check_elb_ips.sh

#!/usr/bin/env bash
 
VERBOSE=1
IPS_FILE="/tmp/elb_ips"
BACKEND_VCL="/etc/varnish/backend.vcl"
# Does not keep newlines, but it's ok
read -r -d '' BACKEND_TEMPLATE <<'EOF'
backend internal_elb_{ELB_NUM} {
    .host = "{ELB_IP}";
    .port = "80";
}
EOF
 
mapfile ELB_IPS < <(dig +short $1 | grep -v '[[:alpha:]]' | sort -u)
 
function write_ip_to_file {
    rm $IPS_FILE
    IPS=("${@}")
    for IP in ${IPS[@]}; do
        echo $IP >> $IPS_FILE
    done
}
 
function verbose {
    if [ $VERBOSE -eq 1 ]; then
      echo $1 >&2
    fi
}
 
if [ ! -f "$IPS_FILE" ]; then
    verbose "ELB IPS are not stored yet, creating..."
    write_ip_to_file ${ELB_IPS[@]}
elif ! diff -q <(printf "%s" "${ELB_IPS[@]}") $IPS_FILE > /dev/null; then
    verbose "IPS have changed, updating.
    write_ip_to_file ${ELB_IPS[@]}
    tmp_file=$(tempfile)
    verbose "Creating backends..."
    for idx in ${!ELB_IPS[@]}; do
        ip=$(echo ${ELB_IPS[$idx]} | tr -d '\n ')
        DEFINED_BACKEND=$(echo $BACKEND_TEMPLATE | sed -e "s/{ELB_NUM}/$idx/" -e "s/{ELB_IP}/$ip/")
        verbose "+ $DEFINED_BACKEND"
        echo $DEFINED_BACKEND >> $tmp_file
    done
 
    verbose "Creating director..."
    echo "director elb round-robin {" >> $tmp_file
    for idx in ${!ELB_IPS[@]}; do
        echo "{ .backend = internal_elb_$idx; }" >> $tmp_file
    done
    echo "}" >> $tmp_file
 
    verbose "Moving new backend file"
    mv $tmp_file $BACKEND_VCL
 
    verbose "Reloading Varnish"
    service varnish reload
else
    verbose "ELB ips have not changed, exiting..."
fi