Add implementation summary document

Co-authored-by: tgrabiec <283695+tgrabiec@users.noreply.github.com>

IMPLEMENTATION_SUMMARY.md

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+# Tablet Migration RPC Decoupling - Implementation Summary
+
+## What Was Changed
+
+### Problem Statement
+The topology_coordinator's `handle_tablet_migration()` function was tightly coupled with RPC sending, making it difficult to test in unit tests without setting up a full multi-node cluster.
+
+### Solution
+Introduced an abstraction layer (`tablet_migration_rpc_handler`) that decouples the coordinator logic from actual RPC operations.
+
+## Files Added
+
+### 1. `service/tablet_migration_rpc_handler.hh`
+Interface defining the RPC operations needed for tablet migration:
+- `tablet_repair()` - Repair a tablet on a replica
+- `tablet_stream_data()` - Stream tablet data to a replica
+- `tablet_cleanup()` - Clean up tablet data on a replica  
+- `repair_update_compaction_ctrl()` - Update compaction controller after repair
+
+### 2. `test/lib/local_tablet_rpc_simulator.hh`
+Test implementation that calls local RPC handlers instead of sending network RPCs.
+
+### 3. `docs/dev/tablet-migration-rpc-decoupling.md`
+Documentation explaining the architecture and how to use it.
+
+## Files Modified
+
+### 1. `service/topology_coordinator.cc`
+- Added `messaging_tablet_rpc_handler` class - production implementation that wraps `messaging_service` RPCs
+- Updated `handle_tablet_migration()` to use `_tablet_rpc_handler` interface instead of direct RPC calls
+- Updated constructor to accept optional RPC handler and initialize it
+- Modified 5 locations where tablet RPCs were called:
+  - rebuild_repair stage → `tablet_repair()`
+  - streaming stage → `tablet_stream_data()`
+  - cleanup stage → `tablet_cleanup()` (2 locations)
+  - repair stage → `tablet_repair()`
+  - end_repair stage → `repair_update_compaction_ctrl()`
+
+### 2. `service/topology_coordinator.hh`
+- Added forward declaration for `tablet_migration_rpc_handler`
+- Updated `run_topology_coordinator()` signature to accept optional `tablet_rpc_handler` parameter
+
+## How It Works
+
+### Production Mode (Default)
+```cpp
+run_topology_coordinator(...);
+// Automatically uses messaging_tablet_rpc_handler
+```
+
+The topology coordinator creates a `messaging_tablet_rpc_handler` that sends actual network RPCs.
+
+### Test Mode
+```cpp
+local_tablet_rpc_simulator simulator(storage_service);
+run_topology_coordinator(..., std::make_unique<local_tablet_rpc_simulator>(simulator));
+```
+
+Tests can provide a `local_tablet_rpc_simulator` that calls local RPC handlers directly, bypassing the network layer.
+
+## Benefits
+
+1. **Testability**: Can now test `handle_tablet_migration()` logic in unit tests
+2. **Separation of Concerns**: Coordinator logic (group0 transitions, barriers) is separated from replica logic (RPCs)
+3. **Minimal Changes**: The refactoring is minimal and surgical - only abstracts the RPC layer
+4. **Backward Compatible**: Existing code continues to work without changes
+
+## What Can Now Be Tested
+
+With this refactoring, tests can now:
+1. Verify that tablet migrations progress through the correct stages
+2. Test barrier coordination between stages
+3. Test rollback logic when migrations fail
+4. Test the interaction between migrations and topology changes
+5. Simulate various failure scenarios at the RPC level
+
+All without needing multiple nodes or network communication.
+
+## Future Work
+
+This refactoring enables future enhancements:
+1. More comprehensive topology coordinator unit tests
+2. Fault injection at the RPC level in tests
+3. Better simulation of network conditions
+4. Potential extraction of more coordinator logic into testable components