Add RFC-0018 for Java connector federation in C++ workers

RFC-0018-java-connector-federation.md

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+# **RFC-0018 for Java Connector Federation in C++ Workers**
+
+## Java Connector Federation via Arrow Flight
+
+Proposers
+
+* Tim Meehan
+* Bryan Cutler
+* Pratik Dabre
+
+## Related Issues
+
+* [PR #25242](https://github.com/prestodb/presto/pull/25242): Custom Serialization Framework (ConnectorCodecProvider)
+* Arrow Flight Connector in presto-native-execution
+
+## Summary
+
+Enable C++ workers to execute queries using existing single-node Java connector implementations through a dedicated Arrow Federation connector that forwards requests to an Arrow Flight server, providing a migration path for single-node connectors not yet ported to C++.
+
+## Background
+
+Presto's C++ workers provide performance improvements but require connectors to be rewritten in C++. Many single-node connectors (JDBC, MongoDB, Elasticsearch, Cassandra, Redis) are systems where the data transfer, not computation, is the bottleneck. Rewriting these connectors in C++ does not improve performance, yet they require a complete reimplementation.
+
+By creating a dedicated Arrow Federation connector in C++ that derives from the existing Arrow Flight connector, we can forward splits from single-node Java connectors to a Flight server that executes them using the original Java connector implementations.
+
+### Goals
+
+* Enable single-node Java connectors to work with C++ workers without code duplication
+* Provide a simple federation mechanism through a dedicated Arrow Federation connector
+* Support connectors like JDBC, MongoDB, Elasticsearch, and other single-node systems
+
+### Non-goals
+
+* Distributed connectors (Hive, Iceberg, Delta Lake) where parallelism and C++ execution improve performance
+* Modifying the coordinator-worker protocol
+
+## Proposed Implementation
+
+### 1. Architecture Overview
+
+```mermaid
+graph LR
+    subgraph "Coordinator - Java"
+        A[Single-node Connector<br/>e.g. JDBC, MongoDB<br/>generates split]
+    end
+
+    subgraph "C++ Worker"
+        B[Arrow Federation<br/>Connector<br/>wraps & forwards]
+    end
+
+    subgraph "Flight Server - Java"
+        C[Same Connector<br/>executes split]
+    end
+
+    A -->|Connector Split| B
+    B -->|Flight Request<br/>with split + columns| C
+    C -->|Arrow<br/>RecordBatches| B
+```
+
+### 2. Module Changes
+
+#### presto-native-execution: Arrow Federation Connector
+
+##### Protocol Definition
+
+The Arrow Federation connector uses a dedicated protocol to handle splits from single-node connectors:
+
+```cpp
+// ArrowFederationProtocol.h
+namespace facebook::presto::protocol::arrow_federation {
+using ArrowFederationProtocol = ConnectorProtocolTemplate<
+    ArrowFederationTableHandle,
+    ArrowFederationTableLayoutHandle,
+    ArrowFederationColumnHandle,
+    NotImplemented,
+    NotImplemented,
+    ArrowFederationSplit,
+    NotImplemented,
+    ArrowFederationTransactionHandle,
+    NotImplemented,
+    NotImplemented>;
+} // namespace facebook::presto::protocol::arrow_federation
+```
+
+```cpp
+// presto_protocol_arrow_federation.h
+namespace facebook::presto::protocol::arrow_federation {
+struct ArrowFederationSplit : public ConnectorSplit {
+  String splitBytes = {};
+
+  ArrowFederationSplit() noexcept;
+};
+
+void to_json(json& j, const ArrowFederationSplit& p) {
+  auto split = folly::base64Decode(p.splitBytes);
+  j = json::parse(split);
+  j["@type"] = "arrow-flight";
+}
+
+void from_json(const json& j, ArrowFederationSplit& p) {
+  p._type = j["@type"];
+  p.splitBytes = folly::base64Encode(j.dump());
+}
+} // namespace facebook::presto::protocol::arrow_federation
+```
+
+##### Federation Connector Implementation
+
+The Arrow Federation connector extends the Arrow Flight connector to wrap single-node connector splits and forward them:
+
+```cpp
+class ArrowFederationDataSource : public ArrowFlightDataSource {
+ public:
+  ArrowFederationDataSource(
+      const velox::RowTypePtr& outputType,
+      const velox::connector::ColumnHandleMap& columnHandles,
+      std::shared_ptr<Authenticator> authenticator,
+      const velox::connector::ConnectorQueryCtx* connectorQueryCtx,
+      const std::shared_ptr<ArrowFlightConfig>& flightConfig,
+      const std::shared_ptr<arrow::flight::FlightClientOptions>& clientOpts);
+
+  void addSplit(
+      std::shared_ptr<velox::connector::ConnectorSplit> split) override;
+};
+
+struct ArrowFederationSplit : public velox::connector::ConnectorSplit {
+  /// @param splitBytes Base64 Serialized Split for Arrow Federation Flight Server
+  /// Can contain any single-node connector split (JDBC, MongoDB, Elasticsearch, etc.)
