Customizing responses

By default a shed request gets a plain-text 503 with the status text as its body. Options on brownout.New change the response, count shed requests, or let you make the decision yourself.

c := brownout.New(
    brownout.WithDefaultStatus(http.StatusTooManyRequests),
    brownout.WithOnShed(func(r *http.Request, rule brownout.Rule) {
        shedCounter.WithLabelValues(rule.ID).Inc()
    }),
    brownout.WithResponder(func(w http.ResponseWriter, r *http.Request, rule brownout.Rule) {
        w.Header().Set("Content-Type", "application/json")
        w.WriteHeader(rule.StatusCode)
        json.NewEncoder(w).Encode(map[string]string{"error": "temporarily unavailable"})
    }),
)

Default status

WithDefaultStatus(code) sets the status for rules that don't set status_code. A rule's own status_code still wins.

Your own response

WithResponder replaces the response for shed requests. The rule it receives always has StatusCode filled in, with the rule's own value or the default. The metrics example returns JSON error bodies this way.

Counting shed requests

WithOnShed is called for every shed request, before the response is written. Use it for metrics or logs. It runs on the request path, so keep it fast.

Degrade instead of reject

c.Check(r) returns the matching rule and whether the request should be shed, without writing anything. Call it in your handler to serve a cheaper response instead of an error:

if _, shed := c.Check(r); shed {
    renderWithoutRecommendations(w, r) // skip the expensive call
    return
}

The degrade example does this. Requests checked with Check don't need to pass through Middleware.

Testing with a fixed clock

WithClock(func() time.Time) replaces the clock used for expiry and TTLs, so tests can move time forward without sleeping.