#Locale

Examples use Indian conventions: ₹ amounts, IST, dd/mm/yyyy, Aadhaar and DPDP Act where a standard mentions identity or privacy law. Keep them when you copy an example.

#Purpose

Rules for throttling requests to protect capacity and to slow abuse. Rate limiting is a control on volume, not a substitute for authentication or authorisation.

Get the key right before the algorithm. Limiting the wrong dimension is the most common failure: per-IP alone does not stop distributed credential stuffing against one account, and per-account alone lets one host spray many accounts.

#Choosing the key

Key	Protects against	Weakness
API key or user id	Per-tenant fairness, quota	Requires authentication first
Account identifier	Credential stuffing on one account	Attacker rotates accounts
Client IP	Single noisy source	NAT and CGNAT share IPs; IPv6 rotates cheaply
IP + account	Login endpoints	—
Endpoint cost class	Expensive operations	Needs per-route configuration

For login, limit per account and per IP independently — a request is rejected if either budget is exhausted.

Never trust X-Forwarded-For blindly. It is client-settable; an attacker prepends a fake address and evades every per-IP limit. Configure trust proxy to the exact number of proxies you run and take the correct position from the right.

jsapp.set("trust proxy", 1);      // exactly one proxy in front — not `true`

Never key on User-Agent, a cookie the client controls, or a request body field. All are attacker-chosen.

#Algorithms

Algorithm	Behaviour	Use
Token bucket	Steady refill, allows bursts up to capacity	Default — matches real traffic
Sliding window log	Exact; stores each timestamp	Low volume, strict accuracy
Sliding window counter	Approximate, cheap	High volume
Fixed window	Simple counter per interval	Avoid — see below
Leaky bucket	Smooths output to a constant rate	Queue-shaped workloads

Avoid fixed windows. They permit a double burst at the boundary: a client spends the whole budget at 00:59 and the whole next budget at 01:01, sending two allowances within two seconds.

js// Token bucket in Redis — atomic, so concurrent requests cannot both pass.
const ALLOW = `
local key, rate, burst, now = KEYS[1], tonumber(ARGV[1]), tonumber(ARGV[2]), tonumber(ARGV[3])
local b = redis.call('HMGET', key, 'tokens', 'ts')
local tokens = tonumber(b[1]) or burst
local ts = tonumber(b[2]) or now
tokens = math.min(burst, tokens + (now - ts) * rate)
if tokens < 1 then
  redis.call('HMSET', key, 'tokens', tokens, 'ts', now)
  return 0
end
redis.call('HMSET', key, 'tokens', tokens - 1, 'ts', now)
redis.call('EXPIRE', key, math.ceil(burst / rate) * 2)
return 1
`;
const allowed = await redis.eval(ALLOW, 1, `rl:${key}`, rate, burst, Date.now() / 1000);

The Lua script matters: read-then-write from application code is a race, and under concurrency more requests pass than the limit permits.

#Distributed state

An in-memory counter per process means the real limit is limit × instances, and it resets on every deploy. Acceptable for a single instance; wrong for anything scaled.
Use a shared store — Redis, or the platform's own limiter at the edge.
Prefer limiting at the edge (CDN, API gateway) for volumetric abuse: it never reaches your origin. Keep application-level limits for per-account and per-endpoint rules the edge cannot see.
Fail open or closed deliberately. If Redis is down, decide in advance whether to allow (availability) or deny (protection), and log the decision. Silently allowing because an exception was swallowed is the common accident.

#Responding

makefileHTTP/1.1 429 Too Many Requests
RateLimit-Limit: 100
RateLimit-Remaining: 0
RateLimit-Reset: 30
Retry-After: 30

Return 429, not 403. 403 tells a client it is forbidden forever.
Always send Retry-After. Without it, well-behaved clients retry immediately and make the situation worse.
Expose remaining budget so clients can self-pace.
Apply jitter to any server-suggested backoff, or every throttled client returns simultaneously.
Never leak whether an account exists through differing limits — see Security/authentication.

#Tuning

Measure real traffic before setting a limit. A limit below the p99 of legitimate use is an outage you scheduled for yourself.
Set different limits per endpoint class: a search or export endpoint costs orders of magnitude more than a health check.
Run in observe-only first, logging what would have been rejected.
Exempt health checks and internal service traffic explicitly, by credential — never by IP range alone.
Alert on sustained 429 rates. A spike is either an attack or a broken client, and both are worth knowing about.

#Anti-patterns

Anti-pattern	Why it fails	Fix
Trusting `X-Forwarded-For`	Client-settable; trivially spoofed	Configure `trust proxy` precisely
In-memory counters across instances	Real limit is `limit × instances`	Shared store
Read-then-write without atomicity	Concurrent requests overshoot	Lua script or atomic primitive
Fixed window	Double burst at the boundary	Token bucket
Per-IP only on login	Distributed stuffing passes	Also limit per account
`403` instead of `429`	Reads as permanent denial	`429` with `Retry-After`
No `Retry-After`	Clients retry immediately	Always send it
Fail-open by swallowed exception	Limit silently disappears	Decide and log the mode
Limit set without measuring	Blocks legitimate users	Observe first
Permanent lockout on failures	Self-inflicted denial of service	Exponential backoff

#Checklist

Verify: Limits are keyed on account and IP independently for authentication routes
Verify: trust proxy is set to the exact proxy count; X-Forwarded-For is not trusted raw
Verify: Algorithm is token bucket or sliding window, never fixed window
Verify: Counter updates are atomic under concurrency
Verify: State is shared across instances, not per-process memory
Verify: Store-unavailable behaviour is a deliberate, logged decision
Verify: Responses return 429 with Retry-After and remaining budget
Verify: Backoff guidance includes jitter
Verify: Limits differ by endpoint cost class
Verify: Limits were derived from measured traffic and trialled in observe-only mode
Verify: Health checks and internal traffic are exempted by credential
Verify: Sustained 429 rates raise an alert

Rate Limiting