add enforce_target_node on batch

scylla/src/statement/batch.rs

@@ -3,7 +3,8 @@ use std::sync::Arc;
 use crate::history::HistoryListener;
 use crate::retry_policy::RetryPolicy;
 use crate::statement::{prepared_statement::PreparedStatement, query::Query};
-use crate::transport::execution_profile::ExecutionProfileHandle;
+use crate::transport::{execution_profile::ExecutionProfileHandle, Node};
+use crate::Session;
 
 use super::StatementConfig;
 pub use super::{Consistency, SerialConsistency};
@@ -144,6 +145,82 @@ impl Batch {
     pub fn get_execution_profile_handle(&self) -> Option<&ExecutionProfileHandle> {
         self.config.execution_profile_handle.as_ref()
     }
+
+    /// Associates the batch with a new execution profile that will have a load balancing policy
+    /// that will enforce the use of the provided [`Node`] to the extent possible.
+    ///
+    /// This should typically be used in conjunction with [`Session::shard_for_statement`], where
+    /// you would constitute a batch by assigning to the same batch all the statements that would be executed in
+    /// the same shard.
+    ///
+    /// Since it is not guaranteed that subsequent calls to the load balancer would re-assign the statement
+    /// to the same node, you should use this method to enforce the use of the original node that was envisioned by
+    /// `shard_for_statement` for the batch:
+    ///
+    /// ```rust
+    /// # use scylla::Session;
+    /// # use std::error::Error;
+    /// # async fn check_only_compiles(session: &Session) -> Result<(), Box<dyn Error>> {
+    /// use scylla::{
+    ///     batch::Batch,
+    ///     frame::value::{SerializedValues, ValueList},
+    /// };
+    ///
+    /// let prepared_statement = session
+    ///     .prepare("INSERT INTO ks.tab(a, b) VALUES(?, ?)")
+    ///     .await?;
+    ///
+    /// let serialized_values: SerializedValues = (1, 2).serialized()?.into_owned();
+    /// let shard = session.shard_for_statement(&prepared_statement, &serialized_values)?;
+    ///
+    /// // Send that to a task that will handle statements targeted to the same shard
+    ///
+    /// // On that task:
+    /// // Constitute a batch with all the statements that would be executed in the same shard
+    ///
+    /// let mut batch: Batch = Default::default();
+    /// if let Some((node, _shard_idx)) = shard {
+    ///     batch.enforce_target_node(&node, &session);
+    /// }
+    /// let mut batch_values = Vec::new();
+    ///
+    /// // As the task handling statements targeted to this shard receives them,
+    /// // it appends them to the batch
+    /// batch.append_statement(prepared_statement);
+    /// batch_values.push(serialized_values);
+    ///
+    /// // Run the batch
+    /// session.batch(&batch, batch_values).await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    ///
+    /// If the target node is not available anymore at the time of executing the statement, it will fallback to the
+    /// original load balancing policy:
+    /// - Either that currently set on the [`Batch`], if any
+    /// - Or that of the [`Session`] if there isn't one on the `Batch`
+    pub fn enforce_target_node<'a>(
+        &mut self,
+        node: &Arc<Node>,
+        base_execution_profile_from_session: &Session,
+    ) {
+        let execution_profile_handle = self.get_execution_profile_handle().unwrap_or_else(|| {
+            base_execution_profile_from_session.get_default_execution_profile_handle()
+        });
+        self.set_execution_profile_handle(Some(
+            execution_profile_handle
+                .pointee_to_builder()
+                .load_balancing_policy(Arc::new(
+                    crate::load_balancing::EnforceTargetNodePolicy::new(
+                        node,
+                        execution_profile_handle.load_balancing_policy(),
+                    ),
+                ))
+                .build()
+                .into_handle(),
+        ))
+    }
 }
 
 impl Default for Batch {

scylla/src/transport/execution_profile.rs

@@ -485,4 +485,8 @@ impl ExecutionProfileHandle {
     pub fn map_to_another_profile(&mut self, profile: ExecutionProfile) {
         self.0 .0.store(profile.0)
     }
+
+    pub fn load_balancing_policy(&self) -> Arc<dyn LoadBalancingPolicy> {
+        self.0 .0.load().load_balancing_policy.clone()
+    }
 }

scylla/src/transport/load_balancing/enforce_node.rs

@@ -0,0 +1,42 @@
+use super::{FallbackPlan, LoadBalancingPolicy, NodeRef, RoutingInfo};
+use crate::transport::{cluster::ClusterData, Node};
+use std::sync::Arc;
+
+#[derive(Debug)]
+pub struct EnforceTargetNodePolicy {
+    target_node: uuid::Uuid,
+    fallback: Arc<dyn LoadBalancingPolicy>,
+}
+
+impl EnforceTargetNodePolicy {
+    pub fn new(target_node: &Arc<Node>, fallback: Arc<dyn LoadBalancingPolicy>) -> Self {
+        Self {
+            target_node: target_node.host_id,
+            fallback,
+        }
+    }
+}
+impl LoadBalancingPolicy for EnforceTargetNodePolicy {
+    fn pick<'a>(&'a self, query: &'a RoutingInfo, cluster: &'a ClusterData) -> Option<NodeRef<'a>> {
+        cluster
+            .known_peers
+            .get(&self.target_node)
+            .or_else(|| self.fallback.pick(query, cluster))
+    }
+
+    fn fallback<'a>(
+        &'a self,
+        query: &'a RoutingInfo,
+        cluster: &'a ClusterData,
+    ) -> FallbackPlan<'a> {
+        self.fallback.fallback(query, cluster)
+    }
+
+    fn name(&self) -> String {
+        format!(
+            "Enforce target node Load balancing policy - Node: {} - fallback: {}",
+            self.target_node,
+            self.fallback.name()
+        )
+    }
+}

scylla/src/transport/load_balancing/mod.rs

@@ -9,9 +9,13 @@ use scylla_cql::{errors::QueryError, frame::types};
 use std::time::Duration;
 
 mod default;
+mod enforce_node;
 mod plan;
-pub use default::{DefaultPolicy, DefaultPolicyBuilder, LatencyAwarenessBuilder};
 pub use plan::Plan;
+pub use {
+    default::{DefaultPolicy, DefaultPolicyBuilder, LatencyAwarenessBuilder},
+    enforce_node::EnforceTargetNodePolicy,
+};
 
 /// Represents info about statement that can be used by load balancing policies.
 #[derive(Default, Clone, Debug)]

scylla/src/transport/session.rs

@@ -1812,11 +1812,11 @@ impl Session {
             .map(|partition_key| prepared.get_partitioner_name().hash(&partition_key)))
     }
 
-    /// Get the first node/shard that the load balancer would target if running this query
+    /// Get a node/shard that the load balancer would potentially target if running this query
     ///
-    /// This may help constituting shard-aware batches
+    /// This may help constituting shard-aware batches (see [`Batch::enforce_target_node`])
     #[allow(clippy::type_complexity)]
-    pub fn first_shard_for_statement(
+    pub fn shard_for_statement(
         &self,
         prepared: &PreparedStatement,
         serialized_values: &SerializedValues,