Documentation
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Overview ¶
Package trace gives access to direct control over the span and trace objects used by the beeline.
Summary ¶
Elementary use of the Honeycomb beeline has few needs - use a wrapper to cover most of the basic info about a given event and then augment that event with a few fields using `beeline.AddField`. Occasionally add an additional span to the trace that is automatically created and add fields to that. However, as the beeline starts being used in more sophisticated applications, or new wrappers are being written, more direct control is needed to manipulate the spans and traces generated by the beeline. Using the `trace` package enables this more sophisticated use.
The types of use that are enabled by using the trace package:
1) creating asynchronous spans; spans that outlive the main trace. These spans are especially useful for goroutines that manage some background task after the main user-facing chore has been completed. Examples of this are sending an email or persisting accepted data.
2) creating traces with upstream parents when you are a downstream service. The existing HTTP wrappers do this for you, but if your trace is getting propagated via kafka or SQS or some other mechanism you may need to do this yourself
3) adding fields that use a different naming scheme. Fields added via the beeline are all namespaced under `app.`, which is convenient when you're adding a few. When you have more complicated code to manage, it can be useful to use your own naming scheme. Adding fields directly to the span or trace objects allows you to specify the full field name with no prefix.
Lifecycle ¶
A trace is made up of spans. A span represents a single unit of work (what makes up a unit of work is up to the application). Spans come in two flavors, synchronous (the default) and asynchronous. Synchronous spans finish before their parents, async spans don't. Which you should use depends on the structure of code - if the outer function will block until the inner function returns, a sync span is appropriate. If the inner function is called in a goroutine and expected to outlive the outer function, an async span fits better.
Spans should be created as children from an existing span. If there is no current span, first create a new trace then get its root span and use that to create subsequent spans. The beeline `StartSpan()` takes care of all of this for you, but if you're using the trace package directly you need to manage that bookkeeping.
When should you create a new span? There are no strict rules, but there are a few heuristics. If there is a process that will repeat in a loop (a batch or something) and each run through the loop is important, make a span. If the code being instrumented has enough attributes that are relevant to it directly to warrant its own bag of data, make a new span. If all you're interested in is a simple timer, it's often cleaner to add that timer to the current existing span.
Spans must have `Send()` called in order to be sent to Honeycomb. Every span that is created should have a corresponding `Send()` call. When `Send()` is called a few things happen. First, there is some trace-level accounting that is done (eg adding trace level fields, determining position in the trace, finishing the running timer, etc.). When that finishes the presend and sampler hooks are called. Finally, the span is dispatched to Honeycomb.
Any span that calls out to another service can serialize the current state of the trace into a string suitable for including as an HTTP header (or other similar method for encoding as part of a message). That serialized form can be fed into the downstream service that will use it to start a new trace using the same trace ID. When you look at one of these traces in Honeycomb, you will see any spans created by the downstream service appear as children of the span that serialized its state. The serialized state includes the trace ID and the ID of the span that serialized state, as well as an encoded form of all trace level fields.
Putting all this together, this is a visualization of a request that spawns two goroutines, each of which must return before the root span can return. Each of those also has a synchronous span as a child. One of those also kicks off an async span to save some state and it does not block returning the result to the original caller.
|----------- root span -----------|
\---- sync child ----|
\----|
\------ async child ---------|
\--- sync child -------|
\-------------|
Sampling ¶
The default sampling applied by the beeline samples entire traces. For example, if you set a sample rate to 10, then one out of 10 traces will be sent, and all spans in that trace will be sent (or none at all). If you take advantage of the SamplerHook, it is up to you and your implementation to decide whether to sample entire traces or individual spans. If traces are incomplete (i.e. some spans are kept and others dropped), the Honeycomb UI will show missing traces where there are children of dropped spans. Any dropped spans that have no children will be entirely absent from the UI.
Use ¶
While easiest to use the `beeline` package and existing wrappers to do most of the legwork for you, here is the general flow of interaction with traces.
- start of the request
When a request starts or program execution begins, create a trace with `NewTrace`. If the program is downstream of something that is also traced, capture the serialized trace headers and pass them in to the trace creation to connect the two. The `NewTrace` function puts the trace and root span in the context for you.
- during work
As your program flows the most common pattern will be to start a span at the beginning of a function and then immediately defer sending that span.
func myFunc(ctx context.Context) {
ctx, span := beeline.StartSpan(ctx) // use the beeline if you can
// parentSpan := trace.GetSpanFromContext(ctx).CreateChild() // or do it manually
// not shown here - if you do it manually, check for nil to avoid panic
defer span.Send()
...
span.AddField("app.fancy_feast_flavor", "paté")
...
}
- wrapping up
When each span finishes and gets sent, it also sends any synchronous children. Any synchronous spans that were unsent when their parent finished will get sent by the parent and will have an additional field (`meta.sent_by_parent`) added to indicate that they were unsent. Sending unsent spans is likely indicative of either an opportunity to use an async span or a bug in the program where a span accidentally does not get sent.
