opentracing

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OpenTracing API for Go

This package is a Go platform API for OpenTracing.

Required Reading

In order to understand the Go platform API, one must first be familiar with the OpenTracing project and terminology more generally.

API overview for those adding instrumentation

Everyday consumers of this opentracing package really only need to worry about a couple of key abstractions: the StartSpan function, the Span interface, and binding a Tracer at main()-time. Here are code snippets demonstrating some important use cases.

Singleton initialization

The simplest starting point is ./default_tracer.go. As early as possible, call

    import "github.com/opentracing/opentracing-go"
    import ".../some_tracing_impl"

    func main() {
        opentracing.InitGlobalTracer(
            // tracing impl specific:
            some_tracing_impl.New(...),
        )
        ...
    }
Non-Singleton initialization

If you prefer direct control to singletons, manage ownership of the opentracing.Tracer implementation explicitly.

Creating a Span given an existing Go context.Context

If you use context.Context in your application, OpenTracing's Go library will happily rely on it for Span propagation. To start a new (blocking child) Span, you can use StartSpanFromContext.

    func xyz(ctx context.Context, ...) {
        ...
        span, ctx := opentracing.StartSpanFromContext(ctx, "operation_name")
        defer span.Finish()
        span.LogEvent("xyz_called")
        ...
    }
Starting an empty trace by creating a "root span"

It's always possible to create a "root" Span with no parent or other causal reference.

    func xyz() {
        ...
        sp := opentracing.StartSpan("operation_name")
        defer sp.Finish()
        sp.LogEvent("xyz_called")
        ...
    }
Creating a (child) Span given an existing (parent) Span
    func xyz(parentSpan opentracing.Span, ...) {
        ...
        sp := opentracing.StartSpan(
            "operation_name",
            opentracing.ChildOf(parentSpan.Context()))
        defer sp.Finish()
        sp.LogEvent("xyz_called")
        ...
    }
Serializing to the wire
    func makeSomeRequest(ctx context.Context) ... {
        if span := opentracing.SpanFromContext(ctx); span != nil {
            httpClient := &http.Client{}
            httpReq, _ := http.NewRequest("GET", "http://myservice/", nil)

            // Transmit the span's TraceContext as HTTP headers on our
            // outbound request.
            tracer.Inject(
                span.Context(),
                opentracing.HTTPHeaders,
                opentracing.HTTPHeadersCarrier(httpReq.Header))

            resp, err := httpClient.Do(httpReq)
            ...
        }
        ...
    }
Deserializing from the wire
    http.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
        var serverSpan opentracing.Span
        appSpecificOperationName := ...
        wireContext, err := opentracing.GlobalTracer().Extract(
            opentracing.HTTPHeaders,
            opentracing.HTTPHeadersCarrier(req.Header))
        if err != nil {
            // Optionally record something about err here
        }

        // Create the span referring to the RPC client if available.
        // If wireContext == nil, a root span will be created.
        serverSpan = opentracing.StartSpan(
            appSpecificOperationName,
            ext.RPCServerOption(wireContext))

        defer serverSpan.Finish()

        ctx := opentracing.ContextWithSpan(context.Background(), serverSpan)
        ...
    }
Goroutine-safety

The entire public API is goroutine-safe and does not require external synchronization.

API pointers for those implementing a tracing system

Tracing system implementors may be able to reuse or copy-paste-modify the basictracer package, found here. In particular, see basictracer.New(...).

API compatibility

For the time being, "mild" backwards-incompatible changes may be made without changing the major version number. As OpenTracing and opentracing-go mature, backwards compatibility will become more of a priority.

Documentation

Index

Constants

This section is empty.

Variables

View Source 
var (
	// ErrUnsupportedFormat occurs when the `format` passed to Tracer.Inject() or
	// Tracer.Extract() is not recognized by the Tracer implementation.
	ErrUnsupportedFormat = errors.New("opentracing: Unknown or unsupported Inject/Extract format")

	// ErrSpanContextNotFound occurs when the `carrier` passed to
	// Tracer.Extract() is valid and uncorrupted but has insufficient
	// information to extract a SpanContext.
	ErrSpanContextNotFound = errors.New("opentracing: SpanContext not found in Extract carrier")

	// ErrInvalidSpanContext errors occur when Tracer.Inject() is asked to
	// operate on a SpanContext which it is not prepared to handle (for
	// example, since it was created by a different tracer implementation).
	ErrInvalidSpanContext = errors.New("opentracing: SpanContext type incompatible with tracer")

