diff --git a/headers/enduro2d/high/_high.hpp b/headers/enduro2d/high/_high.hpp
index ebc25fb5..bf7d0ca0 100644
--- a/headers/enduro2d/high/_high.hpp
+++ b/headers/enduro2d/high/_high.hpp
@@ -50,11 +50,10 @@ namespace e2d
     class content_asset;
     class asset;
 
-    template < typename T >
-    class asset_cache;
-    class asset_cache_base;
-
     class library;
+    class asset_cache;
+    class asset_group;
+    class asset_dependencies;
 
     class atlas;
     class flipbook;
diff --git a/headers/enduro2d/high/asset.hpp b/headers/enduro2d/high/asset.hpp
index 4a3a6d60..cc8bdd8d 100644
--- a/headers/enduro2d/high/asset.hpp
+++ b/headers/enduro2d/high/asset.hpp
@@ -10,6 +10,8 @@
 
 #include "_high.hpp"
 
+#include "address.hpp"
+
 namespace e2d
 {
     //
@@ -22,17 +24,6 @@ namespace e2d
         }
     };
 
-    //
-    // bad_asset_factory_operation
-    //
-
-    class bad_asset_factory_operation final : public exception {
-    public:
-        const char* what() const noexcept final {
-            return "bad asset factory operation";
-        }
-    };
-
     //
     // asset
     //
@@ -45,9 +36,9 @@ namespace e2d
         : private noncopyable
         , public ref_counter<asset> {
     public:
-        asset();
-        virtual ~asset() noexcept;
-        virtual asset_ptr find_nested_asset(str_view name) const noexcept = 0;
+        asset() = default;
+        virtual ~asset() noexcept = default;
+        virtual asset_ptr find_nested_asset(str_view nested_address) const noexcept = 0;
     };
 
     //
@@ -73,9 +64,9 @@ namespace e2d
 
         const Content& content() const noexcept;
 
-        template < typename T >
-        intrusive_ptr<T> find_nested_asset(str_view name) const noexcept;
-        asset_ptr find_nested_asset(str_view name) const noexcept override;
+        template < typename NestedAsset >
+        typename NestedAsset::ptr find_nested_asset(str_view nested_address) const noexcept;
+        asset_ptr find_nested_asset(str_view nested_address) const noexcept override;
     private:
         Content content_;
         nested_content nested_content_;
@@ -87,60 +78,57 @@ namespace e2d
 
     class asset_cache_base : private noncopyable {
     public:
-        asset_cache_base();
-        virtual ~asset_cache_base() noexcept;
+        asset_cache_base() = default;
+        virtual ~asset_cache_base() noexcept = default;
 
-        static std::size_t unload_all_unused_assets() noexcept;
-        virtual std::size_t unload_self_unused_assets() noexcept = 0;
+        virtual std::size_t asset_count() const noexcept = 0;
+        virtual std::size_t unload_unused_assets() noexcept = 0;
+    };
+
+    //
+    // typed_asset_cache
+    //
+
+    template < typename Asset >
+    class typed_asset_cache : public asset_cache_base {
+    public:
+        using asset_ptr = typename Asset::ptr;
+    public:
+        typed_asset_cache() = default;
+        ~typed_asset_cache() noexcept final = default;
+
+        asset_ptr find(str_hash address) const noexcept;
+        void store(str_hash address, const asset_ptr& asset);
+
+        std::size_t asset_count() const noexcept override;
+        std::size_t unload_unused_assets() noexcept override;
     private:
-        static std::mutex mutex_;
-        static hash_set<asset_cache_base*> caches_;
+        hash_map<str_hash, asset_ptr> assets_;
     };
 
     //
     // asset_cache
     //
 
-    template < typename Asset >
-    class asset_cache : public asset_cache_base
-                      , public module<asset_cache<Asset>> {
+    class asset_cache final {
     public:
-        using asset_ptr = typename Asset::ptr;
-    public:
-        asset_cache(library& l);
-        ~asset_cache() noexcept final;
-
-        asset_ptr find(str_hash address) const noexcept;
-        void store(str_hash address, const asset_ptr& asset);
-
-        void clear() noexcept;
-        std::size_t asset_count() const noexcept;
-
-        std::size_t unload_self_unused_assets() noexcept override;
-    private:
-        library& library_;
-        mutable std::mutex mutex_;
-        hash_map<str_hash, asset_ptr> assets_;
-    };
-
-    //
-    // asset_factory
-    //
-
-    class asset_factory : public module<asset_factory> {
-    public:
-        using asset_creator = std::function<
-            stdex::promise<asset_ptr>(const library& library, str_view address)>;
-    public:
-        asset_factory();
-        ~asset_factory() noexcept final;
+        asset_cache() = default;
+        ~asset_cache() noexcept = default;
 
         template < typename Asset >
-        asset_factory& register_asset(str_hash type);
-        asset_factory& register_creator(str_hash type, asset_creator creator);
+        void store(str_hash address, const typename Asset::ptr& asset);
+
+        template < typename Asset >
+        typename Asset::ptr find(str_hash address) const noexcept;
+
+        template < typename Asset >
+        std::size_t asset_count() const noexcept;
+        std::size_t asset_count() const noexcept;
+
+        std::size_t unload_unused_assets() noexcept;
     private:
-        std::mutex mutex_;
-        hash_map<str_hash, asset_creator> creators_;
+        using asset_cache_uptr = std::unique_ptr<asset_cache_base>;
+        hash_map<utils::type_family_id, asset_cache_uptr> caches_;
     };
 }
 
diff --git a/headers/enduro2d/high/asset.inl b/headers/enduro2d/high/asset.inl
index 3f9ecf22..0a52d10c 100644
--- a/headers/enduro2d/high/asset.inl
+++ b/headers/enduro2d/high/asset.inl
@@ -56,33 +56,29 @@ namespace e2d
     }
 
     template < typename Asset, typename Content >
-    template < typename T >
-    intrusive_ptr<T> content_asset<Asset, Content>::find_nested_asset(str_view name) const noexcept {
-        return dynamic_pointer_cast<T>(find_nested_asset(name));
+    template < typename NestedAsset >
+    typename NestedAsset::ptr content_asset<Asset, Content>::find_nested_asset(str_view nested_address) const noexcept {
+        return dynamic_pointer_cast<NestedAsset>(find_nested_asset(nested_address));
     }
 
     template < typename Asset, typename Content >
-    asset_ptr content_asset<Asset, Content>::find_nested_asset(str_view name) const noexcept {
-        const auto iter = nested_content_.find(name);
-        return iter != nested_content_.end()
+    asset_ptr content_asset<Asset, Content>::find_nested_asset(str_view address) const noexcept {
+        const auto iter = nested_content_.find(make_hash(address::parent(address)));
+        if ( iter == nested_content_.end() ) {
+            return nullptr;
+        }
+        const str nested_asset = address::nested(address);
+        return nested_asset.empty()
             ? iter->second
-            : nullptr;
+            : iter->second->find_nested_asset(nested_asset);
     }
 
