If a service is using the rolling update (ECS) deployment type, the
minimum healthy percent represents a lower limit on the number of
tasks in a service that must remain in the RUNNING state during a
deployment, as a percentage of the desired number of tasks (rounded up
to the nearest integer). The parameter also applies while any
container instances are in the DRAINING state if the service contains
tasks using the EC2 launch type. This parameter enables you to deploy
without using additional cluster capacity. For example, if your
service has a desired number of four tasks and a minimum healthy
percent of 50%, the scheduler may stop two existing tasks to free up
cluster capacity before starting two new tasks. Tasks for services
that do not use a load balancer are considered healthy if they are in
the RUNNING state. Tasks for services that do use a load balancer are
considered healthy if they are in the RUNNING state and they are
reported as healthy by the load balancer. The default value for
minimum healthy percent is 100%.
(https://stackoverflow.com/a/40741816/433570, https://docs.aws.amazon.com/AmazonECS/latest/developerguide/update-service.html)
So If I have single instance running,
If I have minimum percentage below 50%, the running task will be killed before creating new task.
If I have minimum percentage above or equal to 50% , new deployment won't be deployed, because it is stuck at having 1 instance.
I have to temporarily increase the desired instance count to 2 and do a deployement if I want to deploy a task without service downtime. correct?
Related
I'm facing the following problem when creating new instances and increasing the container desired count at the same time. Since the instances are not running when I increase the desired count, I get a "service XXX was unable to place a task because no container instance met all of its requirements.". A few seconds later the new instances are up, however, the cluster still has "Desire count: 30, Pending count: 0, Running count: 3". In other words, the cluster does not "know" that there are new instances and no new containers are created.
How can I avoid this situation? Is there a parameter that instructs the cluster to monitor the instance count other than immediately after an increase in desired count?
In this case its an expected behavior of ECS, reason being that ECS service scheduler includes circuit breaker logic that throttles how often tasks are placed if they repeatedly fail to launch.
When a new container instance in spined up it takes some time to get
register to the Cluster and it looks like service is getting throttled
because time taken from increase in desired count to registration of
container instances to the cluster.
Having said that, if you wait for ~15 minutes after scaling number of instance in the cluster, Service scheduler will start placing the task on new container instances.
To avoid this situation, ECS Cluster should be autoscaled based on Custer reservation metric, by this ECS cluster will have additional capacity beforehand to accommodate new task count.
and here is a tutorial on scaling ECS cluster.
I have an ECS cluster with a service in it that is running a task I have defined. It's just a simple flask server as I'm learning how to use ECS. Now I'm trying to understand how to update my app and have it seamlessly deploy.
I start with the flask server returning Hello, World! (rev=1).
I modify my app.py locally to say Hello, World! (rev=2)
I rebuild the docker image, and push to ECR
Since my image is still named image_name:latest, I can simply update the service and force a new deployment with: aws ecs update-service --force-new-deployment --cluster hello-cluster --service hello-service
My minimum percent is set to 100 and my maximum is set to 200% (using rolling updates), so I'm assuming that a new EC2 instance should be set up while the old one is being shutdown. What I observe (continually refreshing the ELB HTTP endpoint) is that that the rev=? in the message alternates back and forth: (rev=1) then (rev=2) without fail (round robin, not randomly).
Then after a little bit (maybe 30 secs?) the flipping stops and the new message appears: Hello, World! (rev=2)
Throughout this process I've noticed that no more EC2 instances have been started. So all this must have been happening on the same instance.
What is going on here? Is this the correct way to update an application in ECS?
This is the normal behavior and it's linked to how you configured your minimum and maximum healthy percent.
A minimum healthy percent of 100% means that at every moment there must be at least 1 task running (for a service that should run 1 instance of your task). A maximum healthy percent of 200% means that you don't allow more than 2 tasks running at the same time (again for a service that should run 1 instance of your task). This means that during a service update ECS will first launch a new task (reaching the maximum of 200% and avoiding to go below 100%) and when this new task is considered healthy it will remove the old one (back to 100%). This explains why both tasks are running at the same time for a short period of time (and are load balanced).
