feat(core): add validationOnly mode to prevent change execution

core/flamingock-core-commons/src/main/java/io/flamingock/internal/common/core/error/PendingChangesException.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2026 Flamingock (https://www.flamingock.io)
+ *
+ * Licensed 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.
+ */
+package io.flamingock.internal.common.core.error;
+
+/**
+ * Exception thrown when Flamingock runs in validation-only mode and detects pending changes.
+ */
+public class PendingChangesException extends FlamingockException {
+
+    private final int pendingCount;
+
+    public PendingChangesException(int pendingCount) {
+        super("Flamingock validationOnly=true: %d pending change(s) detected. Apply them before running in validation-only mode.", pendingCount);
+        this.pendingCount = pendingCount;
+    }
+
+    public int getPendingCount() {
+        return pendingCount;
+    }
+}

core/flamingock-core-commons/src/test/java/io/flamingock/internal/common/core/error/PendingChangesExceptionTest.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2026 Flamingock (https://www.flamingock.io)
+ *
+ * Licensed 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.
+ */
+package io.flamingock.internal.common.core.error;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertInstanceOf;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class PendingChangesExceptionTest {
+
+    @Test
+    @DisplayName("Should return the pending count passed to the constructor")
+    void shouldReturnPendingCount() {
+        // Given
+        int pendingCount = 5;
+
+        // When
+        PendingChangesException exception = new PendingChangesException(pendingCount);
+
+        // Then
+        assertEquals(pendingCount, exception.getPendingCount());
+    }
+
+    @Test
+    @DisplayName("Should include the pending count in the exception message")
+    void shouldIncludeCountInMessage() {
+        // Given
+        int pendingCount = 5;
+
+        // When
+        PendingChangesException exception = new PendingChangesException(pendingCount);
+
+        // Then
+        assertTrue(exception.getMessage().contains("5"),
+                "Message should contain the pending count as a string");
+    }
+
+    @Test
+    @DisplayName("Should be an instance of FlamingockException")
+    void shouldExtendFlamingockException() {
+        // Given / When
+        PendingChangesException exception = new PendingChangesException(3);
+
+        // Then
+        assertInstanceOf(FlamingockException.class, exception);
+    }
+
+    @Test
+    @DisplayName("Should work correctly with zero pending changes")
+    void shouldWorkWithZeroPendingCount() {
+        // Given / When
+        PendingChangesException exception = new PendingChangesException(0);
+
+        // Then
+        assertEquals(0, exception.getPendingCount());
+        assertTrue(exception.getMessage().contains("0"));
+    }
+
+    @Test
+    @DisplayName("Should work correctly with a large pending count")
+    void shouldWorkWithLargePendingCount() {
+        // Given
+        int largeCount = 999;
+
+        // When
+        PendingChangesException exception = new PendingChangesException(largeCount);
+
+        // Then
+        assertEquals(largeCount, exception.getPendingCount());
+        assertTrue(exception.getMessage().contains("999"));
+    }
+}

core/flamingock-core/src/main/java/io/flamingock/internal/core/builder/AbstractChangeRunnerBuilder.java

@@ -359,6 +359,15 @@ public HOLDER setEnabled(boolean enabled) {
         return getSelf();
     }
 
+    public HOLDER setValidationOnly(boolean validationOnly) {
+        coreConfiguration.setValidationOnly(validationOnly);
+        return getSelf();
+    }
+
+    public boolean isValidationOnly() {
+        return coreConfiguration.isValidationOnly();
+    }
+
     @Override
     public HOLDER setServiceIdentifier(String serviceIdentifier) {
         coreConfiguration.setServiceIdentifier(serviceIdentifier);

core/flamingock-core/src/main/java/io/flamingock/internal/core/configuration/core/CoreConfigurable.java

@@ -38,6 +38,10 @@ public interface CoreConfigurable {
 
     void setEnabled(boolean enabled);
 
+    void setValidationOnly(boolean validationOnly);
+
+    boolean isValidationOnly();
+
     void setServiceIdentifier(String serviceIdentifier);
 
     void setMetadata(Map<String, Object> metadata);

core/flamingock-core/src/main/java/io/flamingock/internal/core/configuration/core/CoreConfiguration.java

@@ -35,6 +35,11 @@ public class CoreConfiguration implements CoreConfigurable {
      */
     private boolean enabled = true;
 
+    /**
+     * If true, Flamingock will only validate that no pending changes exist without applying them. Default false
+     */
+    private boolean validationOnly = false;
+
     /**
      * Service identifier.
      */
@@ -91,6 +96,11 @@ public void setEnabled(boolean enabled) {
         this.enabled = enabled;
     }
 
