LLD-013 · LOW LEVEL DESIGN

Rate Limiter

Exact time accounting, atomic admission, safe cleanup and versioned policy changes.

Local Java 21 reference verified: 547 checks. Learner drafts are not executed. Author self-review; independent review pending.

Existing introductory lesson · Version 1

Understand the problem

Build a local admission controller that allows a short burst while enforcing a refill budget over time. It must atomically refill and consume tokens for concurrent callers, preserve fractional credit, bound tracked-key memory and change policy without granting a fresh budget to existing callers.

Before you start

Concurrency and correctness
Hash maps and numeric bounds
Monotonic time vs wall clock

Scope and explicit decisions

One Limiter instance in one JVM. Every key shares the configured capacity/rate policy; budgets are independent per key. It is not a global distributed quota.
capacity, tokensPerSecond and request cost are positive longs. Cost above capacity is invalid because the request can never be admitted; it does not receive a misleading finite retry time.
The trusted time adapter supplies nonnegative elapsed nanoseconds from one origin, monotonic across all calls. Tests pass time directly. A runtime adapter must sample time inside its serialized admission boundary to avoid stale sampled timestamps arriving out of order.
One token equals 1,000,000,000 fixed-point units. Integer tokensPerSecond times elapsed nanoseconds yields exact refill units. BigInteger protects intermediate products from long overflow.
The first request for a new key starts full. Existing-key rejection still stores accrued fractional credit and last observed time. wholeTokens floors the remaining fraction only in the response.
retryAfterNanos is a ceiling estimate under unchanged policy and no competing consumers; it is not a reservation or promise of future admission. Values beyond long range saturate to Long.MAX_VALUE.
At maxKeys, a new key is rejected with an explicit capacity error. Existing keys continue to work. Cleanup removes only fully refilled buckets, whose recreation is equivalent to retained state.
Increasing policy capacity never tops up existing buckets. At cutover refill under the old rate, clamp to the new capacity, then use the new rate. New keys after cutover start at the new full capacity.

Worked examples

Input: capacity=10, rate=2/sec. At t=0 request cost 10, then cost 1.

Outcome: First allowed with zero tokens. Second rejected with retryAfterNanos=500000000.

The first request spends the burst; refill restores one token every half-second.

Input: Empty bucket, old rate=2/sec; change to rate=4/sec, capacity=20 at t=0.5sec.

Outcome: Existing key has one token at cutover, not two or twenty. At t=0.75sec it earns one more under the new rate.

Historical time is evaluated using the old policy and capacity increase is not a budget grant.

Try before revealing

Hint 1

Separate the maximum burst from the rate at which credit returns.

Hint 2

Write the balance equation with units. What survives a rejected request at half a token?

Hint 3

Put refill, check, spend and save under one owner, including cleanup and policy changes.

Requirements and invariants

R1 — A request is admitted only if the refilled budget covers its entire cost. allow protects I1.

R2 — Concurrent requests share one atomic refill-and-consume decision. allow protects I2.

R3 — Key count stays at or below maxKeys without resetting depleted budgets. evictFull protects I3.

R4 — Policy change is version-guarded and preserves accumulated credit. replacePolicy protects I4.

I1: For every bucket, 0 <= fixed-point units <= capacity × 1,000,000,000.

I2: Each accepted cost is deducted once from the same synchronized state used by all contenders.

I3: The map contains at most maxKeys; eviction grants no extra tokens compared with keeping a full bucket.

I4: At policy cutover, old policy earns all pre-cutover credit; existing budgets are clamped, never replenished merely by increasing capacity.

Derive the design

Fixed window: correct for its contract, insufficient for smooth refill

A fixed-window limiter counts requests in each one-second interval and resets at the next boundary. It is compact and predictable, but ten requests at 999,999,999 ns followed by ten at 1,000,000,000 ns all pass for capacity ten. That is not a coding defect: the window contract permits it. If the requirement is a burst budget with gradual recovery, derive token bucket rather than pretending fixed windows satisfy it.

Alternative: A sliding log records timestamps and gives an exact rolling-window count but uses memory proportional to accepted requests in the window and cleanup work. A sliding counter approximates that count. A leaky-bucket queue smooths output by delaying work; this token bucket rejects immediately rather than queuing.

Turn requirements into quantities

Ask whether users require an exact rolling-window count, a sustained rate with bursts, or delayed smooth output. Those are different contracts. Here capacity describes burst, rate describes recovery, and cost can represent weighted work.

