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Published: Jan 15, 2020 License: BSD-3-Clause Imports: 35 Imported by: 0



Package tracer contains Datadog's core tracing client. It is used to trace requests as they flow across web servers, databases and microservices, giving developers visibility into bottlenecks and troublesome requests. To start the tracer, simply call the start method along with an optional set of options. By default, the trace agent is considered to be found at "localhost:8126". In a setup where this would be different (let's say, we could do:

defer tracer.Stop()

The tracing client can perform trace sampling. While the trace agent already samples traces to reduce bandwidth usage, client sampling reduces performance overhead. To make use of it, the package comes with a ready-to-use rate sampler that can be passed to the tracer. To use it and keep only 30% of the requests, one would do:

s := tracer.NewRateSampler(0.3)

All spans created by the tracer contain a context hereby referred to as the span context. Note that this is different from Go's context. The span context is used to package essential information from a span, which is needed when creating child spans that inherit from it. Thus, a child span is created from a span's span context. The span context can originate from within the same process, but also a different process or even a different machine in the case of distributed tracing.

To make use of distributed tracing, a span's context may be injected via a carrier into a transport (HTTP, RPC, etc.) to be extracted on the other end and used to create spans that are direct descendants of it. A couple of carrier interfaces which should cover most of the use-case scenarios are readily provided, such as HTTPCarrier and TextMapCarrier. Users are free to create their own, which will work with our propagation algorithm as long as they implement the TextMapReader and TextMapWriter interfaces. An example alternate implementation is the MDCarrier in our gRPC integration.

As an example, injecting a span's context into an HTTP request would look like this:

req, err := http.NewRequest("GET", "", nil)
// ...
err := tracer.Inject(span.Context(), tracer.HTTPHeadersCarrier(req.Header))
// ...

Then, on the server side, to continue the trace one would do:

sctx, err := tracer.Extract(tracer.HTTPHeadersCarrier(req.Header))
// ...
span := tracer.StartSpan("child.span", tracer.ChildOf(sctx))

In the same manner, any means can be used as a carrier to inject a context into a transport. Go's context can also be used as a means to transport spans within the same process. The methods StartSpanFromContext, ContextWithSpan and SpanFromContext exist for this reason.

Some libraries and frameworks are supported out-of-the-box by using one of our integrations. You can see a list of supported integrations here:


A basic example demonstrating how to start the tracer, as well as how to create a root span and a child span that is a descendant of it.

// Start the tracer and defer the Stop method.
defer Stop()

// Start a root span.
span := StartSpan("")
defer span.Finish()

// Create a child of it, computing the time needed to read a file.
child := StartSpan("read.file", ChildOf(span.Context()))
child.SetTag(ext.ResourceName, "test.json")

// Perform an operation.
_, err := ioutil.ReadFile("~/test.json")

// We may finish the child span using the returned error. If it's
// nil, it will be disregarded.
if err != nil {




View Source
const (
	// DefaultBaggageHeaderPrefix specifies the prefix that will be used in
	// HTTP headers or text maps to prefix baggage keys.
	DefaultBaggageHeaderPrefix = "ot-baggage-"

	// DefaultTraceIDHeader specifies the key that will be used in HTTP headers
	// or text maps to store the trace ID.
	DefaultTraceIDHeader = "x-datadog-trace-id"

	// DefaultParentIDHeader specifies the key that will be used in HTTP headers
	// or text maps to store the parent ID.
	DefaultParentIDHeader = "x-datadog-parent-id"

	// DefaultPriorityHeader specifies the key that will be used in HTTP headers
	// or text maps to store the sampling priority value.
	DefaultPriorityHeader = "x-datadog-sampling-priority"


View Source
var (
	// ErrInvalidCarrier is returned when the carrier provided to the propagator
	// does not implemented the correct interfaces.
	ErrInvalidCarrier = errors.New("invalid carrier")

	// ErrInvalidSpanContext is returned when the span context found in the
	// carrier is not of the expected type.
	ErrInvalidSpanContext = errors.New("invalid span context")

	// ErrSpanContextCorrupted is returned when there was a problem parsing
	// the information found in the carrier.
	ErrSpanContextCorrupted = errors.New("span context corrupted")

	// ErrSpanContextNotFound represents missing information in the given carrier.
	ErrSpanContextNotFound = errors.New("span context not found")


func ContextWithSpan

func ContextWithSpan(ctx context.Context, s Span) context.Context

ContextWithSpan returns a copy of the given context which includes the span s.

func Extract

func Extract(carrier interface{}) (ddtrace.SpanContext, error)

Extract extracts a SpanContext from the carrier. The carrier is expected to implement TextMapReader, otherwise an error is returned. If the tracer is not started, calling this function is a no-op.

func Inject

func Inject(ctx ddtrace.SpanContext, carrier interface{}) error

Inject injects the given SpanContext into the carrier. The carrier is expected to implement TextMapWriter, otherwise an error is returned. If the tracer is not started, calling this function is a no-op.

func Start

func Start(opts ...StartOption)

