review comments resolved

Author: shobhitsg

google-cloud-spanner/src/main/java/com/google/cloud/spanner/Options.java

@@ -173,18 +173,24 @@ public static TransactionOption optimisticLock() {
    *
    * <ul>
    *   <li>{@link ReadLockMode#PESSIMISTIC}: Read locks are acquired immediately on read. This mode
-   *       primarily applies to transactions with {@code SERIALIZABLE} isolation.
+   *       only applies to {@code SERIALIZABLE} isolation. This mode prevents concurrent
+   *       modifications by locking data throughout the transaction. This reduces commit-time aborts
+   *       due to conflicts but can increase how long transactions wait for locks and the overall
+   *       contention.
    *   <li>{@link ReadLockMode#OPTIMISTIC}: Locks for reads within the transaction are not acquired
-   *       on read. Instead the locks are acquired on a commit to validate that read/queried data
-   *       has not changed since the transaction started. If a conflict is detected, the transaction
-   *       will fail. This mode applies to transactions with {@code SERIALIZABLE} isolation.
+   *       on read. Instead the locks are acquired on commit to validate that read/queried data has
+   *       not changed since the transaction started. If a conflict is detected, the transaction
+   *       will fail. This mode only applies to {@code SERIALIZABLE} isolation. This mode defers
+   *       locking until commit, which can reduce contention and improve throughput. However, be
+   *       aware that this increases the risk of transaction aborts if there's significant write
+   *       competition on the same data.
    *   <li>{@link ReadLockMode#READ_LOCK_MODE_UNSPECIFIED}: This is the default if no mode is set.
    *       The locking behavior depends on the isolation level:
    *       <ul>
    *         <li>For {@code REPEATABLE_READ} isolation: Locking semantics default to {@code
-   *             OPTIMISTIC}. However, validation checks at commit are only performed for reads
-   *             within queries using {@code SELECT FOR UPDATE}, statements with {@code
-   *             LOCK_SCANNED_RANGES} hints, or DML statements. <br>
+   *             OPTIMISTIC}. However, validation checks at commit are only performed for queries
+   *             using {@code SELECT FOR UPDATE}, statements with {@code LOCK_SCANNED_RANGES} hints,
+   *             and DML statements. <br>
    *             Note: It is an error to explicitly set {@code ReadLockMode} when the isolation
    *             level is {@code REPEATABLE_READ}.
    *         <li>For all other isolation levels: If the read lock mode is not set, it defaults to

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionImpl.java

@@ -45,7 +45,6 @@
 import com.google.spanner.v1.RequestOptions;
 import com.google.spanner.v1.Transaction;
 import com.google.spanner.v1.TransactionOptions;
-import com.google.spanner.v1.TransactionOptions.ReadWrite;
 import java.time.Instant;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -285,8 +284,7 @@ public CommitResponse writeAtLeastOnceWithOptions(
       transactionOptionsBuilder.setIsolationLevel(options.isolationLevel());
     }
     if (options.readLockMode() != null) {
-      transactionOptionsBuilder.setReadWrite(
-          ReadWrite.newBuilder().setReadLockMode(options.readLockMode()));
+      transactionOptionsBuilder.getReadWriteBuilder().setReadLockMode(options.readLockMode());
     }
     requestBuilder.setSingleUseTransaction(
         defaultTransactionOptions().toBuilder().mergeFrom(transactionOptionsBuilder.build()));

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerOptions.java

@@ -69,7 +69,6 @@
 import com.google.spanner.v1.SpannerGrpc;
 import com.google.spanner.v1.TransactionOptions;
 import com.google.spanner.v1.TransactionOptions.IsolationLevel;
-import com.google.spanner.v1.TransactionOptions.ReadWrite;
 import com.google.spanner.v1.TransactionOptions.ReadWrite.ReadLockMode;
 import io.grpc.CallCredentials;
 import io.grpc.CompressorRegistry;
@@ -1729,8 +1728,7 @@ public DefaultReadWriteTransactionOptionsBuilder setIsolationLevel(
 
         public DefaultReadWriteTransactionOptionsBuilder setReadLockMode(
             ReadLockMode readLockMode) {
-          transactionOptionsBuilder.setReadWrite(
-              ReadWrite.newBuilder().setReadLockMode(readLockMode));
+          transactionOptionsBuilder.getReadWriteBuilder().setReadLockMode(readLockMode);
           return this;
         }