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From kxepal <...@git.apache.org>
Subject [GitHub] couchdb-mem3 pull request: change readme for the couchdb project
Date Mon, 16 Mar 2015 06:56:59 GMT
Github user kxepal commented on a diff in the pull request:

    https://github.com/apache/couchdb-mem3/pull/8#discussion_r26462801
  
    --- Diff: README.md ---
    @@ -1,33 +1,42 @@
     ## mem3
     
    -Mem3 is the node membership application for clustered [CouchDB][1].  It is used in [BigCouch][2]
and tracks two very important things for the cluster:
    +Mem3 is the node membership application for clustered [CouchDB][1]. It is used
    +in CouchDB since version 2.0 and tracks two very important things for the
    +cluster:
     
      1. member nodes
      2. node/shards mappings for each database
     
    -Both the nodes and shards are tracked in node-local couch databases.  Shards are heavily
used, so an ETS cache is also maintained for low-latency lookups.  The nodes and shards are
synchronized via continuous CouchDB replication, which serves as 'gossip' in Dynamo parlance.
 The shards ETS cache is kept in sync based on membership and database event listeners.
    -
    -A very important point to make here is that BigCouch does not necessarily divide up each
database into equal shards across the nodes of a cluster.  For instance, in a 20-node cluster,
you may have the need to create a small database with very few documents.  For efficiency
reasons, you may create your database with Q=4 and keep the default of N=3.  This means you
only have 12 shards total, so 8 nodes will hold none of the data for this database.  Given
this feature, we even shard use out across the cluster by altering the 'start' node for the
database's shards.
    -
    -Splitting and merging shards is an immature feature of the system, and will require attention
in the near-term.  We believe we can implement both functions and perform them while the database
remains online.
    +Both the nodes and shards are tracked in node-local couch databases.  Shards
    +are heavily used, so an ETS cache is also maintained for low-latency lookups.
    +The nodes and shards are synchronized via continuous CouchDB replication,
    +which serves as 'gossip' in Dynamo parlance.  The shards ETS cache is kept in
    +sync based on membership and database event listeners.
    +
    +A very important point to make here is that CouchDB >= 2.0 does not
    +necessarily divide up each database into equal shards across the nodes of a
    +cluster.  For instance, in a 20-node cluster, you may have the need to create
    +a small database with very few documents.  For efficiency reasons, you may
    +create your database with Q=4 and keep the default of N=3.  This means you
    +only have 12 shards total, so 8 nodes will hold none of the data for this
    +database.  Given this feature, we even shard use out across the cluster by
    +altering the 'start' node for the database's shards.
    +
    +Splitting and merging shards is an immature feature of the system, and will
    +require attention in the near-term.  We believe we can implement both
    +functions and perform them while the database remains online.
     
     ### Getting Started
     
    -Mem3 requires R13B03 or higher and can be built with [rebar][6], which comes bundled
in the repository.  Rebar needs to be able to find the `couch_db.hrl` header file; one way
to accomplish this is to set ERL_LIBS to point to the apps
    -subdirectory of a bigcouch checkout, e.g.
    +Mem3 requires R13B03 or higher and can be built with [rebar][2], which comes
    +bundled in the repository.  Rebar needs to be able to find the `couch_db.hrl`
    +header file; one way to accomplish this is to set ERL_LIBS to point to the
    +apps subdirectory of a CouchDB checkout, e.g.
     
    -    ERL_LIBS="/usr/local/src/bigcouch/apps" ./rebar compile
    +    ERL_LIBS="/usr/local/src/couchdb/apps" ./rebar compile
     
     ### License
     [Apache 2.0][3]
     
    -### Contact
    - * [http://cloudant.com][4]
    - * [info@cloudant.com][5]
    -
    -[1]: http://couchdb.apache.org
    -[2]: http://github.com/cloudant/bigcouch
    -[3]: http://www.apache.org/licenses/LICENSE-2.0.html
    -[4]: http://cloudant.com
    -[5]: mailto:info@cloudant.com
    -[6]: http://github.com/basho/rebar
    +[1]: http://couchdb.apache.org [2]: http://github.com/rebar/rebar [3]:
    --- End diff --
    
    Oh no! Someone had stole newlines here!


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