Running a Tomcat App on ECS Fargate — A Complete Step-by-...

Introduction

This guide walks you through migrating an existing Tomcat application to AWS ECS Fargate — covering everything from session management and DB migration to Auto Scaling, with a production-ready setup in mind.

Who this is for

Developers migrating a Tomcat app to AWS
Engineers new to ECS Fargate

Final Architecture

Internet
    │
   ALB (port 80/443)
    │
  ECS Fargate
    ├─ ElastiCache (Redis) ← session store
    └─ RDS (PostgreSQL)   ← persistent data

STEP 1: Create Security Groups

Create dedicated security groups for ALB, ECS, Redis, and RDS. Following the principle of least privilege, only allow what is strictly necessary.

# ALB — allow HTTP/HTTPS from the internet
aws ec2 create-security-group \
  --group-name sg-alb \
  --description "ALB SG" \
  --vpc-id vpc-xxxxxxxx

aws ec2 authorize-security-group-ingress \
  --group-id sg-alb --protocol tcp --port 80 --cidr 0.0.0.0/0
aws ec2 authorize-security-group-ingress \
  --group-id sg-alb --protocol tcp --port 443 --cidr 0.0.0.0/0

# ECS — allow 8080 from ALB only
aws ec2 create-security-group \
  --group-name sg-ecs \
  --description "ECS SG" \
  --vpc-id vpc-xxxxxxxx

aws ec2 authorize-security-group-ingress \
  --group-id sg-ecs --protocol tcp --port 8080 \
  --source-group sg-alb

# Redis — allow 6379 from ECS only
aws ec2 create-security-group \
  --group-name sg-redis \
  --description "Redis SG" \
  --vpc-id vpc-xxxxxxxx

aws ec2 authorize-security-group-ingress \
  --group-id sg-redis --protocol tcp --port 6379 \
  --source-group sg-ecs

# RDS — allow 5432 from ECS only
aws ec2 create-security-group \
  --group-name sg-rds \
  --description "RDS SG" \
  --vpc-id vpc-xxxxxxxx

aws ec2 authorize-security-group-ingress \
  --group-id sg-rds --protocol tcp --port 5432 \
  --source-group sg-ecs

Here is a summary of the security group rules:

Security Group	Inbound Rule
sg-alb	0.0.0.0/0 → 80, 443
sg-ecs	sg-alb → 8080
sg-redis	sg-ecs → 6379
sg-rds	sg-ecs → 5432

STEP 2: Store Credentials in Secrets Manager

Store passwords and other sensitive values in Secrets Manager so they never appear as plain text in your task definition.

aws secretsmanager create-secret \
  --name myapp/db-password \
  --secret-string "your-db-password"

aws secretsmanager create-secret \
  --name myapp/db-user \
  --secret-string "your-db-user"

aws secretsmanager create-secret \
  --name myapp/redis-password \
  --secret-string "your-redis-password"

STEP 3: Provision RDS

# Subnet group (private subnets)
aws rds create-db-subnet-group \
  --db-subnet-group-name myapp-rds-subnet \
  --db-subnet-group-description "myapp RDS subnet" \
  --subnet-ids subnet-aaaaaaaa subnet-bbbbbbbb

# RDS instance
aws rds create-db-instance \
  --db-instance-identifier myapp-db \
  --db-instance-class db.t4g.small \
  --engine postgres \
  --engine-version 15.4 \
  --master-username myapp \
  --master-user-password "your-db-password" \
  --db-name myapp \
  --db-subnet-group-name myapp-rds-subnet \
  --vpc-security-group-ids sg-rds \
  --multi-az \
  --storage-type gp3 \
  --allocated-storage 20 \
  --no-publicly-accessible

STEP 4: Provision ElastiCache (Redis)

Because Fargate replaces the underlying instance on every task restart, any session stored in memory will be lost. With multiple tasks running, a request routed to a different task will fail to find the session.

Externalizing sessions to ElastiCache (Redis) solves both problems.

