Using the C++ worker SDK

Overview

The most important units of functionality in the C++ worker API are:

• The worker::Locator, which is responsible for querying for and connecting to remote cloud deployments.
• The worker::Connection, which represents a worker’s connection to a SpatialOS simulated world.
• The worker::Dispatcher, which is responsible for invoking user-provided callbacks based on data received from SpatialOS via the worker::Connection.
• The worker::Entity, which is a container for the component data associated with a single entity.
• The worker::View, which is an optional drop-in replacement for the worker::Dispatcher that additionally maintains a map from entity ID to worker::Entity.

For the full API reference, have a look at <improbable/worker.h>.

Providing your components

Since C++ has no reflection capability, the SDK makes no attempt to discover your components automatically. Instead, the list of components you want to work with must be explicitly provided to SDK methods that need to know about them (including standard library components).

This is done by passing an instance of the worker::Components variadic template class:

// MyComponents can be passed to SDK functions.
        const worker::Components<
            my_schema::FooComponent,
            my_schema::BarComponent,
            // Must include standard library components you care about, too.
            improbable::EntityAcl,
            improbable::Position,
            ...> MyComponents;
        

Since worker::Components is a template, you need to include the generated code for every component you want to work with in order to define MyComponents. However, since this class implements the worker::ComponentRegistry interface, this only needs to be done in one source file:

// This function can be called from other translation units without including generated code.
        const worker::ComponentRegistry& MyComponents() {
          static const worker::Components<my_schema::FooComponent, my_schema::BarComponent> components;
          return components;
        }
        

The rest of this page assumes you have defined a MyComponents function like this.

Connecting to SpatialOS

Before it can interact with the simulated world, the worker must connect to SpatialOS.

There are two ways of doing this:

• Instantiate a worker::Connection object directly. This allows to connect a managed worker (i.e running in the cloud and started by the SpatialOS runtime) or a remote worker used for debugging via (spatial cloud connect external). The connection is made through the receptionist service of the targeted deployment for which the IP and port should be passed as arguments to the worker. These values can be filled-in automatically at runtime by using the IMPROBABLE_RECEPTIONIST_HOST and IMPROBABLE_RECEPTIONIST_PORT placeholders in your worker’s launch configuration.
• Use the worker::Locator object to enumerate cloud deployments and connect to a chosen deployment with authentication. This is typically used to connect an external client to a cloud deployment.

The value given for the UseExternalIp field in the NetworkParameters structure is also relevant. The table below summarizes the connection possibilities:

	Using worker::Connection directly	Using worker::Locator
UseExternalIp == true	Local client connecting via spatial cloud connect external proxy	External client connecting to cloud deployment
UseExternalIp == false	Managed cloud worker; local client connecting to local deployment	N/A

#include <improbable/standard_library.h>
        #include <improbable/worker.h>
        #include <example.h>
        
        using namespace improbable;

The example below illustrates a very basic connection setup, where the function takes one argument specifying the worker’s own ID, and an optional second one representing the receptionist IP address. If the latter is not provided, the worker connects to localhost. In either case, it will use TCP and connect using internal IP address.

worker::Connection GetConnection(const std::string& worker_id, const std::string& hostname) {
          worker::ConnectionParameters parameters;
          parameters.WorkerType = "CppDocsWorker";
          parameters.Network.ConnectionType = worker::NetworkConnectionType::kTcp;
          parameters.Network.UseExternalIp = false;
        
          return worker::Connection::ConnectAsync(MyComponents(), hostname, 7777, worker_id, parameters)
              .Get();
        }

Note that worker::Connection objects are not thread-safe.

Handling data received from SpatialOS

It is up to each worker to decide how the basic event loop should be structured, depending on its requirements. The worker should call worker::Connection::GetOpList to get a list of “operations” (for example a new entity, or a component update) from SpatialOS that have been sent since the last time the function was called. This worker::OpList then needs to be passed to a dispatcher in order to invoke user-provided callbacks.

