Add rate limiter exercise #3008

andreatanky · 2025-09-08T13:11:30Z

pull request

Add Practice Exercise: Rate Limiter

Adds a new practice exercise, “rate-limiter”, implementing a deterministic fixed-window rate limiter with an injected monotonic clock
To implement a single method (FixedWindowRateLimiter.allow); the API, clock, and constructor are provided
Includes tests, reference solution, Gradle config, and brief instructions

Reviewer Resources:

kahgoh

Thanks for contributing this exercise! As a concept for a practice exercise, I think this looks good! Some comments below:

exercises/practice/rate-limiter/src/main/java/RateLimiter.java

exercises/practice/rate-limiter/src/main/java/TimeSource.java

exercises/practice/rate-limiter/src/test/java/RateLimiterTest.java

exercises/practice/rate-limiter/.docs/instructions.md

exercises/practice/rate-limiter/src/main/java/FixedWindowRateLimiter.java

exercises/practice/rate-limiter/.docs/instructions.md

…va into add-rate-limiter-exercise

exercises/practice/rate-limiter/src/main/java/RateLimiter.java

kahgoh · 2025-09-15T13:27:15Z

exercises/practice/rate-limiter/src/main/java/TimeSource.java

+    public void advanceNanos(long nanos) {
+        this.now = this.now.plusNanos(nanos);
+    }


I think you might have forgotten to update the tests to use the advance(Duration d) method.

exercises/practice/rate-limiter/src/main/java/RateLimiter.java

exercises/practice/rate-limiter/src/test/java/RateLimiterTest.java

exercises/practice/rate-limiter/.docs/instructions.md

exercises/practice/rate-limiter/src/main/java/RateLimiter.java

exercises/practice/rate-limiter/.docs/instructions.md

kahgoh · 2025-09-20T11:01:27Z

exercises/practice/rate-limiter/src/test/java/RateLimiterTest.java

+        final class ClientId {
+            final String id;
+
+            ClientId(String id) {
+                this.id = id;
+            }
+
+            @Override
+            public boolean equals(Object o) {
+                return (o instanceof ClientId) && ((ClientId) o).id.equals(this.id);
+            }
+
+            @Override
+            public int hashCode() {
+                return id.hashCode();
+            }
+        }


To be honest, I'm not a fan of having to rely on the correct implementation of the equals and hashCode in the test and I think having the UUID, Integer, Long and String variants is more than enough to test the generics side - perhaps we could remove this test case?

Thanks for the feedback — could you share a bit more on what feels uncomfortable about the custom‑object test?
Is it that it couples outcomes to equals/hashCode correctness and can cause confusing failures unrelated to the rate‑limiter logic?

I have removed this custom object test case to reduce possible confusion

Yeah, it is that we'd be relying on the equals and hashCode being implemented correctly for this test to work properly. If this had been for an actual project, I'd be wondering if we need tests to prove that they have been implemented correctly and honor the equals and hashCode contract too. I think an alternative would be to use records which would take care of this for you, but I also felt it was already sufficient to use UUID objects.

exercises/practice/rate-limiter/src/test/java/RateLimiterTest.java

kahgoh · 2025-09-21T12:25:31Z

config.json

+        "uuid": "b4b0c60e-4ce1-488e-948f-bcb6821c773c",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 1


For the difficulty, would you say it is similar in difficulty as Luhn? If so, we could say it is a 4 (a 4 is one of the lower medium difficulty, 3 is easy). I think 1 is too low.

I think low‑medium (4) is reasonable! it only requires per‑key state and fixed‑window boundary handling with java.time, but avoids concurrency and advanced algorithms like sliding windows or token buckets.

Great! The exercises are sorted by difficulty and then name though. Could you please move this entry to between proverb and rotational-cipher in the exercises list?

Shifted! Thanks!

config.json

exercises/practice/rate-limiter/.meta/config.json

exercises/practice/rate-limiter/src/main/java/TimeSource.java

exercises/practice/rate-limiter/src/test/java/RateLimiterTest.java

…va into add-rate-limiter-exercise

kahgoh

I think its almost there! I have noticed the CI is failing with some errors (see the results at the bottom). I think one of the errors may be because there are some Gradle files missing. To fix this:

Have a look at resources/exercise-template.
There are some Gradle files and directories that should be copied to your rate-limiter exercise, namely:
- the gradle directory
- gradlew
- gradlew.bat

exercises/practice/rate-limiter/.docs/instructions.md

exercises/practice/rate-limiter/.meta/config.json

kahgoh · 2025-09-22T23:38:23Z

exercises/practice/rate-limiter/src/test/java/RateLimiterTest.java

+    @Test
+    void allowsUpToLimit() {


We've started putting @DisplayName tags in the tests in the other PRs (example in suggestion). Could you please do the same?