     //
-    // asset_cache
+    // typed_asset_cache
     //
 
     template < typename T >
-    asset_cache<T>::asset_cache(library& l)
-    : library_(l) {}
-
-    template < typename T >
-    asset_cache<T>::~asset_cache() noexcept = default;
-
-    template < typename T >
-    typename asset_cache<T>::asset_ptr asset_cache<T>::find(str_hash address) const noexcept {
-        std::lock_guard<std::mutex> guard(mutex_);
+    typename typed_asset_cache<T>::asset_ptr typed_asset_cache<T>::find(str_hash address) const noexcept {
         const auto iter = assets_.find(address);
         return iter != assets_.end()
             ? iter->second
@@ -90,26 +86,17 @@ namespace e2d
     }
 
     template < typename T >
-    void asset_cache<T>::store(str_hash address, const asset_ptr& asset) {
-        std::lock_guard<std::mutex> guard(mutex_);
+    void typed_asset_cache<T>::store(str_hash address, const asset_ptr& asset) {
         assets_[address] = asset;
     }
 
     template < typename T >
-    void asset_cache<T>::clear() noexcept {
-        std::lock_guard<std::mutex> guard(mutex_);
-        assets_.clear();
-    }
-
-    template < typename T >
-    std::size_t asset_cache<T>::asset_count() const noexcept {
-        std::lock_guard<std::mutex> guard(mutex_);
+    std::size_t typed_asset_cache<T>::asset_count() const noexcept {
         return assets_.size();
     }
 
     template < typename T >
-    std::size_t asset_cache<T>::unload_self_unused_assets() noexcept {
-        std::lock_guard<std::mutex> guard(mutex_);
+    std::size_t typed_asset_cache<T>::unload_unused_assets() noexcept {
         std::size_t result = 0u;
         for ( auto iter = assets_.begin(); iter != assets_.end(); ) {
             if ( !iter->second || 1 == iter->second->use_count() ) {
@@ -123,27 +110,61 @@ namespace e2d
     }
 
     //
-    // asset_factory
+    // asset_cache
     //
 
     template < typename Asset >
-    asset_factory& asset_factory::register_asset(str_hash type) {
-        return register_creator(type, [](const library& library, str_view address) {
-            return Asset::load_async(library, address)
-                .then([](const typename Asset::load_result& result){
-                    return static_pointer_cast<asset>(result);
-                });
-        });
+    void asset_cache::store(str_hash address, const typename Asset::ptr& asset) {
+        const auto family = utils::type_family<Asset>::id();
+        const auto iter = caches_.find(family);
+        typed_asset_cache<Asset>* cache = iter != caches_.end() && iter->second
+            ? static_cast<typed_asset_cache<Asset>*>(iter->second.get())
+            : nullptr;
+        if ( !cache ) {
+            cache = static_cast<typed_asset_cache<Asset>*>(caches_.emplace(
+                family,
+                std::make_unique<typed_asset_cache<Asset>>()).first->second.get());
+        }
+        cache->store(address, asset);
     }
 
-    inline asset_factory& asset_factory::register_creator(str_hash type, asset_creator creator) {
-        std::lock_guard<std::mutex> guard(mutex_);
-        bool success = creators_.insert(
-            std::make_pair(type, std::move(creator))).second;
-        if ( !success ) {
-            throw bad_asset_factory_operation();
-        }
-        return *this;
+    template < typename Asset >
+    typename Asset::ptr asset_cache::find(str_hash address) const noexcept {
+        const auto iter = caches_.find(utils::type_family<Asset>::id());
+        const typed_asset_cache<Asset>* cache = iter != caches_.end() && iter->second
+            ? static_cast<const typed_asset_cache<Asset>*>(iter->second.get())
+            : nullptr;
+        return cache
+            ? cache->find(address)
+            : nullptr;
+    }
+
+    template < typename Asset >
+    std::size_t asset_cache::asset_count() const noexcept {
+        const auto iter = caches_.find(utils::type_family<Asset>::id());
+        return iter != caches_.end() && iter->second
+            ? iter->second->asset_count()
+            : 0u;
+    }
+
+    inline std::size_t asset_cache::asset_count() const noexcept {
+        return std::accumulate(
+            caches_.begin(), caches_.end(), std::size_t(0),
+            [](std::size_t acc, const auto& p){
+                return p.second
+                    ? acc + p.second->asset_count()
+                    : acc;
+            });
+    }
+
+    inline std::size_t asset_cache::unload_unused_assets() noexcept {
+        return std::accumulate(
+            caches_.begin(), caches_.end(), std::size_t(0),
+            [](std::size_t acc, const auto& p){
+                return p.second
+                    ? acc + p.second->unload_unused_assets()
+                    : acc;
+            });
     }
 }
 
diff --git a/headers/enduro2d/high/library.hpp b/headers/enduro2d/high/library.hpp
index 9ec56e5e..29a6816d 100644
--- a/headers/enduro2d/high/library.hpp
+++ b/headers/enduro2d/high/library.hpp
@@ -11,32 +11,73 @@
 #include "_high.hpp"
 
 #include "asset.hpp"
-#include "address.hpp"
 
 namespace e2d
 {
     //
-    // bad_library_operation
+    // library_cancelled_exception
     //
 
-    class bad_library_operation final : public exception {
-    public:
-        const char* what() const noexcept final {
-            return "bad library operation";
+    class library_cancelled_exception : public exception {
+        const char* what() const noexcept override {
+            return "library cancelled exception";
         }
     };
 
