Replication & Upgrades - PostgreSQL 9 Administration - page 325

F delay_threshold specifies the accepted replication delay of the Standby against
the primary server in bytes. If the delay exceeds delay_threshold then load
balancing is interrupted momentarily and all queries are sent to the Master. If you
don't want this, just set it to zero.
F log_Standby_delay provides an option to log the Standby delay, so you can review
it over time. If always, we log the delay every time health checking is performed. If
if_over_threshold is specified, then we compare against delay_threshold
before logging.
F health_check_period defines the time between measurements of the Standby
delay. health_check_user specifies the username that will be used for health
check access.
There are additional parameters for automated failover and other features. You may want to
read the pgpool.conf.sample-stream file for more detail on those and the many other
parameters you can control.
Pgpool can be controlled using the command line, like the following:
pgpool -f pgpool.conf -a pool_hba.conf
pgpool -m fast stop
pgpool -f pgpool.conf -a pool_hba.conf reload
The replication delay can be requested dynamically by issuing the following:
SHOW POOL_STATUS;
which is a dummy SQL command that is intercepted by pgpool and returns information about
the dynamic status. Other SHOW commands provide various other feedback.
How it works...
Pgpool parses SQL statements to understand their type and contents. The parsing needs
to go quite deep to understand whether SELECT statements contain volatile function calls.
Pgpool doesn't know about user-defined functions, though you can explicitly include or exclude
functions using the new parameters white_function_list (to include) and black_
function_list (to exclude).
Pgpool implements the health checking feature in a separate worker process, so it can
continue to work without blocking incoming SQL.
Special thanks to Tatsuo Ishii for providing detailed feedback on my questions about the new
features of pgpool II 3.0, which was released shortly before publication.