diff --git a/udf/worker/proto/common.proto b/udf/worker/proto/common.proto
new file mode 100644
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+++ b/udf/worker/proto/common.proto
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+syntax = "proto3";
+
+package org.apache.spark.udf.worker;
+
+option java_package = "org.apache.spark.udf.worker";
+
+option java_multiple_files = true;
+
+// The UDF in & output data format.
+enum UDFWorkerDataFormat {
+    UDF_WORKER_DATA_FORMAT_UNSPECIFIED = 0;
+    
+    // The worker accepts and produces Apache arrow batches.
+    ARROW = 1;
+}
+
+// The UDF execution type/shape.
+message UDFShape {
+    oneof shape {
+        SparkUDFShapes spark = 1;
+    }
+}
+
+enum SparkUDFShapes {
+    SPARK_UDF_SHAPE_UNSPECIFIED = 0;
+    
+    // UDF receives a row with 0+ columns as input
+    // and produces a single, scalar value as output
+    EXPRESSION = 1;
+
+    // UDF receives a iterator to batch of rows as input and
+    // produces iterator to a batch of rows as output.
+    MAP_PARTITIONS = 2;
+}
diff --git a/udf/worker/proto/worker_spec.proto b/udf/worker/proto/worker_spec.proto
new file mode 100644
index 0000000000000..6c9a4d3d10b48
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+++ b/udf/worker/proto/worker_spec.proto
@@ -0,0 +1,202 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+syntax = "proto3";
+
+import "udf/worker/proto/src/main/protobuf/common.proto";
+
+package org.apache.spark.udf.worker;
+
+option java_package = "org.apache.spark.udf.worker";
+option java_multiple_files = true;
+
+///
+/// Worker specification
+///
+message UDFWorkerSpecification {
+    WorkerEnvironment environment = 1;
+    WorkerCapabilities capabilities = 2;
+
+    // How to create new workers.
+    // At the moment, only direct creation is supported.
+    // This can be extended with indirect/provisioned creation in the future.
+    oneof worker {
+        DirectWorker direct = 3;
+    }
+}
+
+message WorkerEnvironment {
+    // Callable that is responsible for environment setup.
+    optional ProcessCallable installation = 1;
+    // Callable, which is called to verify that an environment
+    // is suitable to start the UDF worker. This callable
+    // needs to verify that
+    // - The worker code itself is present
+    // - Any needed dependencies are present
+    optional ProcessCallable environment_verification = 2;
+    // Callable, which is invoked after the worker has been terminated.
+    // This can be used to cleanup dependencies or temporary resources.
+    // Be careful not to cleanup resources that could be used by
+    // other workers running in parallel.
+    optional ProcessCallable environment_cleanup = 3;
+}
+
+// Capabilities used for query planning
+message WorkerCapabilities {
+    // The data formats that the worker supports for UDF data in- & output.
+    // Every worker MUST at least support ARROW. 
+    //
+    // It is expected that for each UDF execution, the input format 
+    // always matches the output format.
+    //
+    // If a worker supports multiple data formats, the engine will select
+    // the most suitable one for each UDF invocation. Which format was chosen
+    // is reported by the engine as part of the UDF protocol's init message.
+    repeated UDFWorkerDataFormat supported_data_formats = 1;
+
+    // Which types of UDFs this worker supports.
+    // This should list all supported Shapes.
+    // Of which shape a specific UDF is will be communicated
+    // in the initial message of the UDF protocol.
+    // 
+    // If a execution for an unsupported UDF type is requested
+    // the query will fail during query planning.
+    repeated UDFShape supported_udf_shapes = 2;
+
+    // Whether multiple, concurrent UDF
+    // connections are supported by this worker
+    // (for example via multi-threading).
+    // 
+    // In the first implementation of the engine-side
+    // worker specification, this property will not be used.
+    // 
+    // Usage of this property, can be enabled in the future if the
+    // engine implements more advanced resource management (Tbd).
