XREAD for reading entries in Redis - Step-by-Step Execution

Concept Flow - XREAD for reading entries

Start XREAD command

↓

Specify stream(s) and ID(s)

↓

Redis checks streams for new entries

↓

If entries found

Yes↓

Return entries after given ID

↓

End

No↓

Block or return empty (if BLOCK option used)

↓

End

XREAD reads new entries from one or more streams after specified IDs, optionally blocking until new entries arrive.

Execution Sample

Redis

XREAD COUNT 2 STREAMS mystream 0

Reads up to 2 entries from 'mystream' starting from the beginning (ID 0).

Execution Table

Step	Action	Stream	ID Given	Entries Found	Output
1	Start XREAD command	mystream	0	Check stream	Waiting for entries
2	Check entries after ID 0	mystream	0	2 entries found	[["mystream", [["1-0", {"field1":"value1"}], ["2-0", {"field2":"value2"}]]]]
3	Return entries	mystream	0	2 entries returned	Output sent to client
4	End				XREAD completes

💡 All requested entries returned or no more entries after given ID

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
Stream	mystream	mystream	mystream	mystream
ID Given	0	0	0	0
Entries Found	none	2 entries	2 entries	2 entries
Output	none	pending	sent	sent

Key Moments - 2 Insights

Why does XREAD return entries only after the given ID?

XREAD returns entries with IDs strictly greater than the given ID to avoid sending duplicates. See execution_table step 2 where entries after ID 0 are found.

What happens if no new entries are found after the given ID?

If no new entries exist, XREAD returns empty or blocks if BLOCK option is used. This is shown in concept_flow where no entries lead to waiting or empty return.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output at step 2?

A No entries found

B 2 entries found and prepared to return

C Command ends with error

D Waiting for new entries

💡 Hint

Check the 'Entries Found' and 'Output' columns in execution_table row 2

At which step does XREAD send the entries back to the client?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Action' and 'Output' columns in execution_table row 3

If the ID given was '2-0', what would happen in step 2?

A Entries with ID 1-0 and 2-0 returned

B Entries with ID 2-0 and above returned

C Only entries with ID greater than 2-0 returned

D No entries returned because 2-0 is invalid

💡 Hint

XREAD returns entries strictly greater than the given ID, see key_moments explanation

Concept Snapshot

XREAD reads entries from Redis streams after given IDs.
Syntax: XREAD [COUNT count] [BLOCK milliseconds] STREAMS key [key ...] ID [ID ...]
Returns entries with IDs strictly greater than given IDs.
Can read multiple streams at once.
Optionally blocks until new entries arrive.
Useful for consuming stream data incrementally.

Full Transcript

XREAD is a Redis command to read entries from one or more streams starting after specified IDs. It checks the streams for entries with IDs greater than those given. If entries exist, it returns them to the client. If none exist, it can return empty or block waiting for new entries if the BLOCK option is used. This allows clients to read stream data incrementally without duplicates. The example shows reading 2 entries from 'mystream' starting from ID 0, returning two entries with IDs 1-0 and 2-0. The flow includes starting the command, checking streams, returning entries, and ending. Variables like stream name, ID given, entries found, and output status change during execution. Key points include that entries returned are strictly after the given ID to avoid duplicates, and that no entries found leads to empty or blocking behavior. The quiz tests understanding of output at each step and behavior when changing IDs.

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