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.