Bridge configuration

The bridge field of the worker configuration file specifies the worker type’s runtime settings. It has the following overall structure:

"bridge": {
            "worker_attribute_set": {},
            "entity_interest": {},
            "streaming_query": [],
            "component_delivery": {},
            "component_settings": {}
        }

Worker attribute sets

Required field.

A worker attribute set describes which layer a worker type belongs to (in other words, what it’s responsible for simulating). It’s used to work out which components the worker type’s instances can have access to, and it must be a single string. See the worker attribute sets page for details.

For example, a UnityWorker would have the following attribute set:

"worker_attribute_set": {
            "attributes": ["physics"]
        }

In addition to the attribute set defined here, each worker instance automatically has the attribute "workerId:<worker ID>". For example, the worker instance MyCSharpWorker12 would have the attribute "workerId:MyCSharpWorker12" added to it.

Entity interest

Required field.

The entity interest field is about defining which entities a worker instance of this type gets component updates for.

If a worker instance is authoritative over an entity, it gets an update whenever that entity’s components change. But it also needs to know about the entities nearby.

By default, worker instances get component updates for all entities they’re authoritative over, as well as for all other entities in the same chunk (the grid squares of a SpatialOS world). But chunks are square, and so this doesn’t quite work.

You can use the entity interest field to set a radius (in world units, see note below). This is a radius around entities that a worker instance is authoritative over. If there’s another chunk with any point inside this radius, the worker instance will get component updates for entities inside that chunk.

For example, to make sure a worker instance gets updates from entities within 10 world units of any entity the worker instance has write access to, use the following entity_interest setting:

"entity_interest": {
            "range_entity_interest": {
                "radius": 10
            }
        }

Positions and distances are expressed in an arbitrary unit called “world unit”. It is up to the worker instances that run the simulation to interpret them consistently.

Streaming queries

Optional field.

What streaming queries are

SpatialOS is based on the idea that, usually, worker instances only need to know about in the entities they’re authoritative over, plus entities that are nearby (ie within the entity interest radius). For example, a player might need to see entities up to 50m away from them, but not any further away.

But sometimes, worker instances need information about other entities. For example, entities that are far away but should be visible (such as mountains), or entities for global communication that aren’t physically located anywhere (for example, a global weather system). To allow a worker instance to receive updates to these entities, you can use streaming queries.

Entity queries or streaming queries?

If you want information about an entity, but you don’t need regular updates about it, use an entity query instead.

If you need regular updates about an entity, it’s better to use streaming queries: they return information periodically, and they only return the new information about a component, rather than all the information about it.

Query-based interest

If you’re using the new Runtime, you can use query-based interest instead of streaming queries. This lets you write queries to express precisely which components on other entities you are interested in, meaning that the worker instance only receives updates relevant to what it is simulating.

How streaming queries work

Using streaming queries is similar to checking out. If you set up a streaming query for a component, for entities with that component, the worker instance gets a local representation of an entity, and receives updates about it. Effectively, to the worker instance, it looks like the entity is within its checkout area.

The worker instance gets updates to this entity’s components at specific time intervals. You can configure this (see below).

Limitations

Streaming queries have some limitations:

The worker instance doesn’t receive all of the updates for each component of an entity:
- It only gets updates at the time interval you set (see below).
- It doesn’t receive any event updates.
You can only set the time interval globally for all streaming queries within a deployment.
You can’t change streaming queries at runtime.
You have to specify components to query, rather than entities.

Overhead of streaming queries

Streaming queries generally have more processing overhead than checking out entities in the standard way (using interest), so use them sparingly.
The processing cost of a streaming query is similar to the cost of an entity query. If you’d expect an entity query to be expensive (for example, if it searches a large area), a similar streaming query would be expensive too.
Each streaming query has its own overhead, which means you can optimise by merging several streaming queries into one.
The more worker instances you have, the more times your streaming queries will be run, and so the higher their overall cost will be.

Setting up a streaming query

First, you need to pick a component. By default, for every entity with that component, a streaming query will return updates to all components on that entity. However, it’s good practice to improve efficiency and bandwidth use by narrowing down which updates are returned.

