It is planned in PostgreSQL 9.2 to have support for cascading replication, which is the ability to add slaves under other slaves. In 9.2, slave-slave connections are only asynchronous.
This feature has been introduced by this commit.
commit 5286105800c7d5902f98f32e11b209c471c0c69c
Author: Simon Riggs
Date: Tue Jul 19 03:40:03 2011 +0100
Cascading replication feature for streaming log-based replication.
Standby servers can now have WALSender processes, which can work with
either WALReceiver or archive_commands to pass data. Fully updated
docs, including new conceptual terms of sending server, upstream and
downstream servers. WALSenders terminated when promote to master.
Fujii Masao, review, rework and doc rewrite by Simon Riggs
So let’s try to use it with the following configuration.
Cascading replication graph
Data files of master, slaves and archive folder are located in $HOME/bin/postgres.
You need to checkout the master branch of PostgreSQL GIT to try this feature.
The settings are pretty similar to what you can find here.
However, you have to take care of the following settings in postgresql.conf:
- Master
wal_level = hot_standby
archive_mode = on
archive_command = 'cp -i %p $HOME/bin/postgres/archive/%f'
max_wal_senders = 10
- Slave 1
hot_standby = on
port = 15432 - Slave 2
hot_standby = on
port = 15433 - Slave 1-1
hot_standby = on
port = 25432 - Slave 1-2
hot_standby = on
port = 25433
You can setup slave1, and slave2 as synchronously with master by using synchronous_standby_names in postgresql.conf of master.
This is set a little bit later after initializing the master backup.
You need to change recovery.conf of slave 1 and slave 2 with those parameters:
standby_mode = on
primary_conninfo = 'host=localhost port=5432 application_name=$SLAVE_NAME'
restore_command = 'cp -i $HOME/bin/postgres/archive/%f %p'
$SLAVE_NAME being slave1 for slave 1, and slave2 for slave 2.
Then as, slave 1-1 and 1-2 have to connect to slave 1, use the same values as above for standby_mode and restore_command, but setup primary_conninfo like this:
primary_conninfo = 'host=localhost port=15432 application_name=$SLAVE_NAME'
$SLAVE_NAME being slave11 for slave 1-1, and slave12 for slave 1-2.
Here is the dirty script used to start up the system. This has been written quickly, so sorry for the bad quality 
.
#!/bin/bash
#Master has port 5432
#Slave1 has port 15432
#Slave2 has port 15433
#Slave1 has port 25432
#Slave2 has port 25433
PSQL_FOLDER=$HOME/bin/postgres
PSQL_BIN=$PSQL_FOLDER/bin #Binary folder
PSQL_CONFIG=$PSQL_FOLDER/config #Folder containing all the configuration files
PSQL_MASTER=$PSQL_FOLDER/master #Master data folder
PSQL_SLAVE1=$PSQL_FOLDER/slave1 #Slave 1 data folder
PSQL_SLAVE2=$PSQL_FOLDER/slave2 #Slave 2 data folder
PSQL_SLAVE11=$PSQL_FOLDER/slave11 #Slave 11 data folder
PSQL_SLAVE12=$PSQL_FOLDER/slave12 #Slave 12 data folder
PSQL_ARCHIVE=$PSQL_FOLDER/archive #Archive folder
#clean up, take down violently all the processes
killall postgres
rm -rf $PSQL_MASTER $PSQL_SLAVE1 $PSQL_SLAVE2 $PSQL_SLAVE11 $PSQL_SLAVE12 $PSQL_ARCHIVE
mkdir $PSQL_MASTER $PSQL_SLAVE1 $PSQL_SLAVE2 $PSQL_SLAVE11 $PSQL_SLAVE12 $PSQL_ARCHIVE
sleep 1
#Initialize master
$PSQL_BIN/initdb -D $PSQL_MASTER
cp $PSQL_CONFIG/postgresql.conf.master $PSQL_MASTER/postgresql.conf
cp $PSQL_CONFIG/pg_hba.conf.master $PSQL_MASTER/pg_hba.conf
#Start master
$PSQL_BIN/postgres -D $PSQL_MASTER &
#Wait a little before server start up, let it finish initialization
