Message ordering and sessions in Azure - Step-by-Step Execution

Process Flow - Message ordering and sessions

Send messages with SessionId

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Messages arrive at queue/topic

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Service groups messages by SessionId

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Receiver accepts a session

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Messages delivered in order within session

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Session state can be stored and updated

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Receiver completes session processing

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End

Messages are sent with a session ID. The service groups messages by this ID. Receivers accept sessions and get messages in order. Session state can be stored and updated during processing.

Execution Sample

Azure

Send message1 with SessionId='A'
Send message2 with SessionId='A'
Send message3 with SessionId='B'
Receiver accepts SessionId='A'
Receiver processes messages in order

This example shows sending messages with session IDs and a receiver processing messages in order for a specific session.

Process Table

Step	Action	SessionId	Message Order	Receiver State	Output
1	Send message1	A	1	None	Message1 queued
2	Send message2	A	2	None	Message2 queued
3	Send message3	B	1	None	Message3 queued
4	Receiver accepts session	A	-	Session A accepted	Ready to receive messages
5	Receiver receives message1	A	1	Processing message1	Message1 delivered
6	Receiver receives message2	A	2	Processing message2	Message2 delivered
7	Receiver completes session	A	-	Session A closed	Session processing done
8	Receiver tries to receive from session B	B	-	Session B not accepted	No messages delivered

💡 Receiver stops after completing session A; messages in session B remain until accepted.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8
Queue Messages	Empty	Message1 (A, order 1)	Message1, Message2 (A, order 2)	Message1, Message2 (A), Message3 (B, order 1)	Same	Message2 (A), Message3 (B)	Message3 (B)	Message3 (B)	Message3 (B)
Receiver Session	None	None	None	None	Session A accepted	Session A processing message1	Session A processing message2	Session A closed	Session A closed

Key Moments - 3 Insights

Why does the receiver only get messages from one session at a time?

Because the receiver accepts a specific session (see Step 4 in execution_table), it processes messages only from that session to maintain order.

What happens to messages in other sessions while one session is being processed?

Messages in other sessions remain in the queue until their session is accepted by a receiver (see Step 8). They are not delivered out of order.

How is message order guaranteed within a session?

Messages with the same SessionId are delivered in the order they were sent (see Steps 5 and 6), ensuring ordered processing.

Visual Quiz - 3 Questions

Look at the execution_table at Step 5. What message does the receiver process?

A Message2 with SessionId 'A'

B Message1 with SessionId 'A'

C Message3 with SessionId 'B'

D No message

💡 Hint

Check the 'Action' and 'Output' columns at Step 5 in execution_table.

At which step does the receiver close the session?

A Step 7

B Step 8

C Step 6

D Step 4

💡 Hint

Look for 'Session closed' in the 'Receiver State' column in execution_table.

If the receiver accepts session 'B' at Step 4 instead of 'A', what changes in the execution_table?

A Messages from session 'A' are delivered first

B No messages are delivered

C Messages from session 'B' are delivered in order

D Messages from both sessions are delivered simultaneously

💡 Hint

Refer to how session acceptance controls message delivery order in execution_table Steps 4-6.

Concept Snapshot

Message ordering and sessions in Azure Service Bus:
- Messages sent with SessionId are grouped.
- Receiver accepts one session at a time.
- Messages within a session are delivered in order.
- Session state can be stored and updated.
- Other sessions wait until accepted.

Full Transcript

In Azure Service Bus, messages can be sent with a SessionId to group related messages. The service groups messages by this SessionId. A receiver accepts a session and processes messages from that session in the order they were sent. This ensures message ordering within the session. The receiver can store and update session state during processing. Messages from other sessions remain queued until their session is accepted. This process guarantees ordered and grouped message processing.