How do reservoir size and fixed rate work together?

The reservoir is the number of traces per second that are guaranteed to be sampled, regardless of traffic volume. After the reservoir is exhausted, the fixed rate (a percentage between 0 and 1) determines what fraction of additional requests are sampled. For example, a reservoir of 5 with a fixed rate of 0.10 means: the first 5 requests per second are always traced, and 10% of all additional requests are traced. For low-traffic services, the reservoir alone may capture all requests. For high-traffic services, the fixed rate controls the sampling percentage.

How does X-Ray choose which sampling rule to apply?

X-Ray evaluates rules in priority order (lowest number first). Each rule specifies matching criteria: service name, service type, HTTP method, URL path, resource ARN, and host. The first rule whose criteria match the incoming request is applied. If no custom rule matches, the default rule (priority 10000) is used. Assign lower priority numbers to more specific rules (e.g., priority 1 for /api/payments, priority 100 for /api/*, priority 10000 for the default catch-all).

X-Ray Sampling Rule Builder

MonitoringAWS

Build AWS X-Ray sampling rules for controlling trace collection rates.

X-Ray Configuration

Build X-Ray sampling rule configs to control tracing rates per service, method, and URL path.

Required Fields

SamplingRule.RuleNameSamplingRule.PrioritySamplingRule.FixedRateSamplingRule.ReservoirSizeSamplingRule.ServiceNameSamplingRule.URLPath

{
  "SamplingRule": {
    "RuleName": "high-priority-api",
    "RuleARN": "*",
    "ResourceARN": "*",
    "Priority": 100,
    "FixedRate": 0.1,
    "ReservoirSize": 10,
    "ServiceName": "order-service",
    "ServiceType": "AWS::ECS::Container",
    "Host": "*",
    "HTTPMethod": "POST",
    "URLPath": "/api/orders/*",
    "Version": 1,
    "Attributes": {
      "environment": "production"
    }
  },
  "AdditionalRules": [
    {
      "RuleName": "health-check-suppress",
      "Priority": 50,
      "FixedRate": 0.0,
      "ReservoirSize": 0,
      "ServiceName": "*",
      "ServiceType": "*",
      "Host": "*",
      "HTTPMethod": "GET",
      "URLPath": "/health",
      "ResourceARN": "*",
      "Version": 1
    },
    {
      "RuleName": "default-low-rate",
      "Priority": 10000,
      "FixedRate": 0.01,
      "ReservoirSize": 1,
      "ServiceName": "*",
      "ServiceType": "*",
      "Host": "*",
      "HTTPMethod": "*",
      "URLPath": "*",
      "ResourceARN": "*",
      "Version": 1
    }
  ]
}

Generated Output

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About This Tool

AWS X-Ray sampling rules control which requests are traced, allowing you to balance observability coverage with cost. Without custom rules, X-Ray applies a default rule that traces the first request each second and 5% of additional requests. Custom sampling rules let you set different rates for different services, URLs, HTTP methods, or other attributes. The X-Ray Sampling Rule Builder helps you define rules with proper priority ordering, reservoir sizes (guaranteed traces per second), and fixed rates (percentage of additional requests to sample).

When to Use This Tool

•Creating high-priority sampling rules that trace 100% of requests to critical API endpoints like payment processing
•Defining low-rate sampling for high-volume health check endpoints to reduce X-Ray costs without losing visibility
•Building service-specific rules that increase sampling for new deployments while keeping steady-state sampling low

Frequently Asked Questions

How do reservoir size and fixed rate work together?: The reservoir is the number of traces per second that are guaranteed to be sampled, regardless of traffic volume. After the reservoir is exhausted, the fixed rate (a percentage between 0 and 1) determines what fraction of additional requests are sampled. For example, a reservoir of 5 with a fixed rate of 0.10 means: the first 5 requests per second are always traced, and 10% of all additional requests are traced. For low-traffic services, the reservoir alone may capture all requests. For high-traffic services, the fixed rate controls the sampling percentage.
How does X-Ray choose which sampling rule to apply?: X-Ray evaluates rules in priority order (lowest number first). Each rule specifies matching criteria: service name, service type, HTTP method, URL path, resource ARN, and host. The first rule whose criteria match the incoming request is applied. If no custom rule matches, the default rule (priority 10000) is used. Assign lower priority numbers to more specific rules (e.g., priority 1 for /api/payments, priority 100 for /api/*, priority 10000 for the default catch-all).

Related Learning Guides

CloudWatch & Observability Guide24 min read

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