Grapevine

• Grapevine was an early distributed system.
• Think of Grapevine as a distributed email system. Literally every application is forced to fit this model.
• Components:
  • highly replicated distributed registration database ("DNS") which provides name server lookups; hierarchical registries ("domain name hierarchy"). Bootstrapping is fun.
  • message servers: accept, queue, forward, & deliver (with duplicate elimination) incoming email ("sendwhale"). Any message server will accept & route any incoming message. "bounce message" sent when delivery impossible. Replication of mail servers, but not of messages. Also sends current inbox to authenticated users, who make a copy (yes, another copy!) for themselves.
  • inbox: a user's buffer where he receives email ("mail spool file"). Can be replicated.
• Implementation experience:
  • updating the registration database: a damned pain in the neck. Grapevine trades off atomicity for simplicity. (Changing a decentralized distributed database is nontrivial.) Updates propagate via email (!), using flooding algorithm.
  • naming: naming is distinct from addressing; you send email to "Dave", and the servers figure out where "Dave" is located; you don't send email to e.g. "daw@cs.berkeley.edu". Creating new names requires manual intervention to screen the appropriateness of the names. (By the way, that's a clue that somewhere folks are doing name-based authentication...)
  • locating a user's inbox: cache lookups of names to inbox sites; cache list of inaccessible message servers (but this is just a hint, since any message server will accept any email).
  • hindsight: should've allowed dynamic addition of new fields in the Grapevine distributed database. Replication for availability worked well to stop hardware failures, but there wasn't enough diversity in software implementations to survive a software bug. (a cute interaction with the reliable message transport protocol.) Testing with lots of users provided interesting insights.
• Key point: notice the similarities, and differences, between Grapevine and "modern-day" distributed systems.