XACK for acknowledging messages in Redis - Step-by-Step Execution

Concept Flow - XACK for acknowledging messages

Consumer reads message from stream

↓

Process message

↓

Send XACK command with stream, group, message ID

↓

Redis marks message as acknowledged

↓

Message removed from pending list

↓

Consumer can receive next message

The consumer reads a message, processes it, then sends XACK to Redis to mark it acknowledged, removing it from the pending list.

Execution Sample

Redis

XACK mystream mygroup 1609459200000-0

Acknowledges the message with ID 1609459200000-0 in group 'mygroup' on stream 'mystream'.

Execution Table

Step	Action	Input	Redis Response	Effect
1	Consumer reads message	Read message ID 1609459200000-0	Message delivered	Message moved to pending list
2	Consumer processes message	Process message data	N/A	Message still pending
3	Consumer sends XACK	XACK mystream mygroup 1609459200000-0	1	Message removed from pending list
4	Consumer ready for next message	N/A	Next message available	Consumer can read next message

💡 Message acknowledged and removed from pending list, so no longer pending.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
pending_list	empty	contains 1609459200000-0	contains 1609459200000-0	empty	empty
message_status	unread	pending	processing	acknowledged	acknowledged

Key Moments - 2 Insights

Why does the message stay in the pending list after reading but before XACK?

Because reading a message moves it to the pending list until the consumer confirms processing by sending XACK (see execution_table step 1 and 2).

What happens if the consumer never sends XACK?

The message remains in the pending list and can be claimed by other consumers later (not shown here but implied by step 3 importance).

Visual Quiz - 3 Questions

Look at the execution_table, what is the Redis response when XACK is sent?

A "1"

B "Message delivered"

C "Next message available"

D "N/A"

💡 Hint

Check execution_table row 3 under Redis Response.

At which step does the message get removed from the pending list?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the Effect column in execution_table rows.

If the consumer processes the message but never sends XACK, what happens to the message_status variable after Step 3?

A "acknowledged"

B "processing"

C "pending"

D "unread"

💡 Hint

Refer to variable_tracker for message_status changes.

Concept Snapshot

XACK command acknowledges a message in a Redis stream consumer group.
Syntax: XACK <stream> <group> <message-id>
Marks message as processed, removing it from the pending list.
Without XACK, message stays pending and can be re-delivered.
Used after consumer processes the message successfully.

Full Transcript

In Redis streams, when a consumer reads a message, it moves to a pending list. The consumer must send the XACK command with the stream name, group name, and message ID to acknowledge processing. This removes the message from the pending list, so Redis knows it was handled. If the consumer does not send XACK, the message remains pending and can be delivered again. The execution flow shows reading the message, processing it, sending XACK, and then the message is removed from pending. Variables track the message status and pending list contents step-by-step.