.. _doc_thread_safe_apis:

Thread-safe APIs
================

Threads
-------

Threads are used to balance processing power across CPUs and cores.
Godot supports multithreading, but not in the whole engine.

Below is a list of ways multithreading can be used in different areas of Godot.

Global scope
------------

Most :ref:`Global Scope<class_@GlobalScope>` singletons are thread-safe by default.
Accessing servers from threads is supported. However, for the
:ref:`rendering <doc_thread_safe_apis_rendering>` and
:ref:`physics <doc_thread_safe_apis_physics>` servers,
thread-safe operation must be enabled in the project settings first.

This makes singletons ideal for code that creates dozens of thousands of instances
in servers and controls them from threads. Of course, it requires a bit more
code, as this is used directly and not within the scene tree.

Scene tree
----------

Interacting with the active scene tree is **not** thread-safe. Make sure
to use mutexes when sending data between threads. If you want to call
functions or set properties from a thread, you may use
:ref:`call_deferred <class_Object_method_call_deferred>` or :ref:`set_deferred <class_Object_method_set_deferred>`:

.. tabs::
 .. code-tab:: gdscript

    # Unsafe:
    node.add_child(child_node)
    # Safe:
    node.add_child.call_deferred(child_node)

 .. code-tab:: csharp

    // Unsafe:
    node.AddChild(childNode);
    // Safe:
    node.CallDeferred(Node.MethodName.AddChild, childNode);

However, creating scene chunks (nodes in tree arrangement) outside the active
tree is fine. This way, parts of a scene can be built or instantiated
in a thread, then added in the main thread:

.. tabs::
 .. code-tab:: gdscript

    var enemy_scene = load("res://enemy_scene.scn")
    var enemy = enemy_scene.instantiate()
    enemy.add_child(weapon) # Set a weapon.
    world.add_child.call_deferred(enemy)

 .. code-tab:: csharp

    PackedScene enemyScene = GD.Load<PackedScene>("res://EnemyScene.scn");
    Node enemy = enemyScene.Instantiate<Node>();
    enemy.AddChild(weapon);
    world.CallDeferred(Node.MethodName.AddChild, enemy);

Still, this is only really useful if you have **one** thread loading data.
Attempting to load or create scene chunks from multiple threads may work,
but you risk resources (which are only loaded once in Godot) being tweaked
by the multiple threads, resulting in unexpected behaviors or crashes.

Only use more than one thread to generate scene data if you *really* know what
you are doing and you are sure that a single resource is not being used or
set in multiple ones. Otherwise, you are safer just using the servers API
(which is fully thread-safe) directly and not touching scene or resources.

.. _doc_thread_safe_apis_rendering:

Rendering
---------

Instancing nodes that render anything in 2D or 3D (such as :ref:`class_Sprite2D`
or :ref:`class_MeshInstance3D`) is *not* thread-safe by default. To run the
rendering driver on a separate thread, set the
:ref:`Rendering > Driver > Thread Model <class_ProjectSettings_property_rendering/driver/threads/thread_model>`
project setting to **Separate**.

Note that the **Separate** thread model has several known bugs, so it may not be usable
in all scenarios.

.. warning::

    You should avoid calling functions involving direct interaction with the GPU
    on other threads, such as creating new textures or modifying and retrieving
    image data. These operations can lead to performance stalls because they require
    synchronization with the :ref:`RenderingServer<class_RenderingServer>`,
    as data needs to be transmitted to or updated on the GPU.

.. _doc_thread_safe_apis_physics:

Physics
-------

Physics simulation is *not* thread-safe by default. To run the physics servers
on separate threads (making them thread-safe), enable the following project settings:

- **PhysicsServer2D:** :ref:`Physics > 2D > Run on Separate Thread <class_ProjectSettings_property_physics/2d/run_on_separate_thread>`.
- **PhysicsServer3D:** :ref:`Physics > 3D > Run on Separate Thread <class_ProjectSettings_property_physics/3d/run_on_separate_thread>`.

GDScript arrays and dictionaries
--------------------------------

In GDScript, reading and writing elements from multiple threads is OK, but anything
that changes the container size (resizing, adding, or removing elements) requires
locking a :ref:`mutex <doc_using_multiple_threads_mutexes>`.

Resources
---------

Modifying a unique resource from multiple threads is not supported. However,
handling references on multiple threads *is* supported. Hence loading resources
on a thread is as well - scenes, textures, meshes, etc - can be loaded and manipulated
on a thread and then added to the active scene on the main thread. The limitation here
is as described above: one must be careful not to load the same resource from
multiple threads at once. Therefore, it's easiest to use **one** thread for loading
and modifying resources, and then the main thread for adding them.