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When a Scene in ARENA is loaded, its current state is fetched from a data store service that tracks the persisted state of the Scene. In this section, we detail some features of this essential ARENA service.

img Figure 3. Scene objects are first loaded from a data store service and then updated over PubSub.

Object Persistence

Simply adding persist: true to the top level MQTT message for any create action and the object will be saved. A client then can make a HTTP request to the URL the server this service is running on to retrieve a list of initially loaded objects upon entering any scene.

If an update message contains an explicit persist: false, then the data therein will not be saved in persistence.


Adding a ttl (int seconds) to the top level MQTT message for any create action with persist:true signals that the object will be automatically deleted from persistence after set duration, as well as a corresponding delete action message sent over pubsub.


Templates are special scenes that can be instantiated in entirety in other scenes.

Templates are crafted in a scene name prefixed with the @ symbol, e.g. @myTemplate. The creation process is exactly the same as any other scene with C(R)UD actions on pubsub, with the exception that ttl values are not enforced. That is to say, the objects do not expire inside @template scenes, but rather activated upon instantiation.

When a template is loaded, a parent container is first created in the target scene. This parent container follows the object ID naming scheme: templateId::instanceId, e.g. myTemplate::instance_0.

Then every object inside the designated @template scene is replicated as descendants of the parent container. In this way, the parent can be repositioned, rotated, or scaled to adjust the template all at once. The objects within the template follow the naming scheme templateId::instanceId::objectId, e.g. myTemplate::instance_0::cube1.

To load an instance of a template, send message to your desired target scene:


The data object should also contain position, rotation, or scale directives if intended not to all default to 0 (or 1 for scale) values. You may also set a parent to attach the entire parent container to another existing object in the target scene.

After the template load, all objects behave as typical in any scene.

Implementation and Source

The persistence service listens on the pubsub message bus for ARENA objects to save to a mongodb store.