XADD for adding entries in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does this XADD command return?

Consider the Redis command:
XADD mystream * sensor-id 1234 temperature 19.8
What is the output of this command?

Redis

XADD mystream * sensor-id 1234 temperature 19.8

A An error message because * is invalid here

B An integer representing the number of fields added, e.g., 2

C The string 'OK' indicating success

D A string representing the ID of the added entry, e.g., '1657891234567-0'

💡 Hint

XADD returns the ID of the new entry added to the stream.

The XADD command adds an entry to the stream and returns the entry's ID, which is a string with a timestamp and sequence number.

📝 Syntax

intermediate

Which XADD command is syntactically correct?

Choose the syntactically valid Redis XADD command to add an entry with fields 'user'='alice' and 'score'='42' to stream 'game_scores'.

A XADD game_scores * user alice score 42

B XADD game_scores * user=alice score=42

C XADD game_scores user alice score 42 *

D XADD game_scores * (user alice) (score 42)

💡 Hint

The * must come immediately after the stream name, followed by field-value pairs without extra symbols.

The correct syntax is: XADD [ ...]. Option A follows this exactly.

❓ optimization

advanced

How to limit stream length efficiently with XADD?

You want to add entries to a Redis stream 'logs' but keep only the latest 1000 entries to save memory. Which XADD option achieves this efficiently?

A XADD logs MAXLEN=1000 * sensor temp

B XADD logs MAXLEN ~ 1000 * sensor temp

C XADD logs MAXLEN 1000~ * sensor temp

D XADD logs MAXLEN 1000 * sensor temp

💡 Hint

Using ~ after MAXLEN allows approximate trimming which is faster.

Option B uses 'MAXLEN ~ 1000' which trims the stream approximately to 1000 entries, improving performance over exact trimming.

🔧 Debug

advanced

Why does this XADD command cause an error?

Given the command:
XADD mystream 1657891234567-0 sensor 12
Why might this cause an error?

Redis

XADD mystream 1657891234567-0 sensor 12

A The ID 1657891234567-0 already exists in the stream, causing a duplicate ID error

B The field 'sensor' is missing a value

C The ID format is invalid because it must be '*' for auto-generated IDs

D The stream 'mystream' does not exist yet

💡 Hint

Manually specifying an ID that already exists causes an error.

XADD with a manual ID fails if that ID already exists in the stream, causing a duplicate ID error.

🧠 Conceptual

expert

What is the purpose of the '*' ID in XADD?

In Redis Streams, what does using '*' as the ID in the XADD command do?

A It causes Redis to reject the command with a syntax error

B It deletes the oldest entry in the stream before adding the new one

C It tells Redis to auto-generate a unique ID based on the current timestamp and sequence number

D It sets the ID of the new entry to zero

💡 Hint

The * is a special placeholder for IDs in Redis Streams.

Using '*' lets Redis create a unique ID automatically, combining the current time and a sequence number to avoid collisions.