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			     S E M I N A I R E
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			     I N R I A - Paris
                 2 rue Simone Iff (ou: 41 rue du Charolais)
                          Salle Lions 1, bâtiment C

                           Lundi 25 janvier, 10h30

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                              Jonathan Protzenko
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                              Microsoft Research

                    ======================================
                        The global sequence protocol:
                    a memory model for distributed systems
                    ======================================

In the context of distributed memory models, one has to choose between
consistency and performance. Sequentially consistent systems offer a
compelling abstraction for the programmer, but require complex and costly
synchronization between clients in order to guarantee consistency. If one is
willing to give up strong consistency, then /eventually consistent/ models
provide an alternative. Eventual consistency means the system /eventually/
reaches a state where all clients agree on the same sequence of operations;
before that, though, each client may have local state that disagrees with
other clients.

Eventual consistency may model a processor with local caches and a central
memory; it may also model a distributed cloud storage system with a central
server and a variety of clients. Starting from a functional setting, I will
introduce a very abstract model where clients send functions that fold over an
initial state; then, I will move on to a more concrete, implementable model
called the Global Sequence Protocol and show how to use it in a concrete
application.