diff --git a/raft/src/main/java/net/kuujo/copycat/raft/StateMachine.java b/raft/src/main/java/net/kuujo/copycat/raft/StateMachine.java
index 1625a76a93..16ebd40930 100644
--- a/raft/src/main/java/net/kuujo/copycat/raft/StateMachine.java
+++ b/raft/src/main/java/net/kuujo/copycat/raft/StateMachine.java
@@ -24,6 +24,8 @@
 import java.lang.reflect.Method;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.function.BiPredicate;
+import java.util.function.Function;
 
 /**
  * Raft state machine.
@@ -32,10 +34,10 @@
  */
 public abstract class StateMachine {
   private final Logger LOGGER = LoggerFactory.getLogger(getClass());
-  private final Map<Compaction.Type, Map<Class<? extends Command>, Method>> filters = new HashMap<>();
-  private Map<Compaction.Type, Method> allFilters = new HashMap<>();
-  private final Map<Class<? extends Operation>, Method> operations = new HashMap<>();
-  private Method allOperation;
+  private final Map<Compaction.Type, Map<Class<? extends Command>, BiPredicate<Commit<?>, Compaction>>> filters = new HashMap<>();
+  private Map<Compaction.Type, BiPredicate<Commit<?>, Compaction>> allFilters = new HashMap<>();
+  private final Map<Class<? extends Operation>, Function<Commit<?>, ?>> operations = new HashMap<>();
+  private Function<Commit<?>, ?> allOperation;
 
   protected StateMachine() {
     init();
@@ -79,37 +81,64 @@ private void declareFilters(Method method) {
       for (Class<? extends Command> command : filter.value()) {
         if (command == Filter.All.class) {
           if (!allFilters.containsKey(filter.compaction())) {
-            allFilters.put(filter.compaction(), method);
+            if (method.getParameterCount() == 1) {
+              allFilters.put(filter.compaction(), wrapFilter(method));
+            }
           }
         } else {
-          Map<Class<? extends Command>, Method> filters = this.filters.get(filter.compaction());
+          Map<Class<? extends Command>, BiPredicate<Commit<?>, Compaction>> filters = this.filters.get(filter.compaction());
           if (filters == null) {
             filters = new HashMap<>();
             this.filters.put(filter.compaction(), filters);
           }
           if (!filters.containsKey(command)) {
-            filters.put(command, method);
+            filters.put(command, wrapFilter(method));
           }
         }
       }
     }
   }
 
+  /**
+   * Wraps a filter method.
+   */
+  private BiPredicate<Commit<?>, Compaction> wrapFilter(Method method) {
+    if (method.getParameterCount() == 1) {
+      return (commit, compaction) -> {
+        try {
+          return (boolean) method.invoke(this, commit);
+        } catch (IllegalAccessException | InvocationTargetException e) {
+          throw new ApplicationException("failed to filter command", e);
+        }
+      };
+    } else if (method.getParameterCount() == 2) {
+      return (commit, compaction) -> {
+        try {
+          return (boolean) method.invoke(this, commit, compaction);
+        } catch (IllegalAccessException | InvocationTargetException e) {
+          throw new ApplicationException("failed to filter command", e);
+        }
+      };
+    } else {
+      throw new IllegalStateException("invalid filter method: too many parameters");
+    }
+  }
+
   /**
    * Finds the filter method for the given command.
    */
-  private Method findFilter(Class<? extends Command> type, Compaction.Type compaction) {
-    Map<Class<? extends Command>, Method> filters = this.filters.get(compaction);
+  private BiPredicate<Commit<?>, Compaction> findFilter(Class<? extends Command> type, Compaction.Type compaction) {
+    Map<Class<? extends Command>, BiPredicate<Commit<?>, Compaction>> filters = this.filters.get(compaction);
     if (filters == null) {
-      Method method = allFilters.get(compaction);
-      if (method == null) {
+      BiPredicate<Commit<?>, Compaction> filter = allFilters.get(compaction);
+      if (filter == null) {
         throw new IllegalArgumentException("unknown command type: " + type);
       }
-      return method;
+      return filter;
     }
 