Preserve fractions before optimizing arithmetic

With two tokens per second, a quarter second is half a token. Store tokens in billionths so nanosecond elapsed time times an integer rate has an exact representation. Floor only the response display, never the internal balance.

Choose the atomic boundary before the data structure

The operation is refill-and-consume, not get and put. The synchronized method protects both the bucket and its policy/time context. Use expectedVersion to prevent a stale administrator from overwriting a newer policy.

Make resource exhaustion an explicit API result

Rate-limited is not the same as identity-store full. This example throws a distinct capacity error for a new key. Production should map it to a deliberate overload response and measure saturation; do not silently grant untracked requests.

Only reclaim equivalent state

A full retained bucket and a freshly created bucket start with the same credit. A depleted retained bucket and a new bucket do not. That equivalence is the proof for evictFull, and the shared monitor prevents a concurrent consumption between the check and deletion.

Responsibilities and ownership

Limiter

Owns: Bucket map, policy version, observed time, key limit

Methods: allow, evictFull, replacePolicy

Single monitor is the authoritative local admission owner.

Policy

Owns: Version, capacity and integer refill rate

Methods: Validated immutable record

No mutable configuration read midway through a request.

Bucket

Owns: Exact fixed-point balance and last update time

Methods: Private immutable record

Never exposed for direct caller mutation.

Decision

Owns: Allowed result, floored remaining tokens, ceiling retry estimate

Methods: Immutable accessors

An estimate is not an allocation of future capacity.

FixedWindow

Owns: One-key window index and used count

Methods: allow(now)

Correct comparison baseline with a different burst guarantee.

Admission ownership

Cleanup and policy replacement use the same owner. Network calls are outside this local contract.

Trusted key / time adapter
↓ Validate identity, cost and elapsed time
Limiter monitor
↓ Refill → compare → consume → save
Bucket map + policy
↓ Return allowed / remaining / retry estimate
Immutable Decision

Atomic budget decision

These are alternative outcomes from the comparison, not sequential steps. Both store fractional state; the table below provides the exact execution order.

Refilled bounded credit

Compare against cost × UNIT

Enough: deduct full cost

Return rejection with ceiling retry estimate

Insufficient: retain credit

Data model, schema and relationships

A policy defines capacity/refill meaning; a bucket owns token balance and elapsed-time state for one scoped key. A decision is an observation, not another independent mutable balance.

Implementation boundary: Token bucket state lives in a local map with policy-version changes. Persistence design would need one authoritative atomic update across processes; a cache replica is not sufficient.

These are proposed logical schemas with field contracts, not executable database migrations. PK identifies a row, FK references an existing parent, UK is a uniqueness rule. A composite PK identifies the combination. Nullable references are marked with ?. A foreign key protects identity; cross-row limits, timing and state transitions also require the transaction rules below.

limit_policy: relationship map

Connectors name the relationship; multiplicities state parents per child and children per parent. 0..1 means optional, 1 required, 0..* any number. Self-links show two roles of the same entity.

limit_subject: relationship map

Connectors name the relationship; multiplicities state parents per child and children per parent. 0..1 means optional, 1 required, 0..* any number. Self-links show two roles of the same entity.

bucket: relationship map

Connectors name the relationship; multiplicities state parents per child and children per parent. 0..1 means optional, 1 required, 0..* any number. Self-links show two roles of the same entity.

Entity definitions and field contracts

limit_policy

Immutable policy interpretation.

Field	Type / key	Meaning and reference
`id`	text · PK	Policy/version identity
`capacity`	bigint · VALUE	Maximum burst tokens
`refill`	bigint · VALUE	Declared rate units

Code mapping: Policy record.

Positive bounded rate/capacity and explicit policy migration semantics.

limit_subject

Quota scope.

Field	Type / key	Meaning and reference
`id`	text · PK	Tenant/user/route compound identity
`scope`	text · VALUE	Declared isolation boundary

Code mapping: Bucket key input.

Normalize key and bound distinct-key admission/memory.

bucket

Current refill/consumption owner.

Field	Type / key	Meaning and reference
`id`	uuid · PK	Bucket identity
`policy_id`	text · FK	Pinned policy · limit_policy.id
`subject_id`	text · FK	Quota subject · limit_subject.id
`tokens`	bigint · VALUE	Available fixed-point tokens
`last_tick`	bigint · VALUE	Trusted elapsed time

Code mapping: Mutable local bucket.

