wohali commented on a change in pull request #268: Rewrite sharding documentation URL: https://github.com/apache/couchdb-documentation/pull/268#discussion_r203377375
########## File path: src/cluster/sharding.rst ########## @@ -12,290 +12,490 @@ .. _cluster/sharding: -======== -Sharding -======== +================ +Shard Management +================ .. _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 "shard replicas" or +just "replicas") to different nodes in a cluster gives the data greater +durability against node loss. CouchDB clusters automatically shard +databases and distribute the subsections of documents that compose each +shard among nodes. Modifying cluster membership and sharding 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. The default value for n is 3, +and for q is 8. 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. We recommend in the general case that the number of +nodes in your cluster should be a multiple of n, so that shards are +distributed evenly. - 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 Review comment: The Dynamo paper's primary example is n=3, but it doesn't go into why. ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: us...@infra.apache.org With regards, Apache Git Services
