dockeractd

React to docker daemon's events

WHAT IS THIS?

There's always something you'd like to trigger upon bringing up a new container. There's Consul and the likes to do such things, but these are ... big. They make sense when you want to scale, but there are times when all you really need to do is just updating that one local file or something like that.

dockeractd is basically the answer for that. It just listens for events emitted by the local docker daemon, and calls the registered handler. That's all. Nothing more. No networks to configure, no ports to open, no agents to install.

A typical usage would be something like this:

$ dockeractd -exec=/path/to/handler

This registered dockeracd to listen to /var/run/docker.sock, and when an event arrives, the handler specified in -exec will be executed.

The handler can be anything: a shell script, a compiled binary, whatever. It receives a JSON string from the standard input, which consists of two top level keys:

{
  "Event": { ... },
  "Container": { ... }
}

The Container key may be null depending on the event type. Available events depend on the API version that your docker daemon supports.

An example handler shell script my look like this:

#!/bin/bash

WORKFILE=/tmp/work.json
cat > $WORKFILE

event=$(jq -r .Event.Status $WORKFILE)
if [ "$event" != "start" ]
    rm -f $WORKFILE
    exit 0
fi

ip=$(jq -r .Container.NetworkSettings.IPAddress $WORKFILE)
if [ "$ip" != "null" ]; then
    echo "Container's IP is $ip"
fi

rm -f $WORKFILE

This just prints out the IP address of the newly started container.