System design URL shortener: Spring Boot 4.1.1, base62 ids, Redis cache-aside, Redis Lua token bucket, 24 tests and live transcripts

output/01-create-and-redirect.txt

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+$ curl -si -X POST /api/v1/urls -d '{"url":"https://ankurm.com/java-interview-preparation-guide/"}'
+HTTP/1.1 201 
+X-RateLimit-Remaining: 4
+Location: http://localhost:8080/6MelkkB
+{"code":"6MelkkB","shortUrl":"http://localhost:8080/6MelkkB","expiresAt":null}
+
+$ curl -si -X GET /6MelkkB
+HTTP/1.1 302 
+Location: https://ankurm.com/java-interview-preparation-guide/
+

output/02-cache-aside-and-clicks.txt

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+code = 7qqChen
+$ redis-cli EXISTS url:7qqChen            # before the first redirect
+0
+--- after 1st redirect (cache miss, database read, cache filled)
+$ redis-cli GET url:7qqChen
+https://example.com/cache-demo
+$ redis-cli TTL url:7qqChen
+3600
+cache hits +0, misses +1
+--- after 9 more redirects
+cache hits +9, misses +1
+$ redis-cli GET clicks:7qqChen
+10
+$ curl -s http://127.0.0.1:8080/api/v1/urls/7qqChen/stats
+{"code":"7qqChen","longUrl":"https://example.com/cache-demo","clicks":10,...

output/03-negative-cache.txt

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+$ curl -si -X GET /Zq9Zq9
+HTTP/1.1 404 
+{"detail":"no such code: Zq9Zq9","instance":"/Zq9Zq9","status":404,"title":"Not Found"}
+
+$ curl -si -X GET /Zq9Zq9
+HTTP/1.1 404 
+{"detail":"no such code: Zq9Zq9","instance":"/Zq9Zq9","status":404,"title":"Not Found"}
+
+cache misses: before=2 after-first-404=3 after-second-404=3
+$ redis-cli GET url:Zq9Zq9   # the negative-cache marker
+-
+$ redis-cli TTL url:Zq9Zq9
+60

output/04-alias-and-validation.txt

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+$ curl -si -X POST /api/v1/urls -d '{"url":"https://example.com/one","alias":"demo-link"}'
+HTTP/1.1 201 
+X-RateLimit-Remaining: 4
+Location: http://localhost:8080/demo-link
+{"code":"demo-link","shortUrl":"http://localhost:8080/demo-link","expiresAt":null}
+
+$ curl -si -X POST /api/v1/urls -d '{"url":"https://example.com/two","alias":"demo-link"}'
+HTTP/1.1 409 
+X-RateLimit-Remaining: 3
+{"detail":"alias already taken: demo-link","instance":"/api/v1/urls","status":409,"title":"Conflict"}
+
+$ curl -si -X POST /api/v1/urls -d '{"url":"ftp://example.com/file"}'
+HTTP/1.1 400 
+X-RateLimit-Remaining: 2
+{"detail":"only http and https URLs are allowed","instance":"/api/v1/urls","status":400,"title":"Bad Request"}
+
+$ curl -si -X POST /api/v1/urls -d '{"url":"https://example.com","alias":"api"}'
+HTTP/1.1 400 
+X-RateLimit-Remaining: 1
+{"detail":"alias must be 4-32 chars of A-Z a-z 0-9 _ -","instance":"/api/v1/urls","status":400,"title":"Bad Request"}
+

output/05-rate-limit.txt

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+capacity 5, refill 0.5 tokens/second (one every 2 s)
+request 1 -> HTTP/1.1 201  X-RateLimit-Remaining: 4
+request 2 -> HTTP/1.1 201  X-RateLimit-Remaining: 3
+request 3 -> HTTP/1.1 201  X-RateLimit-Remaining: 2
+request 4 -> HTTP/1.1 201  X-RateLimit-Remaining: 1
+request 5 -> HTTP/1.1 201  X-RateLimit-Remaining: 0
+request 6 -> HTTP/1.1 429  X-RateLimit-Remaining: 0 Retry-After: 2
+request 7 -> HTTP/1.1 429  X-RateLimit-Remaining: 0 Retry-After: 2
+$ redis-cli HGETALL rl:127.0.0.1
+tokens
+0.046000000000000082
+ts
+1792656760797
+--- sleeping 2.2 s
+request 8 -> HTTP/1.1 201  X-RateLimit-Remaining: 0
+--- redirects are not limited: 30 GETs in a row
+302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 302 