+    @Override
+    public void setValidationOnly(boolean validationOnly) {
+        this.validationOnly = validationOnly;
+    }
+
     @Override
     public void setServiceIdentifier(String serviceIdentifier) {
         this.serviceIdentifier = serviceIdentifier;
@@ -126,6 +136,11 @@ public boolean isEnabled() {
         return enabled;
     }
 
+    @Override
+    public boolean isValidationOnly() {
+        return validationOnly;
+    }
+
     @Override
     public String getServiceIdentifier() {
         return serviceIdentifier;

core/flamingock-core/src/main/java/io/flamingock/internal/core/operation/OperationFactory.java

@@ -16,6 +16,7 @@
 package io.flamingock.internal.core.operation;
 
 import io.flamingock.internal.common.core.context.ContextResolver;
+import io.flamingock.internal.common.core.operation.OperationType;
 import io.flamingock.internal.common.core.recovery.Resolution;
 import io.flamingock.internal.core.builder.args.FlamingockArguments;
 import io.flamingock.internal.core.configuration.core.CoreConfigurable;
@@ -31,6 +32,7 @@
 import io.flamingock.internal.core.operation.execute.ExecuteArgs;
 import io.flamingock.internal.core.operation.execute.ExecuteOperation;
 import io.flamingock.internal.core.operation.execute.ExecuteResult;
+import io.flamingock.internal.core.operation.execute.ValidateOperation;
 import io.flamingock.internal.core.operation.issue.IssueGetArgs;
 import io.flamingock.internal.core.operation.issue.IssueGetOperation;
 import io.flamingock.internal.core.operation.issue.IssueGetResult;
@@ -95,9 +97,15 @@ public OperationFactory(RunnerId runnerId,
     }
 
     public RunnableOperation<?, ?> getOperation() {
-        switch (flamingockArgs.getOperation()) {
+        OperationType operationType = flamingockArgs.getOperation();
+        if (operationType == OperationType.EXECUTE_APPLY && coreConfiguration.isValidationOnly()) {
+            operationType = OperationType.EXECUTE_VALIDATE;
+        }
+        switch (operationType) {
             case EXECUTE_APPLY:
                 return getExecuteOperation();
+            case EXECUTE_VALIDATE:
+                return getValidateOperation();
             case AUDIT_LIST:
                 return getAuditListOperation();
             case AUDIT_FIX:
@@ -107,7 +115,7 @@ public OperationFactory(RunnerId runnerId,
             case ISSUE_GET:
                 return getIssueGetOperation();
             default:
-                throw new UnsupportedOperationException(String.format("Operation %s not supported", flamingockArgs.getOperation()));
+                throw new UnsupportedOperationException(String.format("Operation %s not supported", operationType));
         }
     }
 
@@ -153,6 +161,16 @@ private RunnableOperation<ExecuteArgs, ExecuteResult> getExecuteOperation() {
         return new RunnableOperation<>(executeOperation, new ExecuteArgs(pipeline));
     }
 
+    private RunnableOperation<ExecuteArgs, ExecuteResult> getValidateOperation() {
+        ValidateOperation validateOperation = new ValidateOperation(
+                runnerId,
+                executionPlanner,
+                eventPublisher,
+                isThrowExceptionIfCannotObtainLock,
+                finalizer);
+        return new RunnableOperation<>(validateOperation, new ExecuteArgs(pipeline));
+    }
+
     private static OrphanExecutionContext buildExecutionContext(CoreConfigurable configuration) {
         return new OrphanExecutionContext(StringUtil.hostname(), configuration.getMetadata());
     }

core/flamingock-core/src/main/java/io/flamingock/internal/core/operation/execute/ValidateOperation.java

