14 August 2017

Delving into SpatialOS

SpatialOS is a cloud computing platform developed by the UK based Improbable that can be used for running large-scale simulated worlds, such as a massively multiplayer game (MMO), a virtual city, or a model of the brain. It is a technology that I first heard of in early 2016 and it has been on my radar since, and so I decided to look into it on the most recent DadenU day by working through some of the tutorials to see what it was all about.

There are a few core concepts to SpatialOS that are essential to understanding how it works. The two main concepts are Entities and Workers.

Each object that is simulated in an SpatialOS world are represented by what are called Entities. This could be a tree, a rock, a nerve cell, or a pirate ship. Each of these entities can be made up of components, which define certain persistent properties, events, and commands. An example would be a player character entity that defined a "health component" - this would have a value property, an event for what happened when it reached 0, and perhaps some commands that can modify the property in specific ways.

My ship in the watery world
All of the processing performed in the simulated world, such as visualising the world or modifying component properties, is performed by Workers.These are services that can be scaled by SpatialOS depending on resource demands. There are both server-side workers, handled by SpatialOS, and client side workers - the application that a user will interact with.

You are able to develop, debug, and test applications developed on SpatialOS on your local machine, allowing for small scale messing around to be done fairly painlessly. My plan was to work through the tutorials in the documentation so that I could get a feel of how to use the technology. The first lesson in the Pirates Tutorial series focuses on setting up the machine to run a local instance of SpatialOS and the tutorial project itself.

A command line package manager called chocolatey is used to install the SpatialOS command line interface (CLI) and stores the location in an environment variable. The source code for the tutorial includes a Unity Worker and a Unity Client. Included in the project is a scene with an empty ocean environment. All other objects, such as the islands and the fish are generated by a worker when the project is launched, and the player ship is generated by a client when it connects. The CLI was used to build the worker and launch SpatialOS locallyWith that the 'server-side' of the game was running and all that was left was for a client to connect to it. 

There are several ways that a client can be run, but the most useful for local development using Unity is to run through the editor interface. Pressing play will launch a local client that allows you to sail around an ocean as a ship. 

Observing pirate ships and fish using the Inspector tool
SpatialOS has an interesting web-base tool called the Inspector that lets you see all of the entities and workers in the running simulation. It displays the areas of the game world that each individual worker and client are currently processing - you even have the ability to remove a worker from the simulation, however SpatialOS will start a new worker instance if it feels that it needs one - and as there is only one required in the tutorial a new one was launched if I deleted the existing worker.

All of the entity types listed can be colour coded so that they are easier to follow when observed in the 2D top down view. There is a 3D option but I couldn't seem to get it to work on my browser. All of the components that make up the entity can be viewed as well, which leads me to believe that the inspector could be a fairly useful monitoring tool during development. The inspector is available on deployments on the cloud as well as locally. 

Other lessons in the tutorial take you through the basics step by step. The world was very empty to begin with and was in dire need of some more entities, so the second lesson takes you through the process of creating one from scratch. This is a two step process - the first is to write an entity template, and the last is to then use the template to spawn the entity within the game world.

Building the pirate ship entity template
The tutorial project uses a factory method pattern to generate the templates for each entity, so to create our AI pirate ships all we needed to do was create our own factory method for it. The entity object is generated using the builder pattern, and there are some components that are required in every entity generated - a position and a metadata component. The pattern also requires that you set the persistence of the entity, and that you set the permissions on the access control list (ACL) before any additional components are added.

Spawning of the entities in the tutorial occur at two distinct stages - at runtime when a player connects, and at the beginning when the world is created in what is known as a snapshot. A snapshot is a representation of the state of the game world at any specific point in time, and when you launch the project to SpatialOS you can define a snapshot to load from.

Every game world requires an initial load state and this is what a snapshot provides. In the case of the tutorial, the player ship template is used to spawn a ship when a user connects, and the pirate ship template is used to spawn ships in the snapshot we defined as default. To define a snapshot we created a custom Unity menu item to populate a dictionary with a list of all of the entities we want to spawn, including a whole bunch of our new pirate ships. Once the worker is rebuilt the client will not be able to see a whole host of static pirate ships within the ocean environment.

Generating a snapshot that includes pirate ships
Getting the pirate ships to move in the environment was next. The tutorial focused on the manipulation of a component's properties by creating a script that will write values to the ShipControls component of the pirate ship entity.

