2.1 Pulsar’s physical architecture2.1.1 Pulsar’s layered architecture2.1.2 Stateless serving layer2.1.3 Stream storage layer2.1.4 Metadata storage2.2 Pulsar’s logical architecture2.2.1 Tenants, namespaces, and topics2.2.2 Addressing topics in Pulsar2.2.3 Producers, consumers, and subscriptions2.2.4 Subscription types2.3 Message retention and expiration2.3.1 Data retention2.3.2 Backlog quotas2.3.3 Message expiration2.3.4 Message backlog vs. message expiration2.4 Tiered storageSummary