The following snippet shows a (very) simple example implementation of an event loop that processes operations from SpatialOS 60 times per second:

void RunEventLoop(worker::Connection& connection, const worker::Dispatcher& dispatcher) {
          constexpr unsigned kFramesPerSecond = 60;
          constexpr std::chrono::duration<double> kFramePeriodSeconds(
              1. / static_cast<double>(kFramesPerSecond));
        
          while (true) {
            auto start_time = std::chrono::steady_clock::now();
            auto op_list = connection.GetOpList(0 /* non-blocking */);
            // Invoke user-provided callbacks.
            dispatcher.Process(op_list);
        
            // Do other work here...
        
            auto stop_time = std::chrono::steady_clock::now();
            auto wait_for = kFramePeriodSeconds - (stop_time - start_time);
            std::this_thread::sleep_for(wait_for);
          }
        }

Note that all callbacks provided to the worker::Dispatcher will be invoked only when worker::Dispatcher::Process is called, and only on the thread that is currently calling worker::Dispatcher::Process: the user has complete control over callback timing and threading decisions.

If the connection fails (when Connection.IsConnected() == false), any error messages explaining the cause will be returned by GetOpList(). Make sure to process this list in order to receive all available diagnostic information.

Dispatcher callbacks

Several kinds of callbacks can be registered on the worker::Dispatcher. Each method takes an arbitrary callable std::function<void(const Param&)>, where the parameter type Param depends on the particular kind of callback being registered. The following table has the details:

Method	Parameter type (and fields)	Invoked when…
OnDisconnect	worker::DisconnectOp (std::string Reason)	the Connection is no longer connected and can no longer be used. Check the log for errors relating to the disconnection.
OnFlagUpdate	worker::FlagUpdateOp (std::string Name, Option<std::string> Value)	a worker flag has been created, deleted or when its value has changed.
OnLogMessage	worker::LogMessageOp (worker::LogLevel Level, std::string Message)	the SDK issues a log message for the worker to print. This does not include messages sent using Connection.SendLogMessage.
OnMetrics	worker::MetricsOp (worker::Metrics Metrics)	the SDK reports built-in internal metrics.
OnCriticalSection	worker::CriticalSectionOp (bool InCriticalSection)	a critical section is about to be entered or has just been left.
OnAddEntity	worker::AddEntityOp (worker::EntityId EntityId)	an entity is added to the worker’s view of the simulation.
OnRemoveEntity	worker::RemoveEntityOp (worker::EntityId EntityId)	an entity is removed from the worker’s view of the simulation.
OnReserveEntityIdResponse	worker::ReserveEntityIdResponseOp (worker::RequestId<worker::ReserveEntityIdRequest> RequestId, worker::StatusCode StatusCode, std::string Message, Option<worker::EntityId> EntityId)	the worker has received a response for an entity ID reservation it had requested previously.
OnReserveEntityIdsResponse	worker::ReserveEntityIdsResponseOp (worker::RequestId<worker::ReserveEntityIdsRequest> RequestId, worker::StatusCode StatusCode, std::string Message, Option<worker::EntityId> EntityId, std::size_t NumberOfEntityIds)	the worker has received a response for a reservation of an entity ID range it had requested previously.
OnCreateEntityResponse	worker::CreateEntityResponseOp (worker::RequestId<worker::CreateEntityRequest> RequestId, worker::StatusCode StatusCode, std::string Message, Option<worker::EntityId> EntityId)	the worker has received a response for an entity creation it had requested previously.
OnDeleteEntityResponse	worker::DeleteEntityResponseOp (worker::RequestId<DeleteEntityRequest> RequestId, worker::EntityId EntityId, worker::StatusCode StatusCode, std::string Message)	the worker has received a response for an entity deletion it had requested previously.
OnEntityQueryResponse	worker::EntityQueryResponseOp (worker::RequestId<EntityQueryRequest> RequestId, worker::StatusCode StatusCode, std::string Message, std::size_t ResultCount, worker::Map<worker::EntityId, worker::Entity> Result>)	the worker has received a response for an entity query it had requested previously.
OnAddComponent<T>	worker::AddComponentOp<T> (worker::EntityId EntityId, T::Data Data)	a component is added to an existing entity in the worker’s view of the simulation.
OnRemoveComponent<T>	worker::RemoveComponentOp (worker::EntityId EntityId)	a component is removed from an existing entity in the worker’s view of the simulation.
OnAuthorityChange<T>	worker::AuthorityChangeOp (worker::EntityId EntityId, worker::Authority Authority)	the worker’s authority state over a component is changed to Authoritative, Not Authoritative or Authority Loss Imminent.
OnComponentUpdate<T>	worker::ComponentUpdateOp<T> (worker::EntityId EntityId, T::Update Update)	a component for an entity in the worker’s view of the simulation has been updated.
OnCommandRequest<T>	worker::CommandRequestOp<T> (worker::RequestId<IncomingCommandRequest<T>> RequestId, worker::EntityId EntityId, std::uint32_t TimeoutMillis, std::string CallerWorkerId, List<std::string> CallerAttributeSet, T::Request Request)	the worker has received a command request for a component on an entity over which it has authority.
OnCommandResponse<T>	worker::CommandResponseOp<T> (worker::RequestId<OutgoingCommandRequest<T>> RequestId, worker::EntityId EntityId, worker::StatusCode StatusCode, Option<T::Response> Response)	the worker has received a command response for a request it issued previously.