Start starts the tracer with the given set of options. It will stop and replace any running tracer, meaning that calling it several times will result in a restart of the tracer by replacing the current instance with a new one.

func Stop

func Stop()

Stop stops the started tracer. Subsequent calls are valid but become no-op.


type FinishOption

type FinishOption = ddtrace.FinishOption

FinishOption is a configuration option for FinishSpan. It is aliased in order to help godoc group all the functions returning it together. It is considered more correct to refer to it as the type as the origin, ddtrace.FinishOption.

func FinishTime

func FinishTime(t time.Time) FinishOption

FinishTime sets the given time as the finishing time for the span. By default, the current time is used.

func NoDebugStack

func NoDebugStack() FinishOption

NoDebugStack prevents any error presented using the WithError finishing option from generating a stack trace. This is useful in situations where errors are frequent and performance is critical.

func StackFrames

func StackFrames(n, skip uint) FinishOption

StackFrames limits the number of stack frames included into erroneous spans to n, starting from skip.

func WithError

func WithError(err error) FinishOption

WithError marks the span as having had an error. It uses the information from err to set tags such as the error message, error type and stack trace. It has no effect if the error is nil.

type HTTPHeadersCarrier

type HTTPHeadersCarrier http.Header

HTTPHeadersCarrier wraps an http.Header as a TextMapWriter and TextMapReader, allowing it to be used using the provided Propagator implementation.

func (HTTPHeadersCarrier) ForeachKey

func (c HTTPHeadersCarrier) ForeachKey(handler func(key, val string) error) error

ForeachKey implements TextMapReader.

func (HTTPHeadersCarrier) Set

func (c HTTPHeadersCarrier) Set(key, val string)

Set implements TextMapWriter.

type Propagator

type Propagator interface {
	// Inject takes the SpanContext and injects it into the carrier.
	Inject(context ddtrace.SpanContext, carrier interface{}) error

	// Extract returns the SpanContext from the given carrier.
	Extract(carrier interface{}) (ddtrace.SpanContext, error)

Propagator implementations should be able to inject and extract SpanContexts into an implementation specific carrier.

func NewPropagator

func NewPropagator(cfg *PropagatorConfig) Propagator

NewPropagator returns a new propagator which uses TextMap to inject and extract values. It propagates trace and span IDs and baggage. To use the defaults, nil may be provided in place of the config.

type PropagatorConfig

type PropagatorConfig struct {
	// BaggagePrefix specifies the prefix that will be used to store baggage
	// items in a map. It defaults to DefaultBaggageHeaderPrefix.
	BaggagePrefix string

	// TraceHeader specifies the map key that will be used to store the trace ID.
	// It defaults to DefaultTraceIDHeader.
	TraceHeader string

	// ParentHeader specifies the map key that will be used to store the parent ID.
	// It defaults to DefaultParentIDHeader.
	ParentHeader string

	// PriorityHeader specifies the map key that will be used to store the sampling priority.
	// It deafults to DefaultPriorityHeader.
	PriorityHeader string

PropagatorConfig defines the configuration for initializing a propagator.

type RateSampler

type RateSampler interface {

	// Rate returns the current sample rate.
	Rate() float64

	// SetRate sets a new sample rate.
	SetRate(rate float64)

RateSampler is a sampler implementation which randomly selects spans using a provided rate. For example, a rate of 0.75 will permit 75% of the spans. RateSampler implementations should be safe for concurrent use.

func NewAllSampler

func NewAllSampler() RateSampler

NewAllSampler is a short-hand for NewRateSampler(1). It is all-permissive.

func NewRateSampler

func NewRateSampler(rate float64) RateSampler

NewRateSampler returns an initialized RateSampler with a given sample rate.

type Sampler

type Sampler interface {
	// Sample returns true if the given span should be sampled.
	Sample(span Span) bool

Sampler is the generic interface of any sampler. It must be safe for concurrent use.

type Span

type Span = ddtrace.Span

Span is an alias for ddtrace.Span. It is here to allow godoc to group methods returning ddtrace.Span. It is recommended and is considered more correct to refer to this type as ddtrace.Span instead.

func SpanFromContext

func SpanFromContext(ctx context.Context) (Span, bool)

SpanFromContext returns the span contained in the given context. A second return value indicates if a span was found in the context. If no span is found, a no-op span is returned.

func StartSpan

func StartSpan(operationName string, opts ...StartSpanOption) Span

StartSpan starts a new span with the given operation name and set of options. If the tracer is not started, calling this function is a no-op.

func StartSpanFromContext

func StartSpanFromContext(ctx context.Context, operationName string, opts ...StartSpanOption) (Span, context.Context)

StartSpanFromContext returns a new span with the given operation name and options. If a span is found in the context, it will be used as the parent of the resulting span. If the ChildOf option is passed, the span from context will take precedence over it as the parent span.

type StartOption

type StartOption func(*config)

StartOption represents a function that can be provided as a parameter to Start.

func WithAgentAddr

func WithAgentAddr(addr string) StartOption

WithAgentAddr sets the address where the agent is located. The default is localhost:8126. It should contain both host and port.

