wohali commented on a change in pull request #268: Rewrite sharding 
documentation
URL: 
https://github.com/apache/couchdb-documentation/pull/268#discussion_r179922734
 
 

 ##########
 File path: src/cluster/sharding.rst
 ##########
 @@ -18,43 +18,170 @@ Sharding
 
 .. _cluster/sharding/scaling-out:
 
-Scaling out
-===========
+Introduction
+------------
 
-Normally you start small and grow over time. In the beginning you might do just
-fine with one node, but as your data and number of clients grows, you need to
-scale out.
+A
+`shard <https://en.wikipedia.org/wiki/Shard_(database_architecture)>`__
+is a horizontal partition of data in a database. Partitioning data into
+shards and distributing copies of each shard (called "replicas") to
+different nodes in a cluster gives the data greater durability against
+node loss. CouchDB clusters automatically shard and distribute data
+among nodes, but modifying cluster membership and customizing shard
+behavior must be done manually.
 
-For simplicity we will start fresh and small.
+Shards and Replicas
+~~~~~~~~~~~~~~~~~~~
 
-Start ``node1`` and add a database to it. To keep it simple we will have 2
-shards and no replicas.
+How many shards and replicas each database has can be set at the global
+level, or on a per-database basis. The relevant parameters are *q* and
+*n*.
 
-.. code-block:: bash
+*q* is the number of database shards to maintain. *n* is the number of
+copies of each document to distribute. With q=8, the database is split
+into 8 shards. With n=3, the cluster distributes three replicas of each
+shard. Altogether, that's 24 shards for a single database. In a default
+3-node cluster, each node would receive 8 shards. In a 4-node cluster,
+each node would receive 6 shards.
 
-    curl -X PUT "http://xxx.xxx.xxx.xxx:5984/small?n=1&q=2"; --user daboss
+CouchDB nodes have a ``etc/default.ini`` file with a section named
+``[cluster]`` which looks like this:
 
-If you look in the directory ``data/shards`` you will find the 2 shards.
+::
 
-.. code-block:: text
+    [cluster]
+    q=8
+    n=3
 
-    data/
-    +-- shards/
-    |   +-- 00000000-7fffffff/
-    |   |    -- small.1425202577.couch
-    |   +-- 80000000-ffffffff/
-    |        -- small.1425202577.couch
+These settings can be modified to set sharding defaults for all
+databases, or they can be set on a per-database basis by specifying the
+``q`` and ``n`` query parameters when the database is created. For
+example:
 
-Now, check the node-local ``_dbs_`` database. Here, the metadata for each
-database is stored. As the database is called ``small``, there is a document
-called ``small`` there. Let us look in it. Yes, you can get it with curl too:
+.. code:: bash
 
-.. code-block:: javascript
+    $ curl -X PUT "$COUCH_URL/database-name?q=4&n=2"
 
-    curl -X GET "http://xxx.xxx.xxx.xxx:5986/_dbs/small";
+That creates a database that is split into 4 shards and 2 replicas,
+yielding 8 shards distributed throughout the cluster.
+
+Quorum
+------
+
+When a CouchDB cluster serves reads and writes, it proxies the request
+to nodes with relevant shards and responds once enough nodes have
+responded to establish
+`quorum <https://en.wikipedia.org/wiki/Quorum_(distributed_computing)>`__.
 
 Review comment:
   Wikipedia uses the phrase "to obtain a quorum," for what it's worth.

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