+  ArrowFederationSplit(const std::string& splitBytes)
+      : ConnectorSplit(connectorId),
+        splitBytes_(splitBytes) {}
+
+  const std::string splitBytes_;
+};
+```
+
+##### Request Creation and Split Processing
+
+The federation connector wraps the connector split with column information and forwards it as a Flight request:
+
+```cpp
+void ArrowFederationDataSource::addSplit(
+    std::shared_ptr<ConnectorSplit> split) {
+  auto federationSplit = std::dynamic_pointer_cast<ArrowFederationSplit>(split);
+  VELOX_CHECK(
+      federationSplit,
+      "ArrowFederationDataSource received wrong type of split");
+
+  // Create request with split and column mapping
+  nlohmann::json request = {
+      {"split", federationSplit->splitBytes_},
+      {"columns", this->columnMapping_}
+  };
+
+  // Create FlightEndpoint with request as ticket
+  arrow::flight::FlightEndpoint flightEndpoint;
+  flightEndpoint.ticket = request.dump();
+
+  std::string flightEndpointBytes;
+  AFC_ASSIGN_OR_RAISE(
+      flightEndpointBytes,
+      flightEndpoint.SerializeToString());
+
+  // Create ArrowFlightSplit and forward to parent class
+  auto flightSplit = std::make_shared<ArrowFlightSplit>(
+      connectorId, flightEndpointBytes);
+
+  // Let ArrowFlightDataSource handle the actual Flight communication
+  ArrowFlightDataSource::addSplit(flightSplit);
+}
+```
+
+The Arrow Federation connector leverages the existing Arrow Flight connector's infrastructure for communication with the Flight server, using the locally configured `arrow-flight.server` and `arrow-flight.server.port` properties.
+
+#### presto-flight-server (new)
+Flight server that loads connectors using Presto's plugin loading approach, adapted from the main Presto server implementation.
+
+The Flight server uses standard Presto configuration structure:
+- `jvm.config`: JVM heap size and flags (e.g., `-Xmx4G -XX:+UseG1GC`)
+- `config.properties`: Flight server configuration including:
+  - `catalog.config-dir`: Path to catalog configurations (default: `etc/catalog`)
+  - `plugin.dir`: Path to plugin directory
+  - Other Flight server-specific properties
+- Catalog directory (as specified by `catalog.config-dir`): Connector configurations (jdbc.properties, mongodb.properties, etc.)
+
+```java
+public class FlightServerPluginManager {
+  private final Map<String, ConnectorFactory> connectorFactories = new ConcurrentHashMap<>();
+  private final Map<String, Connector> connectors = new ConcurrentHashMap<>();
+
+  public void loadPlugins() throws Exception {
+    // Use same approach as PluginManager to load from installed plugins dir
+    for (File pluginDir : listFiles(installedPluginsDir)) {
+      loadPlugin(pluginDir);
+    }
+  }
+
+  private void loadPlugin(File pluginDir) {
+    URLClassLoader pluginClassLoader = buildClassLoader(pluginDir);
+    ServiceLoader<Plugin> serviceLoader = ServiceLoader.load(Plugin.class, pluginClassLoader);
+
+    for (Plugin plugin : serviceLoader) {
+      for (ConnectorFactory factory : plugin.getConnectorFactories()) {
+        log.info("Registering connector %s", factory.getName());
+        connectorFactories.put(factory.getName(), factory);
+      }
+    }
+  }
+
+  public Connector getConnector(String connectorName) {
+    // Create connector instances from factories as needed
+    return connectors.computeIfAbsent(connectorName, name -> {
+      ConnectorFactory factory = connectorFactories.get(name);
+      return factory.create(name, config, connectorContext);
+    });
+  }
+}
+
+public class ConnectorFlightProducer extends FlightProducer {
+  private final FlightServerPluginManager pluginManager;
+
+  @Override
+  public void getStream(CallContext context, Ticket ticket,
+          ServerStreamListener listener) {
+    // Parse enhanced ticket with all execution context
+    // Format: [version][connectorId_length][connectorId][params_length][params_serialized]
+    String connectorId = extractConnectorId(ticket);
+    PageSourceParameters params = extractParameters(ticket);
+
+    // Get connector and deserialize all execution context
+    Connector connector = pluginManager.getConnector(connectorId);
+    ConnectorCodecProvider codecProvider = connector.getConnectorCodecProvider();
+
+    // Deserialize transaction handle
+    ConnectorTransactionHandle transactionHandle =
+            codecProvider.getTransactionHandleCodec()
+                    .deserialize(params.getTransactionHandle().toByteArray());
+
+    // Deserialize split
+    ConnectorSplit split = codecProvider.getConnectorSplitCodec()
+            .deserialize(params.getSplit().toByteArray());
+
+    // Deserialize column handles
+    List<ColumnHandle> columns = deserializeColumns(params, codecProvider);
+
+    // Reconstruct session from overrides (user, timezone, locale, query_id, etc.)
