Use AWS SNS to communicate between ECS Tasks

It turns out my original premise (needing to know the task container's own internal IP address for service discovery) is very flawed - that IP address is only usable within a single EC2 Container Instance. If you have multiple container instances (which you probably should have) then those task container IP's are basically useless.

The alternative solution that I came up with is to follow the pattern suggested for Application Load Balancers running HTTP / HTTPS - have a port mapping with 0 as the host port, pointing to the port within the docker instance that I need to use. By doing this, Docker will assign a random host port, which I can then find by using the AWS SDK - in particular, the "describeTasks" function available on the ECS module. See here for details: http://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/ECS.html#describeTasks-property

This is the fundamental basis for my roll-your-own service discovery mechanism - there are a lot of other details necessary to do this in a complete fashion. I used Lambda functions calling out to the AWS SDK as well as a PostgreSQL database in order to keep my list of host containers up to date (something a bit like a dynamic DNS registry). Part of the trick is you need to know the IP and port for each of the containers, but describeTasks only returns the port. Here is a handy NodeJS function that I wrote which takes a container name and looks for all IP addresses and ports found within the cluster for containers of that name:

var Q = require('q');
/**
 * @param {String} - cluster - name of the cluster to query, e.g. "sqlfiddle3"
 * @param {String} - containerType - name of the container to search for within the cluster
 * @returns {Promise} - promise resolved with a list of ip/port combinations found for this container name, like so:
    [
      {
        "connection_meta": "{\"type\":\"ecs\",\"taskArn\":\"arn:aws:ecs:u..\"}",
        "port": 32769,
        "ip": "10.0.1.49"
      }
    ]
 *
 */
exports.getAllHostsForContainerType = (cluster, containerType) => {
    var AWS = require('aws-sdk'),
        ecs = new AWS.ECS({"apiVersion": '2014-11-13'}),
        ec2 = new AWS.EC2({"apiVersion": '2016-11-15'});

    return ecs.listTasks({ cluster }).promise()
    .then((taskList) => ecs.describeTasks({ cluster, tasks: taskList.taskArns }).promise())
    .then((taskDetails) => {
        var containersForName = taskDetails.tasks
            .filter((taskDetail) =>
                taskDetail.containers.filter(
                    (container) => container.name === containerType
                ).length > 0
            )
            .map((taskDetail) =>
                taskDetail.containers.map((container) => {
                    container.containerInstanceArn = taskDetail.containerInstanceArn;
                    return container;
                })
            )
            .reduce((final, containers) =>
                final.concat(containers)
            , []);

        return containersForName.length ? (ecs.describeContainerInstances({ cluster,
            containerInstances: containersForName.map(
                (containerDetails) => containerDetails.containerInstanceArn
            )
        }).promise()
        .then((containerInstanceList) => {

            containersForName.forEach((containerDetails) => {
                containerDetails.containerInstanceDetails = containerInstanceList.containerInstances.filter((instance) =>
                    instance.containerInstanceArn === containerDetails.containerInstanceArn
                )[0];
            });

            return ec2.describeInstances({
                InstanceIds: containerInstanceList.containerInstances.map((instance) =>
                    instance.ec2InstanceId
                )
            }).promise();
        })
        .then((instanceDetails) => {
            var instanceList = instanceDetails.Reservations.reduce(
                (final, res) => final.concat(res.Instances), []
            );

            containersForName.forEach((containerDetails) => {
                if (containerDetails.containerInstanceDetails) {
                    containerDetails.containerInstanceDetails.ec2Instance = instanceList.filter(
                        (instance) => instance.InstanceId === containerDetails.containerInstanceDetails.ec2InstanceId
                    )[0];
                }
            });
            return containersForName;
        })) : [];
    })
    .then(
        (containersForName) => containersForName.map(
            (container) => ({
                connection_meta: JSON.stringify({
                    type: "ecs",
                    taskArn: container.taskArn
                }),
                // assumes that this container has exactly one network binding
                port: container.networkBindings[0].hostPort,
                ip: container.containerInstanceDetails.ec2Instance.PrivateIpAddress
            })
        )
    );
};

Note that this uses the 'Q' promise library - you'll need to declare that as a dependency in your package.json.

The rest of my custom solution for handing ECS service discovery using Lambda functions can be found here: https://github.com/jakefeasel/sqlfiddle3#setting-up-in-amazon-web-services

This is a 2017 question that has popped up again.

As of 2022, the best option here is to use Fargate to provision the container rather than EC2. ECS will spin up the required container on Fargate, use it as long as required, then it will be deprovisioned once the task stops.