@@ -0,0 +1,150 @@
+/*
+ * Copyright 2026 Flamingock (https://www.flamingock.io)
+ *
+ * Licensed 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.
+ */
+package io.flamingock.internal.core.operation.execute;
+
+import io.flamingock.internal.common.core.error.FlamingockException;
+import io.flamingock.internal.common.core.error.PendingChangesException;
+import io.flamingock.internal.common.core.response.data.ExecuteResponseData;
+import io.flamingock.internal.core.event.EventPublisher;
+import io.flamingock.internal.core.event.model.impl.PipelineCompletedEvent;
+import io.flamingock.internal.core.event.model.impl.PipelineFailedEvent;
+import io.flamingock.internal.core.event.model.impl.PipelineStartedEvent;
+import io.flamingock.internal.core.operation.Operation;
+import io.flamingock.internal.core.operation.result.ExecutionResultBuilder;
+import io.flamingock.internal.core.pipeline.execution.ExecutableStage;
+import io.flamingock.internal.core.pipeline.loaded.LoadedPipeline;
+import io.flamingock.internal.core.pipeline.loaded.stage.AbstractLoadedStage;
+import io.flamingock.internal.core.plan.ExecutionPlan;
+import io.flamingock.internal.core.plan.ExecutionPlanner;
+import io.flamingock.internal.core.external.store.lock.LockException;
+import io.flamingock.internal.core.task.executable.ExecutableTask;
+import io.flamingock.internal.util.id.RunnerId;
+import io.flamingock.internal.util.log.FlamingockLoggerFactory;
+import org.slf4j.Logger;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Validates the pipeline without executing any changes.
+ * If pending changes exist, throws {@link PendingChangesException}.
+ */
+public class ValidateOperation implements Operation<ExecuteArgs, ExecuteResult> {
+
+    private static final Logger logger = FlamingockLoggerFactory.getLogger("PipelineRunner");
+
+    private final RunnerId runnerId;
+
+    private final ExecutionPlanner executionPlanner;
+
+    private final EventPublisher eventPublisher;
+
+    private final boolean throwExceptionIfCannotObtainLock;
+
+    private final Runnable finalizer;
+
+    public ValidateOperation(RunnerId runnerId,
+                             ExecutionPlanner executionPlanner,
+                             EventPublisher eventPublisher,
+                             boolean throwExceptionIfCannotObtainLock,
+                             Runnable finalizer) {
+        this.runnerId = runnerId;
+        this.executionPlanner = executionPlanner;
+        this.eventPublisher = eventPublisher;
+        this.throwExceptionIfCannotObtainLock = throwExceptionIfCannotObtainLock;
+        this.finalizer = finalizer;
+    }
+
+    @Override
+    public ExecuteResult execute(ExecuteArgs args) {
+        ExecuteResponseData result;
+        try {
+            result = this.validate(args.getPipeline());
+        } catch (FlamingockException flamingockException) {
+            throw flamingockException;
+        } catch (Throwable throwable) {
+            throw new FlamingockException(throwable);
+        } finally {
+            finalizer.run();
+        }
+        return new ExecuteResult(result);
+    }
+
+    private static List<AbstractLoadedStage> validateAndGetExecutableStages(LoadedPipeline pipeline) {
+        pipeline.validate();
+        List<AbstractLoadedStage> stages = new ArrayList<>();
+        if (pipeline.getSystemStage().isPresent()) {
+            stages.add(pipeline.getSystemStage().get());
+        }
+        stages.addAll(pipeline.getStages());
+        return stages;
+    }
+
+    private ExecuteResponseData validate(LoadedPipeline pipeline) throws FlamingockException {
+        List<AbstractLoadedStage> allStages = validateAndGetExecutableStages(pipeline);
+        int stageCount = allStages.size();
+        long changeCount = allStages.stream()
+                .mapToLong(stage -> stage.getTasks().size())
+                .sum();
+        logger.info("Flamingock validation started [stages={} changes={}]", stageCount, changeCount);
+
+        eventPublisher.publish(new PipelineStartedEvent());
+        ExecutionResultBuilder resultBuilder = new ExecutionResultBuilder().startTimer();
+
+        do {
+            List<AbstractLoadedStage> stages = validateAndGetExecutableStages(pipeline);
+            try (ExecutionPlan execution = executionPlanner.getNextExecution(stages)) {
+                execution.validate();
+
+                if (execution.isExecutionRequired()) {
+                    int pendingCount = countPendingTasks(execution);
+                    throw new PendingChangesException(pendingCount);
+                } else {
+                    break;
+                }
+            } catch (LockException exception) {
+                eventPublisher.publish(new PipelineFailedEvent(exception));
+                if (throwExceptionIfCannotObtainLock) {
+                    logger.debug("Required process lock not acquired - ABORTING VALIDATION", exception);
+                    throw exception;
+                } else {
+                    logger.warn("Process lock not acquired but throwExceptionIfCannotObtainLock=false - CONTINUING WITHOUT LOCK", exception);
+                }
+                break;
+            }
+        } while (true);
+
+        resultBuilder.stopTimer().noChanges();
+        ExecuteResponseData result = resultBuilder.build();
+
+        logger.info("Flamingock validation completed — no pending changes detected");
+        eventPublisher.publish(new PipelineCompletedEvent());
+
+        return result;
+    }
+
+    private static int countPendingTasks(ExecutionPlan execution) {
+        int count = 0;
+        for (ExecutableStage stage : execution.getPipeline().getExecutableStages()) {
+            for (ExecutableTask task : stage.getTasks()) {
+                if (!task.isAlreadyApplied()) {
+                    count++;
+                }
+            }
+        }
+        return count;
+    }
+}