	// ErrInvalidCarrier errors occur when Tracer.Inject() or Tracer.Extract()
	// implementations expect a different type of `carrier` than they are
	// given.
	ErrInvalidCarrier = errors.New("opentracing: Invalid Inject/Extract carrier")

	// ErrSpanContextCorrupted occurs when the `carrier` passed to
	// Tracer.Extract() is of the expected type but is corrupted.
	ErrSpanContextCorrupted = errors.New("opentracing: SpanContext data corrupted in Extract carrier")
)

Functions

func ContextWithSpan 

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

ContextWithSpan returns a new `context.Context` that holds a reference to `span`'s SpanContext.

func InitGlobalTracer 

func InitGlobalTracer(tracer Tracer)

InitGlobalTracer sets the [singleton] opentracing.Tracer returned by GlobalTracer(). Those who use GlobalTracer (rather than directly manage an opentracing.Tracer instance) should call InitGlobalTracer as early as possible in main(), prior to calling the `StartSpan` global func below. Prior to calling `InitGlobalTracer`, any Spans started via the `StartSpan` (etc) globals are noops.

Types

type BuiltinFormat 

type BuiltinFormat byte

BuiltinFormat is used to demarcate the values within package `opentracing` that are intended for use with the Tracer.Inject() and Tracer.Extract() methods. 

const (
	// Binary represents SpanContexts as opaque binary data.
	//
	// For Tracer.Inject(): the carrier must be an `io.Writer`.
	//
	// For Tracer.Extract(): the carrier must be an `io.Reader`.
	Binary BuiltinFormat = iota

	// TextMap represents SpanContexts as key:value string pairs.
	//
	// Unlike HTTPHeaders, the TextMap format does not restrict the key or
	// value character sets in any way.
	//
	// For Tracer.Inject(): the carrier must be a `TextMapWriter`.
	//
	// For Tracer.Extract(): the carrier must be a `TextMapReader`.
	TextMap

	// HTTPHeaders represents SpanContexts as HTTP header string pairs.
	//
	// Unlike TextMap, the HTTPHeaders format requires that the keys and values
	// be valid as HTTP headers as-is (i.e., character casing may be unstable
	// and special characters are disallowed in keys, values should be
	// URL-escaped, etc).
	//
	// For Tracer.Inject(): the carrier must be a `TextMapWriter`.
	//
	// For Tracer.Extract(): the carrier must be a `TextMapReader`.
	//
	// See HTTPHeaderCarrier for an implementation of both TextMapWriter
	// and TextMapReader that defers to an http.Header instance for storage.
	// For example, Inject():
	//
	//    carrier := opentracing.HTTPHeadersCarrier(httpReq.Header)
	//    err := span.Tracer().Inject(
	//        span, opentracing.HTTPHeaders, carrier)
	//
	// Or Extract():
	//
	//    carrier := opentracing.HTTPHeadersCarrier(httpReq.Header)
	//    span, err := tracer.Extract(
	//        opentracing.HTTPHeaders, carrier)
	//
	HTTPHeaders
)

type FinishOptions 

type FinishOptions struct {
	// FinishTime overrides the Span's finish time, or implicitly becomes
	// time.Now() if FinishTime.IsZero().
	//
	// FinishTime must resolve to a timestamp that's >= the Span's StartTime
	// (per StartSpanOptions).
	FinishTime time.Time

	// BulkLogData allows the caller to specify the contents of many Log()
	// calls with a single slice. May be nil.
	//
	// None of the LogData.Timestamp values may be .IsZero() (i.e., they must
	// be set explicitly). Also, they must be >= the Span's start timestamp and
	// <= the FinishTime (or time.Now() if FinishTime.IsZero()). Otherwise the
	// behavior of FinishWithOptions() is undefined.
	//
	// If specified, the caller hands off ownership of BulkLogData at
	// FinishWithOptions() invocation time.
	BulkLogData []LogData
}

FinishOptions allows Span.FinishWithOptions callers to override the finish timestamp and provide log data via a bulk interface.

type HTTPHeadersCarrier 

type HTTPHeadersCarrier http.Header

HTTPHeadersCarrier satisfies both TextMapWriter and TextMapReader.

func (HTTPHeadersCarrier) ForeachKey 

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

ForeachKey conforms to the TextMapReader interface.

func (HTTPHeadersCarrier) Set 

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

Set conforms to the TextMapWriter interface.

