Elastic Beanstalk

PaaS for deploying and scaling web applications without managing servers

AWS Elastic Beanstalk is a Platform-as-a-Service (PaaS) that handles the undifferentiated heavy lifting of deployment infrastructure. You upload your application code, and Beanstalk automatically provisions EC2 instances, load balancers, Auto Scaling groups, RDS databases, and CloudWatch alarms. You retain full control over the underlying resources.

Core Concepts

Component	Description
Application	A logical collection of Elastic Beanstalk components - like a folder. Not the code itself.
Application Version	A specific labeled iteration of deployable code, stored in S3. You can deploy any version to any environment.
Environment	A running collection of AWS resources (EC2, ELB, ASG) running one application version at a time.
Environment Tier	Web server tier (serves HTTP requests via ELB) or Worker tier (processes jobs from an SQS queue).
Platform	The OS + runtime + web server combination. e.g., "64bit Amazon Linux 2 v5.x running Node.js 18".
Saved Configuration	A named snapshot of environment settings that can be applied to new or existing environments.

Beanstalk supports these application platforms: Node.js, Python, Ruby, PHP, Go, Java (Tomcat or SE), .NET on Linux, .NET on Windows, and Docker (single or multi-container).

Deployment Policies

How Beanstalk deploys new application versions to instances affects downtime, speed, and rollback capability. This is a common interview question.

Policy	Downtime	Deployment Speed	Rollback	Notes
All at once	Yes	Fastest	Manual redeployment	Deploy to all instances simultaneously. Only for dev/test.
Rolling	No	Slow	Manual redeployment	Updates a batch at a time. Some instances serve old version during deploy.
Rolling with additional batch	No	Slow	Manual redeployment	Launches extra instances first so capacity is maintained throughout.
Immutable	No	Slowest	Fast (terminate new ASG)	Launches a new ASG with new instances, then swaps. Safest for production.
Traffic splitting	No	Slow	Fast (redirect traffic)	Canary-style - send a % of traffic to new instances, monitor, then shift fully.
Blue/Green (swap URLs)	No	Fast	Instant (swap URLs back)	Two separate environments. Swap CNAME. Not built-in - requires manual steps.

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For zero-downtime production deployments with the fastest safe rollback, use Immutable or Blue/Green. Immutable is built-in; Blue/Green requires managing two environments.

Customizing the Environment

.ebextensions/ directory: place YAML/JSON config files in your deployment bundle to customize EC2 instances (install packages, run commands, set environment variables, create files, configure the load balancer)
.platform/ hooks: shell scripts that run at specific lifecycle events (prebuild, predeploy, postdeploy) for deeper OS-level customization
Environment variables: set at the environment level in the console or EB CLI - available as process env variables in your code
Managed updates: Beanstalk can automatically apply platform updates (OS patches, runtime minor versions) during a weekly maintenance window
Enhanced health reporting: detailed health metrics per instance and environment, color-coded (Green/Yellow/Orange/Red) with root cause hints

yaml

# .ebextensions/install-deps.config
packages:
  yum:
    git: []
    jq: []

commands:
  01_set_timezone:
    command: "ln -sf /usr/share/zoneinfo/Asia/Kolkata /etc/localtime"

container_commands:
  01_migrate:
    command: "python manage.py migrate"
    leader_only: true

EB CLI Workflow

bash

# Initialize Beanstalk in your project
eb init my-app --platform "python-3.11" --region ap-south-1

# Create an environment and deploy
eb create my-app-prod --instance-type t3.small --single

# Deploy latest code to existing environment
eb deploy my-app-prod

# SSH into an instance
eb ssh

# View logs
eb logs

# Swap environment URLs (Blue/Green)
eb swap my-app-blue --destination_name my-app-green

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Interview Focus Points

1Deployment policies - differences between Immutable, Rolling, and Blue/Green
2Web server tier vs Worker tier - when would you use a worker tier?
3.ebextensions - what can you configure and when would you use it?
4How Beanstalk differs from ECS and when you would choose one over the other
5Managed updates - what gets updated and how to avoid unexpected downtime
6How to perform a zero-downtime deployment with instant rollback capability