Application Load Balancer (ALB) is a Layer 7 load balancer service within AWS Elastic Load Balancing that plays a crucial role in ensuring reliability and business continuity for web applications. ALB operates at the application layer, enabling intelligent routing decisions based on HTTP/HTTPS requ…Application Load Balancer (ALB) is a Layer 7 load balancer service within AWS Elastic Load Balancing that plays a crucial role in ensuring reliability and business continuity for web applications. ALB operates at the application layer, enabling intelligent routing decisions based on HTTP/HTTPS request content including headers, paths, query strings, and host names.
For SysOps Administrators, ALB provides several key features that enhance system reliability. Content-based routing allows traffic distribution across multiple target groups based on URL paths or hostnames, enabling microservices architectures where different services handle specific request types. Target groups can include EC2 instances, containers, IP addresses, or Lambda functions, providing deployment flexibility.
Health checks are fundamental to ALB's reliability features. ALB continuously monitors registered targets and routes traffic only to healthy instances. Administrators can configure health check intervals, thresholds, and timeout values to match application requirements. When targets fail health checks, ALB automatically stops sending traffic to them, maintaining application availability.
Cross-zone load balancing distributes traffic evenly across all registered targets in enabled Availability Zones, improving fault tolerance. ALB supports integration with AWS WAF for security, AWS Certificate Manager for SSL/TLS termination, and Amazon CloudWatch for monitoring and alerting.
Connection draining (deregistration delay) ensures in-flight requests complete before removing instances from service during scaling events or maintenance. Sticky sessions maintain user session affinity when required by applications.
For business continuity, ALB integrates with Auto Scaling groups to handle traffic fluctuations and maintain performance during demand spikes. Access logs capture detailed request information for troubleshooting and compliance. ALB also supports authentication through integration with Amazon Cognito or OIDC-compliant identity providers.
SysOps Administrators should monitor key metrics including request count, target response time, healthy host count, and HTTP error codes to maintain optimal application performance and availability.
Application Load Balancer (ALB) - Complete Guide
Why Application Load Balancer is Important
Application Load Balancer is a critical component for achieving high availability and reliability in AWS architectures. It operates at Layer 7 (Application Layer) of the OSI model, enabling intelligent routing decisions based on HTTP/HTTPS traffic content. For the AWS SysOps Administrator Associate exam, understanding ALB is essential because it directly relates to maintaining business continuity and ensuring applications remain available during failures or high-traffic scenarios.
What is an Application Load Balancer?
An Application Load Balancer is an AWS managed load balancing service that distributes incoming application traffic across multiple targets such as EC2 instances, containers, IP addresses, and Lambda functions. Key characteristics include:
• Layer 7 Load Balancing - Routes traffic based on content of HTTP/HTTPS requests • Target Groups - Logical groupings of targets that receive traffic • Listeners - Processes that check for connection requests using configured protocols and ports • Rules - Define how requests are routed to target groups • Health Checks - Monitors the health of registered targets
How Application Load Balancer Works
1. Traffic Reception: The ALB receives incoming requests on configured listeners (typically ports 80 or 443)
2. Rule Evaluation: Listener rules are evaluated in priority order to determine which target group should receive the request
3. Routing Decisions: Traffic can be routed based on: • Path-based routing (/api/*, /images/*) • Host-based routing (api.example.com, www.example.com) • HTTP headers and methods • Query string parameters • Source IP addresses
4. Target Selection: ALB uses a round-robin algorithm to select healthy targets within the chosen target group
5. Health Monitoring: Continuous health checks ensure traffic only goes to healthy targets
Key Features for Reliability
• Cross-Zone Load Balancing - Distributes traffic evenly across all registered targets in all enabled Availability Zones • Connection Draining (Deregistration Delay) - Allows in-flight requests to complete before deregistering targets • Sticky Sessions - Routes requests from the same client to the same target • SSL/TLS Termination - Offloads encryption/decryption from backend servers • Integration with AWS WAF - Protects against common web exploits • Access Logs - Captures detailed information about requests for troubleshooting
Exam Tips: Answering Questions on Application Load Balancer
Tip 1: Know the Layer ALB operates at Layer 7. If a question mentions HTTP headers, cookies, URL paths, or host-based routing, ALB is likely the answer. Network Load Balancer (NLB) operates at Layer 4.
Tip 2: Understand Target Types ALB supports EC2 instances, IP addresses, Lambda functions, and containers. Questions about routing to Lambda functions specifically require ALB.
Tip 3: Health Check Configuration Know the default health check settings: 30-second interval, 5 consecutive successes for healthy, 2 consecutive failures for unhealthy. Understand how to troubleshoot unhealthy targets.
Tip 4: Cross-Zone Load Balancing For ALB, cross-zone load balancing is enabled by default and cannot be disabled at the load balancer level. This ensures even distribution across AZs.
Tip 5: Sticky Sessions Scenarios When questions mention maintaining session state or user-specific data on backend servers, sticky sessions (session affinity) is the solution.
Tip 6: Security Groups ALB requires security groups. Remember that backend instances should allow traffic from the ALB security group, not from client IPs.
Tip 7: HTTPS and Certificates ALB integrates with AWS Certificate Manager (ACM) for SSL/TLS certificates. For HTTPS listeners, you must deploy at least one certificate.
Tip 8: Access Logs vs CloudWatch Access logs provide detailed request-level information stored in S3. CloudWatch metrics provide aggregated performance data. Know when to use each for troubleshooting.
Tip 9: 5xx vs 4xx Errors 502 Bad Gateway often indicates backend issues. 503 Service Unavailable suggests no healthy targets. 504 Gateway Timeout indicates the target did not respond in time.
Tip 10: Deregistration Delay Default is 300 seconds. Questions about graceful shutdown or allowing requests to complete during scaling events relate to this setting.