Suggested change

@Test

void allowsUpToLimit() {

@Test

@DisplayName("Allows up to window limit")

void allowsUpToLimit() {

Updated with the tests with @DisplayName

…va into add-rate-limiter-exercise

Co-authored-by: Kah Goh <[email protected]>

…va into add-rate-limiter-exercise

kahgoh

Sorry, there's still a couple of errors from the CI.

exercises/practice/rate-limiter/.docs/instructions.md

config.json

kahgoh

Thanks, it is passing the CI now! I noticed some more really minor things while doing a last pass over the changes.

exercises/practice/rate-limiter/src/test/java/RateLimiterTest.java

exercises/practice/rate-limiter/src/main/java/RateLimiter.java

…java Co-authored-by: Kah Goh <[email protected]>

Co-authored-by: Kah Goh <[email protected]>

andreatanky · 2025-09-24T00:34:14Z

Appreciate the thorough review!

config.json

Co-authored-by: Kah Goh <[email protected]>

kahgoh · 2025-09-24T12:12:01Z

Thanks for sticking with this one! Looks good now!

andreatanky added 4 commits September 8, 2025 21:08

Run configlet create

0513d96

Update rate limiter implementation

bfe4dbb

Add instructions

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Add instructions

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andreatanky marked this pull request as ready for review September 12, 2025 14:39

andreatanky changed the title ~~[Draft] Add rate limiter exercise~~ Add rate limiter exercise Sep 12, 2025

Merge branch 'main' into add-rate-limiter-exercise

4f5abf3

kahgoh requested changes Sep 13, 2025

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andreatanky added 5 commits September 14, 2025 10:37

Remove interfaces

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Update instructions

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Merge branch 'add-rate-limiter-exercise' of github.com:andreatanky/ja…

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…va into add-rate-limiter-exercise

Add github handle to authors array

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Add blurb

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andreatanky requested a review from kahgoh September 14, 2025 14:13

Merge branch 'main' into add-rate-limiter-exercise

d9520b7

kahgoh requested changes Sep 16, 2025

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andreatanky added 9 commits September 18, 2025 17:44

Update long to use Duration

63ea8a3

Update advance method to use Duration in tests

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Remove unused advanceNanos method

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Add non string tests

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Break down test cases

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Update to one sentence per line

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Update WindowState long type to use Instant

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Remove advanceNanos

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Merge branch 'main' into add-rate-limiter-exercise

a61ef4d

andreatanky requested a review from kahgoh September 20, 2025 04:21

kahgoh requested changes Sep 21, 2025

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kahgoh reviewed Sep 21, 2025

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exercises/practice/rate-limiter/src/test/java/RateLimiterTest.java Show resolved Hide resolved

andreatanky added 2 commits September 22, 2025 08:45

Update naming of key to clientId

50b280c

Merge branch 'add-rate-limiter-exercise' of github.com:andreatanky/ja…

5971500

…va into add-rate-limiter-exercise

Merge branch 'main' into add-rate-limiter-exercise

ae8d97e

andreatanky requested a review from kahgoh September 22, 2025 11:41

kahgoh reviewed Sep 22, 2025

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kahgoh reviewed Sep 22, 2025

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andreatanky and others added 8 commits September 23, 2025 08:42

Shift exercise in config.json

60736bb

Merge branch 'add-rate-limiter-exercise' of github.com:andreatanky/ja…

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…va into add-rate-limiter-exercise

Copy gradle wrapper

d418567

Update exercises/practice/rate-limiter/.docs/instructions.md

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Co-authored-by: Kah Goh <[email protected]>

Update exercises/practice/rate-limiter/.docs/instructions.md

7907757

Co-authored-by: Kah Goh <[email protected]>

Add display name annotation

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Merge branch 'add-rate-limiter-exercise' of github.com:andreatanky/ja…

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…va into add-rate-limiter-exercise

Run configlet to format config.json

6ccd97a

andreatanky requested a review from kahgoh September 23, 2025 01:20

kahgoh reviewed Sep 23, 2025

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Reformat files

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andreatanky and others added 5 commits September 24, 2025 08:29

Update exercises/practice/rate-limiter/src/test/java/RateLimiterTest.…

a8a6048

…java Co-authored-by: Kah Goh <[email protected]>

Update exercises/practice/rate-limiter/src/test/java/RateLimiterTest.…

6b8e310

…java Co-authored-by: Kah Goh <[email protected]>

Update exercises/practice/rate-limiter/src/test/java/RateLimiterTest.…

b3667bc

…java Co-authored-by: Kah Goh <[email protected]>

Update exercises/practice/rate-limiter/src/main/java/RateLimiter.java

0b158f7

Co-authored-by: Kah Goh <[email protected]>

Merge branch 'main' into add-rate-limiter-exercise

d5b13e9

andreatanky requested a review from kahgoh September 24, 2025 02:36

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Update config.json

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Co-authored-by: Kah Goh <[email protected]>

andreatanky requested a review from kahgoh September 24, 2025 09:46

kahgoh approved these changes Sep 24, 2025

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kahgoh merged commit 85eaced into exercism:main Sep 24, 2025
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bibekparajuli123 pushed a commit to bibekparajuli123/java that referenced this pull request Oct 4, 2025

Add rate limiter exercise (exercism#3008)

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Add rate limiter exercise #3008

Add rate limiter exercise #3008

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andreatanky commented Sep 8, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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pull request

Add Practice Exercise: Rate Limiter

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andreatanky commented Sep 24, 2025

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