Unique(subject_id,policy_id) under explicit migration; 0<=tokens<=capacity.

limit_decision

Optional audit sample.

Field	Type / key	Meaning and reference
`id`	uuid · PK	Decision identity
`bucket_id`	uuid · FK	Authority used · bucket.id
`allowed`	boolean · VALUE	Decision result

Code mapping: Optional observability extension.

Audit sampling must not become an unbounded synchronous hot path.

Relationship cardinalities and their reasons

Parent → child	Multiplicity	FK / why
limit_policy → bucket configures	1 parent per child; 0..* children per parent	`bucket.policy_id → limit_policy.id` Bucket values must be interpreted under their policy version.
limit_subject → bucket owns quota	1 parent per child; 0..* children per parent	`bucket.subject_id → limit_subject.id` A subject may use distinct policies; migration cannot create accidental extra burst.
bucket → limit_decision produces decisions	1 parent per child; 0..* children per parent	`limit_decision.bucket_id → bucket.id` Audits observe decisions and must not mutate token truth.

Transaction boundaries

Refill/clamp, compare requested cost and deduct under one bucket owner.
Policy change must transform/reset/clamp state under a documented rule, atomically with active policy identity.

Indexes and uniqueness

Unique bucket scoped key for direct lookup; expire idle buckets only under a safe lifecycle contract.
Avoid writing every decision to a globally contended index; sample or aggregate asynchronous metrics.

Schema traps and failure cases

Per-process buckets permit aggregate traffic above a supposed global quota.
Wall-clock rollback or overflow in refill arithmetic must not mint tokens.

Trace the records through one operation

Capacity 10 admits a burst of 10 under policy v1.
Elapsed refill restores only up to capacity.
Concurrent last-token callers serialize; one succeeds and the other is denied.

Method contracts

`allow(key, cost, elapsedNanos) → Decision`

Reject invalid input or time reversal before mutation. Refill, compare, consume and save while synchronized. Key-capacity failure is distinct from ordinary rate rejection.

`evictFull(elapsedNanos) → removedCount`

Scan K retained keys and remove only those whose refilled balance equals capacity at this instant. An exhausted bucket cannot be dropped to reset its budget.

`replacePolicy(next, expectedVersion, elapsedNanos)`

Reject stale expectedVersion or non-increasing next.version. Materialize old-policy refill into a temporary map, clamp balances to new capacity, then atomically replace configuration and state.

`keyCount() → int`

Synchronized observation of retained buckets; cannot exceed configured maxKeys.

What, when, why and how

Each concept is attached to the requirement it protects, its implementation, an alternative and a test.

Token bucket

what: A bounded budget grows with elapsed time and is spent by accepted requests.
when: The product permits a burst but needs a sustained refill rate.
why: Capacity and rate control different behavior: ten immediately, then two per second.
how: Refill units=min(capacity×UNIT,oldUnits+elapsedNanos×tokensPerSecond). Consume cost×UNIT only if sufficient. BigInteger keeps arithmetic exact for long-sized parameters.

Code: refill, allow · Invariants: I1

Alternatives, cost and failure example

Alternative: Fixed windows are simpler when boundary bursts are acceptable; sliding logs suit exact rolling-window rules.

Cost: BigInteger allocation costs more than bounded fixed-width arithmetic. Operand sizes here are bounded by long inputs/products, but performance must still be measured.

Counterexample: Round each refill to whole tokens: repeated calls at quarter-second intervals discard credit and may starve a caller.

Test: At rate two, reject at 250ms with half-token retained; accept at 500ms.

Monotonic time and injectable clock

what: Measure elapsed duration using a source that cannot move backwards, supplied as a dependency.
when: Refill or expiry depends on time and tests must avoid sleeping.
why: Wall-clock corrections can create negative refill or an unintended fresh burst.
how: This API accepts trusted elapsed nanoseconds and rejects values older than the last observed service time. A real adapter uses a monotonic origin and samples within the serialized boundary.

Code: validateTime, refill · Invariants: I1

Alternatives, cost and failure example

Alternative: Inject a LongSupplier sampled inside allow instead of accepting now; this reduces caller misuse. A shared store must define its own time authority.

Cost: Explicit time makes tests deterministic but adds a contract obligation. Raw System.nanoTime values may be negative; normalize against an origin and handle long-running wrap according to a declared horizon.

