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How New Relic distributed tracing works

Here are some technical details about how New Relic distributed tracing works:

Tip

For instructions about setting up distributed tracing, see Overview: Enable distributed tracing.

Trace sampling

How your traces are sampled will depend on your setup and the New Relic tracing tool you're using. For example, you may be using a third-party telemetry service (like OpenTelemetry) to implement sampling of traces before your data gets to us or, if you're using Infinite Tracing, you'd probably send us all your trace data and use our sampling.

We have a few sampling strategies available:

Head-based sampling (standard distributed tracing)

With the exception of our Infinite Tracing feature, most of our tracing tools use a head-based sampling approach. This applies filters to individual spans before all spans in a trace arrive, which means decisions about whether to accept spans is made at the beginning (the "head") of the filtering process. We use this sampling strategy to capture a representative sample of activity while avoiding storage and performance issues.

Here are some details about how head-based sampling is implemented in our standard distributed tracing tools:

Tail-based sampling (Infinite Tracing)

Our Infinite Tracing feature uses a tail-based sampling approach. "Tail-based sampling" means that trace-retention decisions are done at the tail end of processing after all the spans in a trace have arrived.

With Infinite Tracing, you can send us 100% of your trace data from your application or third-party telemetry service, and Infinite Tracing will figure out which trace data is most important. And you can configure the sampling to ensure the traces important to you are retained.

No sampling

Some of our tools don't use sampling. Sampling details for these tools:

How trace data is structured

Understanding the structure of a distributed trace can help you:

A distributed trace has a tree-like structure, with "child" spans that refer to one "parent" span. This diagram shows some important span relationships in a trace:

New Relic distributed tracing trace structure diagram

This diagram shows how spans in a distributed trace relate to each other.

This diagram shows several important concepts:

  • Trace root. The first service or process in a trace is referred to as the root service or process.
  • Process boundaries. A process represents the execution of a logical piece of code. Examples of a process include a backend service or Lambda function. Spans within a process are categorized as one of the following:
    • Entry span: the first span in a process.
    • Exit span: a span is a considered an exit span if it a) is the parent of an entry span, or b) has http. or db. attributes and therefore represents an external call.
    • In-process span: a span that represents an internal method call or function and that is not an exit or entry span.
  • Client spans. A client span represents a call to another entity or external dependency. Currently, there are two client span types:
    • Datastore. If a client span has any attributes prefixed with db. (like db.statement), it's categorized as a datastore span.
    • External. If a client span has any attributes prefixed with http. (like http.url) or has a child span in another process, it's categorized as an external span. This is a general category for any external calls that are not datastore queries.
  • Trace duration. A trace's total duration is determined by the length of time from the start of the earliest span to the completion of the last span.

You can query span relationship data with the NerdGraph GraphiQL explorer at https://api.newrelic.com/graphiql.

How trace data is stored

Understanding how we store trace data can help you query your trace data.

We save trace data as:

  • Span: A span represents operations that are part of a distributed trace. The operations that a span can represent include browser-side interactions, datastore queries, calls to other services, method-level timing, and Lambda functions. One example: in an HTTP service, a span is created at the start of an HTTP request and completed when the HTTP server returns a response. Span attributes contain important information about that operation (such as duration, host data, etc.), including trace-relationship details (such as traceId, guid). For span-related data, see span attributes.
  • Transaction: If an entity in a trace is monitored by an agent, a request to that entity generates a single Transaction event. Transactions allow trace data to be tied to other New Relic features. For transaction-related data, see transaction attributes.
  • Contextual metadata. We store metadata that shows calculations about a trace and the relationships between its spans. To query this data, use the NerdGraph GraphiQL explorer.

How trace context is passed between applications

We support the W3C Trace Context standard, which makes it easier to trace transactions across networks and services. When you enable distributed tracing, New Relic agents add HTTP headers to a service's outbound requests. HTTP headers act like passports on an international trip: They identify your software traces and carry important information as they travel through various networks, processes, and security systems.

The headers also contain information that helps us link the spans together later: metadata like the trace ID, span ID, the New Relic account ID, and sampling information. This header information is passed along each span of a trace, unless the progress is stopped by something like middleware or agents that don't recognize the header format (see Figure 1).

Diagram of a failed trace with proprietary headers.

Figure 1

To address the problem of header propagation, we support the W3C Trace Context specification that requires two standardized headers. Our latest W3C New Relic agents send and receive these two required headers, and by default, they also send and receive the header of the prior New Relic agent:

  • W3C (traceparent): The primary header that identifies the entire trace (trace ID) and the calling service (span id).
  • W3C (tracestate): A required header that carries vendor-specific information and tracks where a trace has been.
  • New Relic (newrelic): The original, proprietary header that is still sent to maintain backward compatibility with prior New Relic agents.

This combination of three headers allows traces to be propagated across services instrumented with these types of agents:

  • W3C New Relic agents
  • Non-W3C New Relic agents
  • W3C Trace Context-compatible agents

Important

If your requests only touch W3C Trace Context-compatible agents, you can opt to turn off the New Relic header. See the agent configuration documentation for details about turning off the newrelic header.

The scenarios below show various types of successful header propagation.

For more help

If you need more help, check out these support and learning resources:

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