The standard schema library

This page describes the standard schema library. The standard schema library is a set of schema types and components provided alongside SpatialOS.

The standard library components are special in the sense that they are read by SpatialOS at runtime to control built-in behaviour and runtime features, but are otherwise no different from any component defined in a project’s schema. To a worker, they look just like any other component, and can be manipulated in exactly the same way.

Some of these components are required: SpatialOS will refuse to create an entity without the improbable.Position or improbable.EntityAcl components, for example.

The standard schema library is defined in an a file called improbable/standard_library.schema, which can be imported by any other schema file as usual in order to reuse its type definitions. The library is usually provided by the standard_library (previously WorkerSdkSchema) dependency in the project’s spatialos.json.

Each type and component defined by the standard library is explained here. All of them reside in the improbable schema package.

Position (required)

The most important standard library component is improbable.Position. Read about the Position component in the glossary.

It is defined like so:

type Coordinates {
  double x = 1;
  double y = 2;
  double z = 3;
}

component Position {
  id = 54;
  Coordinates coords = 1;
}

EntityAcl (required)

ACL here stands for “access control list”. The EntityAcl component determines:

• which types of workers have read access to an entity
• for each component on the entity, which type of worker can have write access

The definition of EntityAcl is as follows:

type WorkerAttributeSet {
  list<string> attribute = 1;
}

type WorkerRequirementSet {
  list<WorkerAttributeSet> attribute_set = 1;
}

component EntityAcl {
  id = 50;

  WorkerRequirementSet read_acl = 1;
  map<uint32, WorkerRequirementSet> component_write_acl = 2;
}

EntityAcl contains two fields: read_acl and component_write_acl.

read_acl

In the read_acl field, you specify a worker requirement set. This is made up of one or more worker attribute sets, and determines which worker types have read access to the components on the entity.

For example, if you specified physics and AI in the read_acl component, then all worker instances that are in the physics or AI layer will be able to read from all the components on this entity.

Note: You can’t specify both attributes within the same worker attribute set. You need to specify each worker attribute set separately, for example:

ReadACL = WorkerRequirementSet { WorkerAttributeSet { "physics" }, WorkerAttributeSet { "AI" } }

component_write_acl

In the component_write_acl field you also specify a worker requirement set, this time for each component on the entity. The worker requirement set must contain one worker attribute set per component.

Because a worker type’s worker attribute set specifies the layer that the worker type belongs to, you’re effectively using the component_write_acl field to specify the layer that you want each entity component to belong to (in other words, to specify that worker instances of a specific type should simulate this component on this entity).

For example, if there’s a worker type UnrealWorker with the worker attribute set physics, and an entity with a component_write_acl that specifies that the physics layer should simulate the Position component for that entity, then an instance of UnrealWorker will always simulate the Position component for that entity.

Which worker instance gets write access at which time depends on your load balancing strategy for the layer:

• If there are several running instances of a worker type that has this layer as its worker attribute set, SpatialOS grants write access to one of those instances. (If there’s only one, that instance gets write access.)

• If there are no such running instances, but there is a worker configuration file for a worker type that has this layer as its worker attribute set, SpatialOS launches new worker instances based on the worker configuration file and load balancing strategy in an attempt to make sure there is a worker instance to simulate this component.

Persistence (optional)

The improbable.Persistence component is extremely simple, with no fields at all:

component Persistence {
  id = 55;
}

Read about the Persistence component in the glossary.

This component is optional, and has a single purpose: to mark entities for persistence. All entities with this component will be saved in simulation snapshots, and all entities without it will not be. Note that, for consistency, the snapshot APIs in the Worker SDK in C++, C# and Java will not allow any entities without the Persistence component to be written out to a snapshot.

It’s a good idea to think about what sort of entities make sense to be saved in a simulation snapshot, and add the component as appropriate, even if the project isn’t making use of snapshots from live deployments yet. Examples of entities that might make sense to exclude from snapshots are entities representing connected clients or players (which will no longer exist if a deployment is restarted from a snapshot), or short-lived entities (perhaps representing things like bullets or explosions in a game).

For managing persistence at the component field level, use transient fields.

Metadata (optional)

The improbable.Metadata component is for storing metadata related to an entity:

component Metadata {
  id = 53;

  string entity_type = 1;
}

The only built-in special behaviour of the entity_type field is that it’s used by the SpatialOS inspector to categorise entities (the type is displayed beside each entity when hovering over it with the mouse, and so on). However, this field can also be used as a general entity type field for other purposes: to associate SpatialOS entities with game assets, for example.

Interest (optional)

The improbable.Interest lets you specify interest based on the components you’re authoritative over, using queries. It provides a mapping from a component ID (such as 54 for Position) to a list of queries.

component Interest {
   id = 58;
   map<uint32, ComponentInterest> component_interest = 1;
}

type ComponentInterest {
   ... 

   list<Query> queries = 1;
}

You use the improbable.Interest component to specify query-based interest.

Table of component IDs

Component	ID
Position	54
EntityAcl	50
Persistence	55
Metadata	53
Interest	58

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