XREAD for reading entries in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read entries from a Redis stream named 'mystream' starting from the beginning.

Redis

XREAD COUNT 10 STREAMS mystream [1]

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

A0

B$

C1

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '0' will skip existing entries.

Using '-' is invalid as a stream ID.

The '0' means reading from the start of the stream, so XREAD returns all entries from the beginning.

2fill in blank

medium

Complete the code to read entries from multiple streams 'stream1' and 'stream2' starting from their last IDs.

Redis

XREAD STREAMS stream1 stream2 [1] [1]

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

A>

B0

C$

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '0' will read all entries from the start, not just new ones.

Using '-' is not a valid stream ID.

The '>' ID means reading only new entries added after the command runs for each stream.

3fill in blank

hard

Fix the error in the code to read 5 entries from 'mystream' starting after ID '1609459200000-0'.

Redis

XREAD COUNT 5 STREAMS mystream [1]

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

A1609459200000-

B1609459200000-0

C1609459200000-1

D1609459200000

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same ID returns the entry with that ID again.

Using incomplete IDs causes syntax errors.

The ID '1609459200000-1' reads entries after '1609459200000-0'. Using the same ID would return that entry again.

4fill in blank

hard

Fill both blanks to read entries from 'streamA' and 'streamB' starting from the beginning and the last entry respectively.

Redis

XREAD STREAMS streamA streamB [1] [2]

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

A0

B>

C1609459200000-0

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' for both streams reads only new entries, missing old ones.

Using '-' is invalid.

Use '0' to read all entries from 'streamA' from the start, and '>' to read only new entries from 'streamB'.

5fill in blank

hard

Fill all three blanks to read 3 entries from 's1' and 's2' starting from '0-0' and from the latest entry respectively.

Redis

XREAD COUNT [1] STREAMS s1 s2 [2] [3]

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

A5

B3

C>

D0-0

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT 5 reads more entries than requested.

Swapping the stream IDs causes wrong reading points.

Read 3 entries (COUNT 3), from 's1' starting from '0-0' (which means from the beginning), and from 's2' starting at the latest entry ('>')

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