HCOLL-81 make it clear that mapForKey is read only as it uses a Function and updateForKey is read/write and takes a mutator.

src/main/java/net/openhft/chronicle/map/StatelessChronicleMap.java

@@ -91,7 +91,8 @@ class StatelessChronicleMap<K, V> implements ChronicleMap<K, V>, Closeable {
         PUT_ALL,
         PUT_ALL_WITHOUT_ACC,
         HASH_CODE,
-        MAP_FOR_KEY
+        MAP_FOR_KEY,
+        UPDATE_FOR_KEY
     }
 
     private long transactionID;
@@ -470,6 +471,19 @@ class StatelessChronicleMap<K, V> implements ChronicleMap<K, V>, Closeable {
         return (R) keyValueSerializer.readObject(reader);
     }
 
+    @Override
+    public synchronized <R> R updateForKey(K key, @NotNull Mutator<? super V, R> mutator) {
+
+        final ThreadLocalCopies local = keyValueSerializer.threadLocalCopies();
+        final long sizeLocation = writeEventAnSkip(UPDATE_FOR_KEY);
+
+        writeKey(key, local);
+        writeObject(mutator);
+
+        final Bytes reader = blockingFetch(sizeLocation);
+        return (R) keyValueSerializer.readObject(reader);
+    }
+
 
     private <R> void writeObject(@NotNull Object function) {
         long start = bytes.position();