# Subnet group
aws elasticache create-cache-subnet-group \
  --cache-subnet-group-name myapp-redis-subnet \
  --cache-subnet-group-description "myapp redis subnet" \
  --subnet-ids subnet-aaaaaaaa subnet-bbbbbbbb

# Parameter group
aws elasticache create-cache-parameter-group \
  --cache-parameter-group-name myapp-redis-params \
  --cache-parameter-group-family redis7 \
  --description "myapp redis params"

aws elasticache modify-cache-parameter-group \
  --cache-parameter-group-name myapp-redis-params \
  --parameter-name-values \
    ParameterName=maxmemory-policy,ParameterValue=allkeys-lru \
    ParameterName=timeout,ParameterValue=300

# Redis cluster (Multi-AZ, encrypted)
aws elasticache create-replication-group \
  --replication-group-id myapp-redis \
  --description "myapp session store" \
  --engine redis \
  --engine-version 7.0 \
  --cache-node-type cache.t4g.medium \
  --num-cache-clusters 2 \
  --multi-az-enabled \
  --automatic-failover-enabled \
  --cache-subnet-group-name myapp-redis-subnet \
  --cache-parameter-group-name myapp-redis-params \
  --security-group-ids sg-redis \
  --at-rest-encryption-enabled \
  --transit-encryption-enabled \
  --auth-token "your-redis-password"

Spring Session configuration

Add the dependencies to pom.xml:

<dependency>
    <groupId>org.springframework.session</groupId>
    <artifactId>spring-session-data-redis</artifactId>
</dependency>
<dependency>
    <groupId>io.lettuce</groupId>
    <artifactId>lettuce-core</artifactId>
</dependency>

Configure application.yml:

spring:
  session:
    store-type: redis
    timeout: 30m
  data:
    redis:
      host: ${REDIS_HOST}
      port: ${REDIS_PORT}
      password: ${REDIS_PASSWORD}
      ssl:
        enabled: true
      lettuce:
        pool:
          max-active: 8
          max-idle: 8
          min-idle: 2

STEP 5: Build Docker Images and Push to ECR

Dockerfile (application)

FROM tomcat:10.1-jdk17

RUN rm -rf /usr/local/tomcat/webapps/*
COPY target/myapp.war /usr/local/tomcat/webapps/ROOT.war

EXPOSE 8080
CMD ["catalina.sh", "run"]

Dockerfile.migration (Flyway)

FROM flyway/flyway:10-alpine

COPY db/migration /flyway/sql
COPY flyway.conf /flyway/conf/flyway.conf

ENTRYPOINT ["flyway", "migrate"]

flyway.conf:

flyway.url=jdbc:postgresql://${DB_HOST}:5432/${DB_NAME}
flyway.user=${DB_USER}
flyway.password=${DB_PASSWORD}
flyway.locations=filesystem:/flyway/sql
flyway.baselineOnMigrate=true

Push to ECR

# Create repositories
aws ecr create-repository --repository-name myapp
aws ecr create-repository --repository-name myapp-migration

# Login
aws ecr get-login-password --region ap-northeast-1 \
  | docker login --username AWS --password-stdin \
    <account_id>.dkr.ecr.ap-northeast-1.amazonaws.com

# Application image
docker build -t myapp .
docker tag myapp:latest \
  <account_id>.dkr.ecr.ap-northeast-1.amazonaws.com/myapp:latest
docker push \
  <account_id>.dkr.ecr.ap-northeast-1.amazonaws.com/myapp:latest

# Migration image
docker build -f Dockerfile.migration -t myapp-migration .
docker tag myapp-migration:latest \
  <account_id>.dkr.ecr.ap-northeast-1.amazonaws.com/myapp-migration:latest
docker push \
  <account_id>.dkr.ecr.ap-northeast-1.amazonaws.com/myapp-migration:latest

STEP 6: Create CloudWatch Logs Groups

aws logs create-log-group --log-group-name /ecs/myapp/migration
aws logs create-log-group --log-group-name /ecs/myapp/tomcat

STEP 7: Create the IAM Execution Role

The ECS agent needs this role to pull images from ECR and fetch secrets from Secrets Manager.

aws iam create-role \
  --role-name ecsTaskExecutionRole \
  --assume-role-policy-document '{
    "Version": "2012-10-17",
    "Statement": [{
      "Effect": "Allow",
      "Principal": { "Service": "ecs-tasks.amazonaws.com" },
      "Action": "sts:AssumeRole"
    }]
  }'

aws iam attach-role-policy \
  --role-name ecsTaskExecutionRole \
  --policy-arn arn:aws:iam::aws:policy/service-role/AmazonECSTaskExecutionRolePolicy

aws iam attach-role-policy \
  --role-name ecsTaskExecutionRole \
  --policy-arn arn:aws:iam::aws:policy/SecretsManagerReadWrite

STEP 8: Register the Task Definition

The key point here is dependsOn with condition: SUCCESS — Tomcat will not start until the migration container exits cleanly.