+    //
+    // loading_asset_base
+    //
+
+    class loading_asset_base;
+    using loading_asset_base_iptr = intrusive_ptr<loading_asset_base>;
+
+    class loading_asset_base
+        : private noncopyable
+        , public ref_counter<loading_asset_base> {
+    public:
+        loading_asset_base() = default;
+        virtual ~loading_asset_base() noexcept = default;
+
+        virtual void cancel() noexcept = 0;
+        virtual str_hash address() const noexcept = 0;
+    };
+
+    //
+    // loading_asset
+    //
+
+    template < typename Asset >
+    class loading_asset : public loading_asset_base {
+    public:
+        using ptr = intrusive_ptr<loading_asset>;
+        using promise_type = typename Asset::load_async_result;
+    public:
+        loading_asset(str_hash address, promise_type promise);
+        ~loading_asset() noexcept override = default;
+
+        void cancel() noexcept override;
+        str_hash address() const noexcept override;
+
+        const promise_type& promise() const noexcept;
+    private:
+        str_hash address_;
+        promise_type promise_;
+    };
+
     //
     // library
     //
 
     class library final : public module<library> {
     public:
-        library(const url& root);
+        library(const url& root, deferrer& deferrer);
         ~library() noexcept final;
 
         const url& root() const noexcept;
+        const asset_cache& cache() const noexcept;
+
         std::size_t unload_unused_assets() noexcept;
+        std::size_t loading_asset_count() const noexcept;
 
         template < typename Asset >
         typename Asset::load_result load_main_asset(str_view address) const;
@@ -49,8 +90,103 @@ namespace e2d
 
         template < typename Asset, typename Nested = Asset >
         typename Nested::load_async_result load_asset_async(str_view address) const;
+    private:
+        template < typename Asset >
+        vector<loading_asset_base_iptr>::iterator
+        find_loading_asset_iter_(str_hash address) const noexcept;
+
+        template < typename Asset >
+        typename loading_asset<Asset>::ptr
+        find_loading_asset_(str_hash address) const noexcept;
+
+        template < typename Asset >
+        void remove_loading_asset_(str_hash address) const noexcept;
+
+        void cancel_all_loading_assets_() noexcept;
     private:
         url root_;
+        deferrer& deferrer_;
+        std::atomic<bool> cancelled_{false};
+    private:
+        mutable asset_cache cache_;
+        mutable std::recursive_mutex mutex_;
+        mutable vector<loading_asset_base_iptr> loading_assets_;
+    };
+
+    //
+    // asset_group
+    //
+
+    class asset_group {
+    public:
+        asset_group() = default;
+        ~asset_group() noexcept = default;
+
+        template < typename Iter >
+        asset_group& add_assets(Iter first, Iter last);
+
+        template < typename Container >
+        asset_group& add_assets(Container&& container);
+
+        asset_group& add_asset(str_view address, const asset_ptr& asset);
+
+        template < typename Asset, typename Nested = Asset >
+        typename Nested::load_result find_asset(str_view address) const;
+    private:
+        vector<std::pair<str, asset_ptr>> assets_;
+    };
+
+    //
+    // asset_dependency_base
+    //
+
+    class asset_dependency_base;
+    using asset_dependency_base_iptr = intrusive_ptr<asset_dependency_base>;
+
+    class asset_dependency_base
+        : private noncopyable
+        , public ref_counter<asset_dependency_base> {
+    public:
+        asset_dependency_base() = default;
+        virtual ~asset_dependency_base() noexcept = default;
+
+        virtual const str& main_address() const noexcept = 0;
+        virtual stdex::promise<asset_ptr> load_async(const library& library) = 0;
+    };
+
+    //
+    // asset_dependency
+    //
+
+    template < typename Asset >
+    class asset_dependency : public asset_dependency_base {
+    public:
+        using asset_type = Asset;
+        using load_result = typename Asset::load_result;
+    public:
+        asset_dependency(str_view address);
+        ~asset_dependency() noexcept override;
+
+        const str& main_address() const noexcept override;
+        stdex::promise<asset_ptr> load_async(const library& library) override;
+    private:
+        str main_address_;
+    };
+
+    //
+    // asset_dependencies
+    //
+
+    class asset_dependencies final {
+    public:
+        asset_dependencies() = default;
+        ~asset_dependencies() noexcept = default;
+
+        template < typename Asset >
+        asset_dependencies& add_dependency(str_view address);
+        stdex::promise<asset_group> load_async(const library& library);
+    private:
+        vector<asset_dependency_base_iptr> dependencies_;
     };
 }
 
diff --git a/headers/enduro2d/high/library.inl b/headers/enduro2d/high/library.inl
index 0c451ae4..f517dbd5 100644
--- a/headers/enduro2d/high/library.inl
+++ b/headers/enduro2d/high/library.inl
@@ -13,74 +13,277 @@
 
 namespace e2d
 {
+    //
+    // loading_asset
+    //
+
+    template < typename Asset >
+    loading_asset<Asset>::loading_asset(str_hash address, promise_type promise)
+    : address_(address)
+    , promise_(std::move(promise)) {}
+
+    template < typename Asset >
+    void loading_asset<Asset>::cancel() noexcept {
+        promise_.reject(library_cancelled_exception());
+    }
+
+    template < typename Asset >
+    str_hash loading_asset<Asset>::address() const noexcept {
+        return address_;
+    }
+
+    template < typename Asset >
+    const typename loading_asset<Asset>::promise_type& loading_asset<Asset>::promise() const noexcept {
+        return promise_;
+    }
+
+    //
+    // library
+    //
+
+    inline library::library(const url& root, deferrer& deferrer)
+    : root_(root)
+    , deferrer_(deferrer) {}
+
+    inline library::~library() noexcept {
+        std::unique_lock<std::recursive_mutex> lock(mutex_);
+        cancelled_.store(true);
+        cancel_all_loading_assets_();
+    }
+
+    inline const url& library::root() const noexcept {
+        return root_;
+    }
+
+    inline const asset_cache& library::cache() const noexcept {
+        return cache_;
+    }
+
+    inline std::size_t library::unload_unused_assets() noexcept {
+        return cache_.unload_unused_assets();
+    }
+
+    inline std::size_t library::loading_asset_count() const noexcept {
+        std::lock_guard<std::recursive_mutex> guard(mutex_);
+        return loading_assets_.size();
+    }
+
     template < typename Asset >
     typename Asset::load_result library::load_main_asset(str_view address) const {
         auto p = load_main_asset_async<Asset>(address);
-
-        if ( modules::is_initialized<deferrer>() ) {
-            the<deferrer>().active_safe_wait_promise(p);
-        }
-
+        deferrer_.active_safe_wait_promise(p);
         return p.get_or_default(nullptr);
     }
 
     template < typename Asset >
     typename Asset::load_async_result library::load_main_asset_async(str_view address) const {
-        const auto main_address = address::parent(address);
-        const auto main_address_hash = make_hash(main_address);
-
-        if ( !modules::is_initialized<asset_cache<Asset>>() ) {
-            return Asset::load_async(*this, main_address);
+        if ( cancelled_ ) {
+            return stdex::make_rejected_promise<typename Asset::load_result>(library_cancelled_exception());
         }
 