output/06-expiry.txt

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+created ArYvqWu with ttlSeconds=2
+t=0s    -> 302
+$ redis-cli TTL url:ArYvqWu   # capped at the link's remaining life, not the 1 h default
+2
+t=3s    -> 410

output/07-id-generation.txt

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+--- instance A: three links
+ id |  code   | len 
+----+---------+-----
+  1 | 6MelkkB |   7
+  2 | 7qqChen |   7
+  3 | J4Wn4LP |   7
+(3 rows)
+
+--- instance A stopped and restarted (ids 4..100 of its first block are never used)
+ id  |  code   | len 
+-----+---------+-----
+   1 | 6MelkkB |   7
+   2 | 7qqChen |   7
+   3 | J4Wn4LP |   7
+ 101 | IZKi2wO |   7
+(4 rows)
+
+--- two instances running at once never overlap: B starts while A is alive
+ id  |  code   
+-----+---------
+ 102 | DDCfjej
+ 201 | Globq13
+(2 rows)
+

output/08-snowflake.txt

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+code=7sWzEmdQLw length=10
+code=7sWzF1xlNQ length=10
+code=7sWzF7djyi length=10
+         id         |    code    
+--------------------+------------
+ 106666141551718400 | 7sWzEmdQLw
+ 106666141778210816 | 7sWzF1xlNQ
+ 106666141862096896 | 7sWzF7djyi
+(3 rows)
+

output/09-test-results.txt

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+PASS Base62Test.knownValues
+PASS Base62Test.roundTripsAcrossTheRange
+PASS Base62Test.rejectsGarbage
+PASS Base62Test.sevenCharactersCoverAboutThreePointFiveTrillion
+PASS CodeScramblerTest.noTwoIdsShareACodeAcrossTwoMillionConsecutiveIds
+PASS CodeScramblerTest.everyCodeIsExactlySevenCharacters
+PASS CodeScramblerTest.idsBeyondTheFortyBitDomainFallBackToPlainBase62
+PASS CodeScramblerTest.consecutiveIdsDoNotGiveConsecutiveCodes
+PASS RateLimitTest.burstOfFiveThenRejectedThenRefilled
+PASS RateLimitTest.redirectsAreNeverRateLimited
+PASS SnowflakeIdGeneratorTest.idsIncreaseWithinOneThread
+PASS SnowflakeIdGeneratorTest.base62CodeIsLongerThanForASequenceId
+PASS SnowflakeIdGeneratorTest.refusesToIssueIdsWhenTheClockGoesBackwards
+PASS SnowflakeIdGeneratorTest.idsAreUniqueAcrossEightThreads
+PASS SnowflakeIdGeneratorTest.nodeIdIsEncodedInTheId
+PASS SnowflakeIdGeneratorTest.rejectsOutOfRangeNodeId
+PASS SnowflakeProfileTest.snowflakeIdsGiveLongerCodes
+PASS UrlShortenerApiTest.sameLongUrlGetsAnIndependentCodeEachTime
+PASS UrlShortenerApiTest.customAliasWorksAndSecondUseIsAConflict
+PASS UrlShortenerApiTest.unknownCodeIs404AndTheMissIsNegativelyCached
+PASS UrlShortenerApiTest.createsAShortLinkAndRedirectsWith302
+PASS UrlShortenerApiTest.countsClicks
+PASS UrlShortenerApiTest.rejectsBadInput
+PASS UrlShortenerApiTest.expiredLinkIs410
+24 tests, 0 failures