impl: backend migration example

src/rust/rust_kvs/examples/migration.rs

@@ -0,0 +1,97 @@
+// *******************************************************************************
+// Copyright (c) 2026 Contributors to the Eclipse Foundation
+//
+// See the NOTICE file(s) distributed with this work for additional
+// information regarding copyright ownership.
+//
+// This program and the accompanying materials are made available under the
+// terms of the Apache License Version 2.0 which is available at
+// <https://www.apache.org/licenses/LICENSE-2.0>
+//
+// SPDX-License-Identifier: Apache-2.0
+// *******************************************************************************
+
+//! Example for migrations between backends.
+
+use rust_kvs::prelude::*;
+
+/// Example custom backend.
+/// Returns some data on `load_kvs`.
+#[derive(PartialEq)]
+struct FromBackend;
+
+impl KvsBackend for FromBackend {
+    fn load_kvs(&self, _instance_id: InstanceId, _snapshot_id: SnapshotId) -> Result<KvsMap, ErrorCode> {
+        Ok(KvsMap::from([(
+            "example_key".to_string(),
+            KvsValue::String("example_value".to_string()),
+        )]))
+    }
+
+    fn load_defaults(&self, _instance_id: InstanceId) -> Result<KvsMap, ErrorCode> {
+        unimplemented!()
+    }
+
+    fn flush(&self, _instance_id: InstanceId, _kvs_map: &KvsMap) -> Result<(), ErrorCode> {
+        unimplemented!()
+    }
+
+    fn snapshot_count(&self, _instance_id: InstanceId) -> usize {
+        unimplemented!()
+    }
+
+    fn snapshot_max_count(&self) -> usize {
+        unimplemented!()
+    }
+
+    fn snapshot_restore(&self, _instance_id: InstanceId, _snapshot_id: SnapshotId) -> Result<KvsMap, ErrorCode> {
+        unimplemented!()
+    }
+}
+
+/// Example custom backend.
+/// Prints provided data to stdout.
+#[derive(PartialEq)]
+struct ToBackend;
+
+impl KvsBackend for ToBackend {
+    fn load_kvs(&self, _instance_id: InstanceId, _snapshot_id: SnapshotId) -> Result<KvsMap, ErrorCode> {
+        unimplemented!()
+    }
+
+    fn load_defaults(&self, _instance_id: InstanceId) -> Result<KvsMap, ErrorCode> {
+        unimplemented!()
+    }
+
+    fn flush(&self, _instance_id: InstanceId, kvs_map: &KvsMap) -> Result<(), ErrorCode> {
+        println!("{kvs_map:?}");
+        Ok(())
+    }
+
+    fn snapshot_count(&self, _instance_id: InstanceId) -> usize {
+        unimplemented!()
+    }
+
+    fn snapshot_max_count(&self) -> usize {
+        unimplemented!()
+    }
+
+    fn snapshot_restore(&self, _instance_id: InstanceId, _snapshot_id: SnapshotId) -> Result<KvsMap, ErrorCode> {
+        unimplemented!()
+    }
+}
+
+fn main() -> Result<(), ErrorCode> {
+    // Load `KvsMap` from first backend.
+    // `instance_id` and `snapshot_id` normally must be provided, but aren't used in this example.
+    let from_backend = FromBackend;
+    let instance_id = InstanceId(0);
+    let snapshot_id = SnapshotId(0);
+    let data = from_backend.load_kvs(instance_id, snapshot_id).unwrap();
+
+    // Save `KvsMap` in second backend.
+    let to_backend = ToBackend;
+    to_backend.flush(instance_id, &data).unwrap();
+
+    Ok(())
+}