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.

Practice

1. What does the Redis command XREAD primarily do?

easy

A. Deletes entries from a stream

B. Lists all keys in the Redis database

C. Creates a new stream with initial entries

D. Reads new entries from one or more streams starting at a given ID

Solution

1. Step 1: Understand the purpose of XREAD

   XREAD is designed to read entries from Redis streams, starting at a specified ID.

2. Step 2: Compare with other commands

   Deleting entries is done by XDEL, creating streams is implicit when adding entries, and listing keys is done by KEYS or SCAN.

3. Final Answer:

   Reads new entries from one or more streams starting at a given ID -> Option D

4. Quick Check:

   XREAD reads stream entries = B [OK]

Hint: XREAD always reads entries from streams starting at an ID [OK]

Common Mistakes:

• Confusing XREAD with commands that delete or create streams
• Thinking XREAD lists keys instead of reading stream data

2. Which of the following is the correct syntax to read from a stream named mystream starting from the ID 0-0 using XREAD?

easy

A. XREAD STREAMS mystream 0-0

B. XREAD mystream STREAMS 0-0

C. XREAD STREAMS 0-0 mystream

D. XREAD 0-0 STREAMS mystream

Solution

1. Step 1: Recall XREAD syntax

   The correct syntax is: XREAD STREAMS key [key ...] ID [ID ...]. The keyword STREAMS comes before the stream names and IDs.

2. Step 2: Match the order for one stream

   For one stream, it is: XREAD STREAMS mystream 0-0.

3. Final Answer:

   XREAD STREAMS mystream 0-0 -> Option A

4. Quick Check:

   STREAMS keyword before stream and ID = D [OK]

Hint: Remember: XREAD STREAMS stream_name start_id [OK]

Common Mistakes:

• Placing STREAMS after the stream name
• Swapping stream name and ID order
• Omitting the STREAMS keyword

3. Given the following Redis stream entries in mystream:

1) 1609459200000-0: {"name": "Alice"}
2) 1609459201000-0: {"name": "Bob"}
3) 1609459202000-0: {"name": "Carol"}

What will be the output of the command XREAD COUNT 2 STREAMS mystream 1609459200000-0?

medium

A. [["mystream", [["1609459201000-0", {"name": "Bob"}], ["1609459202000-0", {"name": "Carol"}]]]]

B. [["mystream", [["1609459200000-0", {"name": "Alice"}], ["1609459201000-0", {"name": "Bob"}]]]]

C. [["mystream", [["1609459202000-0", {"name": "Carol"}]]]]

D. Error: Invalid COUNT usage

Solution

1. Step 1: Understand COUNT and starting ID

   Starting at 1609459200000-0 means reading from the first entry. COUNT 2 limits to 2 entries.

2. Step 2: Select the first two entries

   The first two entries are Alice and Bob with IDs 1609459200000-0 and 1609459201000-0.

3. Final Answer:

   [["mystream", [["1609459200000-0", {"name": "Alice"}], ["1609459201000-0", {"name": "Bob"}]]]] -> Option B

4. Quick Check:

   COUNT limits entries read = A [OK]

Hint: COUNT limits number of entries returned [OK]

Common Mistakes:

• Starting from next ID instead of given ID
• Ignoring COUNT and returning all entries
• Misunderstanding the output format

4. You run the command XREAD STREAMS mystream 0-0 COUNT 2 but get an error. Which of the following is the most likely cause?

medium

A. The COUNT argument is placed after the STREAMS keyword

B. The stream name is missing

C. The ID format is incorrect

D. XREAD does not support COUNT option

Solution

1. Step 1: Recall XREAD syntax

   Modifiers like COUNT must precede the STREAMS keyword: XREAD [COUNT count] STREAMS key id ...

2. Step 2: Analyze the given command

   COUNT 2 is placed after STREAMS mystream 0-0, which is invalid and causes a syntax error.

3. Step 3: Rule out other options

   Stream name present, ID 0-0 valid, COUNT supported.

4. Final Answer:

   The COUNT argument is placed after the STREAMS keyword -> Option A

5. Quick Check:

   COUNT after STREAMS = error A [OK]

Hint: COUNT and BLOCK must precede STREAMS [OK]

Common Mistakes:

• Placing COUNT after STREAMS keyword
• Omitting stream name
• Assuming COUNT is unsupported

5. You want to read new entries from two streams, streamA and streamB, starting from IDs 0-0 and 1609459200000-0 respectively, and wait up to 5000 milliseconds if no new entries arrive. Which command correctly achieves this?

hard

A. XREAD STREAMS streamA streamB 0-0 1609459200000-0 COUNT 10 BLOCK 5000

B. XREAD STREAMS streamA streamB 0-0 1609459200000-0 BLOCK 5000 COUNT 10

C. XREAD COUNT 10 BLOCK 5000 STREAMS streamA streamB 0-0 1609459200000-0

D. XREAD BLOCK 5000 STREAMS streamA streamB 0-0 1609459200000-0 COUNT 10

Solution

1. Step 1: Recall correct XREAD order

   XREAD [COUNT count] [BLOCK ms] STREAMS key [key ...] id [id ...]. Modifiers precede STREAMS.

2. Step 2: Match given streams and IDs

   streamA with 0-0, streamB with 1609459200000-0.

3. Step 3: Confirm options placement

   COUNT 10 BLOCK 5000 before STREAMS in XREAD COUNT 10 BLOCK 5000 STREAMS streamA streamB 0-0 1609459200000-0 is correct.

4. Final Answer:

   XREAD COUNT 10 BLOCK 5000 STREAMS streamA streamB 0-0 1609459200000-0 -> Option C

5. Quick Check:

   COUNT/BLOCK before STREAMS = C [OK]

Hint: COUNT and BLOCK before STREAMS keyword [OK]

Common Mistakes:

• Placing COUNT or BLOCK after STREAMS keyword
• Mixing order of streams and IDs
• Omitting BLOCK for waiting