Calculate the replication lag between postgresql master and one or more slaves in streaming replication mode.

pglag.sh

#!/bin/bash
#
# Show replication lag for one or more postgresql slaves in streaming replication.
#
# dsimmons@squiz.co.uk
# 2012-01-09
#

psql="which psql"
psql_extra_opts=""
export PGCONNECT_TIMEOUT=3

#
# Converts specified xlog number to byte location
#
_xlog_to_bytes()
{
	logid="${1%%/*}"
	offset="${1##*/}"
	echo $((0xFFFFFF * 0x$logid + 0x$offset))
}

_get_xlog_loc()
{
	local pgfunc

	if _is_up $1; then

		if _in_recovery $1; then
			pgfunc="pg_last_xlog_replay_location"
		else
			pgfunc="pg_current_xlog_location"
		fi

		xlog_loc=$(psql -h $1 -Atc "SELECT $pgfunc();")
		if [ $? -gt 0 ]; then
			echo "ERROR: Failed getting xlog location from node $1" >&2
			return 1
		fi
		echo $xlog_loc
		return 0
	else
		return 1
	fi
}

_in_recovery()
{
	local recovery

	# Retrieve previously cached value
	recovery=$(eval echo \$$1_in_recovery)

	if [ ! -n "$recovery" ]; then
		recovery=$(psql -h $1 -Atc "SELECT pg_is_in_recovery();" 2>/dev/null)

		# Cache result
		eval "$1_in_recovery=\"$recovery\""
	fi

	# If pg_is_in_recovery() returns false then we're a master
	if [ "$recovery" == "f" ]; then
		return 1
	else
		# Slave. or unable to detect
		return 0
	fi

}

_is_up()
{
	local state=$(eval echo \$$1_is_up)

	if [ -n "$state" ]; then
		# Return cached value
		return $state
	else
		# Check if node is responding
		psql -h $1 -Atc "SELECT 1;" &>/dev/null

		# Cache result
		eval "$1_is_up=\"$?\""
		return $(eval echo \$$1_is_up)
	fi
}

if [ $# -lt 1 ]; then
	echo "Usage: $0 [--master <master host>] <slave host [...]>" >&2
	exit 99
fi

# If a master node was manually specified at the command-line, lets try and go
# with that (provided it's up and really is a master)
if [ "$1" == "--master" ]; then

	shift
	master=$1

	if ! _is_up $master; then
		echo "WARNING: Master specified ($master) is down" >&2
		master=""
	elif _in_recovery $master; then
		echo "WARNING: Master specified ($master) is not a master:" >&2
		master=""
	fi
else
	# Try to automatically detect the master
	for s in "$@"; do
		if ! _in_recovery $s; then
			if [ -n "$master" ]; then
				echo "WARNING: There is more than one master. To get lag calculcation, specify a master with the --master switch" >&2
				master=""
				break
			fi
			master=$s
		fi
	done
fi

# Get master xlog location (if there is a master)
if [ -n "$master" ]; then
	master_xlog_loc=$(_get_xlog_loc $master) || exit $?
fi

echo
echo "Replication status ($(date +%c))"
echo "----------------------------------------------------"

for s in "$@"; do

	role=""
	name=""
	xlog_loc=""
	bytes_lag=""

	name=$s

	if _is_up $s; then

		# Set state
		if _in_recovery $s; then
			role="Slave"
		else
			role="Master"
		fi

		# Set xlog loc
		xlog_loc=$(_get_xlog_loc $s)

		# Calculate number of bytes behind
		if [ -n "$master" ] && [ "$s" != "$master" ]; then
			bytes_lag=$(($(_xlog_to_bytes $master_xlog_loc) - $(_xlog_to_bytes $xlog_loc)))
			bytes_lag="($bytes_lag bytes lag)"
		fi
	else
		xlog_loc="down"
	fi

	printf "%-8s %s: %s %s\n" "$role" "$s" "$xlog_loc" "$bytes_lag"
done

echo

Set env variables if needed:

[sysadmin@DBLB001 ~]$ export PGUSER=postgres
[sysadmin@DBLB001 ~]$ 

Usage:

[sysadmin@DBLB001 ~]$ ./pglag.sh 
Usage: ./pglag.sh [--master <master host>] <slave host [...]>
[sysadmin@DBLB001 ~]$ 

Eg.:

[sysadmin@DBLB001 ~]$ ./pglag.sh --master DB001 DB002 DB003 DB004

Replication status (Mon 09 Jan 2012 11:52:13 GMT)
-------------------------------------------------
Master  DB001: 0/A866CEC0
Slave   DB002: 0/A866CEC0 (0 bytes lag)
Slave   DB003: 0/A866CEC0 (0 bytes lag)
Slave   DB004: 0/A866CEC0 (0 bytes lag)

[sysadmin@DBLB001 ~]$ 

Omit --master option and the script will attempt to detect the master automatically.