ScyllaDB Docs Scylla Rust Driver Load balancing DefaultPolicy

DefaultPolicy¶

DefaultPolicy is the default load balancing policy in Scylla Rust Driver. It can be configured to be datacenter-aware and token-aware. Datacenter failover for queries with non-local consistency mode is also supported.

Creating a DefaultPolicy¶

DefaultPolicy can be created only using DefaultPolicyBuilder. The builder() method of DefaultPolicy returns a new instance of DefaultPolicyBuilder with the following default values:

preferred_datacenter: None
is_token_aware: true
permit_dc_failover: false
latency_awareness: None

You can use the builder methods to configure the desired settings and create a DefaultPolicy instance:

use scylla::load_balancing::DefaultPolicy;

let default_policy = DefaultPolicy::builder()
        .prefer_datacenter("dc1".to_string())
        .token_aware(true)
        .permit_dc_failover(true)
        .build();

Semantics of `DefaultPolicy`¶

Preferred Datacenter¶

The preferred_datacenter field in DefaultPolicy allows the load balancing policy to prioritize nodes based on their location. When a preferred datacenter is set, the policy will treat nodes in that datacenter as “local” nodes, and nodes in other datacenters as “remote” nodes. This affects the order in which nodes are returned by the policy when selecting replicas for read or write operations. If no preferred datacenter is specified, the policy will treat all nodes as local nodes.

When datacenter failover is disabled (permit_dc_failover is set to false), the default policy will only include local nodes in load balancing plans. Remote nodes will be excluded, even if they are alive and available to serve requests.

Datacenter Failover¶

In the event of a datacenter outage or network failure, the nodes in that datacenter may become unavailable, and clients may no longer be able to access the data stored on those nodes. To address this, the DefaultPolicy supports datacenter failover, which allows to route requests to nodes in other datacenters if the local nodes are unavailable.

Datacenter failover can be enabled in DefaultPolicy by permit_dc_failover setting in the builder. When this flag is set, the policy will prefer to return alive remote replicas if datacenter failover is permitted and possible due to consistency constraints.

Token awareness¶

Token awareness refers to a mechanism by which the driver is aware of the token range assigned to each node in the cluster. Tokens are assigned to nodes to partition the data and distribute it across the cluster.

When a user wants to read or write data, the driver can use token awareness to route the request to the correct node based on the token range of the data being accessed. This can help to minimize network traffic and improve performance by ensuring that the data is accessed locally as much as possible.

In the case of DefaultPolicy, token awareness is enabled by default, meaning that the policy will prefer to return alive local replicas if the token is available. This means that if the client is requesting data that falls within the token range of a particular node, the policy will try to route the request to that node first, assuming it is alive and responsive.

Token awareness can significantly improve the performance and scalability of applications built on Scylla. By using token awareness, users can ensure that data is accessed locally as much as possible, reducing network overhead and improving throughput.

Please note that for token awareness to be applied, a statement must be prepared before being executed.

Latency awareness¶

Latency awareness is a mechanism that penalises nodes whose measured recent average latency classifies it as falling behind the others.

Every update_rate the global minimum average latency is computed, and all nodes whose average latency is worse than exclusion_threshold times the global minimum average latency become penalised for retry_period. Penalisation involves putting those nodes at the very end of the query plan. As it is often not truly beneficial to prefer faster non-replica than replicas lagging behind the non-replicas, this mechanism may as well worsen latencies and/or throughput.

Warning

Using latency awareness is NOT recommended, unless prior benchmarks prove its beneficial impact on the specific workload’s performance. Use with caution.

Creating a latency aware DefaultPolicy¶

use scylla::load_balancing::{
    LatencyAwarenessBuilder, DefaultPolicy
};
use std::time::Duration;

let latency_awareness_builder = LatencyAwarenessBuilder::new()
    .exclusion_threshold(3.)
    .update_rate(Duration::from_secs(3))
    .retry_period(Duration::from_secs(30))
    .minimum_measurements(200);

let policy = DefaultPolicy::builder()
        // Here further customisation is, of course, possible.
        // e.g.: .prefer_datacenter(...)
        .latency_awareness(latency_awareness_builder)
        .build();

use scylla::load_balancing::DefaultPolicy;

let default_policy = DefaultPolicy::builder()
        .prefer_datacenter("dc1".to_string())
        .token_aware(true)
        .permit_dc_failover(true)
        .build();

Node order in produced plans¶

The DefaultPolicy prefers to return nodes in the following order:

Alive local replicas (if token is available & token awareness is enabled)
Alive remote replicas (if datacenter failover is permitted & possible due to consistency constraints)
Alive local nodes
Alive remote nodes (if datacenter failover is permitted & possible due to consistency constraints)
Enabled down nodes And only if latency awareness is enabled:
Penalised: alive local replicas, alive remote replicas, … (in order as above).

If no preferred datacenter is specified, all nodes are treated as local ones.

Replicas in the same priority groups are shuffled. Non-replicas are randomly rotated (similarly to a round robin with a random index).