Tracing
- Go course
- Advanced
- Lesson 61/68
- 4 min
Learn the distributed tracing concept.
Distributed tracing
When a request passes through several services, logs and metrics don't say exactly where it slowed down. In tracing each piece of work is a span; nested spans form a trace:
trace a1b2 ─ GET /siparis 120 ms
├─ stok-servisi 30 ms
└─ odeme-servisi 80 ms
└─ SQL INSERT 65 msThis view immediately shows that the latency comes from the SQL query in the payment service.
Propagation with context
In Go the active span travels inside context.Context. That's why every traced function takes ctx as its first parameter and passes it down:
func odemeAl(ctx context.Context, tutar int) error {
ctx, span := tracer.Start(ctx, "odemeAl")
defer span.End()
return repo.Kaydet(ctx, tutar) // the child span opens here
}Starting a new chain with context.Background() breaks the trace.
Practice A simple span
Write your own mini span: start(name string) func() records the start time and returns a function that prints name + " bitti" when called. Use it in a function with defer start("islem")().
Starter code
package main
import (
"fmt"
"time"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"fmt"
"time"
)
func start(ad string) func() {
bas := time.Now()
return func() {
_ = time.Since(bas)
fmt.Println(ad + " bitti")
}
}
func islem() {
defer start("islem")()
fmt.Println("çalışıyor")
}
func main() {
islem()
}Expected output
çalışıyor islem bitti
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How is the trace ID carried from one service to another?
- By writing it to the database
- Through the log file
- It isn't; each service creates a new one
- In a request header (e.g. traceparent) Correct answer
Explanation: The W3C Trace Context standard defines the traceparent header; the receiving service finds the parent span from it and opens a child span.
2 End the span
ctx, span := tracer.Start(ctx, "kaydet")
____ span.End()ctx, span := tracer.Start(ctx, "kaydet")
defer span.End()Explanation: defer span.End() closes the span's duration when the function ends.
3 Tracing concepts
| Term | Meaning |
|---|---|
Trace |
A request's whole journey |
Span |
A single unit of work |
traceparent |
Header between services |
ctx |
Carries the span through functions |