refactor(templates): introduce TemplateField hierarchy (#861)

- Add **TemplateField** interface with `validate()` as base
  for all template field types
- **TemplatePayload** extends TemplateField, adds `getInfo()`
  for transactional metadata
- Relax SHARED_CONFIG bound from `TemplatePayload` to
  `TemplateField` across the template hierarchy
- Config fields now need only validation, not tx semantics
- Binary-compatible: existing TemplatePayload impls still
  satisfy the relaxed bound

Author: dieppa

CLAUDE.md

@@ -515,6 +515,7 @@ See `docs/TEMPLATES_EVOLUTION_PROPOSALS.md` for comprehensive proposals includin
 ## Commit Message Convention
 
 All commits must follow [Conventional Commits](https://www.conventionalcommits.org/) with a well-structured body suitable for changelog extraction from `git log`.
+Don't add co-authored by claude
 
 ### Format
 

core/flamingock-core-api/src/main/java/io/flamingock/api/template/AbstractChangeTemplate.java

@@ -39,7 +39,7 @@
  * @param <ROLLBACK_FIELD> rollback payload type
  * @see io.flamingock.api.annotations.ChangeTemplate
  */
-public abstract class AbstractChangeTemplate<SHARED_CONFIGURATION_FIELD extends TemplatePayload, APPLY_FIELD extends TemplatePayload, ROLLBACK_FIELD extends TemplatePayload> implements ChangeTemplate<SHARED_CONFIGURATION_FIELD, APPLY_FIELD, ROLLBACK_FIELD> {
+public abstract class AbstractChangeTemplate<SHARED_CONFIGURATION_FIELD extends TemplateField, APPLY_FIELD extends TemplatePayload, ROLLBACK_FIELD extends TemplatePayload> implements ChangeTemplate<SHARED_CONFIGURATION_FIELD, APPLY_FIELD, ROLLBACK_FIELD> {
 
     private final Class<SHARED_CONFIGURATION_FIELD> configurationClass;
     private final Class<APPLY_FIELD> applyPayloadClass;

core/flamingock-core-api/src/main/java/io/flamingock/api/template/ChangeTemplate.java

@@ -28,7 +28,7 @@
  * @see AbstractChangeTemplate
  * @see io.flamingock.api.annotations.ChangeTemplate
  */
-public interface ChangeTemplate<SHARED_CONFIG_FIELD extends TemplatePayload, APPLY_FIELD extends TemplatePayload, ROLLBACK_FIELD extends TemplatePayload> extends ReflectionMetadataProvider {
+public interface ChangeTemplate<SHARED_CONFIG_FIELD extends TemplateField, APPLY_FIELD extends TemplatePayload, ROLLBACK_FIELD extends TemplatePayload> extends ReflectionMetadataProvider {
 
     void setChangeId(String changeId);
 

core/flamingock-core-api/src/main/java/io/flamingock/api/template/TemplateField.java

@@ -0,0 +1,37 @@
+/*
+ * 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.api.template;
+
+import java.util.List;
+
+/**
+ * Base contract for all template field types (configuration, apply, rollback).
+ *
+ * <p>Provides structural validation at pipeline load time, before any change executes.
+ * Configuration fields extend this directly, while apply/rollback payloads extend
+ * {@link TemplatePayload} which adds transactional metadata via {@code getInfo()}.
+ */
+public interface TemplateField {
+
+    /**
+     * Validates this field using the supplied change-level context
+     * and returns any errors found.
+     *
+     * @param context change-level metadata available during validation
+     * @return list of validation errors, empty if field is valid
+     */
+    List<TemplatePayloadValidationError> validate(TemplateValidationContext context);
+}

core/flamingock-core-api/src/main/java/io/flamingock/api/template/TemplatePayload.java

@@ -20,19 +20,11 @@
 /**
  * Contract for template payload types (APPLY and ROLLBACK generics).
  *
- * <p>All template payload types must implement this interface to enable
- * structural validation at pipeline load time, before any change executes.
+ * <p>Extends {@link TemplateField} to inherit structural validation, and adds
+ * transactional metadata via {@link #getInfo()}. Configuration fields use
+ * {@code TemplateField} directly since they have no transactional semantics.
  */
-public interface TemplatePayload {
-
-    /**
-     * Validates this payload using the supplied change-level context
-     * and returns any errors found.
-     *
-     * @param context change-level metadata available during validation
-     * @return list of validation errors, empty if payload is valid
-     */
-    List<TemplatePayloadValidationError> validate(TemplateValidationContext context);
+public interface TemplatePayload extends TemplateField {
 
     /**
      * Returns metadata about this payload so the framework can make

core/flamingock-core-api/src/main/java/io/flamingock/api/template/wrappers/TemplateVoid.java

@@ -24,12 +24,11 @@
 import java.util.List;
 
 /**
- * A {@link TemplatePayload} sentinel representing "no configuration needed".
+ * A {@link TemplatePayload} sentinel representing "no value needed".
  *
- * <p>Replaces {@code Void} as the CONFIG type parameter in templates that
- * have no shared configuration. Unlike {@code Void}, this class implements
- * {@code TemplatePayload}, satisfying the {@code CONFIG extends TemplatePayload}
- * bound on the template system.
+ * <p>Replaces {@code Void} as a type parameter in templates that have no shared
+ * configuration or no rollback. Implements {@code TemplatePayload} so it can be
+ * used in any template type parameter position (CONFIG, APPLY, or ROLLBACK).
  */
 public class TemplateVoid implements TemplatePayload {
 

core/flamingock-core/src/main/java/io/flamingock/internal/core/task/executable/AbstractTemplateExecutableTask.java

