feat(spanner): add BatchWrite transaction

src/spanner/src/batch_write_transaction.rs

@@ -0,0 +1,207 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use crate::client::DatabaseClient;
+use crate::model::BatchWriteRequest;
+use crate::model::BatchWriteResponse;
+use crate::mutation::MutationGroup;
+use crate::server_streaming::stream::BatchWriteStream;
+use gaxi::prost::FromProto;
+
+/// A builder for [BatchWriteTransaction].
+pub struct BatchWriteTransactionBuilder {
+    client: DatabaseClient,
+}
+
+impl BatchWriteTransactionBuilder {
+    pub(crate) fn new(client: DatabaseClient) -> Self {
+        Self { client }
+    }
+
+    /// Builds the [BatchWriteTransaction].
+    pub fn build(self) -> BatchWriteTransaction {
+        let session_name = self.client.session_name();
+        BatchWriteTransaction {
+            session_name,
+            client: self.client,
+        }
+    }
+}
+
+/// A transaction for executing batch writes.
+///
+/// Batch writes are not guaranteed to be atomic across mutation groups.
+/// All mutations within a group are applied atomically.
+pub struct BatchWriteTransaction {
+    session_name: String,
+    client: DatabaseClient,
+}
+
+impl BatchWriteTransaction {
+    /// Executes the batch write and returns a stream of responses.
+    ///
+    /// # Example
+    /// ```
+    /// # use google_cloud_spanner::client::{Mutation, Spanner, MutationGroup};
+    /// # use google_cloud_gax::error::rpc::Code;
+    /// # async fn sample() -> Result<(), Box<dyn std::error::Error>> {
+    /// let client = Spanner::builder().build().await?;
+    /// let db = client.database_client("projects/p/instances/i/databases/d").build().await?;
+    ///
+    /// let mutation = Mutation::new_insert_builder("Users")
+    ///     .set("UserId").to(&1)
+    ///     .build();
+    /// let group = MutationGroup::new(vec![mutation]);
+    ///
+    /// let tx = db.batch_write_transaction().build();
+    /// let mut stream = tx.execute_streaming(vec![group]).await?;
+    ///
+    /// while let Some(response) = stream.next_message().await {
+    ///     let response = response?;
+    ///     if let Some(status) = response.status.as_ref().filter(|s| s.code != Code::Ok as i32) {
+    ///         eprintln!("Error applying groups {:?}: {}", response.indexes, status.message);
+    ///     } else {
+    ///         println!("Applied groups: {:?}", response.indexes);
+    ///     }
+    /// }
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// This method sends the mutation groups to Spanner and returns the responses as a stream.
+    /// Each response includes a status code that indicates whether the mutation groups that
+    /// it references were applied successfully.
+    /// The method does not handle any errors, including retryable errors like Aborted.
+    /// The caller is responsible for handling any errors and for retrying the transaction in
+    /// case it is aborted by Spanner.
+    pub async fn execute_streaming<I>(self, groups: I) -> crate::Result<BatchWriteResponseStream>
+    where
+        I: IntoIterator<Item = MutationGroup>,
+    {
+        let req = BatchWriteRequest::new()
+            .set_session(self.session_name.clone())
+            .set_mutation_groups(groups.into_iter().map(|g| g.build_proto()));
+
+        let stream = self
+            .client
+            .spanner
+            .batch_write(req, crate::RequestOptions::default())
+            .send()
+            .await?;
+        Ok(BatchWriteResponseStream { inner: stream })
+    }
+}
+
+/// A stream of [BatchWriteResponse] messages.
+pub struct BatchWriteResponseStream {
+    pub(crate) inner: BatchWriteStream,
+}
+
+impl BatchWriteResponseStream {
+    /// Fetches the next [BatchWriteResponse] from the stream.
+    ///
+    /// Returns `Some(Ok(BatchWriteResponse))` when a message is successfully received,
+    /// `None` when the stream concludes naturally, or `Some(Err(_))` on RPC errors.
+    pub async fn next_message(&mut self) -> Option<crate::Result<BatchWriteResponse>> {
+        let proto_opt = self.inner.next_message().await?;
+        match proto_opt {
+            Ok(proto) => match proto.cnv() {
+                Ok(model) => Some(Ok(model)),
+                Err(e) => Some(Err(crate::Error::deser(e))),
+            },
+            Err(e) => Some(Err(e)),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::client::{Mutation, Spanner};
+    use crate::result_set::tests::adapt;
+    use anyhow::Result;
+    use gaxi::grpc::tonic::Response;
+    use spanner_grpc_mock::MockSpanner;
+    use spanner_grpc_mock::google::spanner::v1 as mock_v1;
+
+    pub(crate) async fn setup_db_client(
+        mock: MockSpanner,
+    ) -> (DatabaseClient, tokio::task::JoinHandle<()>) {
+        use google_cloud_auth::credentials::anonymous::Builder as Anonymous;
+        let (address, server) = spanner_grpc_mock::start("0.0.0.0:0", mock)
+            .await
+            .expect("Failed to start mock server");
+        let spanner = Spanner::builder()
+            .with_endpoint(address)
+            .with_credentials(Anonymous::new().build())
+            .build()
+            .await
+            .expect("Failed to build client");
+
+        let db_client = spanner
+            .database_client("projects/p/instances/i/databases/d")
+            .build()
+            .await
+            .expect("Failed to create DatabaseClient");
+
+        (db_client, server)
+    }
+
+    #[tokio::test]
+    async fn execute_streaming() -> Result<()> {
+        let mut mock = MockSpanner::new();
+        mock.expect_create_session().returning(|_| {
+            Ok(Response::new(mock_v1::Session {
+                name: "projects/p/instances/i/databases/d/sessions/123".to_string(),
+                ..Default::default()
+            }))
+        });
+
+        mock.expect_batch_write().once().returning(|req| {
+            let req = req.into_inner();
+            assert_eq!(
+                req.session,
+                "projects/p/instances/i/databases/d/sessions/123"
+            );
+            assert_eq!(req.mutation_groups.len(), 1);
+
+            let response = mock_v1::BatchWriteResponse {
+                indexes: vec![0],
+                status: None,
+                commit_timestamp: None,
+            };
+
+            Ok(Response::from(adapt([Ok(response)])))
+        });
+
+        let (db_client, _server) = setup_db_client(mock).await;
+
+        let mutation = Mutation::new_insert_builder("Users")
+            .set("UserId")
+            .to(&1)
+            .build();
+        let group = MutationGroup::new(vec![mutation]);
+
+        let tx = db_client.batch_write_transaction().build();
+        let mut stream = tx.execute_streaming(vec![group]).await?;
+
+        let result = stream
+            .next_message()
+            .await
+            .expect("stream should have yielded a message")?;
+        assert_eq!(result.indexes, vec![0]);
+
+        Ok(())
+    }
+}