{
  "family": "myapp-task",
  "networkMode": "awsvpc",
  "requiresCompatibilities": ["FARGATE"],
  "cpu": "512",
  "memory": "1024",
  "executionRoleArn": "arn:aws:iam::<account_id>:role/ecsTaskExecutionRole",
  "containerDefinitions": [
    {
      "name": "migration",
      "image": "<account_id>.dkr.ecr.ap-northeast-1.amazonaws.com/myapp-migration:latest",
      "essential": false,
      "secrets": [
        { "name": "DB_USER", "valueFrom": "arn:aws:secretsmanager:ap-northeast-1:<account_id>:secret:myapp/db-user" },
        { "name": "DB_PASSWORD", "valueFrom": "arn:aws:secretsmanager:ap-northeast-1:<account_id>:secret:myapp/db-password" }
      ],
      "environment": [
        { "name": "DB_HOST", "value": "myapp-db.xxxx.ap-northeast-1.rds.amazonaws.com" },
        { "name": "DB_NAME", "value": "myapp" }
      ],
      "logConfiguration": {
        "logDriver": "awslogs",
        "options": {
          "awslogs-group": "/ecs/myapp/migration",
          "awslogs-region": "ap-northeast-1",
          "awslogs-stream-prefix": "ecs"
        }
      }
    },
    {
      "name": "tomcat",
      "image": "<account_id>.dkr.ecr.ap-northeast-1.amazonaws.com/myapp:latest",
      "essential": true,
      "dependsOn": [
        { "containerName": "migration", "condition": "SUCCESS" }
      ],
      "portMappings": [
        { "containerPort": 8080, "protocol": "tcp" }
      ],
      "environment": [
        { "name": "DB_HOST", "value": "myapp-db.xxxx.ap-northeast-1.rds.amazonaws.com" },
        { "name": "DB_NAME", "value": "myapp" },
        { "name": "REDIS_HOST", "value": "myapp-redis.xxxx.ng.0001.apne1.cache.amazonaws.com" },
        { "name": "REDIS_PORT", "value": "6379" },
        { "name": "TZ", "value": "Asia/Tokyo" }
      ],
      "secrets": [
        { "name": "DB_USER", "valueFrom": "arn:aws:secretsmanager:ap-northeast-1:<account_id>:secret:myapp/db-user" },
        { "name": "DB_PASSWORD", "valueFrom": "arn:aws:secretsmanager:ap-northeast-1:<account_id>:secret:myapp/db-password" },
        { "name": "REDIS_PASSWORD", "valueFrom": "arn:aws:secretsmanager:ap-northeast-1:<account_id>:secret:myapp/redis-password" }
      ],
      "healthCheck": {
        "command": ["CMD-SHELL", "curl -f http://localhost:8080/health || exit 1"],
        "interval": 30,
        "timeout": 5,
        "retries": 3,
        "startPeriod": 90
      },
      "logConfiguration": {
        "logDriver": "awslogs",
        "options": {
          "awslogs-group": "/ecs/myapp/tomcat",
          "awslogs-region": "ap-northeast-1",
          "awslogs-stream-prefix": "ecs"
        }
      }
    }
  ]
}

aws ecs register-task-definition \
  --cli-input-json file://task-definition.json

Note: Setting essential: false on the migration container means the task keeps running after migration completes. Tomcat uses condition: SUCCESS to wait for a clean exit before starting.

