Time-based event queues in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Retrieve events scheduled before a specific timestamp

Given a Redis sorted set named events where the score is the event timestamp, which command returns all events scheduled before timestamp 1650000000?

A ZRANGEBYSCORE events -inf 1650000000

B ZRANGEBYSCORE events 1650000000 +inf

C ZRANGE events 0 1650000000

D ZREVRANGEBYSCORE events 1650000000 -inf

💡 Hint

Remember that scores in sorted sets represent timestamps and you want all events with scores less than or equal to 1650000000.

ZRANGEBYSCORE with -inf to 1650000000 returns all members with scores up to that timestamp. Option A returns events after that timestamp. Option A reverses the order but with wrong bounds. Option A uses index range, not score range.

📝 Syntax

intermediate

Identify the correct syntax to add a timed event

Which Redis command correctly adds an event named event42 scheduled at timestamp 1680000000 to a sorted set events?

A ZADD events 1680000000 event42

B ZADD events event42 1680000000

C ZADD events (1680000000) event42

D ZADD events event42

💡 Hint

ZADD syntax requires the score first, then the member.

Option A uses correct syntax: ZADD key score member. Option A swaps score and member causing error. Option A uses parentheses incorrectly. Option A misses the score argument.

❓ optimization

advanced

Efficiently removing expired events

You want to remove all events from the events sorted set that have timestamps less than the current time 1690000000. Which command is the most efficient to do this?

A ZREM events 1690000000

B ZREMRANGEBYSCORE events -inf (1690000000

C ZREM events -inf 1690000000

D ZREMRANGEBYRANK events 0 1690000000

💡 Hint

Removing by score range is best for timestamp-based removals.

ZREMRANGEBYSCORE removes all members with scores in the given range efficiently. Option B tries to remove a member named '1690000000' which likely doesn't exist. Option B is invalid syntax. Option B removes by rank, which is unrelated to timestamps.

🔧 Debug

advanced

Why does this time-based event retrieval return no results?

A developer runs this command to get events before timestamp 1700000000: ZRANGEBYSCORE events (1700000000 -inf. Why does it return no results?

A The command syntax is correct; the sorted set is empty

B Parentheses around 1700000000 exclude that score, causing no matches

C The score range is reversed; min should be less than max

D ZRANGEBYSCORE does not support negative infinity

💡 Hint

Check the order of min and max arguments in ZRANGEBYSCORE.

ZRANGEBYSCORE requires the min score first, then max score. Here max is given first, so no results are returned. Parentheses exclude the boundary but do not cause empty results if order is correct. Negative infinity is supported.

🧠 Conceptual

expert

Choosing the best data structure for time-based event queues

Which Redis data structure is best suited for implementing a time-based event queue where events must be processed in order of their scheduled timestamps?

A List because it maintains insertion order and supports push/pop operations

B Set because it stores unique members without order

C Hash because it stores key-value pairs for fast lookup

D Sorted Set (ZSET) because it stores members with scores and supports range queries by score

💡 Hint

Consider which structure supports ordering by timestamp and efficient range queries.

Sorted Sets store members with scores, allowing ordering by timestamp and efficient retrieval of events in time order. Lists maintain insertion order but do not support score-based queries. Hashes and Sets do not maintain order by score.