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Introduction to the Grapevine Distributed System

Introduction to the Grapevine Distributed System. CS-4513 Distributed Computing Systems. Grapevine. A distributed system for e-mail and other distributed activities Implemented at Xerox PARC in ~1978-80 Deployed throughout company Productized in Xerox 8000 series office systems

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Introduction to the Grapevine Distributed System

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  1. Introduction to theGrapevine Distributed System CS-4513Distributed Computing Systems Grapevine

  2. Grapevine • A distributed system for e-mail and other distributed activities • Implemented at Xerox PARC in ~1978-80 • Deployed throughout company • Productized in Xerox 8000 series office systems • Predates most Internet mail systems • Embodies many principles of distributed computing systems • … Grapevine

  3. Grapevine (references) • … • Birrell, et. al., “Grapevine: An Exercise in Distributed Computing,” Communications of ACM, vol. 25, #4, April 1982, pp 260-274. (pdf) • Schroeder, et. al., “Experience with Grapevine: The Growth of a Distributed System,” ACM Transactions on Computer Systems, vol. 2, #1, Feb 1984, pp 3-23. (pdf) Grapevine

  4. Goals • Delivery mechanism for large, dispersed e-mail system • Previous e-mail system were time-sharing based • (now) Familiar model • Senders • Recipients • Distribution lists • Buffered delivery • Independent clients (e-mail agents) Grapevine

  5. Design Goals • No assumptions about … • Message content • Client correctness • Message delivery guarantee • If accepted, message would be delivered to recipient’s inbox • or … returned with error • Failure of server  unavailability of service • Decentralized administration Grapevine

  6. Key Components • Registration Data Base • Distributed • Replicated • Recursively implemented • Message Delivery System • Transports, buffers, delivers • Expands distribution lists • Distributed • Replicated Grapevine

  7. Issues Exposed by Grapevine • Reliable Transport and Storage • Synchronizing and Ordering of Operations • Fault-tolerance • Consistency and Replication • Security and Authentication • … Grapevine

  8. Registration Database • {RName, value} pairs • Two types of entry • Individual • Group • Group entry • value = list of RNames — i.e., the group members • May be distribution list, resource list, access control list, etc. • Individual entry • value = [authenticator, ordered inbox list, contact site, …] • May be human users, servers, etc. Grapevine

  9. Organization • Registry • Subset of the registration database entries • Organizational, geographic, or other convenient partition • RName – two-part name Name.R • R is a registry • Name is a unique name within that particular registry • (Easily expandable to three part names or more) Grapevine

  10. Functions Provided by Grapevine Service Delivery services:– • Accept message: • [sender, password, recipients, message-body]  ok • Message polling: • [individual]  {empty, non-empty} • Retrieve messages: • [name, password]  sequence of messages  ok • … Grapevine

  11. Functions Provided by Grapevine Service (continued) Registry services:– • Authenticate: • [individual, password]  {authentic, bogus} • Membership: • [name, group]  {member, non-member} • Resource location: • [group]  members • [individual]  contact site • [individual]  ordered list of inbox sites • … Grapevine

  12. Grapevine Server • Contains both Registration and Message servers • Independent of each other • Communicate only by internet protocols • Registration server • Contains replicas of one or more Registries • Can accept change request for any of its registries • Propagates changes to other replicas of Registry • Message server • Accepts any message for delivery • Stores inboxes for some individuals • (Individuals typically have more than one inbox) Grapevine

  13. Grapevine “User Package” • Library code for Grapevine client programs • E-mail clients • Other kinds of distributed applications • Handles all naming and addressing issues • Clients never need to know name or address of any Grapevine server • Servers optimized based on “User Package” Grapevine

  14. Grapevine Operation • User P.Q sends message to user X.Y • User X.Y wants authenticated access to server E Grapevine

  15. Message Delivery • Find Message service • Present [sender, password, recipients] to service • Authenticates sender • Validates recipients • (Assume valid if recipient registry not available) • Construct envelope (aka property list) • Sender Name, Return-to, recipient list, postmark • Postmark = [server internet addr, server time stamp] • Accept message-body • Store envelope & message body in reliable storage • Acknowledge receipt of message Grapevine

  16. Message Delivery (continued)Transport system • Recursively enumerate groups in destination list • Find inbox server contact for each recipient • Sort by server, transmit one copy to each server with list of recipient names • Duplicate elimination done at receiving server • E.g., an individual a member of more than one group • Queue if all servers down or inaccessible • Receiving client polls all its inboxes • Downloads from each • Message deleted from inbox only after receipt is acknowledged Grapevine

  17. Replication in Message Delivery • Any server can accept a message for delivery • Individuals typically have more than one inbox • However, • Crashes can delay messages in transit. Grapevine

  18. Registration Database • gv registry • Replicated in all registration servers • Groups in gv • Names of registries • Members are RNames of registration servers holding replicas of the group registry • I.e., • reg is a Registry iff there is a group reg.gv • Server holds a replica of reg iff its name is in reg.gv Grapevine

  19. Registration Database (continued) • gv.gv is a group in gv registry • Its members are RNames of all registration servers • Adding a registration server  adding a member to gv.gv • Adding a new registry r  adding a new group entry r.gv to gv registry • Members of r.gv are a subset of gv.gv Grapevine

  20. Registration Database (continued) • Used to locate resources • Service is a group in the database • Members are RNames of servers • Servers are individuals in the database • Contact site is internet address of server • To find foo.bar • Contact local Grapevine server, ask for bar.gv • Enumerate members, contact one of them, and ask for foo.bar Grapevine

  21. Finding the first Grapevine Server • Ask another name service (e.g., DNS) for GrapevineRServer • Returns a list of addresses • List changes very infrequently or • Broadcast to well-known socket on LAN • Local Grapevine servers listen on socket • Usually quicker Grapevine

  22. Summary • Names in Grapevine • People • Services • Servers • Anything else you want them to be • Registry maps names to … • Groups (i.e., a list of other names) • Individuals (something with an address and/or other properties) Grapevine

  23. Summary (continued) • Highly replicated • Registries exist in multiple servers • Individuals have more than one mailbox • Mail can be delivered to anywhere for any recipient • Registry used to manage itself! • Distributed System issues to be addressed • Synchronizing and Ordering of Operations • Fault-tolerance • Consistency and Replication • Security and Authentication Grapevine

  24. Break Next Topic Grapevine

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