t3: the templating tool

t3 is a tool to populate a template from a set of predefined data. Today, often the best tools available for this job is sed(1) or awk(1). While extremely powerful, these tools are terrible at templating.

t3 is designed for a world with docker, etcd, consul & vault. Or really, a world with curl, jq and URLs like http://169.254.169.254/latest/meta-data/. A world with cloud-init scripts on auto-scaled instances. A world to impatient to type all the letters in kubernetes. t3 is the hero we deserve.

The simplest thing that could possibly work

cat >services.json <<EOF
{
  "statsd": [
    {
      "hostname": "10.0.0.216",
      "port": 8126
    }
  ]
}
EOF

cat >config.mustache <<EOF
{#statsd}
statsd.host={{hostname}}:{{port}}
{/statsd}
EOF

t3 -d services.json -t config.mustache > services.cfg

cat services.cfg
statsd.host=10.0.0.216:8126

That was delightfully simple, but what could this possibly be good for?

Imagine you've got a service called myface and you want to reload its config any time the values in your service config JSON file are updated.

#!/bin/sh
# myface-cfg-update - watch for service updates
#   and safely load them into myface

BASE=myface
TEMP=`mktemp -t $BASE.XXXXXXXXXX` || exit 1

SERVICE_MAP=/var/lib/discoteq/services.json
TEMPLATE=/opt/myface/config.mustache
CONFIG=/etc/myface.cfg
DAEMON=myface

# watch for service info changes in registry
# re-eval config template
# verify generated config is valid
# swap if valid and different
# reload if swapped
fswatch -1 $SERVICE_MAP &&
  t3 -d $SERVICE_MAP -t $TEMPLATE > $TEMP &&
  myface --check-cfg $TEMP &&
  flock $CONFIG diffswp $TEMP $CONFIG &&
  service $DAEMON reload