Time-based event queues in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an event with a timestamp to a Redis sorted set.

Redis

ZADD events [1] "event1"

Drag options to blanks, or click blank then click option'

AZADD

B"event1"

Cevents

D1625097600

Attempts:

3 left

💡 Hint

Common Mistakes

Using the event name as the score instead of the timestamp.

Forgetting to provide the score before the member name.

The ZADD command adds a member with a score (timestamp) to a sorted set. Here, 1625097600 is the Unix timestamp for the event.

2fill in blank

medium

Complete the code to retrieve all events with timestamps up to a given time.

Redis

ZRANGEBYSCORE events -inf [1]

Drag options to blanks, or click blank then click option'

A"event1"

B1625097600

Cnow

D+inf

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string like 'now' instead of a numeric timestamp.

Using '+inf' which would return all events regardless of time.

The ZRANGEBYSCORE command retrieves members with scores between the given min and max. Here, 1625097600 is the max timestamp to get all events up to that time.

3fill in blank

hard

Fix the error in the code to remove events older than a certain timestamp.

Redis

ZREMRANGEBYSCORE events -inf [1]

Drag options to blanks, or click blank then click option'

A1625097600

B"event1"

C+inf

D-inf

Attempts:

3 left

💡 Hint

Common Mistakes

Using a member name instead of a timestamp.

Using '+inf' which would remove all events.

The ZREMRANGEBYSCORE command removes members with scores between the given min and max. To remove old events, use the max timestamp as the cutoff.

4fill in blank

hard

Fill both blanks to add multiple events with their timestamps to the sorted set.

Redis

ZADD events [1] "event1" [2] "event2"

Drag options to blanks, or click blank then click option'

A1625097600

B"event1"

C1625184000

D"event2"

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping event names and timestamps.

Using strings instead of numeric timestamps for scores.

When adding multiple members with ZADD, each score must be followed by its member. Here, 1625097600 and 1625184000 are timestamps for event1 and event2.

5fill in blank

hard

Fill all three blanks to retrieve events within a time range and limit the number of results.

Redis

ZRANGEBYSCORE events [1] [2] LIMIT 0 [3]

Drag options to blanks, or click blank then click option'

A1625097600

B1625184000

C10

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using strings instead of timestamps for min and max.

Confusing the LIMIT count with offset.

The ZRANGEBYSCORE command can take min and max scores to filter events by time. The LIMIT clause restricts the number of results returned. Here, events between timestamps 1625097600 and 1625184000 are retrieved, limited to 10 results.

Practice

1. What is the main purpose of using a Redis sorted set for a time-based event queue?

easy

A. To store events in a hash for quick lookup

B. To store events with timestamps as scores for ordered retrieval

C. To store only unique event names without timestamps

D. To store events without any order

Solution

1. Step 1: Understand Redis sorted sets

   Redis sorted sets store members with a score that determines their order.

2. Step 2: Apply to time-based event queues

   Using timestamps as scores allows events to be retrieved in chronological order.

3. Final Answer:

   To store events with timestamps as scores for ordered retrieval -> Option B

4. Quick Check:

   Sorted sets + timestamps = ordered event queue [OK]

Hint: Remember: sorted sets order by score (timestamp) [OK]

Common Mistakes:

• Thinking sorted sets don't keep order
• Confusing hashes with sorted sets
• Ignoring the timestamp as score

2. Which Redis command correctly adds an event with a timestamp score to a sorted set named events?

easy

A. SADD events 1625097600 "event1"

B. HSET events 1625097600 "event1"

C. ZADD events 1625097600 "event1"

D. LPUSH events 1625097600 "event1"

Solution

1. Step 1: Identify the command for sorted sets

   ZADD adds members with scores to sorted sets; others are for different data types.

2. Step 2: Check syntax correctness

   ZADD events 1625097600 "event1" adds "event1" with score 1625097600 to sorted set events.

3. Final Answer:

   ZADD events 1625097600 "event1" -> Option C

4. Quick Check:

   ZADD = add to sorted set [OK]

Hint: Use ZADD for sorted sets, not SADD or HSET [OK]

Common Mistakes:

• Using SADD which is for sets without scores
• Using HSET which is for hashes
• Using LPUSH which is for lists

3. Given the sorted set events with members and scores:
{"event1": 1000, "event2": 1500, "event3": 2000}
What will the command ZREVRANGEBYSCORE events 1500 -inf return?

medium

A. ["event1"]

B. ["event3", "event2", "event1"]

C. ["event1", "event2"]

D. ["event2", "event1"]

Solution

1. Step 1: Understand ZREVRANGEBYSCORE command

   This command returns members with scores between max and min in descending order.

2. Step 2: Apply to given scores

   Scores ≤ 1500 are event1 (1000) and event2 (1500). Descending order: event2, event1.

3. Final Answer:

   ["event2", "event1"] -> Option D

4. Quick Check:

   Descending scores ≤ 1500 = event2, event1 [OK]

Hint: ZREVRANGEBYSCORE returns descending order by score [OK]

Common Mistakes:

• Confusing ascending with descending order
• Including events with scores above max
• Misreading score boundaries

4. You run this Redis command to fetch due events:
ZREVRANGEBYSCORE events 1625097600 -inf
But you notice the same events are returned repeatedly. What is the likely mistake?

medium

A. Not removing processed events from the sorted set

B. Using ZREVRANGEBYSCORE instead of ZRANGEBYSCORE

C. Using the wrong key name for the sorted set

D. Using a timestamp that is too high

Solution

1. Step 1: Understand event queue behavior

   After processing events, they must be removed to avoid duplicates.

2. Step 2: Identify cause of repeated events

   If events are not removed, the same events appear on every fetch.

3. Final Answer:

   Not removing processed events from the sorted set -> Option A

4. Quick Check:

   Remove events after processing to avoid repeats [OK]

Hint: Always delete processed events from sorted set [OK]

Common Mistakes:

• Thinking order command causes repeats
• Ignoring event removal step
• Assuming wrong key name causes repeats

5. You want to implement a Redis time-based event queue that fetches all events due up to now and removes them atomically to avoid duplicates. Which sequence of commands is best?

hard

A. Use ZRANGEBYSCORE to get events ≤ now, then ZREM to remove them

B. Use ZREVRANGEBYSCORE to get events ≥ now, then ZREM to remove them

C. Use ZRANGEBYSCORE with a future timestamp, then LPUSH to add events

D. Use SPOP to pop events from a set

Solution

1. Step 1: Fetch due events with ZRANGEBYSCORE

   ZRANGEBYSCORE with max = current timestamp fetches all events due up to now.

2. Step 2: Remove fetched events with ZREM

   Removing them prevents duplicates; doing both steps atomically avoids race conditions.

3. Final Answer:

   Use ZRANGEBYSCORE to get events ≤ now, then ZREM to remove them -> Option A

4. Quick Check:

   Fetch ≤ now + remove = atomic event processing [OK]

Hint: Fetch with ZRANGEBYSCORE ≤ now, then remove with ZREM [OK]

Common Mistakes:

• Using ZREVRANGEBYSCORE with wrong score range
• Using sets or lists instead of sorted sets
• Not removing events after fetching