type LogData 

type LogData struct {
	// The timestamp of the log record; if set to the default value (the unix
	// epoch), implementations should use time.Now() implicitly.
	Timestamp time.Time

	// Event (if non-empty) should be the stable name of some notable moment in
	// the lifetime of a Span. For instance, a Span representing a browser page
	// load might add an Event for each of the Performance.timing moments
	// here: https://developer.mozilla.org/en-US/docs/Web/API/PerformanceTiming
	//
	// While it is not a formal requirement, Event strings will be most useful
	// if they are *not* unique; rather, tracing systems should be able to use
	// them to understand how two similar Spans relate from an internal timing
	// perspective.
	Event string

	// Payload is a free-form potentially structured object which Tracer
	// implementations may retain and record all, none, or part of.
	//
	// If included, `Payload` should be restricted to data derived from the
	// instrumented application; in particular, it should not be used to pass
	// semantic flags to a Log() implementation.
	//
	// For example, an RPC system could log the wire contents in both
	// directions, or a SQL library could log the query (with or without
	// parameter bindings); tracing implementations may truncate or otherwise
	// record only a snippet of these payloads (or may strip out PII, etc,
	// etc).
	Payload interface{}
}

LogData is data associated with a Span. Every LogData instance should specify at least one of Event and/or Payload.

type NoopTracer 

type NoopTracer struct{}

A NoopTracer is a trivial implementation of Tracer for which all operations are no-ops.

func (NoopTracer) Extract 

func (n NoopTracer) Extract(format interface{}, carrier interface{}) (SpanContext, error)

Extract belongs to the Tracer interface.

func (NoopTracer) Inject 

func (n NoopTracer) Inject(sp SpanContext, format interface{}, carrier interface{}) error

Inject belongs to the Tracer interface.

func (NoopTracer) StartSpan 

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

StartSpan belongs to the Tracer interface.

type Span 

type Span interface {
	// Sets the end timestamp and finalizes Span state.
	//
	// With the exception of calls to Context() (which are always allowed),
	// Finish() must be the last call made to any span instance, and to do
	// otherwise leads to undefined behavior.
	Finish()
	// FinishWithOptions is like Finish() but with explicit control over
	// timestamps and log data.
	FinishWithOptions(opts FinishOptions)

	// Context() yields the SpanContext for this Span. Note that the return
	// value of Context() is still valid after a call to Span.Finish(), as is
	// a call to Span.Context() after a call to Span.Finish().
	Context() SpanContext

	// Sets or changes the operation name.
	SetOperationName(operationName string) Span

	// Adds a tag to the span.
	//
	// If there is a pre-existing tag set for `key`, it is overwritten.
	//
	// Tag values can be numeric types, strings, or bools. The behavior of
	// other tag value types is undefined at the OpenTracing level. If a
	// tracing system does not know how to handle a particular value type, it
	// may ignore the tag, but shall not panic.
	SetTag(key string, value interface{}) Span

	// LogEvent() is equivalent to
	//
	//   Log(LogData{Event: event})
	//
	LogEvent(event string)

	// LogEventWithPayload() is equivalent to
	//
	//   Log(LogData{Event: event, Payload: payload0})
	//
	LogEventWithPayload(event string, payload interface{})

	// Log() records `data` to this Span.
	//
	// See LogData for semantic details.
	Log(data LogData)

	// SetBaggageItem sets a key:value pair on this Span and its SpanContext
	// that also propagates to descendants of this Span.
	//
	// SetBaggageItem() enables powerful functionality given a full-stack
	// opentracing integration (e.g., arbitrary application data from a mobile
	// app can make it, transparently, all the way into the depths of a storage
	// system), and with it some powerful costs: use this feature with care.
	//
	// IMPORTANT NOTE #1: SetBaggageItem() will only propagate baggage items to
	// *future* causal descendants of the associated Span.
	//
	// IMPORTANT NOTE #2: Use this thoughtfully and with care. Every key and
	// value is copied into every local *and remote* child of the associated
	// Span, and that can add up to a lot of network and cpu overhead.
	//
	// Returns a reference to this Span for chaining.
	SetBaggageItem(restrictedKey, value string) Span

	// Gets the value for a baggage item given its key. Returns the empty string
	// if the value isn't found in this Span.
	BaggageItem(restrictedKey string) string

	// Provides access to the Tracer that created this Span.
	Tracer() Tracer
}

Span represents an active, un-finished span in the OpenTracing system.

