The Problem
Amazon Web Services (AWS) is pretty cool. You ought to know that by now. if you don’t, take a few hours and check out some tutorials and play around.
One of the many services AWS provides is the EC2 Container Service (ECS), where the scheduling and lifecycle management of running Docker containers is handled by the ECS control plane (probably magic cooked up in Seattle over coffee or in Dublin over a pint or seven).
You can read all about its launch here.
One missing feature from the ECS offering in comparison to other container schedulers was the concept of scheduling a service to be run on each host in a cluster, such as a logging or monitoring agent.
This feature allows clusters to grow or shrink and still have the correct services running on each node.
A published workaround was to have each node individually run an instance of the defined task on startup, which works pretty well.
The downside here is is that if a task definition changes, ECS has no way of triggering an update to the running tasks – normal services will stop then start the task with a new definition, and use your logic to maintain some degree of uptime.
To achieve the update, one must terminate/replace the entire ECS Container Instance (the EC2 host) and if you’re using AutoScalingGroups, get a fresh node with the updated task.
Other Solutions
- Docker Swarm calls this a
global service, and will run one instance of the service on every node. - Mesos’ Marathon doesn’t support this yet either, and is in deep discussion on GitHub on how to implement this in their
constraintssyntax. - Kubernetes has a
DaemonSetto run a pod on each node. - The recently-released ECS-focused Blox provides a
daemon-schedulerto accomplish this, but brings along extra components to accomplish the scheduling.
Back to ECS
So imagine my excitement when the ECS team announced the release of their new Task Placement Strategies last week, offering a “One Task Per Host” strategy as part of the Service declaration.
This indeed is awesome and works as advertised, with no extra components, installs, schedulers, etc.
However! Currently each Service requires a “Desired Count” parameter of how many instances of this service you want to run in the cluster.
Given a cluster with 5 ECS Container Instance hosts, setting the Desired Count to 5 ensures that one runs on each host, provided there are resources available (cpu, ram, available port).
If the cluster grows to 6 (autoscaling, manually adding, etc), there’s nothing in the Service definition that will increase the desired count to 6, so this solution is actually worse off than our previous mode of using user-data to run the task at startup.
One approach is to arbitrarily raise the Desired count to a very high number, such as 100 for this cluster, with the consideration that we are unlikely to grow the cluster to this size without realizing it.
The scheduler will periodically examine the cluster for placement, and handle any hosts missing the service.
The problem with this is that it’s not deterministic, and CloudWatch metrics will report these unplaced tasks as Pending, and I have alarms to notify me if tasks aren’t placed in clusters, as this can point to a resource allocation mismatch.
Enter The Players
To accomplish an automated service desired count, we must use some elements to “glue” a few of the systems together with our custom logic.
- ECS
- CloudWatch Events
- Lambda
- IAM (not visually represented, but everything in AWS needs some sort of IAM set up)
Here’s a sequence diagram of the conceptual flow between the components.
Every time there is a change in an ECS Cluster, CloudWatch Events will receive a payload.
Based on a rule we craft to select events classified as “Container Instance State Change”, CW Events will emit an event to the target of your choice, in our case, Lambda.
We could feasibly use a cron-like schedule to fire this every N minutes to inspect, evaluate, and remediate a semi-static set of services/cluster, but having a system that is reactive to change feels preferable to poll/test/repair.
A simple rule that captures all Container Instance changes:
{
"source": [
"aws.ecs"
],
"detail-type": [
"ECS Container Instance State Change"
]
}
You can restrict this to specific clusters by adding the cluster’s ARN to the keys like so:
"detail": {
"clusterArn": [
"arn:aws:ecs:us-east-1:123456789012:cluster/my-specific-cluster",
"arn:aws:ecs:us-east-1:123456789012:cluster/another-cluster"
]
}
If being throttled or cost is a concern here, you may wish to filter to a set of known clusters, but this reduces the reactiveness of the logic to new clusters being brought online.
The Actual Logic
The Lambda function receives the event, performs some basic validation checks to ensure it has enough details to proceed, and then makes a single API call to the ECS endpoint to find our specified service in the cluster that fired the change event.
If no such service is found, we terminate now, and move on.
If the cluster does indeed have this service defined, then we perform another API call to describe the count of registered container instances, and compare that with the value we already have from the service definition call.
If there’s a mismatch, we perform a final third API call to adjust the service definition’s desired task count.
All in all, a maximum total of 3 possible API calls, usually in under 300ms.
In my environment, I want this task to apply to every cluster in my account, as we later on inspect the cluster to see if it has a service definition applied to it, to act upon.
In my ballpark figures with a set of 10 active clusters, the cost for running this logic should be under $2/month – yes, two dollars a month to ensure your cluster has the correct number of tasks for a given service.
Do you own estimation with the Lambda Pricing Calculator.
Conclusions
The code can be found on GitHub, and was developed with test-everything philosophy, where I spent a large amount of time learning how to actually write the code and tests elegantly.
Writing out all of the tests and sequences allowed me to find multiple points of refactoring and increased efficiency from my first implementation, leading to a much cleaner solution.
Taking on a project like this is a great way to increase one’s own technical prowess, leading to the ability to reason about other problems.
While I strongly believe that this feature should be part of the ECS platform and not require any client-side intervention, the ability to take the current offerings and extend them via mechanisms such as Events, Lambda and API calls further demonstrates the flexibility and extensibility of the AWS ecosystem.
The feature launched just over a week ago, and I’ve been able to put together an acceptable solution on my own, using the documentation, tooling, and infrastructure while minimizing costs and making my system more reactive to change.
I look forward to what else the ECS, Lambda and CloudWatch Events team cook up in the future!
