ctxdata

A helper for collecting and emitting metadata throughout a request lifecycle.

When a new request arrives in your program, HTTP server, etc., create a new Data value and inject it into the request context via ctxdata.New.

import "github.com/peterbourgon/ctxdata/v4"

func handler(w http.ResponseWriter, r *http.Request) {
    ctx, d := ctxdata.New(r.Context())

Use the returned context for all further operations on that request.

    otherHandler(w, r.WithContext(ctx))
    result, err := process(ctx, r.Header.Get("X-My-Key"))
    // etc.

Once the Data has been created and injected to the context, use ctxdata.From from any downstream function with access to the context to fetch it and add more metadata about the request.

func otherHandler(w http.ResponseWriter, r *http.Request) {
    d := ctxdata.From(r.Context())
    d.Set("user", r.URL.Query().Get("user"))
    d.Set("corrleation_id", r.Header.Get("X-Correlation-ID"))
    // ...
}

func process(ctx context.Context, key string) {
    begin := time.Now()
    // ...
    ctxdata.From(ctx).Set("process_duration", time.Since(begin))
}

At the end of the request, all of the metadata collected throughout the request's lifecycle will be available.

    fmt.Fprintln(w, "OK")

    for _, kv := range d.GetAllSlice() {
        log.Printf("%s: %s", kv.Key, kv.Val)
    }
}

Here is an example middleware that writes a so-called wide event in JSON to the dst at the end of each request.

func logMiddleware(next http.Handler, dst io.Writer) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        ctx, d := ctxdata.New(r.Context())
        defer func() { json.NewEncoder(dst).Encode(d.GetAllMap()) }()
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}