Spans are created by the Tracer interface.

func SpanFromContext 

func SpanFromContext(ctx context.Context) Span

SpanFromContext returns the `Span` previously associated with `ctx`, or `nil` if no such `Span` could be found.

NOTE: context.Context != SpanContext: the former is Go's intra-process context propagation mechanism, and the latter houses OpenTracing's per-Span identity and baggage information.

func StartSpan 

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

StartSpan defers to `Tracer.StartSpan`. See `GlobalTracer()`.

func StartSpanFromContext 

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

StartSpanFromContext starts and returns a Span with `operationName`, using any Span found within `ctx` as a ChildOfRef. If no such parent could be found, StartSpanFromContext creates a root (parentless) Span.

The second return value is a context.Context object built around the returned Span.

Example usage: 

SomeFunction(ctx context.Context, ...) {
    sp, ctx := opentracing.StartSpanFromContext(ctx, "SomeFunction")
    defer sp.Finish()
    ...
}

type SpanContext 

type SpanContext interface {
	// ForeachBaggageItem grants access to all baggage items stored in the
	// SpanContext.
	// The handler function will be called for each baggage key/value pair.
	// The ordering of items is not guaranteed.
	//
	// The bool return value indicates if the handler wants to continue iterating
	// through the rest of the baggage items; for example if the handler is trying to
	// find some baggage item by pattern matching the name, it can return false
	// as soon as the item is found to stop further iterations.
	ForeachBaggageItem(handler func(k, v string) bool)
}

SpanContext represents Span state that must propagate to descendant Spans and across process boundaries (e.g., a <trace_id, span_id, sampled> tuple).

type SpanReference 

type SpanReference struct {
	Type              SpanReferenceType
	ReferencedContext SpanContext
}

SpanReference is a StartSpanOption that pairs a SpanReferenceType and a referenced SpanContext. See the SpanReferenceType documentation for supported relationships. If SpanReference is created with ReferencedContext==nil, it has no effect. Thus it allows for a more concise syntax for starting spans: 

sc, _ := tracer.Extract(someFormat, someCarrier)
span := tracer.StartSpan("operation", opentracing.ChildOf(sc))

The `ChildOf(sc)` option above will not panic if sc == nil, it will just not add the parent span reference to the options.

func ChildOf 

func ChildOf(sc SpanContext) SpanReference

ChildOf returns a StartSpanOption pointing to a dependent parent span. If sc == nil, the option has no effect.

See ChildOfRef, SpanReference

func FollowsFrom 

func FollowsFrom(sc SpanContext) SpanReference

FollowsFrom returns a StartSpanOption pointing to a parent Span that caused the child Span but does not directly depend on its result in any way. If sc == nil, the option has no effect.

See FollowsFromRef, SpanReference

func (SpanReference) Apply 

func (r SpanReference) Apply(o *StartSpanOptions)

Apply satisfies the StartSpanOption interface.

type SpanReferenceType 

type SpanReferenceType int

SpanReferenceType is an enum type describing different categories of relationships between two Spans. If Span-2 refers to Span-1, the SpanReferenceType describes Span-1 from Span-2's perspective. For example, ChildOfRef means that Span-1 created Span-2.

NOTE: Span-1 and Span-2 do *not* necessarily depend on each other for completion; e.g., Span-2 may be part of a background job enqueued by Span-1, or Span-2 may be sitting in a distributed queue behind Span-1. 

const (
	// ChildOfRef refers to a parent Span that caused *and* somehow depends
	// upon the new child Span. Often (but not always), the parent Span cannot
	// finish until the child Span does.
	//
	// An timing diagram for a ChildOfRef that's blocked on the new Span:
	//
	//     [-Parent Span---------]
	//          [-Child Span----]
	//
	// See http://opentracing.io/spec/
	//
	// See opentracing.ChildOf()
	ChildOfRef SpanReferenceType = iota

	// FollowsFromRef refers to a parent Span that does not depend in any way
	// on the result of the new child Span. For instance, one might use
	// FollowsFromRefs to describe pipeline stages separated by queues,
	// or a fire-and-forget cache insert at the tail end of a web request.
	//
	// A FollowsFromRef Span is part of the same logical trace as the new Span:
	// i.e., the new Span is somehow caused by the work of its FollowsFromRef.
	//
	// All of the following could be valid timing diagrams for children that
	// "FollowFrom" a parent.
	//
	//     [-Parent Span-]  [-Child Span-]
	//
	//
	//     [-Parent Span--]
	//      [-Child Span-]
	//
	//
	//     [-Parent Span-]
	//                 [-Child Span-]
	//
	// See http://opentracing.io/spec/
	//
	// See opentracing.FollowsFrom()
	FollowsFromRef
)

type StartSpanOption 

type StartSpanOption interface {
	Apply(*StartSpanOptions)
}

StartSpanOption instances (zero or more) may be passed to Tracer.StartSpan.