This kind of configuration ensures maximum availability. If you want to avoid this, and can allow a small downtime, you can configure your minimum to 0% and maximum to 100%.
About your EC2 instances: they represent your "cluster" = the hardware that your service use to launch tasks. The process described above happens on this "fixed" hardware.
After pushing a new image of my container I use Terraform apply to update the task definition. This seems to work fine but in the ECS service list of tasks I can see the task as inactive and I have an event:
service blahblah was unable to place a task because no container instance met all of its requirements. The closest matching container-instance [guid here] is already using a port required by your task.
The thing is, the site is still active and working.
This is more of an ECS issue than a Terraform issue because Terraform is updating your task definition and updating the service to use the new task definition but ECS is unable to schedule new tasks on to the container instances because you're (presumably) defining a specific port that the container must run on and directly mapping it to the host or using host networking instead of bridge (or the new aws-vpc CNI plugin).
ECS has a couple of parameters to control the behaviour of an update to the service: minimum healthy percent and maximum healthy percent. By default these are set to 100% and 200% respectively meaning that ECS will attempt to deploy a new task matching the new task definition and wait for it to be considered healthy (such as passing ELB health checks) before terminating the old tasks.
In your case you have as many tasks as you have container instances in your cluster and so when it attempts to schedule a new task on to the cluster it is unable to place it because the port is already bound to by the old task. You could also find yourself in this position if you had placement constraints on your task/service.
Because the minimum healthy percent is set to 100% it is unable to schedule the removal of any of the old tasks that would then free up a placement option for a new task.
You could have more container instances in the cluster than you have instances of the task running which would allow ECS to deploy new tasks before removing old tasks from the other instances or you could change the minimum healthy percent (deployment_minimum_healthy_percent in Terraform's ECS service resource) to a number less than 100 that allows deployments to happen.
For example, if you normally deploy 3 instances of the task in the service then setting the minimum healthy percent to 50% would allow ECS to remove one task from the service before scheduling a new task matching the new task definition. It would then proceed with a rolling upgrade, making sure the new task is healthy before replacing the old task.
Setting the minimum healthy percent to 0% would mean that ECS can stop all of the tasks running before starting new tasks but this would obviously lead to a potential (but not guaranteed) service interruption.
Alternatively you could remove the placement constraint by switching away from host networking if that is viable for your service.
I have a service running on AWS EC2 Container Service (ECS). My setup is a relatively simple one. It operates with a single task definition and the following details:
Desired capacity set at 2
Minimum healthy set at 50%
Maximum available set at 200%
Tasks run with 80% CPU and memory reservations
Initially, I am able to get the necessary EC2 instances registered to the cluster that holds the service without a problem. The associated task then starts running on the two instances. As expected – given the CPU and memory reservations – the tasks take up almost the entirety of the EC2 instances' resources.
Sometimes, I want the task to use a new version of the application it is running. In order to make this happen, I create a revision of the task, de-register the previous revision, and then update the service. Note that I have set the minimum healthy percentage to require 2 * 0.50 = 1 instance running at all times and the maximum healthy percentage to permit up to 2 * 2.00 = 4 instances running.
Accordingly, I expected 1 of the de-registered task instances to be drained and taken offline so that 1 instance of the new revision of the task could be brought online. Then the process would repeat itself, bringing the deployment to a successful state.
Unfortunately, the cluster does nothing. In the events log, it tells me that it cannot place the new tasks, even though the process I have described above would permit it to do so.
How can I get the cluster to perform the behavior that I am expecting? I have only been able to get it to do so when I manually register another EC2 instance to the cluster and then tear it down after the update is complete (which is not desirable).
I have faced the same issue where the tasks used to get stuck and had no space to place them. Below snippet from AWS doc on updating a service helped me to make the below decision.