echo "Master started"
sleep 2
#Initialize slave
#This is used to start the backup so as slave does not have to recover from
#scratch when being build. It definitely accelerates standby start up
$PSQL_BIN/psql postgres -c "select pg_start_backup('backup')"
cp -pr $PSQL_MASTER/* $PSQL_SLAVE1/
cp -pr $PSQL_MASTER/* $PSQL_SLAVE2/
cp -pr $PSQL_MASTER/* $PSQL_SLAVE11/
cp -pr $PSQL_MASTER/* $PSQL_SLAVE12/
$PSQL_BIN/psql postgres -c "select pg_stop_backup()"
echo "Backup performed"
sleep 1
#Setup synchronous commit mode on master
echo "synchronous_standby_names='slave1,slave2'" >> $PSQL_MASTER/postgresql.conf
$PSQL_BIN/pg_ctl reload -D $PSQL_MASTER
echo "Force master to synchronize mode with slave1 (priority 1) and slave 2 (priority 2)"
sleep 1
#Then finish by copying all the configuration files for slaves
cp $PSQL_CONFIG/postgresql.conf.slave1 $PSQL_SLAVE1/postgresql.conf
cp $PSQL_CONFIG/recovery.conf.slave1 $PSQL_SLAVE1/recovery.conf
cp $PSQL_CONFIG/postgresql.conf.slave2 $PSQL_SLAVE2/postgresql.conf
cp $PSQL_CONFIG/recovery.conf.slave2 $PSQL_SLAVE2/recovery.conf
cp $PSQL_CONFIG/postgresql.conf.slave11 $PSQL_SLAVE11/postgresql.conf
cp $PSQL_CONFIG/recovery.conf.slave11 $PSQL_SLAVE11/recovery.conf
cp $PSQL_CONFIG/postgresql.conf.slave12 $PSQL_SLAVE12/postgresql.conf
cp $PSQL_CONFIG/recovery.conf.slave12 $PSQL_SLAVE12/recovery.conf
#Delete unnecessary xlog files and postmaster pid files
rm $PSQL_SLAVE1/postmaster.pid
rm -r $PSQL_SLAVE1/pg_xlog/*
rm $PSQL_SLAVE2/postmaster.pid
rm -r $PSQL_SLAVE2/pg_xlog/*
rm $PSQL_SLAVE11/postmaster.pid
rm -r $PSQL_SLAVE11/pg_xlog/*
rm $PSQL_SLAVE12/postmaster.pid
rm -r $PSQL_SLAVE12/pg_xlog/*
#Start Slave 1
chmod 700 $PSQL_SLAVE1
$PSQL_BIN/postgres -D $PSQL_SLAVE1 &
echo "Slave 1 started"
#Start Slave 2
chmod 700 $PSQL_SLAVE2
$PSQL_BIN/postgres -D $PSQL_SLAVE2 &
echo "Slave 2 started"
#Start Slave 11
chmod 700 $PSQL_SLAVE11
$PSQL_BIN/postgres -D $PSQL_SLAVE11 &
echo "Slave 11 started"
#Start Slave 12
chmod 700 $PSQL_SLAVE12
$PSQL_BIN/postgres -D $PSQL_SLAVE12 &
echo "Slave 12 started"
exit 0
Configuration files and scripts can be found here.
OK now let’s check if it works well.
$ psql postgres
postgres=# select application_name,state,sync_priority,sync_state from pg_stat_replication;
application_name | state | sync_priority | sync_state
------------------+-----------+---------------+------------
slave1 | streaming | 1 | sync
slave2 | streaming | 2 | potential
(2 rows)
On master, slave1 has priority 1 for synchronization (synchronous_standby_nodes has been set up ‘slave1,slave2′). It looks to be correctly synchronized.
Then let’s do the same check from slave1.
$ psql -p 15432 postgres
postgres=# select application_name,state,sync_priority,sync_state from pg_stat_replication;
application_name | state | sync_priority | sync_state
------------------+-----------+---------------+------------
slave12 | streaming | 0 | async
slave11 | streaming | 0 | async
(2 rows)
slave11 and slave12 are correctly linked to slave1. Yippee.
Why not some additional check with some data…
$ #Connection from master
$ psql -p 5432 postgres
postgres=# create table aa (a int);
CREATE TABLE
postgres=# insert into aa values (1),(2);
INSERT 0 2
postgres=# select * from aa;
a
---
1
2
(2 rows)
postgres=# \q
Has the slave of a slave been updated? Connection to let’s say… slave12
$ psql -p 25433 postgres
postgres=# select * from aa;
a
---
1
2
(2 rows)
OK, that rocks. I let you imagine then how to use that as an HA solution 
.
