Trait scylla::transport::load_balancing::LoadBalancingPolicy

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pub trait LoadBalancingPolicy: Send + Sync + Debug {
    // Required methods
    fn pick<'a>(
        &'a self,
        query: &'a RoutingInfo<'_>,
        cluster: &'a ClusterData,
    ) -> Option<(NodeRef<'a>, Option<Shard>)>;
    fn fallback<'a>(
        &'a self,
        query: &'a RoutingInfo<'_>,
        cluster: &'a ClusterData,
    ) -> FallbackPlan<'a>;
    fn name(&self) -> String;

    // Provided methods
    fn on_query_success(
        &self,
        _query: &RoutingInfo<'_>,
        _latency: Duration,
        _node: NodeRef<'_>,
    ) { ... }
    fn on_query_failure(
        &self,
        _query: &RoutingInfo<'_>,
        _latency: Duration,
        _node: NodeRef<'_>,
        _error: &QueryError,
    ) { ... }
}
Expand description

Policy that decides which nodes and shards to contact for each query.

When a query is prepared to be sent to ScyllaDB/Cassandra, a LoadBalancingPolicy implementation constructs a load balancing plan. That plan is a list of targets (target is a node + an optional shard) to which the driver will try to send the query. The first elements of the plan are the targets which are the best to contact (e.g. they might have the lowest latency).

Most queries are sent on the first try, so the query execution layer rarely needs to know more than one target from plan. To better optimize that case, LoadBalancingPolicy has two methods: pick and fallback. pick returns the first target to contact for a given query, fallback returns the rest of the load balancing plan.

fallback is called not only if a send to picked node failed (or when executing speculatively), but also if pick returns None.

Usually the driver needs only the first node from load balancing plan (most queries are send successfully, and there is no need to retry).

This trait is used to produce an iterator of nodes to contact for a given query.

Required Methods§

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fn pick<'a>( &'a self, query: &'a RoutingInfo<'_>, cluster: &'a ClusterData, ) -> Option<(NodeRef<'a>, Option<Shard>)>

Returns the first node to contact for a given query.

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fn fallback<'a>( &'a self, query: &'a RoutingInfo<'_>, cluster: &'a ClusterData, ) -> FallbackPlan<'a>

Returns all contact-appropriate nodes for a given query.

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fn name(&self) -> String

Returns the name of load balancing policy.

Provided Methods§

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fn on_query_success( &self, _query: &RoutingInfo<'_>, _latency: Duration, _node: NodeRef<'_>, )

Invoked each time a query succeeds.

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fn on_query_failure( &self, _query: &RoutingInfo<'_>, _latency: Duration, _node: NodeRef<'_>, _error: &QueryError, )

Invoked each time a query fails.

Implementors§