-        auto& cache = the<asset_cache<Asset>>();
-        if ( auto cached_asset = cache.find(main_address_hash) ) {
+        const str main_address = address::parent(address);
+        const str_hash main_address_hash = make_hash(main_address);
+
+        std::lock_guard<std::recursive_mutex> guard(mutex_);
+
+        if ( auto cached_asset = cache_.find<Asset>(main_address_hash) ) {
             return stdex::make_resolved_promise(std::move(cached_asset));
         }
 
-        return Asset::load_async(*this, main_address)
-            .then([
-                &cache,
-                main_address_hash
-            ](const typename Asset::load_result& new_asset){
-                cache.store(main_address_hash, new_asset);
-                return new_asset;
-            });
+        if ( auto asset = find_loading_asset_<Asset>(main_address_hash) )  {
+            return asset->promise();
+        }
+
+        auto p = Asset::load_async(*this, main_address)
+        .then([
+            this,
+            main_address_hash
+        ](const typename Asset::load_result& new_asset){
+            std::lock_guard<std::recursive_mutex> guard(mutex_);
+            cache_.store<Asset>(main_address_hash, new_asset);
+            remove_loading_asset_<Asset>(main_address_hash);
+            return new_asset;
+        }).except([
+            this,
+            main_address_hash
+        ](std::exception_ptr e) -> typename Asset::load_result {
+            std::lock_guard<std::recursive_mutex> guard(mutex_);
+            remove_loading_asset_<Asset>(main_address_hash);
+            std::rethrow_exception(e);
+        });
+
+        loading_assets_.push_back(new loading_asset<Asset>(address, p));
+        return p;
     }
 
     template < typename Asset, typename Nested >
     typename Nested::load_result library::load_asset(str_view address) const {
         auto p = load_asset_async<Asset, Nested>(address);
-
-        if ( modules::is_initialized<deferrer>() ) {
-            the<deferrer>().active_safe_wait_promise(p);
-        }
-
+        deferrer_.active_safe_wait_promise(p);
         return p.get_or_default(nullptr);
     }
 
     template < typename Asset, typename Nested >
     typename Nested::load_async_result library::load_asset_async(str_view address) const {
-        return load_main_asset_async<Asset>(address::parent(address))
+        return load_main_asset_async<Asset>(address)
             .then([
-                address = str(address)
+                nested_address = address::nested(address)
             ](const typename Asset::load_result& main_asset){
-                asset_ptr nested_asset = main_asset;
-                str nested_address = address::nested(address);
-
-                while ( nested_asset && !nested_address.empty() ) {
-                    nested_asset = nested_asset->find_nested_asset(address::parent(nested_address));
-                    nested_address = address::nested(nested_address);
+                typename Nested::load_result nested_asset = nested_address.empty()
+                    ? dynamic_pointer_cast<Nested>(main_asset)
+                    : main_asset->template find_nested_asset<Nested>(nested_address);
+                if ( nested_asset ) {
+                    return nested_asset;
                 }
-
-                using nested_asset_type = typename Nested::asset_type;
-                if ( auto result = dynamic_pointer_cast<nested_asset_type>(nested_asset) ) {
-                    return result;
-                }
-
                 throw asset_loading_exception();
             });
     }
+
+    template < typename Asset >
+    vector<loading_asset_base_iptr>::iterator
+    library::find_loading_asset_iter_(str_hash address) const noexcept {
+        return std::find_if(
+            loading_assets_.begin(), loading_assets_.end(),
+            [address](const loading_asset_base_iptr& asset) noexcept {
+                return asset->address() == address
+                    && dynamic_pointer_cast<loading_asset<Asset>>(asset);
+            });
+    }
+
+    template < typename Asset >
+    typename loading_asset<Asset>::ptr
+    library::find_loading_asset_(str_hash address) const noexcept {
+        auto iter = find_loading_asset_iter_<Asset>(address);
+        return iter != loading_assets_.end()
+            ? static_pointer_cast<loading_asset<Asset>>(*iter)
+            : nullptr;
+    }
+
+    template < typename Asset >
+    void library::remove_loading_asset_(str_hash address) const noexcept {
+        auto iter = find_loading_asset_iter_<Asset>(address);
+        if ( iter != loading_assets_.end() ) {
+            loading_assets_.erase(iter);
+        }
+    }
+
+    inline void library::cancel_all_loading_assets_() noexcept {
+        for ( const auto& asset : loading_assets_ ) {
+            asset->cancel();
+        }
+        loading_assets_.clear();
+    }
+
+    //
+    // asset_group
+    //
+
+    template < typename Iter >
+    asset_group& asset_group::add_assets(Iter first, Iter last) {
+        for ( auto iter = first; iter != last; ++iter ) {
+            add_asset(iter->first, iter->second);
+        }
+        return *this;
+    }
+
+    template < typename Container >
+    asset_group& asset_group::add_assets(Container&& container) {
+        return add_assets(std::begin(container), std::end(container));
+    }
+
+    inline asset_group& asset_group::add_asset(str_view address, const asset_ptr& asset) {
+        str main_address = address::parent(address);
+        auto iter = std::upper_bound(
+            assets_.begin(), assets_.end(), main_address,
+            [](const str& l, const auto& r){
+                return l < r.first;
+            });
+        assets_.insert(iter, std::make_pair(std::move(main_address), asset));
+        return *this;
+    }
+
+    template < typename Asset, typename Nested >
+    typename Nested::load_result asset_group::find_asset(str_view address) const {
+        const str main_address = address::parent(address);
+        const str nested_address = address::nested(address);
+        auto iter = std::lower_bound(
+            assets_.begin(), assets_.end(), main_address,
+            [](const auto& l, const str& r) noexcept {
+                return l.first < r;
+            });
+        for ( ; iter != assets_.end() && iter->first == main_address; ++iter ) {
+            asset_ptr main_asset = iter->second;
+            typename Asset::load_result typed_main_asset = dynamic_pointer_cast<Asset>(main_asset);
+            if ( !typed_main_asset ) {
+                continue;
+            }
+            typename Nested::load_result nested_asset = nested_address.empty()
+                ? dynamic_pointer_cast<Nested>(typed_main_asset)
+                : typed_main_asset->template find_nested_asset<Nested>(nested_address);
+            if ( nested_asset ) {
+                return nested_asset;
+            }
+        }
+        return nullptr;
+    }
+
+    //
+    // asset_dependency
+    //
+
+    template < typename Asset >
+    asset_dependency<Asset>::asset_dependency(str_view address)
+    : main_address_(address::parent(address)) {}
+
+    template < typename Asset >
+    asset_dependency<Asset>::~asset_dependency() noexcept = default;
+
+    template < typename Asset >
+    const str& asset_dependency<Asset>::main_address() const noexcept {
+        return main_address_;
+    }
+
+    template < typename Asset >
+    stdex::promise<asset_ptr> asset_dependency<Asset>::load_async(const library& library) {
+        return library.load_main_asset_async<Asset>(main_address())
+        .then([](const typename Asset::load_result& main_asset){
+            return asset_ptr(main_asset);
+        });
+    }
+
+    //
+    // asset_dependencies
+    //
+
+    template < typename Asset >
+    asset_dependencies& asset_dependencies::add_dependency(str_view address) {
+        asset_dependency_base_iptr dep(new asset_dependency<Asset>(address));
+        auto iter = std::upper_bound(
+            dependencies_.begin(), dependencies_.end(), dep->main_address(),
+            [](const str& l, const auto& r){
+                return l < r->main_address();
+            });
+        dependencies_.insert(iter, std::move(dep));
+        return *this;
+    }
+
+    inline stdex::promise<asset_group> asset_dependencies::load_async(const library& library) {
+        vector<stdex::promise<std::pair<str, asset_ptr>>> promises;
+        promises.reserve(dependencies_.size());
+        std::transform(
+            dependencies_.begin(), dependencies_.end(), std::back_inserter(promises),
+            [&library](const asset_dependency_base_iptr& dep){
+                return dep->load_async(library)
+                .then([main_address = dep->main_address()](const asset_ptr& asset){
+                    return std::make_pair(main_address, asset);
+                });
+            });
+        return stdex::make_all_promise(std::move(promises))
+        .then([](const vector<std::pair<str, asset_ptr>>& results){
+            return asset_group()
+                .add_assets(results.begin(), results.end());
+        });
+    }
 }
 
