api-gateway-config/marathon-service-discovery.sh

#!/bin/sh
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#
#  Overview:
#    It reads the list of tasks from Marathon and it dynamically generates the Nginx configuration with the upstreams.
#
#    In order to gather the information the script needs to know where to find Marathon
#       so it looks for the $MARATHON_HOST environment variable.
#

TMP_FILE=/tmp/api-gateway-upstreams.http.conf
UPSTREAM_FILE=/etc/api-gateway/environment.conf.d/api-gateway-upstreams.http.conf
marathon_host=$(echo $MARATHON_HOST)

do_log() {
        local _MSG=$1
        echo "`date +'%Y/%m/%d %H:%M:%S'` - marathon-service-discovery: ${_MSG}"
}

fatal_error() {
        local _MSG=$1
        do_log "ERROR: ${_MSG}"
        exit 255
}

info_log() {
        local _MSG=$1
        do_log "${_MSG}"
}

# 1. create the new upstream config
# NOTE: for the moment when tasks expose multiple ports, only the first one is exposed through nginx
curl -s ${marathon_host}/v2/tasks -H "Accept:text/plain" | awk 'NF>2' | grep -v :0 | awk '!seen[$1]++' | awk ' {s=""; for (f=3; f<=NF; f++) s = s  "\n server " $f " fail_timeout=10s;" ; print "upstream " $1 " {"  s  "\n keepalive 16;\n}" }'  > ${TMP_FILE}
# 1.1. check redis upstreams
#
# ASSUMPTION:  there is a redis app named "api-gateway-redis" deployed in marathon and optionally another app named "api-gateway-redis-replica"
#
redis_master=$(cat ${TMP_FILE} | grep api-gateway-redis | wc -l)
redis_replica=$(cat ${TMP_FILE} | grep api-gateway-redis-replica | wc -l)
#      if api-gateway-redis upstream exists but api-gateway-redis-replica does not, then create the replica
if [ ${redis_master} -gt 0 ] && [ ${redis_replica} -eq 0 ]; then
    # clone api-gateway-redis block
    sed -e '/api-gateway-redis/,/}/!d' ${TMP_FILE} | sed 's/-redis/-redis-replica/' >> ${TMP_FILE}
fi

if [ ${redis_master} -eq 0 ]; then
    echo "upstream api-gateway-redis { server 127.0.0.1:6379; }" >> ${TMP_FILE}
fi

# 2 check for changes
cmp -s ${TMP_FILE} ${UPSTREAM_FILE}
changed_upstreams=$?
if [[ \( ${changed_upstreams} -gt 0 \) ]]; then
    cp ${TMP_FILE} ${UPSTREAM_FILE}
fi