Pass a cancel signal through every call chain

How to do it

When a visitor closes the tab or a deadline passes, the server keeps querying and calling other services for an answer nobody reads. Under load that wasted work is what tips a service over. Use the language's own signal. That is AbortSignal in JavaScript, context.Context in Go and CancellationToken in C#. Python's asyncio cancels the task itself. Join a request's signal with a deadline, such as AbortSignal.any with AbortSignal.timeout in JavaScript. Take the deadline from Set timeouts on every call. Close what the work opened, as Release every resource you open says.

Examples

TypeScript example

app.get('/reports/:id', async (req, res) => {
  const controller = new AbortController();
  req.on('close', () => controller.abort());
  const signal = AbortSignal.any([controller.signal, AbortSignal.timeout(10_000)]);
  const report = await buildReport(req.params.id, { signal });
  res.json(report);
});

async function buildReport(id: string, { signal }: { signal: AbortSignal }) {
  const rows = await db.query(REPORT_SQL, [id], { signal });
  const rates = await fetch(`${RATES_URL}?date=${rows.date}`, { signal });
  return summarise(rows, await rates.json());
}

Go example

func (h *Handler) Report(w http.ResponseWriter, r *http.Request) {
	ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
	defer cancel()
	report, err := h.buildReport(ctx, r.PathValue("id"))
	if err != nil {
		http.Error(w, "report failed", http.StatusServiceUnavailable)
		return
	}
	json.NewEncoder(w).Encode(report)
}

func (h *Handler) buildReport(ctx context.Context, id string) (Report, error) {
	rows, err := h.db.QueryContext(ctx, reportSQL, id)
	if err != nil {
		return Report{}, err
	}
	defer rows.Close()
	return summarise(ctx, rows)
}

How to check

A test cancels a request while its first downstream call is in flight. No later downstream call or query starts. In Go, the contextcheck linter reports no function that drops its caller's context.

Sources

• DOM Standard on using AbortSignal in APIs
• .NET on cancellation in managed threads

Related rules

• API-35 Set timeouts on every call in API design
• ARC-32 Release every resource you open in Clean code and architecture