 #endif
diff --git a/headers/enduro2d/utils/_utils.hpp b/headers/enduro2d/utils/_utils.hpp
index 61448008..d89ce9bf 100644
--- a/headers/enduro2d/utils/_utils.hpp
+++ b/headers/enduro2d/utils/_utils.hpp
@@ -39,13 +39,13 @@ namespace e2d
 
 namespace e2d
 {
-    using str   = basic_string<char>;
-    using wstr  = basic_string<wchar_t>;
+    using str = basic_string<char>;
+    using wstr = basic_string<wchar_t>;
     using str16 = basic_string<char16_t>;
     using str32 = basic_string<char32_t>;
 
-    using str_view   = basic_string_view<char>;
-    using wstr_view  = basic_string_view<wchar_t>;
+    using str_view = basic_string_view<char>;
+    using wstr_view = basic_string_view<wchar_t>;
     using str16_view = basic_string_view<char16_t>;
     using str32_view = basic_string_view<char32_t>;
 
@@ -147,4 +147,36 @@ namespace e2d { namespace utils
     constexpr std::underlying_type_t<E> enum_to_underlying(E e) noexcept {
         return static_cast<std::underlying_type_t<E>>(e);
     }
+
+    //
+    // type_family
+    //
+
+    using type_family_id = u32;
+
+    namespace impl
+    {
+        template < typename Void = void >
+        class type_family_base {
+            static_assert(
+                std::is_void<Void>::value &&
+                std::is_unsigned<type_family_id>::value,
+                "unexpected internal error");
+        protected:
+            static type_family_id last_id_;
+        };
+
+        template < typename Void >
+        type_family_id type_family_base<Void>::last_id_ = 0u;
+    }
+
+    template < typename T >
+    class type_family final : public impl::type_family_base<> {
+    public:
+        static type_family_id id() noexcept {
+            static type_family_id self_id = ++last_id_;
+            assert(self_id > 0u && "type_family_id overflow");
+            return self_id;
+        }
+    };
 }}
diff --git a/scripts/pvs_analyze.sh b/scripts/pvs_analyze.sh
new file mode 100755
index 00000000..501d2e5c
--- /dev/null
+++ b/scripts/pvs_analyze.sh
@@ -0,0 +1,15 @@
+#!/bin/bash
+set -e
+BUILD_DIR=`dirname "$BASH_SOURCE"`/../build
+mkdir -p $BUILD_DIR/pvs_analyze
+cd $BUILD_DIR/pvs_analyze
+rm -rf pvs_report
+cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=On ../..
+pvs-studio-analyzer analyze\
+                    -e ../../untests\
+                    -e ../../modules\
+                    -e ../../sources/3rdparty\
+                    -o pvs_report.log\
+                    -j8
+plog-converter -a GA:1,2 -t fullhtml -o pvs_report pvs_report.log
+open pvs_report/index.html
diff --git a/sources/enduro2d/high/asset.cpp b/sources/enduro2d/high/asset.cpp
deleted file mode 100644
index 62eebf11..00000000
--- a/sources/enduro2d/high/asset.cpp
+++ /dev/null
@@ -1,49 +0,0 @@
-/*******************************************************************************
- * This file is part of the "Enduro2D"
- * For conditions of distribution and use, see copyright notice in LICENSE.md
- * Copyright (C) 2018-2019, by Matvey Cherevko (blackmatov@gmail.com)
- ******************************************************************************/
-
-#include <enduro2d/high/asset.hpp>
-
-namespace e2d
-{
-    //
-    // asset
-    //
-
-    asset::asset() = default;
-    asset::~asset() noexcept = default;
-
-    //
-    // asset_cache_base
-    //
-
-    std::mutex asset_cache_base::mutex_;
-    hash_set<asset_cache_base*> asset_cache_base::caches_;
-
-    asset_cache_base::asset_cache_base() {
-        std::lock_guard<std::mutex> guard(mutex_);
-        caches_.insert(this);
-    }
-
-    asset_cache_base::~asset_cache_base() noexcept {
-        std::lock_guard<std::mutex> guard(mutex_);
-        caches_.erase(this);
-    }
-
-    std::size_t asset_cache_base::unload_all_unused_assets() noexcept {
-        std::lock_guard<std::mutex> guard(mutex_);
-        return std::accumulate(caches_.begin(), caches_.end(), std::size_t(0),
-            [](std::size_t acc, asset_cache_base* cache){
-                return acc + cache->unload_self_unused_assets();
-            });
-    }
-
-    //
-    // asset_factory
-    //
-
-    asset_factory::asset_factory() = default;
-    asset_factory::~asset_factory() noexcept = default;
-}
diff --git a/sources/enduro2d/high/library.cpp b/sources/enduro2d/high/library.cpp
deleted file mode 100644
index 4a3573f0..00000000
--- a/sources/enduro2d/high/library.cpp
+++ /dev/null
@@ -1,23 +0,0 @@
-/*******************************************************************************
- * This file is part of the "Enduro2D"
- * For conditions of distribution and use, see copyright notice in LICENSE.md
- * Copyright (C) 2018-2019, by Matvey Cherevko (blackmatov@gmail.com)
- ******************************************************************************/
-
-#include <enduro2d/high/library.hpp>
-
-namespace e2d
-{
-    library::library(const url& root)
-    : root_(root) {}
-
-    library::~library() noexcept = default;
-
-    const url& library::root() const noexcept {
-        return root_;
-    }
-
-    std::size_t library::unload_unused_assets() noexcept {
-        return asset_cache_base::unload_all_unused_assets();
-    }
-}
diff --git a/sources/enduro2d/high/starter.cpp b/sources/enduro2d/high/starter.cpp
index 685db6ab..a9b8f8fe 100644
--- a/sources/enduro2d/high/starter.cpp
+++ b/sources/enduro2d/high/starter.cpp
@@ -9,20 +9,6 @@
 #include <enduro2d/high/world.hpp>
 #include <enduro2d/high/library.hpp>
 