To do this, specify:

which components (on the entities returned by the streaming query) you want updates for
a radius (around entities that a worker instance is authoritative over) that an entity must be within

You always need to include the improbable.Position component, and, if you’re writing a streaming query for a Unity or Unreal worker type, you also need to include the improbable.Metadata component.

For example, the following snippet will send this worker instance:

updates to the components your.game.Status, your.game.Health, and your.game.Flammable for all entities with the component your.game.GloballyVisible
updates to all components from all entities with the component your.game.HighlyVisible, within a radius of 1000 world units around an entity that the worker instance is authoritative over

"streaming_query": [
            {
                "global_component_streaming_query": {
                    "component_name": "your.game.GloballyVisible"
                },
                "components_streamed": {
                    "specific_components": {
                        "components": [
                            "improbable.Position"
                            "your.game.Status",
                            "your.game.Health",
                            "your.game.Flammable"
                        ]
                    }
                }
            },
            {
            "bounded_component_streaming_query": {
                "component_name": "your.game.HighlyVisible",
                "radius": 1000
                }
            }
        ]

Configuring the time interval

To configure the time interval between streaming queries, set streaming_query_interval in your launch configuration file.

The default time interval is 4 seconds.

Component delivery

Optional field.

This field configures the set of components a worker instance checks out when they check out an entity, and whether component updates are allowed to be sent in an unreliable manner.

By default:

only components with checkout_initially set to true are sent to the worker instance
the transmission for all component updates is RELIABLE_ORDERED

Query-based interest

You can use query-based interest instead of changing the component delivery settings. Query-based interest lets you write queries to express precisely which components on other entities you are interested in, meaning that the worker instance only receives updates relevant to what it is simulating.

To use query-based interest, you need to upgrade to the new Runtime (available from SpatialOS version 13.4).

Components to check out initially

To specify the set of components a worker instance will get when they check out an entity, use the following fields of component_delivery:

checkoutAllInitially (boolean). By default, this is set to false, which means only the components with checkout_initially set to true are sent to the worker instance when it checks out an entity.

If set to true, all components are sent to the worker instance when it checks out an entity.
override maps fully-qualified component names to component delivery settings.

Components without an override use the default message delivery settings as specified in the default field. They are only checked out initially if checkoutAllInitially is true.

The 4 permutations of these settings for a particular component C therefore lead to the following behaviour:

checkoutAllInitially is false or isn’t specified and the checkout_initially override is false or isn’t specified for C - C is not checked out
checkoutAllInitially is true and the checkout_initially override is false or isn’t specified for C - C is checked out
checkoutAllInitially is false or isn’t specified and the checkout_initially override is true for C - C is checked out
checkoutAllInitially is true and the checkout_initially override is true for C - C is checked out

Individual component overrides only have an effect when checkoutAllInitially is false or isn’t specified.

The worker instance can also change the set of components it is interested in at runtime - see the API documentation (C++, C#, Java) for more details.

Transmitting component updates unreliably (“quality of service”)

Component updates can be transmitted reliably or unreliably.

To prevent worker instances getting overloaded, you can choose to transmit some component updates unreliably. Quality of service describes the concept in more detail.

The default field of component_delivery sets the default way to transmit component updates. This can be either RELIABLE_ORDERED or UNRELIABLE_ORDERED.

This configuration applies only to messages sent to worker instances, not for messages sent from worker instances.

Examples

This example reliably transmits updates for all components except Position and Rotation, and mandates that the position component is checked out initially:

"component_delivery": {
            "default": "RELIABLE_ORDERED",
            "override": {
                "improbable.Position": {
                    "checkout_initially": true,
                    "message_delivery": "UNRELIABLE_ORDERED"
                },
                "mygame.coordinates.Rotation": {
                    "message_delivery": "UNRELIABLE_ORDERED"
                }
            }
        }

This example mandates that all components are checked out initially (using checkoutAllInitially). Setting checkoutAllInitially to true takes precedence over the individual components’ checkout_initially settings, so we don’t specify them:

"component_delivery": {
            "default": "RELIABLE_ORDERED",
            "checkoutAllInitially": true,
            "override": {
                "improbable.Position": {
                    "message_delivery": "UNRELIABLE_ORDERED"
                },
                "mygame.coordinates.Rotation": {
                    "message_delivery": "UNRELIABLE_ORDERED"
                }
            }
        }

Note that the initial checkouts of mygame.coordinates.Position and mygame.coordinates.Rotation components are not affected by the UNRELIABLE_ORDERED message delivery setting. Only subsequent updates will be unreliable.