@@ -16,6 +16,7 @@
 package io.flamingock.internal.core.task.executable;
 
 import io.flamingock.api.template.ChangeTemplate;
+import io.flamingock.api.template.TemplateField;
 import io.flamingock.api.template.TemplatePayload;
 import io.flamingock.api.template.wrappers.TemplateVoid;
 import io.flamingock.internal.common.core.recovery.action.ChangeAction;
@@ -36,7 +37,7 @@
  * @param <ROLLBACK> the rollback payload type
  * @param <LOADED_CHANGE>        the type of template loaded change
  */
-public abstract class AbstractTemplateExecutableTask<CONFIG extends TemplatePayload, APPLY extends TemplatePayload, ROLLBACK extends TemplatePayload,
+public abstract class AbstractTemplateExecutableTask<CONFIG extends TemplateField, APPLY extends TemplatePayload, ROLLBACK extends TemplatePayload,
         LOADED_CHANGE extends AbstractTemplateLoadedChange<CONFIG, APPLY, ROLLBACK>> extends ReflectionExecutableTask<LOADED_CHANGE> {
     protected final Logger logger = FlamingockLoggerFactory.getLogger("TemplateExecutableChange");
 

core/flamingock-core/src/main/java/io/flamingock/internal/core/task/executable/SimpleTemplateExecutableTask.java

@@ -16,6 +16,7 @@
 package io.flamingock.internal.core.task.executable;
 
 import io.flamingock.api.template.AbstractChangeTemplate;
+import io.flamingock.api.template.TemplateField;
 import io.flamingock.api.template.TemplatePayload;
 import io.flamingock.internal.common.core.error.ChangeExecutionException;
 import io.flamingock.internal.common.core.recovery.action.ChangeAction;
@@ -33,7 +34,7 @@
  * @param <ROLLBACK> the rollback payload type
  */
 @SuppressWarnings({"rawtypes", "unchecked"})
-public class SimpleTemplateExecutableTask<CONFIG extends TemplatePayload, APPLY extends TemplatePayload, ROLLBACK extends TemplatePayload>
+public class SimpleTemplateExecutableTask<CONFIG extends TemplateField, APPLY extends TemplatePayload, ROLLBACK extends TemplatePayload>
         extends AbstractTemplateExecutableTask<CONFIG, APPLY, ROLLBACK,
                 SimpleTemplateLoadedChange<CONFIG, APPLY, ROLLBACK>> {
 

core/flamingock-core/src/main/java/io/flamingock/internal/core/task/executable/SteppableTemplateExecutableTask.java

@@ -16,6 +16,7 @@
 package io.flamingock.internal.core.task.executable;
 
 import io.flamingock.api.template.AbstractChangeTemplate;
+import io.flamingock.api.template.TemplateField;
 import io.flamingock.api.template.TemplatePayload;
 import io.flamingock.api.template.TemplateStep;
 import io.flamingock.internal.common.core.error.ChangeExecutionException;
@@ -36,7 +37,7 @@
  * @param <ROLLBACK> the rollback payload type
  */
 @SuppressWarnings({"rawtypes", "unchecked"})
-public class SteppableTemplateExecutableTask<CONFIG extends TemplatePayload, APPLY extends TemplatePayload, ROLLBACK extends TemplatePayload>
+public class SteppableTemplateExecutableTask<CONFIG extends TemplateField, APPLY extends TemplatePayload, ROLLBACK extends TemplatePayload>
         extends AbstractTemplateExecutableTask<CONFIG, APPLY, ROLLBACK,
         MultiStepTemplateLoadedChange<CONFIG, APPLY, ROLLBACK>> {
 

core/flamingock-core/src/main/java/io/flamingock/internal/core/task/loaded/AbstractTemplateLoadedChange.java

@@ -18,6 +18,7 @@
 import io.flamingock.api.annotations.Apply;
 import io.flamingock.api.annotations.Rollback;
 import io.flamingock.api.template.ChangeTemplate;
+import io.flamingock.api.template.TemplateField;
 import io.flamingock.api.template.TemplatePayload;
 import io.flamingock.api.template.TemplateValidationContext;
 import io.flamingock.internal.common.core.error.validation.ValidationError;
@@ -43,7 +44,7 @@
  * @param <APPLY>    the apply payload type
  * @param <ROLLBACK> the rollback payload type
  */
-public abstract class AbstractTemplateLoadedChange<CONFIG extends TemplatePayload, APPLY extends TemplatePayload, ROLLBACK extends TemplatePayload> extends AbstractLoadedChange {
+public abstract class AbstractTemplateLoadedChange<CONFIG extends TemplateField, APPLY extends TemplatePayload, ROLLBACK extends TemplatePayload> extends AbstractLoadedChange {
 
     protected static final Logger logger = FlamingockLoggerFactory.getLogger(AbstractTemplateLoadedChange.class);