STEP 9: Create the ALB

# Create ALB
aws elbv2 create-load-balancer \
  --name myapp-alb \
  --subnets subnet-aaaaaaaa subnet-bbbbbbbb \
  --security-groups sg-alb \
  --scheme internet-facing \
  --type application

# Create target group
# Important: Fargate requires --target-type ip, not instance
aws elbv2 create-target-group \
  --name myapp-tg \
  --protocol HTTP \
  --port 8080 \
  --vpc-id vpc-xxxxxxxx \
  --target-type ip \
  --health-check-path /health \
  --health-check-interval-seconds 30 \
  --healthy-threshold-count 2 \
  --unhealthy-threshold-count 3

# Create listener
aws elbv2 create-listener \
  --load-balancer-arn <alb-arn> \
  --protocol HTTP \
  --port 80 \
  --default-actions Type=forward,TargetGroupArn=<tg-arn>

# Shorten deregistration delay (default 300s is too long)
aws elbv2 modify-target-group-attributes \
  --target-group-arn <tg-arn> \
  --attributes Key=deregistration_delay.timeout_seconds,Value=30

STEP 10: Create the ECS Cluster and Service

# Create cluster
aws ecs create-cluster --cluster-name myapp-cluster

# Create service
aws ecs create-service \
  --cluster myapp-cluster \
  --service-name myapp-service \
  --task-definition myapp-task \
  --desired-count 8 \
  --launch-type FARGATE \
  --network-configuration "awsvpcConfiguration={
    subnets=[subnet-aaaaaaaa,subnet-bbbbbbbb],
    securityGroups=[sg-ecs],
    assignPublicIp=DISABLED
  }" \
  --load-balancers "targetGroupArn=<tg-arn>,containerName=tomcat,containerPort=8080" \
  --deployment-configuration \
    "minimumHealthyPercent=50,maximumPercent=200,deploymentCircuitBreaker={enable=true,rollback=true}"

Enabling deploymentCircuitBreaker means ECS automatically rolls back to the previous revision if a deployment fails repeatedly.

STEP 11: Configure Auto Scaling

Auto Scaling handles traffic spikes without manual intervention.

aws application-autoscaling register-scalable-target \
  --service-namespace ecs \
  --resource-id service/myapp-cluster/myapp-service \
  --scalable-dimension ecs:service:DesiredCount \
  --min-capacity 4 \
  --max-capacity 12

aws application-autoscaling put-scaling-policy \
  --policy-name cpu-tracking \
  --service-namespace ecs \
  --resource-id service/myapp-cluster/myapp-service \
  --scalable-dimension ecs:service:DesiredCount \
  --policy-type TargetTrackingScaling \
  --target-tracking-scaling-policy-configuration '{
    "TargetValue": 70.0,
    "PredefinedMetricSpecification": {
      "PredefinedMetricType": "ECSServiceAverageCPUUtilization"
    },
    "ScaleOutCooldown": 60,
    "ScaleInCooldown": 300
  }'

Setting	Value	Reason
min-capacity	4	Baseline task count
max-capacity	12	Peak capacity
TargetValue	70%	Scale out when CPU exceeds 70%
ScaleOutCooldown	60s	React quickly to load spikes
ScaleInCooldown	300s	Prevent aggressive scale-in

Verify the Deployment

# Check running tasks
aws ecs list-tasks --cluster myapp-cluster

# Check service status
aws ecs describe-services \
  --cluster myapp-cluster \
  --services myapp-service \
  --query 'services[0].{Status:status,Running:runningCount,Desired:desiredCount}'

# Get the ALB DNS name and open it in a browser
aws elbv2 describe-load-balancers \
  --names myapp-alb \
  --query 'LoadBalancers[0].DNSName'

Common Pitfalls

Problem	Cause	Fix
Task exits immediately after start	Application startup error	Check CloudWatch Logs for the stack trace
Health check never passes	Tomcat takes too long to start	Set `startPeriod` to 90–120 seconds
Cannot pull image from ECR	Missing permissions on `ecsTaskExecutionRole`	Attach `AmazonECSTaskExecutionRolePolicy`
Cannot connect to Redis	Security group misconfiguration	Verify sg-ecs → sg-redis on port 6379
Tomcat does not start after migration	Missing `dependsOn`	Add `condition: SUCCESS` to the task definition
ALB returns 502	Target type set to `instance`	Change to `ip`

Summary

Step	What you do
1	Create security groups
2	Store credentials in Secrets Manager
3	Provision RDS
4	Provision ElastiCache
5	Build images and push to ECR
6	Create CloudWatch Logs groups
7	Create IAM execution role
8	Register task definition
9	Create ALB
10	Create ECS cluster and service
11	Configure Auto Scaling

Fargate eliminates server management and hands off deployment and scaling to AWS. Setting up session management, DB migration, and Auto Scaling from the start dramatically reduces rework later.

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