Component settings

The component settings field allows you to configure various component settings. For now this allows you to set the authority handover timeout period. For more information, see Handing over authority between workers.

The component settings field allows you to configure settings for each individual component or configure a default which is applied to all other components. Below is an example of a typical configuration.

"componentSettings": {
            "perComponentSettings": {
                "improbable.MyFancyComponent": {
                    "authorityHandoverTimeoutMs": 300
                }
            },
            "default": {
                "authorityHandoverTimeoutMs": 400
            }
        }

In the example above, improbable.MyFancyComponent will have an authorityHandoverTimeoutMs of 300 and all other components will default to 400.

If no default is set, components will default to having an authorityHandoverTimeoutMs of 0.

Information about what the authorityHandoverTimeoutMs does can be found in Handing over authority between workers.

Delta-compressed component updates

This lets you configure which components you want to delta-compress when they are updated. If you’re sending large component updates, you might want to enable delta-compressed component updates for a component. This means that, instead of sending the entire new state, the component update will be sent as a description of how the state has changed.

Imagine you have submitted your book to an editor, but you want to add an additional page. A delta-compressed update would be sending that page to the editor with a note saying “insert this page after page X”, instead of sending the entire book including that new page.

This is a tradeoff: it saves bandwidth at the cost of CPU overhead and latency.

By default, no worker types have delta compression applied to component updates.

How delta compression works

Delta compression works using diff caches on both the SpatialOS Runtime and worker instances that store the last component state sent and received for a given component. Updates are sent as a binary diff across the network which, for some property types, can have large bandwidth savings.

This works best for small changes to larger property types, e.g. when appending an element to a long list, as opposed to sending the whole list again.

This feature is still in the very early stages. If you want to try it out, we recommend reading the documentation fully and in detail. In particular, if you aren’t careful about which components you delta-compress, it can crash your deployment, because - used badly - sending delta-compressed updates at too high a frequency can overload the SpatialOS Runtime. Because of this, please don’t use this feature in cloud deployments. If you use it in a cloud deployment, we reserve the right to shut your deployment down.

Usage

You enable delta compression on a per-worker type, per-component basis. For example, to turn on delta-compressed component updates for improbable.ants.ToNest and improbable.ants.ToFood you would need to modify your worker.json like so:

"component_settings": {
            "per_component_settings": {
                "improbable.ants.ToNest": {
                    "encoding": {
                        "type": "BINARY_DELTA_COMPRESSION"
                    },
                },
                "improbable.ants.ToFood": {
                    "encoding": {
                        "type": "BINARY_DELTA_COMPRESSION"
                    },
                }
            }
        }

Best practices

You should only delta-compress component properties that are large with small, infrequent changes, e.g. appending an item to a long list as opposed to changing all items at once. For example, any of the following:

list
map
string
large object types contained within components

We recommend you don’t enable delta compression on:

any of the above types that may change dramatically, for example a List that would have half of the contents replaced.
smaller types, such as single int fields, due to the overhead of calculating the diff.

You can enable delta compression for other component types, but diffs may be abandoned because:

the computed diff actually exceeds the size of the update.
the process hits the approximate cutoff time (default 5ms - we’re looking into making this configurable).
the process detects before diffing starts that one of the above cases is highly likely to occur.

Diffs that aren’t abandoned can still use CPU on the SpatialOS Runtime or worker instance side to compute.

We would strongly recommend you closely monitor behaviour and metrics (e.g. SpatialOS Runtime CPU usage and latency) after enabling this functionality, as it can cause worker instances to stop responding to pings.

Metrics

You can measure delta compression through worker instance metrics via the Inspector. Use this to confirm delta compression is working as intended, and get insight into the bandwidth you’re saving and your CPU/lag overhead due to diffs abandoned.

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