Backup agents are all stateless workers, that coordinate using the database to grab smaller tasks as part of the overall backup. If a backup agent dies, then the work it was assigned will eventually be picked up and completed by some other backup agent.
has links to both the TaskBucket implementation in FDB that’s used to coordinate backup agent work, and the next post down has a link to something similar, but written in python (and smaller, clearer, and probably easier to quickly grok).
This is also all changing in 6.3, and there will be a new backup implementation available that uses a different sort of worker, that instead recruited and managed as part of the cluster. You can read more about that in the design doc.