Access restrictions defined when attaching a component to an entity template determine what kind of worker can read from or write to the component. We can use a custom attribute to determine what worker type we want the script to be available for - i.e, the pirate ship is an NPC so we only want it to be controlled on the server side, so we lock the script using the attribute to only appear on UnityWorker instances.

Only one worker, or client, can have write access to a component at any given time, though more than one worker can read from the component. We add a writer component to the script we have created and ensure that it has the [Require] attribute - this means that the script will only be enabled if the current worker has write access to the component.  

To write to a component you use a send method that takes an update structure, which should contain any updates to the component values that need to happen - in the case of the pirate ship we want to update the speed and the steering values of the ShipControls component to get it to move. The worker was rebuilt again, the local client relaunched, and we had moving pirate ships! There was no decision making so they were rather aimless, but at least they were moving now.

Event data flow
Another important aspect of the components are the ability to fire of events. These are transient and are usually used for one-off or infrequent changes, as there is less bandwidth overhead than modifying properties, which are persistent. To learn about events we were tasked with converting locally spawned cannonballs to be visible on other clients.

Adding events to a component first requires knowledge of how a component is defined in the first place. SpatialOS uses a schema to generate code that workers can then use to read and write to components. These are written in what is called schemalang, which is SpatialOS' own proprietary language. An event is defined in this language using the structure: event type name. For example we defined an event that will be fired when a cannon is fired on the left of the ship like so: event FireLeft fire_left. 

Using our new FireLeft and FireRight events instead
of locally firing cannons
Events are defined within the component, and FireLeft is defined as an empty type outwith the component definition in the following fashion: type FireLeft {}. The custom types are capable of storing data, but that wasn't required for the purposes of the tutorial.

The code needs to be generated once the schema for the component has been written so that we can access the component from within our Unity project. The CLI can generate code in multiple languages (currently C#, C++ and Java). To be able to fire events we need access to the component writer so that when we detect that the user has pressed the "fire cannonballs" key we can fire an event by using the component update structure, like we have done when moving the pirate ships.

The script that contains callbacks that fire the cannons
hen an event is received
Firing an event is only half of the story as nothing will happen if nothing is reacting to the event being fired. In the case of Unity it's as easy as creating a new MonoBehaviour script and giving it a component reader as well as a couple of methods that will contain the code we want to run when we receive an event. These methods must be registered as callbacks to the event through the component reader in the MonoBehaviour script's OnEnable method, and must be removed as a callback in the OnDisable method. This is mostly to prevent unexpected behaviour and stop the script from receiving event information when it is disabled.

Next was a short tutorial that discussed how components are accessed by workers and clients. One of the key terms to understand is checked out. Workers don't know about the entire simulated environment in SpatialOS and instead only know about an allocated sub-set of the environment, called a checkout area. They have read access to, and can receive updates from, any entity within this designated area. I mentioned earlier that more than one worker can have read access to a component, and this is because the checkout areas of a worker can overlap with that of another worker; meaning that an entity may be within the area of multiple workers. This is also the reason that only one worker can have write access to a component at any given time.

The ShipControls component's full schema
The final tutorial that I managed to complete before the day ended walked me through the basics of creating a new component from scratch, in this case a "health" component that could be applied to ships so that cannonball hits would affect them on contact.

As mentioned before the component is defined in schemalang. In the schema file you define the namespace of the component as well as the component itself. Each component must have a unique ID within the project and this is define in the schema file. The properties and events of the component are all defined here (eg the Health component has a "current_health" integer property). You can also define commands here but I believe those are covered in the final tutorial.

After defining the component the code has to be generated once again so that the new component can be accessed within the project. Adding the component to an entity is as easy as modifying the template for whichever entity you wish to add it to. Reducing the health of a ship in the tutorial was as simple as updating the current health of the health component whenever a collision was detected between the ship and a cannonball - using a mixture of Unity's OnTriggerEnter method and a writer to the health component I defined.

Writing to the new Health component
In conclusion I think that SpatialOS was actually fairly simple to use once it was all set up. I did attempt to launch the project locally but I never managed to get it consistently working in the short time I had left. The biggest drawback to the pirates tutorial is it didn't give me much of an idea of the main attraction of SpatialOS, which is the ability for there to be multiple workers running a simulation in tandem; for the entirety of the tutorials there was need for only one worker. I'm very curious to see how SpatialOS as a platform develops in the future, as I feel it could have some interesting applications.

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