Add descriptive error when orchestration type is not registered

client/src/main/java/com/microsoft/durabletask/TaskOrchestrationExecutor.java

@@ -882,7 +882,12 @@ private void processEvent(HistoryEvent e) {
                             // Try getting the default orchestrator
                             factory = TaskOrchestrationExecutor.this.orchestrationFactories.get("*");
                         }
-                        // TODO: Throw if the factory is null (orchestration by that name doesn't exist)
+                        if (factory == null) {
+                            throw new IllegalStateException(String.format(
+                                    "No orchestration factory registered for orchestration type '%s'. This usually means that a worker that doesn't support this orchestration type is connected to this task hub. Make sure the worker has a registered orchestration for '%s'.",
+                                    name,
+                                    name));
+                        }
                         TaskOrchestration orchestrator = factory.create();
                         orchestrator.run(this);
                         break;

client/src/test/java/com/microsoft/durabletask/TaskOrchestrationExecutorTest.java

@@ -0,0 +1,85 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+package com.microsoft.durabletask;
+
+import com.google.protobuf.StringValue;
+import com.google.protobuf.Timestamp;
+import com.microsoft.durabletask.implementation.protobuf.OrchestratorService.*;
+import org.junit.jupiter.api.Test;
+
+import java.time.Duration;
+import java.util.*;
+import java.util.logging.Logger;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Unit tests for TaskOrchestrationExecutor.
+ */
+public class TaskOrchestrationExecutorTest {
+
+    private static final Logger logger = Logger.getLogger(TaskOrchestrationExecutorTest.class.getName());
+
+    @Test
+    void execute_unregisteredOrchestrationType_failsWithDescriptiveMessage() {
+        // Arrange: create executor with no registered orchestrations
+        HashMap<String, TaskOrchestrationFactory> factories = new HashMap<>();
+        TaskOrchestrationExecutor executor = new TaskOrchestrationExecutor(
+                factories,
+                new JacksonDataConverter(),
+                Duration.ofDays(3),
+                logger,
+                null);
+
+        String unknownName = "NonExistentOrchestration";
+
+        // Build history events simulating an orchestration start
+        HistoryEvent orchestratorStarted = HistoryEvent.newBuilder()
+                .setEventId(-1)
+                .setTimestamp(Timestamp.getDefaultInstance())
+                .setOrchestratorStarted(OrchestratorStartedEvent.getDefaultInstance())
+                .build();
+
+        HistoryEvent executionStarted = HistoryEvent.newBuilder()
+                .setEventId(-1)
+                .setTimestamp(Timestamp.getDefaultInstance())
+                .setExecutionStarted(ExecutionStartedEvent.newBuilder()
+                        .setName(unknownName)
+                        .setVersion(StringValue.of(""))
+                        .setInput(StringValue.of("\"test-input\""))
+                        .setOrchestrationInstance(OrchestrationInstance.newBuilder()
+                                .setInstanceId("test-instance-id")
+                                .build())
+                        .build())
+                .build();
+
+        HistoryEvent orchestratorCompleted = HistoryEvent.newBuilder()
+                .setEventId(-1)
+                .setTimestamp(Timestamp.getDefaultInstance())
+                .setOrchestratorCompleted(OrchestratorCompletedEvent.getDefaultInstance())
+                .build();
+
+        List<HistoryEvent> pastEvents = Arrays.asList(orchestratorStarted, executionStarted, orchestratorCompleted);
+        List<HistoryEvent> newEvents = Collections.emptyList();
+
+        // Act
+        TaskOrchestratorResult result = executor.execute(pastEvents, newEvents);
+
+        // Assert: the result should contain a CompleteOrchestrationAction with FAILED status
+        // and a failure message mentioning the unknown orchestration name
+        OrchestratorAction action = result.getActions().iterator().next();
+        assertTrue(action.hasCompleteOrchestration(), "Expected a CompleteOrchestrationAction");
+
+        CompleteOrchestrationAction completeAction = action.getCompleteOrchestration();
+        assertEquals(OrchestrationStatus.ORCHESTRATION_STATUS_FAILED, completeAction.getOrchestrationStatus());
+        assertTrue(completeAction.hasFailureDetails(), "Expected failure details");
+
+        TaskFailureDetails failureDetails = completeAction.getFailureDetails();
+        assertEquals("java.lang.IllegalStateException", failureDetails.getErrorType());
+        assertTrue(failureDetails.getErrorMessage().contains(unknownName),
+                "Error message should contain the orchestration name: " + failureDetails.getErrorMessage());
+        assertTrue(failureDetails.getErrorMessage().contains("worker"),
+                "Error message should mention workers: " + failureDetails.getErrorMessage());
+    }
+}