A swarm is a collection of 5 – 7 Service Nodes that are responsible for the temporary storage of messages for a predefined range of Session IDs. Swarms ensure that your messages are delivered, because even if one service node goes offline — your messages are not lost.
Once a message's time-to-live (TTL) has expired, the swarm will no longer carry a copy of that message.
Swarms make Session’s decentralised network backend more robust and fault-tolerant.