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0999fad279
This commit adds a link to the Limitations section on the Tablets page to the CQL pag, the tablets option. This is actually the place where the user will need the information: when creating a keyspace. In addition, I've reorganized the section for better readability (otherwise, the section about limitations was easy to miss) and moved the section up on the page. Note that I've removed the updated content from the `_common` folder (which I deleted) to the .rst page - we no longer split OSS and Enterprise, so there's no need to keep using the `scylladb_include_flag` directive to include OSS- and Ent-specific content. Fixes https://github.com/scylladb/scylladb/issues/22892 Fixes https://github.com/scylladb/scylladb/issues/22940 Closes scylladb/scylladb#22939
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ReStructuredText
=========================================
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Data Distribution with Tablets
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=========================================
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A ScyllaDB cluster is a group of interconnected nodes. The data of the entire
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cluster has to be distributed as evenly as possible across those nodes.
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ScyllaDB is designed to ensure a balanced distribution of data by storing data
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in tablets. When you add or remove nodes to scale your cluster, add or remove
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a datacenter, or replace a node, tablets are moved between the nodes to keep
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the same number on each node. In addition, tablets are balanced across shards
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in each node.
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This article explains the concept of tablets and how they let you scale your
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cluster quickly and seamlessly.
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Data Distribution
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-------------------
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ScyllaDB distributes data by splitting tables into tablets. Each tablet has
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its replicas on different nodes, depending on the RF (replication factor). Each
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partition of a table is mapped to a single tablet in a deterministic way. When you
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query or update the data, ScyllaDB can quickly identify the tablet that stores
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the relevant partition.
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The following example shows a 3-node cluster with a replication factor (RF) of
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3. The data is stored in a table (Table 1) with two rows. Both rows are mapped
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to one tablet (T1) with replicas on all three nodes.
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.. image:: images/tablets-cluster.png
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.. TODO - Add a section about tablet splitting when there are more triggers,
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like throughput. In 6.0, tablets only split when reaching a threshold size
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(the threshold is based on the average tablet data size).
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Load Balancing
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==================
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ScyllaDB autonomously moves tablets to balance the load. This process
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is managed by a load balancer mechanism and happens independently of
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the administrator. The tablet load balancer decides where to migrate
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the tablets, either within the same node to balance the shards or across
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the nodes to balance the global load in the cluster.
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As a table grows, each tablet can split into two, creating a new tablet.
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The load balancer can migrate the split halves independently to different nodes
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or shards.
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The load-balancing process takes place in the background and is performed
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without any service interruption.
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Scaling Out
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=============
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A tablet can be dynamically migrated to an existing node or a newly added
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empty node. Paired with consistent topology updates with Raft, tablets allow
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you to add multiple nodes simultaneously. After nodes are added to the cluster,
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existing nodes stream data to the new ones, and the system load eventually
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converges to an even distribution as the process completes.
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With tablets enabled, manual cleanup is not required.
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Cleanup is performed automatically per tablet,
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making tablets-based streaming user-independent and safer.
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In addition, tablet cleanup is lightweight and efficient, as it doesn't
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involve rewriting SStables on the existing nodes, which makes data ownership
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changes faster. This dramatically reduces
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the impact of cleanup on the performance of user queries.
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The following diagrams show migrating tablets from heavily loaded nodes A and B
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to a new node.
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.. image:: images/tablets-load-balancing.png
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File-based Streaming
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========================
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Migrating tablets is performed by streaming entire
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SStables, which does not require (de)serializing or processing mutation fragments.
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As a result, less data is streamed over the network, and less CPU is consumed,
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especially for data models that contain small cells.
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File-based streaming is used for tablet migration in all
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:ref:`keyspaces created with tablets enabled <tablets>`.
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.. _tablets-enable-tablets:
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Enabling Tablets
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-------------------
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ScyllaDB now uses tablets by default for data distribution.
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Enabling tablets by default when creating new keyspaces is
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controlled by the :confval:`enable_tablets` option. However, tablets only work if
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supported on all nodes within the cluster.
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When creating a new keyspace with tablets enabled by default, you can still opt-out
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on a per-keyspace basis. The recommended ``NetworkTopologyStrategy`` for keyspaces
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remains *required* even if tablets are disabled.
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You can create a keyspace with tablets
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disabled with the ``tablets = {'enabled': false}`` option:
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.. code:: cql
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CREATE KEYSPACE my_keyspace
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WITH replication = {
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'class': 'NetworkTopologyStrategy',
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'replication_factor': 3
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} AND tablets = {
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'enabled': false
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};
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When creating a new keyspace with tablets disabled by default, you can still opt-in
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on a per-keyspace basis. The recommended ``NetworkTopologyStrategy`` for keyspaces
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remains *required* when using tablets.
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You can create a keyspace with tablets enabled with the ``tablets = {'enabled': true}`` option:
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.. code:: cql
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CREATE KEYSPACE my_keyspace
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WITH replication = {
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'class': 'NetworkTopologyStrategy',
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'replication_factor': 3
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} AND tablets = {
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'enabled': true
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};
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.. warning::
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You cannot ALTER a keyspace to enable or disable tablets.
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The only way to update the tablet support for a keyspace is to DROP it
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(losing the schema and data) and then recreate it after redefining
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the keyspace schema with ``tablets = { 'enabled': false }`` or
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``tablets = { 'enabled': true }``.
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.. _tablets-limitations:
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Limitations and Unsupported Features
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--------------------------------------
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The following ScyllaDB features are not supported if a keyspace has tablets
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enabled:
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* Counters
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* Change Data Capture (CDC)
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* Lightweight Transactions (LWT)
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* Alternator (as it uses LWT)
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If you plan to use any of the above features, CREATE your keyspace
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:ref:`with tablets disabled <tablets-enable-tablets>`.
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The following ScyllaDB features are disabled by default when used with a keyspace
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that has tablets enabled:
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* Materialized Views (MV)
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* Secondary indexes (SI, as it depends on MV)
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To enable MV and SI for tablet keyspaces, use the `--experimental-features=views-with-tablets`
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configuration option. See :ref:`Views with tablets <admin-views-with-tablets>` for details.
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Resharding in keyspaces with tablets enabled has the following limitations:
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* ScyllaDB does not support reducing the number of shards after node restart.
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* ScyllaDB does not reshard data on node restart. Tablet replicas remain
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allocated to the old shards on restart and are subject to background
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load-balancing to additional shards after restart completes and the node
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starts serving CQL.
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