Data Distribution with Tablets¶

Data Distribution¶

ScyllaDB distributes data by splitting tables into tablets. Each tablet has its replicas on different nodes, depending on the RF (replication factor). Each partition of a table is mapped to a single tablet in a deterministic way. When you query or update the data, ScyllaDB can quickly identify the tablet that stores the relevant partition.

The following example shows a 3-node cluster with a replication factor (RF) of 3. The data is stored in a table (Table 1) with two rows. Both rows are mapped to one tablet (T1) with replicas on all three nodes.

Scaling Out¶

A tablet can be dynamically migrated to an existing node or a newly added empty node. Paired with consistent topology updates with Raft, tablets allow you to add multiple nodes simultaneously. After nodes are added to the cluster, existing nodes stream data to the new ones, and the system load eventually converges to an even distribution as the process completes.

With tablets enabled, manual cleanup is not required. Cleanup is performed automatically per tablet, making tablets-based streaming user-independent and safer.

In addition, tablet cleanup is lightweight and efficient, as it doesn’t involve rewriting SStables on the existing nodes, which makes data ownership changes faster. This dramatically reduces the impact of cleanup on the performance of user queries.

The following diagrams show migrating tablets from heavily loaded nodes A and B to a new node.

Enabling Tablets¶

ScyllaDB now uses tablets by default for data distribution. Enabling tablets by default when creating new keyspaces is controlled by the enable_tablets option. However, tablets only work if supported on all nodes within the cluster.

When creating a new keyspace with tablets enabled by default, you can still opt-out on a per-keyspace basis. The recommended NetworkTopologyStrategy for keyspaces remains required even if tablets are disabled.

You can create a keyspace with tablets disabled with the tablets = {'enabled': false} option:

CREATE KEYSPACE my_keyspace
WITH replication = {
    'class': 'NetworkTopologyStrategy',
    'replication_factor': 3
} AND tablets = {
    'enabled': false
};

When creating a new keyspace with tablets disabled by default, you can still opt-in on a per-keyspace basis. The recommended NetworkTopologyStrategy for keyspaces remains required when using tablets.

You can create a keyspace with tablets enabled with the tablets = {'enabled': true} option:

CREATE KEYSPACE my_keyspace
WITH replication = {
    'class': 'NetworkTopologyStrategy',
    'replication_factor': 3
} AND tablets = {
    'enabled': true
};

Limitations and Unsupported Features¶

The following ScyllaDB features are not supported if a keyspace has tablets enabled:

• Counters

• Change Data Capture (CDC)

• Lightweight Transactions (LWT)

• Alternator (as it uses LWT)

If you plan to use any of the above features, CREATE your keyspace with tablets disabled.

Resharding in keyspaces with tablets enabled has the following limitations:

• ScyllaDB does not support reducing the number of shards after node restart.

• ScyllaDB does not reshard data on node restart. Tablet replicas remain allocated to the old shards on restart and are subject to background load-balancing to additional shards after restart completes and the node starts serving CQL.