-#include <enduro2d/high/assets/atlas_asset.hpp>
-#include <enduro2d/high/assets/binary_asset.hpp>
-#include <enduro2d/high/assets/flipbook_asset.hpp>
-#include <enduro2d/high/assets/image_asset.hpp>
-#include <enduro2d/high/assets/material_asset.hpp>
-#include <enduro2d/high/assets/mesh_asset.hpp>
-#include <enduro2d/high/assets/model_asset.hpp>
-#include <enduro2d/high/assets/prefab_asset.hpp>
-#include <enduro2d/high/assets/shader_asset.hpp>
-#include <enduro2d/high/assets/shape_asset.hpp>
-#include <enduro2d/high/assets/sprite_asset.hpp>
-#include <enduro2d/high/assets/text_asset.hpp>
-#include <enduro2d/high/assets/texture_asset.hpp>
-
 #include <enduro2d/high/systems/flipbook_system.hpp>
 #include <enduro2d/high/systems/render_system.hpp>
 
@@ -129,53 +115,12 @@ namespace e2d
 
     starter::starter(int argc, char *argv[], const parameters& params) {
         safe_module_initialize<engine>(argc, argv, params.engine_params());
-        safe_module_initialize<library>(params.library_root());
-        safe_module_initialize<asset_cache<atlas_asset>>(the<library>());
-        safe_module_initialize<asset_cache<binary_asset>>(the<library>());
-        safe_module_initialize<asset_cache<flipbook_asset>>(the<library>());
-        safe_module_initialize<asset_cache<image_asset>>(the<library>());
-        safe_module_initialize<asset_cache<material_asset>>(the<library>());
-        safe_module_initialize<asset_cache<mesh_asset>>(the<library>());
-        safe_module_initialize<asset_cache<model_asset>>(the<library>());
-        safe_module_initialize<asset_cache<prefab_asset>>(the<library>());
-        safe_module_initialize<asset_cache<shader_asset>>(the<library>());
-        safe_module_initialize<asset_cache<shape_asset>>(the<library>());
-        safe_module_initialize<asset_cache<sprite_asset>>(the<library>());
-        safe_module_initialize<asset_cache<text_asset>>(the<library>());
-        safe_module_initialize<asset_cache<texture_asset>>(the<library>());
-        safe_module_initialize<asset_factory>()
-            .register_asset<atlas_asset>("atlas")
-            .register_asset<binary_asset>("binary")
-            .register_asset<flipbook_asset>("flipbook")
-            .register_asset<image_asset>("image")
-            .register_asset<material_asset>("material")
-            .register_asset<mesh_asset>("mesh")
-            .register_asset<model_asset>("model")
-            .register_asset<prefab_asset>("prefab")
-            .register_asset<shader_asset>("shader")
-            .register_asset<shape_asset>("shape")
-            .register_asset<sprite_asset>("sprite")
-            .register_asset<text_asset>("text")
-            .register_asset<texture_asset>("texture");
+        safe_module_initialize<library>(params.library_root(), the<deferrer>());
         safe_module_initialize<world>();
     }
 
     starter::~starter() noexcept {
         modules::shutdown<world>();
-        modules::shutdown<asset_factory>();
-        modules::shutdown<asset_cache<texture_asset>>();
-        modules::shutdown<asset_cache<text_asset>>();
-        modules::shutdown<asset_cache<sprite_asset>>();
-        modules::shutdown<asset_cache<shape_asset>>();
-        modules::shutdown<asset_cache<shader_asset>>();
-        modules::shutdown<asset_cache<prefab_asset>>();
-        modules::shutdown<asset_cache<model_asset>>();
-        modules::shutdown<asset_cache<mesh_asset>>();
-        modules::shutdown<asset_cache<material_asset>>();
-        modules::shutdown<asset_cache<image_asset>>();
-        modules::shutdown<asset_cache<flipbook_asset>>();
-        modules::shutdown<asset_cache<binary_asset>>();
-        modules::shutdown<asset_cache<atlas_asset>>();
         modules::shutdown<library>();
         modules::shutdown<engine>();
     }
diff --git a/untests/sources/untests_high/asset.cpp b/untests/sources/untests_high/asset.cpp
index ce748d05..a3519dfe 100644
--- a/untests/sources/untests_high/asset.cpp
+++ b/untests/sources/untests_high/asset.cpp
@@ -44,36 +44,43 @@ namespace
     };
 }
 
