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/**
* In-process fixed-window per-IP rate limiter (research.md R8). Deliberately
* not a persisted guarantee: state is a plain `Map`, per Functions instance
* — Functions can scale out, so this is friction against a single hot
* source, not a hard control. The real backstops are per-user monthly caps
* (meteringService) and the deployment TPM quota. Applied to analyze and
* billing routes (contracts/metering.md).
*/
const WINDOW_MS = 60_000;
const DEFAULT_ANALYZE_PER_MIN = 30;
const DEFAULT_BILLING_PER_MIN = 10;
interface WindowState {
windowStart: number;
count: number;
}
const buckets = new Map<string, WindowState>();
/** Drops all tracked windows so tests can start from a clean slate. */
export function resetRateLimiterForTests(): void {
buckets.clear();
}
/**
* Fixed-window check-and-increment for an arbitrary key (contracts/metering.md
* defaults: research R8). Returns true when the request is allowed.
*/
export function checkRateLimit(
key: string,
limit: number,
windowMs: number = WINDOW_MS,
now: number = Date.now()
): boolean {
const state = buckets.get(key);
if (!state || now - state.windowStart >= windowMs) {
buckets.set(key, { windowStart: now, count: 1 });
return true;
}
if (state.count >= limit) return false;
state.count++;
return true;
}
/** First address in `x-forwarded-for` (the client, per Azure Functions' proxy chain). */
export function extractClientIp(request: HttpRequest): string {
const header = request.headers.get("x-forwarded-for");
if (!header) return "unknown";
return header.split(",")[0].trim();
}
export type RouteKey = "analyze" | "billing";
function routeLimit(routeKey: RouteKey): number {
const envVar =
routeKey === "analyze" ? "RATE_LIMIT_ANALYZE_PER_MIN" : "RATE_LIMIT_BILLING_PER_MIN";
const parsed = Number(process.env[envVar]);
if (Number.isFinite(parsed) && parsed > 0) return parsed;
return routeKey === "analyze" ? DEFAULT_ANALYZE_PER_MIN : DEFAULT_BILLING_PER_MIN;
}
export type PlainHandler = (
request: HttpRequest,
context: InvocationContext
) => Promise<HttpResponseInit>;
/**
* Wraps a whole route (before withAuth) so unauthenticated bursts are also
* throttled. 429 `RATE_LIMITED` — distinct from metering's
* `USAGE_LIMIT_REACHED`, both are 429s but clients branch on `error.code`
* (contracts/metering.md).
*/
export function withRateLimit(routeKey: RouteKey, handler: PlainHandler): PlainHandler {
return async (request, context) => {
if (request.method === "OPTIONS") return handler(request, context);
const limit = routeLimit(routeKey);
const ip = extractClientIp(request);
if (!checkRateLimit(`${routeKey}:${ip}`, limit)) {
return {
status: 429,
headers: {
"Content-Type": "application/json",
"Access-Control-Allow-Origin": "*",
"Access-Control-Allow-Headers": "Content-Type, x-functions-key, Authorization",
},
jsonBody: {
error: {
code: "RATE_LIMITED",
message: "Too many requests. Please wait a moment and try again.",
},
},
};
}
return handler(request, context);
};
}
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