-    Method method = filters.computeIfAbsent(type, t -> {
-      for (Map.Entry<Class<? extends Command>, Method> entry : filters.entrySet()) {
+    BiPredicate<Commit<?>, Compaction> filter = filters.computeIfAbsent(type, t -> {
+      for (Map.Entry<Class<? extends Command>, BiPredicate<Commit<?>, Compaction>> entry : filters.entrySet()) {
         if (entry.getKey().isAssignableFrom(type)) {
           return entry.getValue();
         }
@@ -117,10 +146,10 @@ private Method findFilter(Class<? extends Command> type, Compaction.Type compact
       return allFilters.get(compaction);
     });
 
-    if (method == null) {
+    if (filter == null) {
       throw new IllegalArgumentException("unknown command type: " + type);
     }
-    return method;
+    return filter;
   }
 
   /**
@@ -132,20 +161,39 @@ private void declareOperations(Method method) {
       method.setAccessible(true);
       for (Class<? extends Operation> operation : apply.value()) {
         if (operation == Apply.All.class) {
-          allOperation = method;
+          allOperation = wrapOperation(method);
         } else if (!operations.containsKey(operation)) {
-          operations.put(operation, method);
+          operations.put(operation, wrapOperation(method));
         }
       }
     }
   }
 
+  /**
+   * Wraps an operation method.
+   */
+  private Function<Commit<?>, ?> wrapOperation(Method method) {
+    if (method.getParameterCount() < 1) {
+      throw new IllegalStateException("invalid operation method: not enough arguments");
+    } else if (method.getParameterCount() > 1) {
+      throw new IllegalStateException("invalid operation method: too many arguments");
+    } else {
+      return commit -> {
+        try {
+          return method.invoke(this, commit);
+        } catch (IllegalAccessException | InvocationTargetException e) {
+          return new ApplicationException("failed to invoke operation", e);
+        }
+      };
+    }
+  }
+
   /**
    * Finds the operation method for the given operation.
    */
-  private Method findOperation(Class<? extends Operation> type) {
-    Method method = operations.computeIfAbsent(type, t -> {
-      for (Map.Entry<Class<? extends Operation>, Method> entry : operations.entrySet()) {
+  private Function<Commit<?>, ?> findOperation(Class<? extends Operation> type) {
+    Function<Commit<?>, ?> operation = operations.computeIfAbsent(type, t -> {
+      for (Map.Entry<Class<? extends Operation>, Function<Commit<?>, ?>> entry : operations.entrySet()) {
         if (entry.getKey().isAssignableFrom(type)) {
           return entry.getValue();
         }
@@ -153,10 +201,10 @@ private Method findOperation(Class<? extends Operation> type) {
       return allOperation;
     });
 
-    if (method == null) {
+    if (operation == null) {
       throw new IllegalArgumentException("unknown operation type: " + type);
     }
-    return method;
+    return operation;
   }
 
   /**
@@ -176,12 +224,8 @@ public void register(Session session) {
    * @return Whether to keep the commit.
    */
   public boolean filter(Commit<? extends Command> commit, Compaction compaction) {
-    LOGGER.debug("filter {}", commit);
-    try {
-      return (boolean) findFilter(commit.type(), compaction.type()).invoke(this, commit, compaction);
-    } catch (IllegalAccessException | InvocationTargetException e) {
-      throw new ApplicationException("failed to filter command", e);
-    }
+    LOGGER.debug("Filtering {}", commit);
+    return findFilter(commit.type(), compaction.type()).test(commit, compaction);
   }
 
   /**
@@ -191,12 +235,8 @@ public boolean filter(Commit<? extends Command> commit, Compaction compaction) {
    * @return The operation result.
    */
   public Object apply(Commit<? extends Operation> commit) {
-    LOGGER.debug("apply {}", commit);
-    try {
-      return findOperation(commit.type()).invoke(this, commit);
-    } catch (IllegalAccessException | InvocationTargetException e) {
-      return new ApplicationException("failed to invoke operation", e);
-    }
+    LOGGER.debug("Applying {}", commit);
+    return findOperation(commit.type()).apply(commit);
   }
 
   /**