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|
package context
import (
"runtime"
"time"
"github.com/docker/distribution/uuid"
)
// WithTrace allocates a traced timing span in a new context. This allows a
// caller to track the time between calling WithTrace and the returned done
// function. When the done function is called, a log message is emitted with a
// "trace.duration" field, corresponding to the elapsed time and a
// "trace.func" field, corresponding to the function that called WithTrace.
//
// The logging keys "trace.id" and "trace.parent.id" are provided to implement
// dapper-like tracing. This function should be complemented with a WithSpan
// method that could be used for tracing distributed RPC calls.
//
// The main benefit of this function is to post-process log messages or
// intercept them in a hook to provide timing data. Trace ids and parent ids
// can also be linked to provide call tracing, if so required.
//
// Here is an example of the usage:
//
// func timedOperation(ctx Context) {
// ctx, done := WithTrace(ctx)
// defer done("this will be the log message")
// // ... function body ...
// }
//
// If the function ran for roughly 1s, such a usage would emit a log message
// as follows:
//
// INFO[0001] this will be the log message trace.duration=1.004575763s trace.func=github.com/docker/distribution/context.traceOperation trace.id=<id> ...
//
// Notice that the function name is automatically resolved, along with the
// package and a trace id is emitted that can be linked with parent ids.
func WithTrace(ctx Context) (Context, func(format string, a ...interface{})) {
if ctx == nil {
ctx = Background()
}
pc, file, line, _ := runtime.Caller(1)
f := runtime.FuncForPC(pc)
ctx = &traced{
Context: ctx,
id: uuid.Generate().String(),
start: time.Now(),
parent: GetStringValue(ctx, "trace.id"),
fnname: f.Name(),
file: file,
line: line,
}
return ctx, func(format string, a ...interface{}) {
GetLogger(ctx,
"trace.duration",
"trace.id",
"trace.parent.id",
"trace.func",
"trace.file",
"trace.line").
Debugf(format, a...)
}
}
// traced represents a context that is traced for function call timing. It
// also provides fast lookup for the various attributes that are available on
// the trace.
type traced struct {
Context
id string
parent string
start time.Time
fnname string
file string
line int
}
func (ts *traced) Value(key interface{}) interface{} {
switch key {
case "trace.start":
return ts.start
case "trace.duration":
return time.Since(ts.start)
case "trace.id":
return ts.id
case "trace.parent.id":
if ts.parent == "" {
return nil // must return nil to signal no parent.
}
return ts.parent
case "trace.func":
return ts.fnname
case "trace.file":
return ts.file
case "trace.line":
return ts.line
}
return ts.Context.Value(key)
}
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