If your service has a desired number of four tasks and a maximum
percent value of 200%, the scheduler may start four new tasks before
stopping the four older tasks (provided that the cluster resources
required to do this are available). The default value for maximum
percent is 200%.
We should have the cluster resources available / container instances available to have the new tasks get started so they can start and the older one can drain.
These are the things i do
Before doing a service update add like 20% capacity to your cluster. You can use the ASG (Autoscaling group) commandline and from the desired capacity add 20% to your cluster. This way you will have some additional instance during deployment.
Once you have the instance the new tasks will start spinning up quickly and the older one will start draining.
But does this mean i will have extra container instances ?
Yes, during the deployment you will add some instances but as the older tasks drain they will hang around. The way to remove them is
Create a MemoryReservationLow alarm (~70% threshold in your case) for like 25 mins (longer duration to be sure that we have over commissioned). As the reservation will go low once you have those extra server not being used they can be removed.
I have seen this before. If your port mapping is attempting to map a static host port to the container within the task, you need more cluster instances.
Also this could be because there is not enough available memory to meet the memory (soft or hard) limit requested by the container within the task.
I already have the experience in Docker and EC2. But I'm new to ECS. Can someone help me to understand what these two parameters actually does, their difference and usage.
Official Docs says:
The minimum healthy percent represents a lower limit on the number of your service's tasks that must remain in the RUNNING state during a deployment, as a percentage of the desired number of tasks (rounded up to the nearest integer). This parameter enables you to deploy without using additional cluster capacity. For example, if your service has a desired number of four tasks and a minimum healthy percent of 50%, the scheduler may stop two existing tasks to free up cluster capacity before starting two new tasks. Tasks for services that do not use a load balancer are considered healthy if they are in the RUNNING state; tasks for services that do use a load balancer are considered healthy if they are in the RUNNING state and the container instance it is hosted on is reported as healthy by the load balancer. The default value for minimum healthy percent is 50% in the console and 100% for the AWS CLI, the AWS SDKs, and the APIs.
The maximum percent parameter represents an upper limit on the number of your service's tasks that are allowed in the RUNNING or PENDING state during a deployment, as a percentage of the desired number of tasks (rounded down to the nearest integer). This parameter enables you to define the deployment batch size. For example, if your service has a desired number of four tasks and a maximum percent value of 200%, the scheduler may start four new tasks before stopping the four older tasks (provided that the cluster resources required to do this are available). The default value for maximum percent is 200%.
I still didn't get a clear idea about these two parameters.
With minimum healthy percent, if my service has a desired number of 4 tasks and a minimum healthy percent of 25%, how many existing/new tasks will the scheduler start/stop?
With maximum percent, if my service has a desired number of 4 tasks and a maximum percent value of 50%, how many existing/new tasks will the scheduler start/stop?
If my service has only one task running, how to set these parameters to get existing task stopped and new task running.
With desired tasks at 4 and minimum health at 25% then during deployment ECS is allowed to stop all tasks except 1 before it starts new tasks
Maximum should be 100+ so that configuration does not make sense. But if you have 4 desired tasks and maximum at 150% then ECS is allowed to start 2 new tasks before it stops other tasks.
If you want to make sure there is never more than 1 task running during redeployment then you need to set desired to 1, minimum to 0% and maximum to 100%.
If you want to make sure there is always at least 1 task running you need to set desired to 1, minimum to 100% and maximum to 200%.
Other example. If you have desired at 4, minimum at 50% and maximum of 150%. Then ECS can decide what it will do during deployment.
If the cluster does not have more resources it can decide to stop 2 tasks, start 2 new tasks in the new version, wait for the new task to be healthy. Then stop the two remaining tasks and start two new ones.
If the cluster has more resources then ECS can decide to start two new tasks before stopping of existing tasks.
Or you could look at it this way. During redeployment ECS needs to run between MinimumPercent/100*desired and MaximumPercent/100*desired tasks. In this case between 2-6 tasks.