func WithAnalytics

func WithAnalytics(on bool) StartOption

WithAnalytics allows specifying whether Trace Search & Analytics should be enabled for integrations.

func WithAnalyticsRate

func WithAnalyticsRate(rate float64) StartOption

WithAnalyticsRate sets the global sampling rate for sampling APM events.

func WithDebugMode

func WithDebugMode(enabled bool) StartOption

WithDebugMode enables debug mode on the tracer, resulting in more verbose logging.

func WithDogstatsdAddress

func WithDogstatsdAddress(addr string) StartOption

WithDogstatsdAddress specifies the address to connect to for sending metrics to the Datadog Agent. If not set, it defaults to "localhost:8125" or to the combination of the environment variables DD_AGENT_HOST and DD_DOGSTATSD_PORT. This option is in effect when WithRuntimeMetrics is enabled.

func WithEnv

func WithEnv(env string) StartOption

WithEnv sets the environment to which all traces started by the tracer will be submitted. The default value is the environment variable DD_ENV, if it is set.

func WithGlobalTag

func WithGlobalTag(k string, v interface{}) StartOption

WithGlobalTag sets a key/value pair which will be set as a tag on all spans created by tracer. This option may be used multiple times.

func WithHTTPRoundTripper

func WithHTTPRoundTripper(r http.RoundTripper) StartOption

WithHTTPRoundTripper allows customizing the underlying HTTP transport for emitting spans. This is useful for advanced customization such as emitting spans to a unix domain socket. The default should be used in most cases.

func WithLogger

func WithLogger(logger ddtrace.Logger) StartOption

WithLogger sets logger as the tracer's error printer.

func WithPrioritySampling

func WithPrioritySampling() StartOption

WithPrioritySampling is deprecated, and priority sampling is enabled by default. When using distributed tracing, the priority sampling value is propagated in order to get all the parts of a distributed trace sampled. To learn more about priority sampling, please visit:

func WithPropagator

func WithPropagator(p Propagator) StartOption

WithPropagator sets an alternative propagator to be used by the tracer.

func WithRuntimeMetrics

func WithRuntimeMetrics() StartOption

WithRuntimeMetrics enables automatic collection of runtime metrics every 10 seconds.

func WithSampler

func WithSampler(s Sampler) StartOption

WithSampler sets the given sampler to be used with the tracer. By default an all-permissive sampler is used.

func WithServiceName

func WithServiceName(name string) StartOption

WithServiceName sets the default service name to be used with the tracer.

type StartSpanOption

type StartSpanOption = ddtrace.StartSpanOption

StartSpanOption is a configuration option for StartSpan. It is aliased in order to help godoc group all the functions returning it together. It is considered more correct to refer to it as the type as the origin, ddtrace.StartSpanOption.

func ChildOf

func ChildOf(ctx ddtrace.SpanContext) StartSpanOption

ChildOf tells StartSpan to use the given span context as a parent for the created span.

func ResourceName

func ResourceName(name string) StartSpanOption

ResourceName sets the given resource name on the started span. A resource could be an SQL query, a URL, an RPC method or something else.

func ServiceName

func ServiceName(name string) StartSpanOption

ServiceName sets the given service name on the started span. For example "http.server".

func SpanType

func SpanType(name string) StartSpanOption

SpanType sets the given span type on the started span. Some examples in the case of the Datadog APM product could be "web", "db" or "cache".

func StartTime

func StartTime(t time.Time) StartSpanOption

StartTime sets a custom time as the start time for the created span. By default a span is started using the creation time.

func Tag

func Tag(k string, v interface{}) StartSpanOption

Tag sets the given key/value pair as a tag on the started Span.

func WithSpanID

func WithSpanID(id uint64) StartSpanOption

WithSpanID sets the SpanID on the started span, instead of using a random number. If there is no parent Span (eg from ChildOf), then the TraceID will also be set to the value given here.

type TextMapCarrier

type TextMapCarrier map[string]string

TextMapCarrier allows the use of a regular map[string]string as both TextMapWriter and TextMapReader, making it compatible with the provided Propagator.

func (TextMapCarrier) ForeachKey

func (c TextMapCarrier) ForeachKey(handler func(key, val string) error) error

ForeachKey conforms to the TextMapReader interface.

func (TextMapCarrier) Set

func (c TextMapCarrier) Set(key, val string)

Set implements TextMapWriter.

type TextMapReader

type TextMapReader interface {
	// ForeachKey iterates over all keys that exist in the underlying
	// carrier. It takes a callback function which will be called
	// using all key/value pairs as arguments. ForeachKey will return
	// the first error returned by the handler.
	ForeachKey(handler func(key, val string) error) error

TextMapReader allows iterating over sets of key/value pairs. Carriers implementing TextMapReader are compatible to be used with Datadog's TextMapPropagator.

type TextMapWriter

type TextMapWriter interface {
	// Set sets the given key/value pair.
	Set(key, val string)

TextMapWriter allows setting key/value pairs of strings on the underlying data structure. Carriers implementing TextMapWriter are compatible to be used with Datadog's TextMapPropagator.

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