# In Order over multiple topics

**URL:** <https://community.hivemq.com/t/in-order-over-multiple-topics/3390>\
**Category:** HiveMQ Client Library\
**Created:** [December 9, 2024, 10:29pm UTC](https://community.hivemq.com/t/in-order-over-multiple-topics/3390 "2024-12-09T22:29:21Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![ounger](https://avatars.discourse-cdn.com/v4/letter/o/5e9695/32.png) [@ounger](https://community.hivemq.com/u/ounger)\
**Post date:** [December 9, 2024, 10:29pm UTC](https://community.hivemq.com/t/in-order-over-multiple-topics/3390/1 "2024-12-09T22:29:21Z")

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How to ensure receiving messages from multiple topics in order?

Lets say I subscribe to two topics A and B and QoS 2 is given:  
client.subscribeWith()  
.topicFilter(“A”)  
.qos(MqttQos.EXACTLY\_ONCE)  
.send();

client.subscribeWith()  
.topicFilter(“B”)  
.qos(MqttQos.EXACTLY\_ONCE)  
.send();

client.publishes().subscribe(SUBSCRIBED, this::onReceive);

When a publisher publishes in this order:

- message MA to topic A
- message MB to topic B  
Will a subscriber as shown above always receive A before B?

Or is the only way setting Receive Maximum on broker and subscriber to 1?  
See in MQTT 5 Reference:

> If both Client and Server set Receive Maximum to 1, they make sure that no more than one message is “in-flight” at any one time. In this case no QoS 1 message will be received after any later one even on re-connection. For example a subscriber might receive them in the order 1,2,3,3,4 but not 1,2,3,2,3,4. Refer to [section 4.9](https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Flow_Control) Flow Control for details of how the Receive Maximum is used.

---

<div class="post-metadata">

**Author:** ![Daria\_H](https://avatars.discourse-cdn.com/v4/letter/d/dfb087/32.png) [@Daria\_H](https://community.hivemq.com/u/Daria_H)\
**Post date:** [February 19, 2025, 11:02am UTC](https://community.hivemq.com/t/in-order-over-multiple-topics/3390/2 "2025-02-19T11:02:47Z")

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Hi @ounger

In MQTT, message ordering is only guaranteed **within** a single topic, not **across** multiple topics. Here’s what happens in your scenario:

### 1. **Default Behavior (No Additional Flow Control)**

- Your client subscribes to **both** topics `A` and `B` with **QoS 2** (Exactly Once).
- The publisher sends:
  1. `MA` to topic `A`
  2. `MB` to topic `B`

- The broker handles these messages separately and may deliver them in **any order** to the client.
- The order of delivery to your subscriber depends on internal processing at the broker and the network.

Thus, the subscriber **is not guaranteed** to always receive `MA` before `MB`, even if they were published in that order.

### 2. **Ensuring Order Across Topics**

To enforce strict ordering across topics, you have two main options:

#### **A. Use a Single Topic**

- Instead of separate topics `A` and `B`, use one topic (e.g., `C`).
- This way, MQTT guarantees **strict message ordering within topic `C`**.

#### **B. Set `Receive Maximum = 1`**

- **Both the broker and the client** must set `Receive Maximum = 1` to ensure only **one** in-flight message at a time.
- This prevents interleaving of messages from different topics.
- This setting is **not ideal** for high-throughput applications because it slows down message processing.

### **Conclusion**

If ordering across topics is critical:

1. **Best Option:** Publish to a single topic and differentiate messages using metadata.
2. **Alternative Option:** Use `Receive Maximum = 1`, but be aware of performance trade-offs.

I hope this helps,  
Best,  
Dasha from The HiveMQ Team