flimzy commented on a change in pull request #268: Rewrite sharding documentation URL: https://github.com/apache/couchdb-documentation/pull/268#discussion_r203283458
########## 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 - 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 +~~~~~~ + +Depending on the size of the cluster, the number of shards per database, +and the number of shard replicas, not every node may have access to +every shard, but every node knows where all the replicas of each shard +can be found through CouchDB's internal shard map. + +Each request that comes in to a CouchDB cluster is handled by any one +random coordinating node. This coordinating node proxies the request to +the other nodes that have the relevant data, which may or may not +include itself. The coordinating node sends a response to the client +once a `quorum +<https://en.wikipedia.org/wiki/Quorum_(distributed_computing)>`__ of +database nodes have responded; 2, by default. + +The size of the required quorum can be configured at +request time by setting the ``r`` parameter for document and view +reads, and the ``w`` parameter for document writes. For example, here +is a request that specifies that at least two nodes must respond in +order to establish a quorum: + +.. code:: bash + + $ curl "$COUCH_URL:5984/<doc>?r=2" + +Here is a similar example for writing a document: + +.. code:: bash + + $ curl -X PUT "$COUCH_URL:5984/<doc>?w=2" -d '{...}' + +Setting ``r`` or ``w`` to be equal to ``n`` (the number of replicas) +means you will only receive a response once all nodes with relevant +shards have responded or timed out, and as such this approach does not +guarantee `ACIDic consistency +<https://en.wikipedia.org/wiki/ACID#Consistency>`__. Setting ``r`` or +``w`` to 1 means you will receive a response after only one relevant +node has responded. + +.. _cluster/sharding/move: + +Moving a shard +-------------- + +This section describes how to manually place and replace shards, and how +to set up placement rules to assign shards to specific nodes. These +activities are critical steps when you determine your cluster is too big +or too small, and want to resize it successfully, or you have noticed +from server metrics that database/shard layout is non-optimal and you +have some "hot spots" that need resolving. + +Consider a three node cluster with q=8 and n=3. Each database has 24 Review comment: "three node" should be hyphenated: > Consider a three-node cluster ... ---------------------------------------------------------------- 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. 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