Pulsar subscription types: which one keeps your message ordering, and which one quietly drops it

In Pulsar, the subscription type decides how messages are shared out among consumers, and with it whether ordering survives. Teams arriving from other platforms often pick the type that scales best and only later discover what it cost them. All four are defined in Pulsar's messaging concepts documentation [1].

The four types

• Exclusive only allows a single consumer to attach to the subscription [1]. It is the default type [1]. With one consumer reading, messages are processed in the order they arrive — at the price of no parallelism.
• Failover lets multiple consumers attach, but picks a master consumer for a non-partitioned topic, or for each partition of a partitioned topic, and that consumer receives the messages [1]. The others stand by, so ordering holds within each partition while still surviving a consumer failure.
• Shared lets multiple consumers attach and delivers messages in a round-robin distribution [1]. The documentation is explicit that Shared subscriptions do not guarantee message ordering [1].
• Key_Shared also distributes messages across consumers, but messages with the same key are delivered to only one consumer at a time, which preserves the order of delivery by key [1].

The wider point

The subscription type is a correctness decision disguised as a scaling knob. Engineers who have run Pulsar in production ask what order the consumers need before they ask how many there should be — which is why it sits in the messaging screen we publish.