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				SECTION8

			      Irene Zhang
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Table of Contents
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1 Administrivia
2 Lab Architecture
3 Shardmaster
.. 3.1 Interface
.. 3.2 Replication
.. 3.3 Balance


1 Administrivia
===============

  - Jeff Dean in EEB 105 at 3:30 = architect of Spanner


2 Lab Architecture
==================

  - Shards that each hold a subset of the keys
  - replica groups of storage servers
  - Each replica group gets assigned a few shards
  - Centralized shardmaster that holds the assignment between shards and
    groups (Part A)
  - Replica groups serves keys for their assigned shards (Part B)


3 Shardmaster
=============

  - system moves through sequence of configurations of mapping between
    shards and groups. first config is 0, all shards = gid 0
  - shardmaster dictates what configuration the system is in


3.1 Interface
~~~~~~~~~~~~~

  - JOIN(gid)
  - LEAVE(gid)
  - MOVE(shard id, gid)
  - QUERY(number) if -1 or bigger than known config, return biggest


3.2 Replication
~~~~~~~~~~~~~~~

  - use your kv store from last lab
  - one instance per config or one instance per change will work
  - so either replace your put/get calls with shardmaster calls
  - or use instances to agree to next config (but remember if you lose,
    you might have to change your next config membership)


3.3 Balance
~~~~~~~~~~~

  - Want to ensure that group sizes are balanced. tests will check!
  - And ensure you move as few shards as possible with each change
  - I just took one shard from the largest group each time, but there
    are other ways to do it
  - small number of shards (~10), so no need to do anything fancy like
    consistent hashing