Tracing

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 ms

This 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.

Exercise 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")().

Show example 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()
}

Test yourself

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)
Show answer

In a request header (e.g. traceparent) — The W3C Trace Context standard defines the traceparent header; the receiving service finds the parent span from it and opens a child span.

End the span

ctx, span := tracer.Start(ctx, "kaydet")
____ span.End()

Show answer

ctx, span := tracer.Start(ctx, "kaydet")
defer span.End()

Tracing concepts

Trace

A request's whole journey
Span

A single unit of work
traceparent

Header between services
ctx

Carries the span through functions

Next lesson: OpenTelemetry

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