Add Snapshot-read-only isolation support

You can read more about this type of isolation here https://ydb.tech/docs/en/concepts/transactions#modes

Author: jorki02

repository-ydb-v2/src/main/java/tech/ydb/yoj/repository/ydb/YdbRepositoryTransaction.java

@@ -233,6 +233,10 @@ private TxControl<?> getTxControl() {
             case ONLINE_CONSISTENT_READ_ONLY -> TxControl.onlineRo().setAllowInconsistentReads(false);
             case ONLINE_INCONSISTENT_READ_ONLY -> TxControl.onlineRo().setAllowInconsistentReads(true);
             case STALE_CONSISTENT_READ_ONLY -> TxControl.staleRo();
+            case SNAPSHOT -> {
+                TxControl<?> txControl = (txId != null ? TxControl.id(txId) : TxControl.snapshotRo());
+                yield txControl.setCommitTx(false);
+            }
         };
     }
 

repository-ydb-v2/src/test/java/tech/ydb/yoj/repository/ydb/YdbRepositoryIntegrationTest.java

@@ -33,6 +33,7 @@
 import tech.ydb.yoj.databind.schema.Column;
 import tech.ydb.yoj.databind.schema.ObjectSchema;
 import tech.ydb.yoj.repository.db.EntitySchema;
+import tech.ydb.yoj.repository.db.IsolationLevel;
 import tech.ydb.yoj.repository.db.Repository;
 import tech.ydb.yoj.repository.db.RepositoryTransaction;
 import tech.ydb.yoj.repository.db.Tx;
@@ -304,6 +305,30 @@ public void transactionLevel() {
         checkSession(sessionManager, firstSession);
     }
 
+    @Test
+    public void snapshotTransactionLevel() {
+        Project expected1 = new Project(new Project.Id("SP1"), "snapshot1");
+        Project expected2 = new Project(new Project.Id("SP2"), "snapshot2");
+
+        db.tx(() -> db.projects().save(expected1));
+        db.tx(() -> db.projects().save(expected2));
+
+        Project actual1 = db.tx(() -> db.projects().find(expected1.getId()));
+        assertThat(actual1).isEqualTo(expected1);
+        Project actual2 = db.readOnly().run(() -> db.projects().find(expected2.getId()));
+        assertThat(actual2).isEqualTo(expected2);
+
+        db.readOnly()
+            .withStatementIsolationLevel(IsolationLevel.SNAPSHOT)
+            .run(() ->  {
+                Project actualSnapshot1 = db.projects().find(expected1.getId());
+                assertThat(actualSnapshot1).isEqualTo(expected1);
+
+                Project actualSnapshot2 = db.projects().find(expected2.getId());
+                assertThat(actualSnapshot2).isEqualTo(expected2);
+            });
+    }
+
     @SneakyThrows
     @Test
     public void truncated() {

repository/src/main/java/tech/ydb/yoj/repository/db/IsolationLevel.java

@@ -32,7 +32,14 @@ public enum IsolationLevel {
      * An <em>almost</em> recent consistent state of the database. Read only.
      * This level is faster then {@code ONLINE_CONSISTENT_READ_ONLY}, but may return stale data.
      */
-    STALE_CONSISTENT_READ_ONLY("SC");
+    STALE_CONSISTENT_READ_ONLY("SC"),
+
+    /**
+     * All the read operations within a transaction access the database snapshot. Read only.
+     * All the data reads are consistent. The snapshot is taken when the transaction begins,
+     * meaning the transaction sees all changes committed before it began.
+     */
+    SNAPSHOT("SP");
 
     @Getter
     private final String txIdSuffix;