StartSpanOption borrows from the "functional options" pattern, per http://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis

type StartSpanOptions 

type StartSpanOptions struct {
	// Zero or more causal references to other Spans (via their SpanContext).
	// If empty, start a "root" Span (i.e., start a new trace).
	References []SpanReference

	// StartTime overrides the Span's start time, or implicitly becomes
	// time.Now() if StartTime.IsZero().
	StartTime time.Time

	// Tags may have zero or more entries; the restrictions on map values are
	// identical to those for Span.SetTag(). May be nil.
	//
	// If specified, the caller hands off ownership of Tags at
	// StartSpan() invocation time.
	Tags map[string]interface{}
}

StartSpanOptions allows Tracer.StartSpan() callers and implementors a mechanism to override the start timestamp, specify Span References, and make a single Tag or multiple Tags available at Span start time.

StartSpan() callers should look at the StartSpanOption interface and implementations available in this package.

Tracer implementations can convert a slice of `StartSpanOption` instances into a `StartSpanOptions` struct like so: 

func StartSpan(opName string, opts ...opentracing.StartSpanOption) {
    sso := opentracing.StartSpanOptions{}
    for _, o := range opts {
        o.Apply(&sso)
    }
    ...
}

type StartTime 

type StartTime time.Time

StartTime is a StartSpanOption that sets an explicit start timestamp for the new Span.

func (StartTime) Apply 

func (t StartTime) Apply(o *StartSpanOptions)

Apply satisfies the StartSpanOption interface.

type Tag 

type Tag struct {
	Key   string
	Value interface{}
}

Tag may be passed as a StartSpanOption to add a tag to new spans, or its Set method may be used to apply the tag to an existing Span, for example:

tracer.StartSpan("opName", Tag{"Key", value}) 

or

Tag{"key", value}.Set(span)

func (Tag) Apply 

func (t Tag) Apply(o *StartSpanOptions)

Apply satisfies the StartSpanOption interface.

func (Tag) Set 

func (t Tag) Set(s Span)

Set applies the tag to an existing Span.

type Tags 

type Tags map[string]interface{}

Tags are a generic map from an arbitrary string key to an opaque value type. The underlying tracing system is responsible for interpreting and serializing the values.

func (Tags) Apply 

func (t Tags) Apply(o *StartSpanOptions)

Apply satisfies the StartSpanOption interface.

type TextMapCarrier 

type TextMapCarrier map[string]string

TextMapCarrier allows the use of regular map[string]string as both TextMapWriter and TextMapReader.

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 Set() of opentracing.TextMapWriter

type TextMapReader 

type TextMapReader interface {
	// ForeachKey returns TextMap contents via repeated calls to the `handler`
	// function. If any call to `handler` returns a non-nil error, ForeachKey
	// terminates and returns that error.
	//
	// NOTE: The backing store for the TextMapReader may contain data unrelated
	// to SpanContext. As such, Inject() and Extract() implementations that
	// call the TextMapWriter and TextMapReader interfaces must agree on a
	// prefix or other convention to distinguish their own key:value pairs.
	//
	// The "foreach" callback pattern reduces unnecessary copying in some cases
	// and also allows implementations to hold locks while the map is read.
	ForeachKey(handler func(key, val string) error) error
}

TextMapReader is the Extract() carrier for the TextMap builtin format. With it, the caller can decode a propagated SpanContext as entries in a map of unicode strings.

type TextMapWriter 

type TextMapWriter interface {
	// Set a key:value pair to the carrier. Multiple calls to Set() for the
	// same key leads to undefined behavior.
	//
	// NOTE: The backing store for the TextMapWriter may contain data unrelated
	// to SpanContext. As such, Inject() and Extract() implementations that
	// call the TextMapWriter and TextMapReader interfaces must agree on a
	// prefix or other convention to distinguish their own key:value pairs.
	Set(key, val string)
}

TextMapWriter is the Inject() carrier for the TextMap builtin format. With it, the caller can encode a SpanContext for propagation as entries in a map of unicode strings.

