In Pulsar, a topic (or a partition) is owned by a single Broker meaning that all reads and writes go through that broker. Brokers cache bookies entries in memory so that it can dispatch messages directly to all consumers. This will avoid a network round-trip and a possible disk read on bookies.
In addition, you should note that a broker must send data over the network for each subscription. This can lead to network saturation if you have a very high throughput.
Moreover Bookies also have a write/reach cache to reduce disk access.
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