Counterexample: Two threads sample 100 and 101; the 101 call arrives first. The older 100 call is rejected instead of silently earning negative credit.

Test: Call with a smaller time and assert error; use exactly 499999999 and 500000000 ns boundary inputs.

Linearizable compound admission

what: Each concurrent call appears to make one indivisible budget decision.
when: Several workers call allow for the same key.
why: A safe map cannot stop two callers from spending the same last token after separate reads.
how: The monitor covers lookup, refill, comparison, deduction and write. It also serializes cleanup and policy replacement.

Code: allow, replacePolicy · Invariants: I2, I4

Alternatives, cost and failure example

Alternative: Per-key locks can improve parallelism, but shared policy changes and key deletion require coordinated generations/lock ownership.

Cost: Different keys serialize in this reference. This is deliberately simple; sharding is justified by measured contention, not aesthetics.

Counterexample: Read one token in two threads, then both write zero: two requests passed using one token.

Test: Release twenty concurrent unit requests at t=0 with capacity ten; exactly ten must succeed.

Bounded state and semantics-preserving eviction

what: Limit retained identities without resetting live budget.
when: A public endpoint can see many distinct keys.
why: Unbounded maps exhaust memory; evicting a depleted key grants a new burst on recreation.
how: Reject new keys when full. evictFull refills for comparison and removes only full buckets. A recreated full bucket has the same budget at eviction time.

Code: evictFull, keyCount · Invariants: I3

Alternatives, cost and failure example

Alternative: TTL cleanup is safe only when TTL guarantees full refill under the relevant policy and no concurrent replacement changes that conclusion.

Cost: Cleanup is O(K) under the monitor and can pause admission. Schedule bounded sweeps in a larger implementation; preserve atomic equivalence.

Counterexample: Evict a zero-token key merely because it is least recently used; the next request creates it with capacity ten.

Test: At t=0.5 depleted keys remain; at t=5.5 they are full and may be safely removed.

Versioned policy cutover

what: Change configuration with a guard and a declared credit-transfer rule.
when: Operations change limits while clients are active.
why: Applying the new refill rate retroactively or topping up capacity gifts unintended tokens.
how: Refill each bucket with OLD rate to the cutover time. Clamp to NEW capacity. Require expectedVersion and next version greater than current, then publish all state atomically.

Code: replacePolicy · Invariants: I4

Alternatives, cost and failure example

Alternative: Resetting all budgets is simpler only if the product explicitly accepts a fresh burst; it must not be hidden behind a generic config update.

Cost: O(K) cutover and temporary O(K) map. A lazy migration needs per-bucket policy versions and more complex accounting.

Counterexample: Changing rate two→four at 0.5s incorrectly credits two tokens instead of the one earned under the old rate.

Test: Test old-rate earning, no top-up on capacity increase, clamp on decrease and stale-version rejection.

Read and run the Java reference

Reveal complete Solution.java

Java 21. Save as Solution.java; compile with javac --release 21. The repository also contains Test.java and the lld:java verification command. This reference implements the local contract above.

