter differences in physical replication

On 2020/02/27 17:23, Peter Eisentraut wrote:
> When certain parameters are changed on a physical replication primary,   this is communicated to standbys using the XLOG_PARAMETER_CHANGE WAL record.  The standby then checks whether its own settings are at least as big as the ones on the primary.  If not, the standby shuts down with a fatal error.
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> The correspondence of settings between primary and standby is required because those settings influence certain shared memory sizings that are required for processing WAL records that the primary might send.  For example, if the primary sends a prepared transaction, the standby must have had max_prepared_transaction set appropriately or it won't be able to process those WAL records.
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> However, fatally shutting down the standby immediately upon receipt of the parameter change record might be a bit of an overreaction.  The resources related to those settings are not required immediately at that point, and might never be required if the activity on the primary does not exhaust all those resources.  An extreme example is raising max_prepared_transactions on the primary but never actually using prepared transactions.
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> Where this becomes a serious problem is if you have many standbys and you do a failover.  If the newly promoted standby happens to have a higher setting for one of the relevant parameters, all the other standbys that have followed it then shut down immediately and won't be able to continue until you change all their settings.
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> If we didn't do the hard shutdown and we just let the standby roll on with recovery, nothing bad will happen and it will eventually produce an appropriate error when those resources are required (e.g., "maximum number of prepared transactions reached").
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> So I think there are better ways to handle this.  It might be reasonable to provide options.  The attached patch doesn't do that but it would be pretty easy.  What the attached patch does is:
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> Upon receipt of XLOG_PARAMETER_CHANGE, we still check the settings but only issue a warning and set a global flag if there is a problem.  Then when we actually hit the resource issue and the flag was set, we issue another warning message with relevant information.  Additionally, at that point we pause recovery instead of shutting down, so a hot standby remains usable.  (That could certainly be configurable.)

+1
> Btw., I think the current setup is slightly buggy.  The MaxBackends value that is used to size shared memory is computed as MaxConnections + autovacuum_max_workers + 1 + max_worker_processes + max_wal_senders, but we don't track autovacuum_max_workers in WAL.

Maybe this is because autovacuum doesn't work during recovery?

Regards,

--
Fujii Masao
NTT DATA CORPORATION
Advanced Platform Technology Group
Research and Development Headquarters