diff --git a/src/clientmon.cpp b/src/clientmon.cpp
index bdcd4e2..0c1f9b0 100644
--- a/src/clientmon.cpp
+++ b/src/clientmon.cpp
@@ -547,47 +547,7 @@ void Connection::handle_MONITOR()
             }
             from_wire_valid(M, rxRegistry, data);
 
-            /* co-iterate data and prototype.
-             * copy   marked from data -> prototype
-             * copy unmarked from prototype -> data
-             */
-            {
-                auto desc = Value::Helper::desc(info->prototype);
-                auto delta = Value::Helper::store_ptr(data);
-                auto complete = Value::Helper::store_ptr(info->prototype);
-                auto N = desc->size();
-                for(size_t i=0u; i < N; i++, delta++, complete++)
-                {
-                    const auto src = delta->valid ? delta : complete;
-                    auto       dst = delta->valid ? complete : delta;
-
-                    switch(delta->code) {
-                    case StoreType::Null:
-                        break;
-                    case StoreType::Bool:
-                    case StoreType::UInteger:
-                    case StoreType::Integer:
-                    case StoreType::Real:
-                        memcpy(&dst->store, &src->store, sizeof(src->store));
-                        break;
-                    case StoreType::String:
-                        dst->as<std::string>() = src->as<std::string>();
-                        break;
-                    case StoreType::Array:
-                        dst->as<shared_array<const void>>() = src->as<shared_array<const void>>();
-                        break;
-                    case StoreType::Compound:
-                    {
-                        std::shared_ptr<impl::FieldStorage> sstore(Value::Helper::store(data),
-                                                                  src);
-                        auto& dfld(dst->as<Value>());
-                        Value::Helper::set_desc(dfld, &desc[i]);
-                        Value::Helper::store(dfld) = std::move(sstore);
-                    }
-                        break;
-                    }
-                }
-            }
+            cache_sync(info->prototype, data);
 
             BitMask overrun;
             from_wire(M, overrun);
diff --git a/src/data.cpp b/src/data.cpp
index e640197..a3bc571 100644
--- a/src/data.cpp
+++ b/src/data.cpp
@@ -1184,6 +1184,52 @@ Value::_Iterator<Value::_IMarked>::operator++() noexcept
     return *this;
 }
 
+void cache_sync(Value& cache, Value& dlt)
+{
+    if(!cache.equalType(dlt))
+        throw std::logic_error(SB()<<__func__<<" requires matching types");
+
+    /* co-iterate data and prototype.
+     * copy   marked from data -> prototype
+     * copy unmarked from prototype -> data
+     */
+    auto desc = Value::Helper::desc(cache);
+    auto delta = Value::Helper::store_ptr(dlt);
+    auto complete = Value::Helper::store_ptr(cache);
+    auto N = desc->size();
+    for(size_t i=0u; i < N; i++, delta++, complete++)
+    {
+        const auto src = delta->valid ? delta : complete;
+        auto       dst = delta->valid ? complete : delta;
+
+        switch(delta->code) {
+        case StoreType::Null:
+            break;
+        case StoreType::Bool:
+        case StoreType::UInteger:
+        case StoreType::Integer:
+        case StoreType::Real:
+            memcpy(&dst->store, &src->store, sizeof(src->store));
+            break;
+        case StoreType::String:
+            dst->as<std::string>() = src->as<std::string>();
+            break;
+        case StoreType::Array:
+            dst->as<shared_array<const void>>() = src->as<shared_array<const void>>();
+            break;
+        case StoreType::Compound:
+        {
+            std::shared_ptr<impl::FieldStorage> sstore(Value::Helper::store(dlt),
+                                                      src);
+            auto& dfld(dst->as<Value>());
+            Value::Helper::set_desc(dfld, &desc[i]);
+            Value::Helper::store(dfld) = std::move(sstore);
+        }
+            break;
+        }
+    }
+}
+
 namespace impl {
 
 void FieldStorage::init(StoreType code)
diff --git a/src/dataimpl.h b/src/dataimpl.h
index 1e6c71c..98c7fd7 100644
--- a/src/dataimpl.h
+++ b/src/dataimpl.h
@@ -41,6 +41,14 @@ struct Value::Helper {
     static std::shared_ptr<const impl::FieldDesc> type(const Value& v);
 };
 
+/* Refresh cache from delta, and populate delta with previous.
+ * Requires matching types.
+ *
+ * copy   marked from delta -> cache
+ * copy unmarked from cache -> delta
+ */
+void cache_sync(Value& cache, Value& delta);
+
 namespace impl {
 struct Buffer;