XREADGROUP for consumer groups in Redis - Step-by-Step Execution

Concept Flow - XREADGROUP for consumer groups

Create Consumer Group

↓

Consumer sends XREADGROUP

↓

Server checks pending messages

↓

Pending msgs?

Yes→Deliver pending msgs

↓

Deliver new msgs

↓

Consumer processes msgs

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Consumer acknowledges msgs with XACK

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Server removes acknowledged msgs from pending

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End

This flow shows how a consumer group reads messages from a stream using XREADGROUP, handling pending and new messages, then acknowledges them.

Execution Sample

Redis

XGROUP CREATE mystream mygroup $
XREADGROUP GROUP mygroup Alice COUNT 2 STREAMS mystream >
XACK mystream mygroup 1609459200000-0 1609459200000-1

Create a group, consumer Alice reads new messages, then acknowledges both messages.

Execution Table

Step	Command	Action	Messages Delivered	Pending Messages	Notes
1	XGROUP CREATE mystream mygroup $	Create consumer group 'mygroup' on 'mystream' starting at latest message	None	None	Group created, no pending messages
2	XREADGROUP GROUP mygroup Alice COUNT 2 STREAMS mystream >	Consumer Alice reads new messages from 'mystream'	2 new messages	2	No pending messages before read, delivers new messages, pending count increases
3	XACK mystream mygroup 1609459200000-0 1609459200000-1	Consumer Alice acknowledges messages with IDs 1609459200000-0 and 1609459200000-1	None	0	Acknowledged messages removed from pending
4	XREADGROUP GROUP mygroup Alice COUNT 2 STREAMS mystream >	Consumer Alice reads again	0 messages	0	No new or pending messages, returns empty
5	End	No more messages to read	None	0	Execution ends

💡 No more new or pending messages to deliver, consumer group read ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Pending Messages	0	0	2	0	0	0
Delivered Messages	0	0	2	0	0	0

Key Moments - 3 Insights

Why does XREADGROUP deliver pending messages before new messages?

Because the server first checks for pending messages (see Step 2 in execution_table). If there are pending messages, it delivers those first to avoid losing unacknowledged messages.

What does the '>' symbol mean in the XREADGROUP command?

It tells Redis to deliver new messages added to the stream after the maximum ID already delivered to the group. Pending messages for the consumer are always delivered first before new messages.

Why does the pending message count decrease after XACK?

Acknowledging a message with XACK (Step 3) tells Redis the consumer processed it, so Redis removes it from the pending list, reducing the count.

Visual Quiz - 3 Questions

Look at the execution_table at Step 2. How many messages does consumer Alice receive?

A 0 messages

B 1 pending message

C 2 new messages

D 3 messages

💡 Hint

Check the 'Messages Delivered' column at Step 2 in the execution_table.

At which step does the pending message count decrease?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Pending Messages' column changes in the variable_tracker and execution_table.

If the consumer used '0' instead of '>' in XREADGROUP, what would change?

A It would read only new messages

B It would read all messages from the beginning

C It would read only messages with ID greater than 0

D It would read only pending messages

💡 Hint

The '>' means new messages; '0' means from the beginning or a specific ID.

Concept Snapshot

XREADGROUP reads messages for a consumer group.
It delivers pending messages first, then new messages.
Use '>' to read new messages only.
Consumers acknowledge messages with XACK.
Pending messages track unacknowledged deliveries.
Helps distribute stream processing among consumers.

Full Transcript

This visual execution trace shows how Redis XREADGROUP works for consumer groups. First, a consumer group is created on a stream. Then a consumer reads messages using XREADGROUP. Redis checks for pending messages (unacknowledged) and delivers them first. If none, it delivers new messages using the '>' ID. The consumer processes and acknowledges messages with XACK, which removes them from pending. The trace shows message delivery counts and pending message tracking step-by-step, clarifying how consumers share stream processing safely.