# Advanced Internode (RPC) Compression

Internode (RPC) compression controls whether traffic between nodes is
compressed. If enabled, it reduces network bandwidth usage.

To further reduce network traffic, you can configure ScyllaDB to use
ZSTD-based compression and shared dictionary compression. You can enable one or
both of these features to limit network throughput and reduce network transfer costs.

## Enabling ZSTD-based Compression

ZSTD-based compression utilizes the ZSTD algorithm for RPC compression.

To enable ZSTD-based compression:

1. Set `internode_compression_enable_advanced` to `true`.
2. Set `internode_compression_zstd_max_cpu_fraction` to
   a non-zero value in the range [0, 1], where:
   * 0 - The node will never use ZSTD for RPC compression.
   * 1 - The node will always use ZSTD for RPC compression (for all messages,
     regardless of CPU consumption).

A good rule of thumb is to start from `0.05`. As a result, the node will spend
not more than 5% of its total CPU on RPC ZSTD compression.

Note that enabling ZSTD increases CPU usage. We recommend monitoring metrics
to ensure additional CPU consumption does not disrupt the workload. If ZSTD
impacts performance, try setting `internode_compression_zstd_max_cpu_fraction`
to a lower value.

## Enabling Shared Dictionary Compression

Shared dictionary compression allows you to limit network throughput and reduce
network transfer costs.

If enabled, the message-by-message compression is replaced with a more
efficient compression based on a dictionary trained on one node and shared
with other nodes. The dictionary is periodically re-trained on the node’s
RPC traffic and then distributed across the cluster.

To enable shared dictionaries:

1. Set `internode_compression_enable_advanced` to `true`.
2. Configure the `rpc_dict_training_when` parameter.
   In typical scenarios, you should set it to `when_leader`.
   As a result, the [Raft leader](https://docs.scylladb.com/manual/master/architecture/raft.md) node will
   train and publish new dictionaries.

```default
internode_compression_enable_advanced: true
rpc_dict_training_when: when_leader
```

#### NOTE
Some dictionary training data may be encrypted using storage-level encryption
(if enabled) instead of database-level encryption, meaning protection is
applied at the storage layer rather than within the database itself.

## Reference

See the *Inter-node settings* section in [Configuration Parameters](https://docs.scylladb.com/manual/master/reference/configuration-parameters.md)
for a complete list of internode compression options.