Auto Scale Fargate Service Based On SQS ApproximateNumberOfMessagesVisible

Question

I would like to scale out my aws fargate containers based on the size of the SQS queue. It appears that I can only scale based on the container's CPU or Memory usage. Is there a way to create a policy that would scale out and in based on queue size? Has anyone been able to scale based on other cloudwatch metrics?

Answer 1

Yes you can do this. You have to use a step scaling policy, and you need to have an alarm created already for your SQS queue depth (ApproximateNumberOfMessagesVisible).

Go to CloudWatch, create a new alarm. We'll call this alarm sqs-queue-depth-high, and have it trigger when the approximate number of messages visible is 1000.

With that done, go to ECS to the service you want to autoscale. Click Update for the service. Add a scaling policy and choose the Step Tracking variety. You'll see there's an option to create a new alarm (which only lets you choose between CPU or MemoryUtilization), or use an existing alarm.

Type sqs-queue-depth-high in the "Use existing alarm" field and press enter, you should see a green checkmark that lets you know the name is valid (i.e. the alarm exists). You'll see new dropdowns where you can adjust the step policy now.

This works for any metric alarm and ECS services. If you're going to be trying to scale this setup out, for multiple environments for example, or making it any more sophisticated than 2 steps, do yourself a favor and jump in with CloudFormation or Terraform to help manage it. Nothing is worse than having to adjust a 5-step alarm across 10 services.

Answer 2

AWS provides a solution for scaling based on SQS queue: https://docs.aws.amazon.com/autoscaling/ec2/userguide/as-using-sqs-queue.html

Main idea

1. Create a CloudWatch Custom Metric sqs-backlog-per-task using formula: sqs-backlog-per-task = sqs-messages-number / running-task-number.
2. Create a Target Tracking Scaling Policy based on the backlogPerInstance metric.

Implementation details

Custom Metric

In my case all the infrastructure (Fargate, SQS, and other resources) is described in CloudFormation stack. So for calculating and logging the custom metric I decided to use AWS Lambda function which is also described in CloudFormation stack and deployed together with the entire infrastructure.

Below you can find code snippets for the AWS Lambda function for logging the following custom metrics:

• sqs-backlog-per-task - used for scaling
• running-task-number - used for scaling optimization and debugging

AWS Lambda function described in AWS SAM syntax in CloudFormation stack (infrastructure.yml):

CustomMetricLoggerFunction:
    Type: AWS::Serverless::Function
    Properties:
      FunctionName: custom-metric-logger
      Handler: custom-metric-logger.handler
      Runtime: nodejs8.10
      MemorySize: 128
      Timeout: 3
      Role: !GetAtt CustomMetricLoggerFunctionRole.Arn
      Environment:
        Variables:
          ECS_CLUSTER_NAME: !Ref Cluster
          ECS_SERVICE_NAME: !GetAtt Service.Name
          SQS_URL: !Ref Queue
      Events:
        Schedule:
          Type: Schedule
          Properties:
            Schedule: 'cron(0/1 * * * ? *)' # every one minute

AWS Lambda Javascript code for calculating and logging (custom-metric-logger.js):

var AWS = require('aws-sdk');

exports.handler = async () => {
  try {
    var sqsMessagesNumber = await getSqsMessagesNumber();
    var runningContainersNumber = await getRunningContainersNumber();

    var backlogPerInstance = sqsMessagesNumber;
    if (runningContainersNumber > 0) {
      backlogPerInstance = parseInt(sqsMessagesNumber / runningContainersNumber);
    }

    await putRunningTaskNumberMetricData(runningContainersNumber);
    await putSqsBacklogPerTaskMetricData(backlogPerInstance);

    return {
      statusCode: 200
    };
  } catch (err) {
    console.log(err);

    return {
      statusCode: 500
    };
  }
};

function getSqsMessagesNumber() {
  return new Promise((resolve, reject) => {
    var data = {
      QueueUrl: process.env.SQS_URL,
      AttributeNames: ['ApproximateNumberOfMessages']
    };

    var sqs = new AWS.SQS();
    sqs.getQueueAttributes(data, (err, data) => {
      if (err) {
        reject(err);
      } else {
        resolve(parseInt(data.Attributes.ApproximateNumberOfMessages));
      }
    });
  });
}

function getRunningContainersNumber() {
  return new Promise((resolve, reject) => {
    var data = {
      services: [
        process.env.ECS_SERVICE_NAME
      ],
      cluster: process.env.ECS_CLUSTER_NAME
    };

    var ecs = new AWS.ECS();
    ecs.describeServices(data, (err, data) => {
      if (err) {
        reject(err);
      } else {
        resolve(data.services[0].runningCount);
      }
    });
  });
}

function putRunningTaskNumberMetricData(value) {
  return new Promise((resolve, reject) => {
    var data = {
      MetricData: [{
        MetricName: 'running-task-number',
        Value: value,
        Unit: 'Count',
        Timestamp: new Date()
      }],
      Namespace: 'fargate-sqs-service'
    };

    var cloudwatch = new AWS.CloudWatch();
    cloudwatch.putMetricData(data, (err, data) => {
      if (err) {
        reject(err);
      } else {
        resolve(data);
      }
    });
  });
}

function putSqsBacklogPerTaskMetricData(value) {
  return new Promise((resolve, reject) => {
    var data = {
      MetricData: [{
        MetricName: 'sqs-backlog-per-task',
        Value: value,
        Unit: 'Count',
        Timestamp: new Date()
      }],
      Namespace: 'fargate-sqs-service'
    };

    var cloudwatch = new AWS.CloudWatch();
    cloudwatch.putMetricData(data, (err, data) => {
      if (err) {
        reject(err);
      } else {
        resolve(data);
      }
    });
  });
}

Target Tracking Scaling Policy

Then based on the sqs-backlog-per-task metric I created Target Tracking Scaling Policy in my Cloud Formation template.

Target Tracking Scaling Policy based on the sqs-backlog-per-task metric (infrastructure.yml):

ServiceScalingPolicy:
    Type: AWS::ApplicationAutoScaling::ScalingPolicy
    Properties:
      PolicyName: service-scaling-policy
      PolicyType: TargetTrackingScaling
      ScalingTargetId: !Ref ServiceScalableTarget
      TargetTrackingScalingPolicyConfiguration:
        ScaleInCooldown: 60
        ScaleOutCooldown: 60
        CustomizedMetricSpecification:
          Namespace: fargate-sqs-service
          MetricName: sqs-backlog-per-task
          Statistic: Average
          Unit: Count
        TargetValue: 2000

As a result AWS Application Auto Scaling creates and manages the CloudWatch alarms that trigger the scaling policy and calculates the scaling adjustment based on the metric and the target value. The scaling policy adds or removes capacity as required to keep the metric at, or close to, the specified target value. In addition to keeping the metric close to the target value, a target tracking scaling policy also adjusts to changes in the metric due to a changing load pattern.