src/spanner/src/client.rs

@@ -20,7 +20,7 @@ pub use crate::database_client::DatabaseClient;
 pub use crate::error::SpannerInternalError;
 pub use crate::from_value::{ConvertError, FromValue};
 pub use crate::key::{Key, KeyRange, KeySet, KeySetBuilder};
-pub use crate::mutation::{Mutation, ValueBinder, WriteBuilder};
+pub use crate::mutation::{Mutation, MutationGroup, ValueBinder, WriteBuilder};
 pub use crate::read::ConfiguredReadRequestBuilder;
 pub use crate::read::ReadRequest;
 pub use crate::read::ReadRequestBuilder;

src/spanner/src/database_client.rs

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 use crate::batch_read_only_transaction::BatchReadOnlyTransactionBuilder;
+use crate::batch_write_transaction::BatchWriteTransactionBuilder;
 use crate::client::Spanner;
 use crate::partitioned_dml_transaction::PartitionedDmlTransactionBuilder;
 use crate::read_only_transaction::{
@@ -193,6 +194,54 @@ impl DatabaseClient {
         crate::write_only_transaction::WriteOnlyTransactionBuilder::new(self.clone())
     }
 
+    /// Returns a builder for a batch write transaction.
+    ///
+    /// # Example
+    /// ```
+    /// # use google_cloud_spanner::client::{Spanner, Mutation, MutationGroup};
+    /// # use google_cloud_gax::error::rpc::Code;
+    /// # async fn sample() -> Result<(), Box<dyn std::error::Error>> {
+    /// let client = Spanner::builder().build().await?;
+    /// let db = client.database_client("projects/p/instances/i/databases/d").build().await?;
+    ///
+    /// let mutation1a = Mutation::new_insert_builder("Users")
+    ///     .set("UserId").to(&1)
+    ///     .build();
+    /// let mutation1b = Mutation::new_insert_builder("UserRoles")
+    ///     .set("UserId").to(&1)
+    ///     .set("Role").to(&"Admin")
+    ///     .build();
+    /// let group1 = MutationGroup::new(vec![mutation1a, mutation1b]);
+    ///
+    /// let mutation2 = Mutation::new_insert_builder("Users")
+    ///     .set("UserId").to(&2)
+    ///     .build();
+    /// let group2 = MutationGroup::new(vec![mutation2]);
+    ///
+    /// let transaction = db.batch_write_transaction().build();
+    /// let mut stream = transaction.execute_streaming(vec![group1, group2]).await?;
+    ///
+    /// while let Some(response) = stream.next_message().await {
+    ///     let response = response?;
+    ///     if let Some(status) = response.status.as_ref().filter(|s| s.code != Code::Ok as i32) {
+    ///         eprintln!("Error applying groups {:?}: {}", response.indexes, status.message);
+    ///     } else {
+    ///         println!("Applied groups: {:?}", response.indexes);
+    ///     }
+    /// }
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// A batch write transaction is used to execute non-atomic writes using mutations.
+    /// Related mutations should be placed in a group. For example, two mutations inserting
+    /// rows with the same primary key prefix in both parent and child tables are related.
+    /// All mutations within a group are applied atomically, but the entire batch is not
+    /// guaranteed to be atomic.
+    pub fn batch_write_transaction(&self) -> BatchWriteTransactionBuilder {
+        BatchWriteTransactionBuilder::new(self.clone())
+    }
+
     pub(crate) fn session_name(&self) -> String {
         self.session_maintainer.session_name()
     }

src/spanner/src/lib.rs

@@ -25,6 +25,9 @@ pub use batch_dml::BatchDmlBuilder;
 pub use batch_read_only_transaction::{
     BatchReadOnlyTransaction, BatchReadOnlyTransactionBuilder, Partition,
 };
+pub use batch_write_transaction::{
+    BatchWriteResponseStream, BatchWriteTransaction, BatchWriteTransactionBuilder,
+};
 pub use error::BatchUpdateError;
 pub use google_cloud_gax::Result;
 pub use google_cloud_gax::error::Error;
@@ -44,6 +47,7 @@ pub mod batch_read_only_transaction;
 pub mod model {
     pub use crate::generated::gapic_dataplane::model::*;
 }
+pub(crate) mod batch_write_transaction;
 pub(crate) mod database_client;
 pub(crate) mod from_value;
 pub(crate) mod key;