Can it be guaranteed that messages from a specific user will go to a specific consumer instance?

Guaranteeing that messages from a specific user reach a particular consumer instance can be achieved by using partitioning and keying mechanisms supported by many messaging systems (e.g., Kafka).

The idea is that messages with the same key (for example, user ID) are always directed to the same partition, and each consumer instance is bound to specific partitions. Thus, messages from a particular user will be processed by the same consumer.

However, this depends on the architecture and capabilities of the message broker. In systems without support for partitioning or with dynamic load distribution, guaranteeing this is difficult.

Example with Kafka:

• When sending a message, specify the key — userID.
• Kafka hashes the key and determines the partition.
• Consumers subscribe to partitions.

This ensures sequential processing of a user's messages by the same consumer.