Houdini 20.0 Reference

Houdini Engine SessionSync

Utility that lets you run a session of Houdini Engine inside Houdini.

On this page



The video above shows the integration of SessionSync into the Houdini Engine for Unity plug-in.

Houdini Engine SessionSync lets you run a session of Houdini Engine inside Houdini. This gives Houdini Engine clients the ability to connect and communicate through the Houdini Engine API (HAPI). SessionSync also lets you visualize the state of a Houdini Engine session through the Houdini viewport and interface.

Any changes to assets and nodes in either application, whether Houdini or a Houdini Engine client, are automatically synchronized so that you can make changes in both applications and see the same results. SessionSync supports the synchronization of the HDA, curve, and input data types and only nodes created by a Houdini Engine client (for example, like the Unity or UE4 plugin) are synchronized. Currently, nodes created or loaded in Houdini are not synchronized, and HDA actions that add or remove promoted parameters are not synchronized unless the HDA is unlocked and then re-saved.


Houdini Engine SessionSync replaces the Houdini Engine Debugger.



For information on how to open the Houdini Engine SessionSync pane, see Houdini Engine SessionSync pane.

Once the SessionSync server has started, you can configure your Houdini Engine client to connect to a running Houdini instance so that it can execute HAPI calls.

Once connected, you can stop or reconnect SessionSync at any time from your Houdini Engine client, or from your SessionSync pane in Houdini.

SessionSync gives you many capabilities that are beneficial to your workflows. SessionSync allows you to:

  • Visualize the state of your loaded assets and HIP files, and troubleshoot any problems you may have with them.

  • Obtain real-time feedback when you are building an asset in Houdini and see its generated output in your Houdini Engine client.

  • Use Houdini tools and interfaces that aren’t available in the Houdini Engine clients (for example, like python states).

  • Synchronize the Houdini viewport with your Houdini Engine client viewports.

  • Better understand your Houdini Engine client’s features and limitations.

Houdini Engine integration

You can make Houdini Engine integrations (for example, like game engine or DCC plugins) support SessionSync. For more information on HAPI SessionSync integration, see the documentation online.

The SideFX Unreal, Unity, Max, and Maya Houdini Engine plugins are able to connect to and use SessionSync. Currently, only the Unity plugin supports the full set of SessionSync features. For more information, see Unity SessionSync.

Additional documentation

  • For Houdini Engine API documentation and examples, see the Houdini Engine documentation online.

  • For information on the SideFX Houdini Engine plugins, see the Maya, UE4, 3ds Max, and Unity Houdini plugin documentation online.


User interface

  • Menus

    Explains each of the items in the main menus.

  • Viewers

    Viewer pane types.

  • Panes

    Documents the options in various panes.

  • Windows

    Documents the options in various user interface windows.


  • Expression functions

    Expression functions let you compute the value of parameters.

  • Expression cookbook

    Tips and tricks for writing expressions to accomplish various effects.

  • Python scripting

    How to script Houdini using Python and the Houdini Object Model.

  • VEX

    VEX is a high-performance expression language used in many places in Houdini, such as writing shaders.

  • HScript commands

    HScript is Houdini’s legacy scripting language.

  • hwebserver

    Functions and classes for running a web server inside a graphical or non-graphical Houdini session.

Command line

Guru level

Plugin installation

  • Houdini packages

    How to write and combine multiple environment variable definition files for different plug-ins, tools, and add-ons.

Houdini Engine