Index ¶
- Variables
- func CopyContext(dest context.Context, src context.Context) (context.Context, error)
- func PutSpanInContext(ctx context.Context, span *Span) context.Context
- func PutTraceInContext(ctx context.Context, trace *Trace) context.Context
- type Config
- type Span
- func (s *Span) AddField(key string, val interface{})
- func (s *Span) AddRollupField(key string, val float64)
- func (s *Span) AddTraceField(key string, val interface{})
- func (s *Span) CreateAsyncChild(ctx context.Context) (context.Context, *Span)
- func (s *Span) CreateChild(ctx context.Context) (context.Context, *Span)
- func (s *Span) GetChildren() []*Span
- func (s *Span) GetParent() *Span
- func (s *Span) IsAsync() bool
- func (s *Span) Send()
- func (s *Span) SerializeHeaders() string
- type Trace
Constants ¶
This section is empty.
Variables ¶
var (
ErrTraceNotFoundInContext = errors.New("beeline trace not found in source context")
)
Functions ¶
func CopyContext ¶ added in v0.3.6
CopyContext takes a context that has a beeline trace and one that doesn't. It copies all the bits necessary to continue the trace from one to the other. This is useful if you need to break context to launch a goroutine that shouldn't be cancelled by the parent's cancellation context. It returns the newly populated context. If it can't find a trace in the source context, it returns the unchanged dest context with an error.
func PutSpanInContext ¶
PutSpanInContext takes an existing context and a span and pushes the span into the context. It will replace any spans that already exist in the context. Spans put in context are retrieved using GetSpanFromContext.
func PutTraceInContext ¶
PutTraceInContext takes an existing context and a trace and pushes the trace into the context. It will replace any traces that already exist in the context. Traces put in context are retrieved using GetTraceFromContext.
Types ¶
type Config ¶
type Config struct {
// SamplerHook is a function to manage sampling on this trace. See the docs
// for `beeline.Config` for a full description.
SamplerHook func(map[string]interface{}) (bool, int)
// PresendHook is a function to mutate spans just before they are sent to
// Honeycomb. See the docs for `beeline.Config` for a full description.
PresendHook func(map[string]interface{})
}
var GlobalConfig Config
type Span ¶
type Span struct {
// contains filtered or unexported fields
}
Span represents a specific task or portion of an application. It has a time and duration, and is linked to parent and children.
func GetSpanFromContext ¶
GetSpanFromContext identifies the currently active span via the span context key. It returns that span, and access to the trace is available via the span or from the context directly. It will return nil if there is no span available.
func (*Span) AddRollupField ¶
AddRollupField adds a key/value pair to this span. If it is called repeatedly on the same span, the values will be summed together. Additionally, this field will be summed across all spans and added to the trace as a total. It is especially useful for doing things like adding the duration spent talking to a specific external service - eg database time. The root span will then get a field that represents the total time spent talking to the database from all of the spans that are part of the trace.
func (*Span) AddTraceField ¶
AddTraceField adds a key/value pair to this span and all others involved in this trace. These fields are also passed along to downstream services. This method is functionally identical to `Trace.AddField()`.
func (*Span) CreateAsyncChild ¶
CreateAsyncChild creates a child of the current span that is expected to outlive the current span (and trace). Async spans are not automatically sent when their parent finishes, but are otherwise identical to synchronous spans.
func (*Span) CreateChild ¶
Span creates a synchronous child of the current span. Spans must finish before their parents.
func (*Span) GetChildren ¶
GetChildren returns a list of all child spans (both synchronous and asynchronous).
func (*Span) IsAsync ¶
IsAsync reveals whether the span is asynchronous (true) or synchronous (false).
func (*Span) Send ¶
func (s *Span) Send()
Send marks a span complete. It does some accounting and then dispatches the span to Honeycomb. Sending a span also triggers sending all synchronous child spans - in other words, if any synchronous child span has not yet been sent, sending the parent will finish and send the children as well.
func (*Span) SerializeHeaders ¶
SerializeHeaders returns the trace ID, current span ID as parent ID, and an encoded form of all trace level fields. This serialized header is intended to be put in an HTTP (or other protocol) header to transmit to downstream services so they may start a new trace that will be connected to this trace. The serialized form may be passed to NewTrace() in order to create a new trace that will be connected to this trace.
type Trace ¶
type Trace struct {
// contains filtered or unexported fields
}
Trace holds some trace level state and the root of the span tree that will be the entire in-process trace. Traces are sent to Honeycomb when the root span is sent. You can send a trace manually, and that will cause all synchronous spans in the trace to be sent and sent. Asynchronous spans must still be sent on their own
func GetTraceFromContext ¶
GetTraceFromContext retrieves a trace from the passed in context or returns nil if no trace exists.
func NewTrace ¶
NewTrace creates a brand new trace. serializedHeaders is optional, and if included, should be the header as written by trace.SerializeHeaders(). When not starting from an upstream trace, pass the empty string here.
func (*Trace) AddField ¶
AddField adds a field to the trace. Every span in the trace will have this field added to it. These fields are also passed along to downstream services. It is useful to add fields here that pertain to the entire trace, to aid in filtering spans at many different areas of the trace together.
func (*Trace) GetRootSpan ¶
GetRootSpan returns the root of the in-process trace. Sending the root span will send the entire trace to Honeycomb. From the root span you can walk the entire span tree using GetChildren (and recursively calling GetChildren on each child).
Source Files