Solution.java Java 21

1import java.math.BigInteger;
2import java.util.*;
3
4/** Bounded local token bucket; time is supplied by a trusted monotonic-clock adapter. */
5public final class Solution {
6    private Solution() {}
7    private static final BigInteger UNIT = BigInteger.valueOf(1_000_000_000L);
8    public record Policy(long version, long capacity, long tokensPerSecond) {
9        public Policy {
10            if (version < 1 || capacity < 1 || tokensPerSecond < 1) throw new IllegalArgumentException("policy");
11        }
12    }
13    public record Decision(boolean allowed, long wholeTokens, long retryAfterNanos) {}
14    private record Bucket(BigInteger units, long lastNanos) {}
15    public static final class Limiter {
16        private final Map<String, Bucket> buckets = new HashMap<>();
17        private final int maxKeys;
18        private Policy policy;
19        private long observedTime;
20        public Limiter(Policy policy, int maxKeys) {
21            this.policy = Objects.requireNonNull(policy);
22            if (maxKeys < 1) throw new IllegalArgumentException("maxKeys");
23            this.maxKeys = maxKeys;
24        }
25        private void validateTime(long now) {
26            if (now < observedTime) throw new IllegalArgumentException("non-monotonic time");
27        }
28        private BigInteger ceiling() { return UNIT.multiply(BigInteger.valueOf(policy.capacity())); }
29        private Bucket refill(Bucket b, long now) {
30            BigInteger elapsed = BigInteger.valueOf(now).subtract(BigInteger.valueOf(b.lastNanos()));
31            BigInteger units = b.units().add(elapsed.multiply(BigInteger.valueOf(policy.tokensPerSecond())));
32            return new Bucket(units.min(ceiling()), now);
33        }
34        public synchronized Decision allow(String key, long cost, long now) {
35            if (key == null || key.isBlank() || cost < 1 || cost > policy.capacity())
36                throw new IllegalArgumentException("key or impossible cost");
37            validateTime(now);
38            Bucket b = buckets.get(key);
39            if (b == null) {
40                if (buckets.size() == maxKeys) throw new IllegalStateException("key capacity reached");
41                b = new Bucket(ceiling(), now);
42            } else b = refill(b, now);
43            BigInteger need = UNIT.multiply(BigInteger.valueOf(cost));
44            boolean allowed = b.units().compareTo(need) >= 0;
45            BigInteger after = allowed ? b.units().subtract(need) : b.units();
46            long retry = 0;
47            if (!allowed) {
48                BigInteger rate = BigInteger.valueOf(policy.tokensPerSecond());
49                BigInteger wait = need.subtract(after).add(rate).subtract(BigInteger.ONE).divide(rate);
50                retry = wait.min(BigInteger.valueOf(Long.MAX_VALUE)).longValueExact();
51            }
52            buckets.put(key, new Bucket(after, now)); observedTime = now;
53            return new Decision(allowed, after.divide(UNIT).longValueExact(), retry);
54        }
55        // Evict only full buckets: recreation grants exactly the budget already available.
56        public synchronized int evictFull(long now) {
57            validateTime(now); int before = buckets.size();
58            buckets.entrySet().removeIf(e -> refill(e.getValue(), now).units().equals(ceiling()));
59            observedTime = now; return before - buckets.size();
60        }
61        // Refill under OLD rate up to cutover, then clamp under NEW capacity. Never grant free tokens.
62        public synchronized void replacePolicy(Policy next, long expectedVersion, long now) {
63            Objects.requireNonNull(next); validateTime(now);
64            if (expectedVersion != policy.version() || next.version() <= policy.version())
65                throw new IllegalStateException("stale policy version");
66            Map<String, Bucket> updated = new HashMap<>();
67            BigInteger newCeiling = UNIT.multiply(BigInteger.valueOf(next.capacity()));
68            for (var e : buckets.entrySet()) {
69                Bucket b = refill(e.getValue(), now);
70                updated.put(e.getKey(), new Bucket(b.units().min(newCeiling), now));
71            }
72            buckets.clear(); buckets.putAll(updated); policy = next; observedTime = now;
73        }
74        public synchronized int keyCount() { return buckets.size(); }
75    }
76    /** Correct fixed-window baseline, with its deliberately different burst contract. */
77    public static final class FixedWindow {
78        private final long capacity;
79        private long window = -1, used, observed;
80        public FixedWindow(long capacity) {
81            if (capacity < 1) throw new IllegalArgumentException("capacity"); this.capacity = capacity;
82        }
83        public synchronized boolean allow(long now) {
84            if (now < observed) throw new IllegalArgumentException("time");
85            long nextWindow = now / 1_000_000_000L;
86            if (nextWindow != window) { window = nextWindow; used = 0; }
87            observed = now;
88            if (used == capacity) return false;
89            used++; return true;
90        }
91    }
92}
93

Read only · 93 linesJava 21 code

refill: exact units and clamp

elapsed is a BigInteger subtraction after monotonic validation. Multiplying by integer tokensPerSecond produces fixed-point units. Clamp to ceiling so a long idle period cannot bank unlimited burst credit.

allow: one transaction over budget

Validate cost and key before reading time. New keys check capacity. Compute need, choose allowed, deduct only on success and save fractional units on either branch. retryAfter uses ceiling division and saturates rather than overflowing.

evictFull: equivalence, not arbitrary LRU

Recompute potential balance at now for each entry. A full result may be removed because recreation is equivalent. Depleted entries stay even if they are old; no unbounded new-key admission is implied.

replacePolicy: build before publish

Validate versions, refill using the still-current old policy into a temporary map and clamp each balance. Publish map, policy and observedTime together under the monitor. Failures before replacement leave the original map authoritative.

