AWS X-Ray

Core concept: X-Ray provides distributed tracing — see how requests flow through your entire application (Lambda → API Gateway → DynamoDB → external services).

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Key Concepts

Concept	Description
Trace	End-to-end path of a request through all services
Segment	One service's portion of a trace (e.g., Lambda execution)
Subsegment	Work within a segment (e.g., a DynamoDB call)
Annotation	Key-value pair indexed for filtering/searching
Metadata	Key-value pair not indexed — for debugging detail
Service Map	Visual graph of all services and their connections
Sampling	% of requests to trace (controls cost)

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Annotations vs Metadata

	Annotations	Metadata
Indexed	✅ Yes — searchable	❌ No
Use for	Filtering traces (userId, orderId)	Debugging data (full request/response)
Type	String, number, boolean	Any (JSON)

Subsegment subsegment = AWSXRay.beginSubsegment("ProcessOrder");
try {
    // Annotations are searchable
    subsegment.putAnnotation("orderId", orderId);
    subsegment.putAnnotation("customerId", customerId);
    
    // Metadata for rich debugging (not searchable)
    subsegment.putMetadata("orderDetails", orderMap);
    
    processOrder(orderId);
} catch (Exception e) {
    subsegment.addException(e);
    throw e;
} finally {
    AWSXRay.endSubsegment();
}

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Sampling Rules

Default sampling: 5% of requests + 1 req/second reservoir

Custom rules (configured in console or via API):

{
  "RuleName": "HighValueOrders",
  "Priority": 1,
  "ReservoirSize": 10,
  "FixedRate": 0.5,
  "URLPath": "/orders/*",
  "ServiceName": "order-service",
  "HTTPMethod": "POST"
}

• ReservoirSize: requests/second to always trace
• FixedRate: % of remaining requests to trace
• Lower Priority number = higher priority

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X-Ray Daemon

The X-Ray SDK sends trace data to the X-Ray daemon, which buffers and sends to the X-Ray API:

Your App → X-Ray SDK → UDP port 2000 → X-Ray Daemon → X-Ray API

Environment	Daemon Location
Lambda	Built-in (enable Active Tracing)
ECS	Sidecar container
EC2	Install manually or via User Data
Elastic Beanstalk	Built-in (enable in config)

# Lambda — enable X-Ray in SAM
MyFunction:
  Type: AWS::Serverless::Function
  Properties:
    Tracing: Active    # PassThrough = disabled

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Java SDK Integration

Maven Dependency

<dependency>
    <groupId>com.amazonaws</groupId>
    <artifactId>aws-xray-recorder-sdk-core</artifactId>
    <version>2.14.0</version>
</dependency>
<!-- AWS SDK instrumentation -->
<dependency>
    <groupId>com.amazonaws</groupId>
    <artifactId>aws-xray-recorder-sdk-aws-sdk-v2-instrumentor</artifactId>
    <version>2.14.0</version>
</dependency>

Spring Boot Integration

@Configuration
public class XRayConfig {
    
    @Bean
    public Filter tracingFilter() {
        return new AWSXRayServletFilter("my-spring-service");
    }
}

// All AWS SDK calls (DynamoDB, S3, SQS...) are auto-instrumented
// when aws-xray-recorder-sdk-aws-sdk-v2-instrumentor is on classpath

Manual Subsegments

@Service
public class PaymentService {
    
    public void processPayment(String orderId) {
        // Creates a subsegment in the current trace
        AWSXRay.createSubsegment("PaymentGateway", (subsegment) -> {
            subsegment.putAnnotation("orderId", orderId);
            
            // Call external payment gateway
            paymentGateway.charge(orderId);
            
            return null;
        });
    }
}

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What X-Ray Traces

With the SDK and instrumentation:

• Incoming HTTP requests (via filter)
• AWS SDK calls (DynamoDB, S3, SQS, SNS, Lambda...)
• SQL queries (JDBC instrumentation)
• HTTP client calls (Apache HttpClient, OkHttp)
• Custom business logic (manual subsegments)

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🎯 DVA-C02 Exam Tips

Quick Exam Rules

• Annotations vs Metadata: Annotations are indexed and can be used in filter expressions (searchable). Metadata is not indexed and is only used to store extra detail.
• Instrumenting other services: To trace outgoing AWS SDK calls from your application, you must wrap or instrument your AWS SDK clients using the X-Ray SDK interceptors.
• Sampling: To control costs or trace specific routes more frequently, adjust Sampling Rules.
• X-Ray Daemon: The X-Ray daemon buffers and sends trace data to the X-Ray API. It can run as a sidecar container in ECS or as a separate process on EC2.
• X-Ray Groups: Use X-Ray Groups to filter and group traces based on specific criteria.
• X-Ray Tracing: Use X-Ray Tracing to trace requests through your application.

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Practice Questions

Q1. A developer needs to search X-Ray traces for all requests where userId = "user-123". What should they use to make this possible?

A) X-Ray Metadata with key userId
B) X-Ray Annotation with key userId
C) CloudWatch Log filter
D) X-Ray Groups

✅ Answer & Explanation

B — Annotations are indexed and searchable. Metadata is stored with the trace but not indexed and cannot be used in filter expressions. Use putAnnotation("userId", userId).

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Q2. A Lambda function has X-Ray Active Tracing enabled. However, DynamoDB calls are not appearing as subsegments. What is missing?

A) X-Ray Daemon is not installed
B) The IAM role lacks xray:PutTraceSegments
C) The X-Ray SDK AWS SDK instrumentor is not on the classpath
D) X-Ray sampling rate is too low

✅ Answer & Explanation

C — To auto-instrument AWS SDK v2 calls, you need aws-xray-recorder-sdk-aws-sdk-v2-instrumentor on the classpath. Without it, DynamoDB/S3/SQS calls won't appear as subsegments.

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Q3. By default, what percentage of requests does X-Ray trace?

A) 100%
B) 10%
C) 5% + 1 request/second reservoir
D) 1%

✅ Answer & Explanation

C — The default sampling rule traces the first request each second (reservoir) and 5% of all additional requests. This balances visibility with cost.

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🔗 Resources

• X-Ray Developer Guide
• X-Ray SDK for Java
• X-Ray Spring Boot Integration
• X-Ray Sampling Rules

Interview Questions

1. How do you define sampling strategy across low-traffic critical paths and high-volume commodity endpoints?
2. What annotation taxonomy would you standardize to support incident debugging without exploding cardinality?
3. A service map shows healthy latency but users report timeouts. How would you triage observability blind spots?
4. How do you combine X-Ray with logs and metrics for reliable root-cause analysis in distributed systems?

Short answer guide:

• Use prioritized sampling rules per endpoint criticality and error budget.
• Keep searchable annotations bounded and domain-oriented.
• Validate missing spans, upstream retries, and client-side latency not visible in traces.
• Correlate trace IDs with structured logs and SLO metrics.