[pgpool-general: 4473] Re: Dynamic configuration of pgpool

Tatsuo Ishii ishii at postgresql.org
Fri Feb 19 18:10:18 JST 2016

Have you actually tried with 127 standbys?

I don't think the configuration is usable because of too much load on
the primary master.

Best regards,
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php

> How do I manage read replicas that appear and disappear dynamically and
> unpredictably in an Amazon Web Services environment? If I understand the
> docs correctly at most, 127 read replicas can appear and disappear before
> we absolutely must restart pgpool, correct?
> Some background:
>   We are running PostgreSQL in AWS. We have an always-on master database,
> and a number of read-only replicas using streaming replication. At the
> moment, we do not use pgpool - we just hardcode the hostnames of the master
> and the read-only replicas in our app code.
>   We want to dynamically scale up and down our read replicas based on daily
> usage patterns, as well as use the cheaper spot EC2 instances that Amazon
> provides - the caveat being that the latter can be shut down if the market
> price goes above what we are willing to pay. When this happens, we would
> either wait for the spot price to come down again, or bring up new
> instances with a higher price.
>   The following part from the manual for the "backend_hostname"
> configuration setting suggests that using pgpool to accomplish this will be
> hard: "New nodes can be added in this parameter by reloading a
> configuration file. However, values cannot be updated so you must restart
> pgpool-II in that case."
> So, if we have, in the config:
> backend_hostname1=A
> and A goes down, then when its replacement B comes up, we can change the
> config in the following way:
> backend_hostname1=A
> backend_hostname2=B
> and tell pgpool to reload the config. I guess we can also remove the
> backend_hostname1 line completely, like this:
> backend_hostname2=B
> We *cannot* edit backend_hostname1 like this and expect it work:
> backend_hostname1=B
> If I understand the code correctly, the biggest number allowed in the
> backend_hostname<backend_number> is MAX_NUM_BACKENDS, which is 128.
> This means that after we add backend_hostname127=ZZZ, we must drop back to
> backend_hostname0=ZZZA and restart pgpool. Is there a way to avoid this?

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