Complexity and limits

allow performs expected O(1) map access plus exact arithmetic on bounded-size numbers derived from long inputs. General BigInteger complexity depends on bit length; this is not a universal constant-time cryptographic primitive.
Space is O(K) with K<=maxKeys. Input key strings also consume memory; a production key resolver must bound key length and normalize authenticated tenant/user identity.
evictFull and replacePolicy are O(K); cutover additionally uses O(K) temporary state. Both hold the monitor, so large K can produce admission latency spikes.
FixedWindow is O(1) time/space for one key; SlidingLog uses O(A) memory for A retained accepted timestamps, with amortized expiry and worst-case cleanup bursts.
A single JVM monitor guarantees local serialization, not fairness, lock-wait bounds, durability or global distributed quota.

Walk through the actual reference

These walkthrough fixtures are emitted only after the Java harness verifies their operations and expected state. Concurrency tests additionally exercise competing callers.

Burst, rejection and fractional refill

Step 1 of 4: At 0 ns consume 10

Allowed; 0 tokens remain

Invariant: Budget never negative

Read the complete trace table

Action	State after action	Invariant
At 0 ns consume 10	Allowed; 0 tokens remain	Budget never negative
At 0 ns consume 1	Rejected; retry after 500000000 ns	No token spent on rejection
At 250000000 ns consume 1	Rejected; half a token accumulated	Fractional credit retained exactly
At 500000000 ns consume 1	Allowed; 0 tokens remain	Refill and consumption form one atomic decision

Mistakes and counterexamples

Reset the balance every second and call it token bucket

Failing case: Ten requests just before and ten just after a window boundary all pass.

Repair: Name this fixed-window behavior; use elapsed refill if burst recovery must be gradual.

Drop fractional refill on each request

Failing case: At rate two/sec, checking every 250ms always rounds 0.5 to zero.

Repair: Retain exact units; round only wholeTokens in the response.

Evict any idle/LRU key under memory pressure

Failing case: An exhausted key is removed and returns immediately to a full bucket.

Repair: Only evict equivalent full state; otherwise reject new keys or use another bounded ownership scheme.

Multiply long capacity by a billion without checks

Failing case: A large valid long capacity overflows negative during fixed-point conversion.

Repair: Use checked bounded arithmetic or BigInteger; test Long.MAX_VALUE parameters.

Treat retryAfter as guaranteed admission

Failing case: Another caller spends the token first, or a policy cutover changes recovery.

Repair: It is an estimate; retry must perform another atomic decision.

Engineer → Senior → Staff

Choose a deep diveEngineerSenior EngineerStaff Engineer

Implement correct fractional refill and explicit input behavior.

Executed: token bucket and fixed-window comparison, weighted costs, exact boundaries and overflow-safe arithmetic.

Change exercise

With capacity one and rate three/sec, calculate the first nanosecond at which a request can pass.

Worked answer guidance

Ceiling(1e9/3)=333333334ns. The preceding nanosecond is insufficient. Keep fractional credit so repeated checks do not destroy progress.

Interview follow-ups

Why does capacity differ from rate?

Capacity permits a short burst; rate controls refill. A capacity of ten and rate two admits ten immediately, but not ten every second.

Would you always use BigInteger?

No. If capacity, elapsed range and rate have tight enforced limits, checked long arithmetic is faster. Explain those bounds and overflow policy before replacing exact arithmetic.

Your design attempt

Each language has a separate draft across all three levels. Saving is device-only and does not sync to your account. Anyone using this browser profile can load it. Loading is always explicit.

Java 21 reference code is provided for local study. Use Run program in the editor to execute your code and test driver. Program runs show output; they do not grade your design.

Requirements, objects, code and tests (20,000 characters maximum)

Solution.java Language

Editable draft · 12 linesJava 21 code

Run program · Java 21

No unsaved changes · 231/20000

Self-review

Can I derive the equation with units and preserve fractional credit?Can I locate the atomic decision and distinguish local from global guarantees?Can I prove eviction gives no extra tokens?Can I explain old-rate refill and new-capacity clamp during cutover?

Checks apply to this visit. Self-review is not a passed submission or verified mastery.

Open focused practice →

Provenance and scope of evidence

Original teaching material with contracts defined by the chapter and Java 21 reference/tests. No claim of a vendor-specific distributed limiter implementation.

Reference verification is not independent editorial review, production deployment verification or a claim that distributed adapters are implemented.