+    //  
+    bool supports_concurrent_udfs = 3;
+
+    // Whether compatible workers may be reused.
+    // If this is not supported, the worker is 
+    // terminated after every single UDF invocation.
+    bool supports_reuse = 4;
+
+    // To be extended with UDF chaining, ...
+}
+
+// The worker that will be created to process UDFs
+message DirectWorker  {
+    // Blocking callable that is terminated by SIGTERM/SIGKILL
+    ProcessCallable runner = 1;
+
+    UDFWorkerProperties properties = 2;
+}
+
+message UDFWorkerProperties {
+    // Maximum amount of time to wait until the worker can accept connections.
+    //
+    // The engine will use this timeout, if it does not exceed a 
+    // engine-configurable maximum time (e.g. 30 seconds).
+    optional int32 initialization_timeout_ms = 1;
+
+    // Used for graceful engine-initiated termination signaled via SIGTERM.
+    // After this time, the worker process should have terminated itself.
+    // Otherwise, the process will be forcefully killed using SIGKILL.
+    //
+    // The engine will use this timeout, if it does not exceed a 
+    // engine-configurable maximum time (e.g. 30 seconds).
+    optional int32 graceful_termination_timeout_ms = 2;
+
+    // The connection this [[UDFWorker]] supports. Note that a single
+    // connection is sufficient to run multiple UDFs and (gRPC) services.
+    //
+    // On [[UDFWorker]] creation, connection information 
+    // is passed to the callable as a string parameter. 
+    // The string format depends on the [[WorkerConnection]]:
+    // 
+    // For example, when using TCP, the callable argument will be:
+    // --connection PORT
+    // Here is a concrete example
+    // --connection 8080
+    // 
+    // For the format of each specific transport type, see the comments below.
+    WorkerConnection connection = 3;
+}
+
+message WorkerConnection {
+    oneof transport {
+        UnixDomainSocket unix_domain_socket = 1;
+        TcpConnection tcp = 2;
+    }
+}
+
+ // Communication between the engine and worker
+// is done using a UNIX domain socket.
+//
+// On [[UDFWorker]] creation, a path to a socket 
+// to listen on is passed as a argument.
+// Examples:
+//  /tmp/channel-uuid.sock
+//  /some/system/path/channel-1234.sock
+message UnixDomainSocket {}
+
+// Communication between the engine and worker
+// is done using a localhost TCP connection. 
+//
+// On [[UDFWorker]] creation, a PORT 
+// is passed as a connection parameter.
+// 
+// It is expected that the worker binds to this
+// port on both IPv4 and IPv6 localhost interfaces.
+// E.g. the worker server should be reachable via
+// 127.0.0.1:PORT and [::1]:PORT.
+//
+// Examples:
+//  8080
+//  1234 
+message TcpConnection {}
+
+message ProcessCallable {
+    // Executable to invoke.
+    // Examples:
+    //   ["python3", "-m"]
+    //   ["worker.bin"]
+    //   ["java", "worker.java"]
+    //   ["bin/bash", "-c"]
+    // This executable should be blocking, until the task is finished.
+    // Termination is requested via a SIGTERM signal.
+    //
+    // Success/Failure can be indicated via exit codes:
+    // Exit code 0    -> Success
+    // Exit code != 0 -> Failure
+    repeated string command = 1;
+
+    // Arguments passed directly to the executable.
+    // Examples:
+    //   ["udf_worker.py"]
+    //   [""]
+    //   ["--max_concurrency", "5"]
+    //   ["\"echo 'Test'\""]
+    //
+    // Every executable will ALWAYS receive a
+    // --id argument. This argument CANNOT be part of the below list of arguments. 
+    // The value of the id argument is a string with the engine-assigned
+    // id of this UDF Worker. This can be used in logs and other state information.
+    repeated string arguments = 2;
+
+    // Environment variables for the invoked process.
+    map<string, string> environment_variables = 3;
+}