type Tracer 

type Tracer interface {

	// Create, start, and return a new Span with the given `operationName` and
	// incorporate the given StartSpanOption `opts`. (Note that `opts` borrows
	// from the "functional options" pattern, per
	// http://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis)
	//
	// A Span with no SpanReference options (e.g., opentracing.ChildOf() or
	// opentracing.FollowsFrom()) becomes the root of its own trace.
	//
	// Examples:
	//
	//     var tracer opentracing.Tracer = ...
	//
	//     // The root-span case:
	//     sp := tracer.StartSpan("GetFeed")
	//
	//     // The vanilla child span case:
	//     sp := tracer.StartSpan(
	//         "GetFeed",
	//         opentracing.ChildOf(parentSpan.Context()))
	//
	//     // All the bells and whistles:
	//     sp := tracer.StartSpan(
	//         "GetFeed",
	//         opentracing.ChildOf(parentSpan.Context()),
	//         opentracing.Tag("user_agent", loggedReq.UserAgent),
	//         opentracing.StartTime(loggedReq.Timestamp),
	//     )
	//
	StartSpan(operationName string, opts ...StartSpanOption) Span

	// Inject() takes the `sm` SpanContext instance and injects it for
	// propagation within `carrier`. The actual type of `carrier` depends on
	// the value of `format`.
	//
	// OpenTracing defines a common set of `format` values (see BuiltinFormat),
	// and each has an expected carrier type.
	//
	// Other packages may declare their own `format` values, much like the keys
	// used by `context.Context` (see
	// https://godoc.org/golang.org/x/net/context#WithValue).
	//
	// Example usage (sans error handling):
	//
	//     carrier := opentracing.HTTPHeadersCarrier(httpReq.Header)
	//     err := tracer.Inject(
	//         span.Context(),
	//         opentracing.HttpHeaders,
	//         carrier)
	//
	// NOTE: All opentracing.Tracer implementations MUST support all
	// BuiltinFormats.
	//
	// Implementations may return opentracing.ErrUnsupportedFormat if `format`
	// is not supported by (or not known by) the implementation.
	//
	// Implementations may return opentracing.ErrInvalidCarrier or any other
	// implementation-specific error if the format is supported but injection
	// fails anyway.
	//
	// See Tracer.Extract().
	Inject(sm SpanContext, format interface{}, carrier interface{}) error

	// Extract() returns a SpanContext instance given `format` and `carrier`.
	//
	// OpenTracing defines a common set of `format` values (see BuiltinFormat),
	// and each has an expected carrier type.
	//
	// Other packages may declare their own `format` values, much like the keys
	// used by `context.Context` (see
	// https://godoc.org/golang.org/x/net/context#WithValue).
	//
	// Example usage (with StartSpan):
	//
	//
	//     carrier := opentracing.HttpHeadersCarrier(httpReq.Header)
	//     clientContext, err := tracer.Extract(opentracing.HttpHeaders, carrier)
	//
	//     // ... assuming the ultimate goal here is to resume the trace with a
	//     // server-side Span:
	//     var serverSpan opentracing.Span
	//     if err == nil {
	//         span = tracer.StartSpan(
	//             rpcMethodName, ext.RPCServerOption(clientContext))
	//     } else {
	//         span = tracer.StartSpan(rpcMethodName)
	//     }
	//
	//
	// NOTE: All opentracing.Tracer implementations MUST support all
	// BuiltinFormats.
	//
	// Return values:
	//  - A successful Extract returns a SpanContext instance and a nil error
	//  - If there was simply no SpanContext to extract in `carrier`, Extract()
	//    returns (nil, opentracing.ErrSpanContextNotFound)
	//  - If `format` is unsupported or unrecognized, Extract() returns (nil,
	//    opentracing.ErrUnsupportedFormat)
	//  - If there are more fundamental problems with the `carrier` object,
	//    Extract() may return opentracing.ErrInvalidCarrier,
	//    opentracing.ErrSpanContextCorrupted, or implementation-specific
	//    errors.
	//
	// See Tracer.Inject().
	Extract(format interface{}, carrier interface{}) (SpanContext, error)
}

Tracer is a simple, thin interface for Span creation and SpanContext propagation.

func GlobalTracer 

func GlobalTracer() Tracer

GlobalTracer returns the global singleton `Tracer` implementation. Before `InitGlobalTracer()` is called, the `GlobalTracer()` is a noop implementation that drops all data handed to it.

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