-TEST_CASE("asset"){
+TEST_CASE("asset") {
     safe_starter_initializer initializer;
     library& l = the<library>();
-    {
-        REQUIRE_FALSE(l.load_asset<fake_asset>("none"));
+    SECTION("nested_assets") {
+        {
+            REQUIRE_FALSE(l.load_asset<fake_asset>("none"));
 
-        auto fa = l.load_asset<fake_asset>("42");
-        REQUIRE(fa);
-        REQUIRE(fa->content() == 42);
+            auto fa = l.load_asset<fake_asset>("42");
+            REQUIRE(fa);
+            REQUIRE(fa->content() == 42);
 
-        REQUIRE_FALSE(fa->find_nested_asset("none"));
-        REQUIRE(fa->find_nested_asset("21"));
-        REQUIRE_FALSE(fa->find_nested_asset<binary_asset>("21"));
-        REQUIRE(fa->find_nested_asset<fake_nested_asset>("21"));
-        REQUIRE(fa->find_nested_asset<fake_nested_asset>("21")->content() == 21);
-        REQUIRE(fa->find_nested_asset<fake_nested_asset>("84")->content() == 84);
-    }
-    {
-        REQUIRE(l.load_asset<fake_asset, fake_nested_asset>("42:/21"));
-        REQUIRE(l.load_asset<fake_asset, fake_nested_asset>("42:/21")->content() == 21);
+            REQUIRE_FALSE(fa->find_nested_asset("none"));
+            REQUIRE(fa->find_nested_asset("21"));
+            REQUIRE(fa->find_nested_asset("21:/2"));
+            REQUIRE_FALSE(fa->find_nested_asset<binary_asset>("21"));
+            REQUIRE_FALSE(fa->find_nested_asset<binary_asset>("21:/2"));
+            REQUIRE(fa->find_nested_asset<fake_nested_asset>("21"));
+            REQUIRE(fa->find_nested_asset<fake_nested_asset>("21:/2"));
+            REQUIRE(fa->find_nested_asset<fake_nested_asset>("21")->content() == 21);
+            REQUIRE(fa->find_nested_asset<fake_nested_asset>("21:/2")->content() == 2);
+            REQUIRE(fa->find_nested_asset<fake_nested_asset>("84")->content() == 84);
+            REQUIRE(fa->find_nested_asset<fake_nested_asset>("84:/8")->content() == 8);
+        }
+        {
+            REQUIRE(l.load_asset<fake_asset, fake_nested_asset>("42:/21"));
+            REQUIRE(l.load_asset<fake_asset, fake_nested_asset>("42:/21")->content() == 21);
 
-        REQUIRE_FALSE(l.load_asset<fake_asset, binary_asset>("42:/21"));
-        REQUIRE_FALSE(l.load_asset<fake_asset, fake_nested_asset>("42:/none"));
-        REQUIRE_FALSE(l.load_asset<fake_asset, fake_nested_asset>("42:/none:/21"));
+            REQUIRE_FALSE(l.load_asset<fake_asset, binary_asset>("42:/21"));
+            REQUIRE_FALSE(l.load_asset<fake_asset, fake_nested_asset>("42:/none"));
+            REQUIRE_FALSE(l.load_asset<fake_asset, fake_nested_asset>("42:/none:/21"));
 
-        REQUIRE(l.load_asset<fake_asset, fake_nested_asset>("42:/21:/2"));
-        REQUIRE(l.load_asset<fake_asset, fake_nested_asset>("42:/21:/2")->content() == 2);
+            REQUIRE(l.load_asset<fake_asset, fake_nested_asset>("42:/21:/2"));
+            REQUIRE(l.load_asset<fake_asset, fake_nested_asset>("42:/21:/2")->content() == 2);
 
-        REQUIRE_FALSE(l.load_asset<fake_asset, binary_asset>("42:/21:/2"));
-        REQUIRE_FALSE(l.load_asset<fake_asset, fake_nested_asset>("42:/21:/none"));
-        REQUIRE_FALSE(l.load_asset<fake_asset, fake_nested_asset>("42:/21:/none:/2"));
+            REQUIRE_FALSE(l.load_asset<fake_asset, binary_asset>("42:/21:/2"));
+            REQUIRE_FALSE(l.load_asset<fake_asset, fake_nested_asset>("42:/21:/none"));
+            REQUIRE_FALSE(l.load_asset<fake_asset, fake_nested_asset>("42:/21:/none:/2"));
+        }
     }
 }
diff --git a/untests/sources/untests_high/library.cpp b/untests/sources/untests_high/library.cpp
index dab0b93f..25622971 100644
--- a/untests/sources/untests_high/library.cpp
+++ b/untests/sources/untests_high/library.cpp
@@ -27,6 +27,44 @@ namespace
 TEST_CASE("library"){
     safe_starter_initializer initializer;
     library& l = the<library>();
+    {
+        binary_asset::ptr b1;
+        binary_asset::ptr b2;
+
+        {
+            auto p1 = l.load_asset_async<binary_asset>("binary_asset.bin");
+            auto p2 = l.load_asset_async<binary_asset>("binary_asset.bin");
+
+            the<deferrer>().active_safe_wait_promise(p1);
+            the<deferrer>().active_safe_wait_promise(p2);
+
+            b1 = p1.get();
+            b2 = p2.get();
+            REQUIRE(b1 == b2);
+        }
+
+        b1.reset();
+        b2.reset();
+
+        std::this_thread::sleep_for(std::chrono::milliseconds(10));
+        REQUIRE(1u == l.unload_unused_assets());
+        REQUIRE(l.cache().asset_count<binary_asset>() == 0);
+    }
+    {
+        {
+            auto p1 = l.load_asset_async<binary_asset>("binary_asset.bin");
+            REQUIRE(l.loading_asset_count() == 1);
+            the<deferrer>().active_safe_wait_promise(p1);
+            REQUIRE(l.loading_asset_count() == 0);
+
+            auto p2 = l.load_asset_async<binary_asset>("none_asset");
+            REQUIRE(l.loading_asset_count() == 1);
+            the<deferrer>().active_safe_wait_promise(p2);
+            REQUIRE(l.loading_asset_count() == 0);
+        }
+        std::this_thread::sleep_for(std::chrono::milliseconds(10));
+        REQUIRE(1u == l.unload_unused_assets());
+    }
     {
         auto text_res = l.load_asset<text_asset>("text_asset.txt");
         REQUIRE(text_res);
@@ -36,15 +74,17 @@ TEST_CASE("library"){
         REQUIRE(text_res_from_cache);
         REQUIRE(text_res_from_cache.get() == text_res.get());
 
