XREAD for reading entries in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does this XREAD command return?

Given a Redis stream named mystream with entries:

1) 1609459200000-0: {"name":"Alice", "age":"30"}
2) 1609459260000-0: {"name":"Bob", "age":"25"}

What is the output of this command?

XREAD COUNT 1 STREAMS mystream 0

Redis

XREAD COUNT 1 STREAMS mystream 0

A [["mystream", [["1609459260000-0", {"name":"Bob", "age":"25"}]]]]

B [["mystream", [["0-0", {"name":"Alice", "age":"30"}]]]]

C [["mystream", [["1609459200000-0", {"name":"Alice", "age":"30"}]]]]

D []

💡 Hint

The COUNT option limits the number of entries returned. The ID '0' means start from the beginning.

The command reads 1 entry from the stream starting at ID '0', so it returns the first entry only.

🧠 Conceptual

intermediate

Understanding the ID parameter in XREAD

In the command XREAD STREAMS mystream 1609459200000-0, what does the ID 1609459200000-0 specify?

A Start reading entries strictly after the entry with ID 1609459200000-0

B Start reading entries from the end of the stream

C Start reading entries from the beginning of the stream

D Start reading entries including the entry with ID 1609459200000-0

💡 Hint

XREAD reads entries with IDs greater than the given ID.

The ID parameter tells XREAD to return entries with IDs strictly greater than the given ID, so it excludes the entry with that ID.

📝 Syntax

advanced

Identify the syntax error in this XREAD command

Which option contains a syntax error when trying to read from stream mystream starting at ID 0?

Redis

XREAD STREAMS mystream 0

A XREAD COUNT 5 STREAMS mystream 0

B XREAD STREAMS mystream

C XREAD STREAMS mystream 0

D XREAD STREAMS mystream 0-0

💡 Hint

XREAD requires an ID after the stream name(s).

Option B is missing the ID parameter after the stream name, causing a syntax error.

❓ optimization

advanced

Optimizing XREAD for multiple streams

You want to read new entries from two streams stream1 and stream2 starting from their latest entries. Which command is the most efficient to do this without reading old entries?

A XREAD STREAMS stream1 stream2 $ $

B XREAD STREAMS stream1 stream2 0 0

C XREAD COUNT 1 STREAMS stream1 stream2 $ $

D XREAD BLOCK 0 STREAMS stream1 stream2 0 0

💡 Hint

The special ID '$' means the latest entry in the stream.

Using '$' as the ID tells XREAD to only return new entries added after the command runs, avoiding old entries.

🔧 Debug

expert

Why does this XREAD command block indefinitely?

Consider the command:

XREAD BLOCK 0 STREAMS mystream 1609459260000-0

Given that mystream has no new entries after ID 1609459260000-0, what is the reason this command blocks indefinitely?

A Because the COUNT option is missing, causing infinite blocking

B Because the ID is invalid and causes the command to hang

C Because the stream is empty and XREAD cannot return anything

D Because BLOCK 0 means wait forever for new entries after the given ID

💡 Hint

BLOCK 0 means wait forever until new data arrives.

BLOCK 0 tells XREAD to wait indefinitely for new entries after the specified ID, so if no new entries arrive, it blocks forever.