+    ConnectorSession session = reconstructSession(params.getSessionOverrides(), connectorId);
+
+    // Optional layout handle
+    ConnectorTableLayoutHandle layout = deserializeLayout(params, codecProvider);
+
+    // Split execution context
+    SplitContext splitContext = new SplitContext(params.getSplitContextNanos());
+
+    // Create page source with complete execution context
+    ConnectorPageSource pageSource = connector.getPageSourceProvider()
+            .createPageSource(
+                    transactionHandle,
+                    session,
+                    split,
+                    layout,
+                    columns,
+                    splitContext,
+                    new RuntimeStats());
+
+    streamPagesAsArrow(pageSource, listener);
+  }
+}
+```
+
+### 3. Configuration
+
+#### Arrow Federation Connector Configuration
+
+The Arrow Federation connector is configured through catalog properties like any other connector. The connector ID (catalog name) must be identical between the coordinator and C++ worker, even though they use different connector implementations. Both must use the same catalog configuration filename (e.g., `mongodb.properties`), which determines the connector ID/catalog name. This ensures splits are routed to the correct connector on the worker and allows the Flight server to load the appropriate Java connector for execution.
+
+Example configuration:
+```properties
+# Coordinator: etc/catalog/mongodb.properties
+connector.name=mongodb
+mongodb.connection-url=mongodb://localhost:27017
+
+# C++ Worker: etc/catalog/mongodb.properties (same filename)
+connector.name=arrow-federation
+arrow-flight.server=flight-server.internal
+arrow-flight.server.port=8815
+```
+
+The Flight server is managed externally as a separate service.
+
+### 4. Protocol Flow
+
+#### Split Scheduling Phase
+1. Coordinator's single-node connector (e.g., JDBC, MongoDB) creates its connector-specific split
+2. Split is scheduled to C++ worker where the Arrow Federation protocol receives it but doesn't interpret it - it simply wraps the raw bytes as `ArrowFederationSplit` and forwards them unchanged to the Flight server
+
+#### Split Processing Phase
+1. C++ worker's Arrow Federation connector receives `ArrowFederationSplit`
+2. Federation connector creates Flight request containing:
+   - Original split bytes
+   - Column mapping information
+3. Request wrapped in FlightEndpoint and forwarded to parent ArrowFlightDataSource
+
+#### Execution Phase
+1. ArrowFlightDataSource sends request to configured Flight server
+2. Flight server deserializes split and executes using Java connector
+3. Results stream back as Arrow RecordBatches
+
+### 5. Dependencies and Prerequisites
+
+#### Arrow Flight Connector
+
+The Arrow Federation connector depends on the existing Arrow Flight connector infrastructure in presto-native-execution. It extends `ArrowFlightDataSource` to reuse the Flight communication mechanisms.
+
+#### Flight Request Format
+
+The federation connector creates a simple JSON request containing:
+- `split`: Base64-encoded serialized connector split (from any single-node connector)
+- `columns`: Column mapping information needed for execution
+
+The Flight server deserializes this request and executes the split using the appropriate Java connector.
+
+## Metrics
+
+* Flight protocol overhead (latency comparison between C++ worker and Flight server metrics)
+* Data transfer throughput between C++ worker and Flight server
+
+## Other Approaches Considered
+
+**JNI Integration**: Direct JVM embedding in C++ workers. Rejected due to memory management and debugging requirements.
+
+**Custom Protocol**: Thrift-based connector protocol. Arrow Flight chosen instead for columnar-native transport, existing ecosystem support, and potential for future cross-language connector implementations.
+
+## Adoption Plan
+
+* Configuration:
+  - Configure catalogs to use `connector.name=arrow-federation` for federated execution
+  - Set `arrow-flight.server` and `arrow-flight.server.port` to point to Flight server
+
+* Flight server deployment:
+  - Deploy `presto-flight-server` as a separate service
+  - Configure with standard Presto server configuration structure
+  - Can be colocated with C++ workers or deployed separately
+
+* Migration:
+  - Connectors can be migrated individually by creating federation catalog configurations
+  - Documentation: Update connector documentation as federation support is added
+
+## Test Plan
+
+* Unit tests for split serialization and request creation
+* Integration tests for end-to-end query execution through Flight server
+* Performance tests comparing direct Java execution vs. federation through Flight