-        REQUIRE(0u == the<asset_cache<text_asset>>().unload_self_unused_assets());
-        REQUIRE(the<asset_cache<text_asset>>().asset_count() == 1);
+        REQUIRE(0u == l.unload_unused_assets());
+        REQUIRE(l.cache().asset_count() == 1);
+        REQUIRE(l.cache().asset_count<text_asset>() == 1);
 
         text_res.reset();
         text_res_from_cache.reset();
 
         std::this_thread::sleep_for(std::chrono::milliseconds(10));
-        REQUIRE(1u == the<asset_cache<text_asset>>().unload_self_unused_assets());
-        REQUIRE(the<asset_cache<text_asset>>().asset_count() == 0);
+        REQUIRE(1u == l.unload_unused_assets());
+        REQUIRE(l.cache().asset_count() == 0);
+        REQUIRE(l.cache().asset_count<text_asset>() == 0);
     }
     {
         auto text_res = l.load_asset<text_asset>("text_asset.txt");
@@ -56,12 +96,14 @@ TEST_CASE("library"){
         REQUIRE(binary_res->content() == buffer("world", 5));
 
         REQUIRE(0u == l.unload_unused_assets());
+        REQUIRE(l.cache().asset_count() == 2);
 
         text_res.reset();
         binary_res.reset();
 
         std::this_thread::sleep_for(std::chrono::milliseconds(10));
         REQUIRE(2u == l.unload_unused_assets());
+        REQUIRE(l.cache().asset_count() == 0);
     }
     {
         auto empty_res = l.load_asset<binary_asset>("empty_asset");
@@ -72,19 +114,19 @@ TEST_CASE("library"){
         REQUIRE(image_res);
         REQUIRE(!image_res->content().empty());
 
-        REQUIRE(the<asset_cache<image_asset>>().find("image.png"));
-        REQUIRE(the<asset_cache<binary_asset>>().find("image.png"));
+        REQUIRE(l.cache().find<image_asset>("image.png"));
+        REQUIRE(l.cache().find<binary_asset>("image.png"));
 
         std::this_thread::sleep_for(std::chrono::milliseconds(10));
-        the<asset_cache<binary_asset>>().unload_self_unused_assets();
-        REQUIRE(the<asset_cache<image_asset>>().find("image.png"));
-        REQUIRE_FALSE(the<asset_cache<binary_asset>>().find("image.png"));
+        l.unload_unused_assets();
+        REQUIRE(l.cache().find<image_asset>("image.png"));
+        REQUIRE_FALSE(l.cache().find<binary_asset>("image.png"));
 
         image_res.reset();
         std::this_thread::sleep_for(std::chrono::milliseconds(10));
-        the<asset_cache<image_asset>>().unload_self_unused_assets();
-        REQUIRE_FALSE(the<asset_cache<image_asset>>().find("image.png"));
-        REQUIRE_FALSE(the<asset_cache<binary_asset>>().find("image.png"));
+        l.unload_unused_assets();
+        REQUIRE_FALSE(l.cache().find<image_asset>("image.png"));
+        REQUIRE_FALSE(l.cache().find<binary_asset>("image.png"));
     }
     {
         if ( modules::is_initialized<render>() ) {
@@ -284,3 +326,39 @@ TEST_CASE("library"){
         }
     }
 }
+
+TEST_CASE("asset_dependencies") {
+    safe_starter_initializer initializer;
+    library& l = the<library>();
+    {
+        auto ad = asset_dependencies()
+            .add_dependency<text_asset>("text_asset.txt")
+            .add_dependency<binary_asset>("binary_asset.bin");
+        auto g1_p = ad.load_async(l);
+        the<deferrer>().active_safe_wait_promise(g1_p);
+        asset_group g1 = g1_p.get();
+        REQUIRE(g1.find_asset<text_asset>("text_asset.txt"));
+        REQUIRE(g1.find_asset<binary_asset>("binary_asset.bin"));
+
+        ad.add_dependency<text_asset>("none_asset");
+        auto g2_p = ad.load_async(l);
+        the<deferrer>().active_safe_wait_promise(g2_p);
+    }
+    {
+        if ( modules::is_initialized<render>() ) {
+            auto ad = asset_dependencies()
+                .add_dependency<atlas_asset>("atlas.json:/sprite");
+            auto g1_p = ad.load_async(l);
+            the<deferrer>().active_safe_wait_promise(g1_p);
+            asset_group g1 = g1_p.get();
+            REQUIRE(g1.find_asset<atlas_asset>("atlas.json")
+                == l.load_asset<atlas_asset>("atlas.json"));
+            REQUIRE(g1.find_asset<atlas_asset, sprite_asset>("atlas.json:/sprite")
+                == l.load_asset<atlas_asset, sprite_asset>("atlas.json:/sprite"));
+
+            ad.add_dependency<sprite_asset>("atlas.json:/sprite");
+            auto g2_p = ad.load_async(l);
+            the<deferrer>().active_safe_wait_promise(g2_p);
+        }
+    }
+}
diff --git a/untests/sources/untests_utils/_utils.cpp b/untests/sources/untests_utils/_utils.cpp
index 95023e6f..99a43b7a 100644
--- a/untests/sources/untests_utils/_utils.cpp
+++ b/untests/sources/untests_utils/_utils.cpp
@@ -49,4 +49,11 @@ TEST_CASE("utils") {
             42u, str1, str1 + std::strlen(str1)
         ) == utils::sdbm_hash(42u, str2));
     }
+    {
+        utils::type_family_id id1 = utils::type_family<str16>::id();
+        utils::type_family_id id2 = utils::type_family<str32>::id();
+        REQUIRE(id1 != id2);
+        REQUIRE(id1 == utils::type_family<str16>::id());
+        REQUIRE(id2 == utils::type_family<str32>::id());
+    }
 }