(Note that some callbacks use a template parameter T which relates to schema-generated code. This is discussed in more detail below.)

Here’s an example of registering callbacks:

void RegisterLambdaCallbacks(worker::Dispatcher& dispatcher) {
          dispatcher.OnAddEntity([&](const worker::AddEntityOp& op) {
            std::cout << "Entity " << op.EntityId << " added." << std::endl;
          });
          dispatcher.OnComponentUpdate<example::Switch>(
              [&](const worker::ComponentUpdateOp<example::Switch>& op) {
                // `op.Update.toggled()` contains a list of SwitchToggled events.
                for (auto it : op.Update.toggled()) {
                  std::cout << "Switch toggled at " << it.time() << std::endl;
                }
              });
        }

Of course, you don’t have to use lambdas:

void RegisterCallbacks(worker::Dispatcher& dispatcher) {
          dispatcher.OnComponentUpdate<improbable::Position>(&PositionUpdated);
        }
        
        void PositionUpdated(const worker::ComponentUpdateOp<improbable::Position>& op) {
          const auto coords = op.Update.coords().data();
          std::cout << "Updated position of entity " << op.EntityId << " to "
                    << " x: " << coords->x() << " y: " << coords->y() << " z: " << coords->z() << std::endl;
        }

If you want to unregister a callback, call Remove() with the Dispatcher::CallbackKey returned from the registration method:

void UnregisterCallbacks(worker::Dispatcher& dispatcher) {
          auto key = dispatcher.OnComponentUpdate<improbable::Position>(
              [&](const worker::ComponentUpdateOp<improbable::Position>& op) {
                std::cout << "Updated position of entity " << op.EntityId << std::endl;
              });
        
          // ...
        
          dispatcher.Remove(key);
        }

Using dynamic component dispatch

The SDK provides two functions for type-safe manipulation of components when the component type is not known statically (however, the complete set of possible components must be statically provided using the worker::Components template class.

These functions are defined as follows:

/**
         * Invokes Accept<T>() on the provided handler for the component in the given
         * list of components whose ID matches the given component ID.
         */
        template <typename... T, typename Handler>
        void ForComponent(const Components<T...>& components, ComponentId component_id, Handler&& handler);
        
        /**
         * Invokes Accept<T>() on the provided handler for each component in the given list of
         * components.
         */
        template <typename... T, typename Handler>
        void ForEachComponent(const Components<T...>& components, Handler&& handler);
        

For example, you could use ForComponent to copy the components of one entity to another without knowledge of the types of the components. To do this, you would first define a class which implements a handler to provide the desired copying functionality:

struct CopyFunc
        {
          const Entity& from;
          Entity& to;
        
          template <typename T>
          void Accept<T>() {
            to.Add<T>(*from.Get<T>());
          }
        }
        

You would then an instance of the CopyFunc class and call the static methods worker::ForComponent or worker::ForEachComponent to apply the functionality defined in Accept to the entity’s components:

// Probably defined elsewhere.
        using MyComponents = worker::Components<my_schema::Foo, my_schema::Bar>;
        
        // Copy all components from source_entity to target_entity.
        CopyFunc copy_func{source_entity, target_entity};
        for (auto component_id : source_entity.GetComponentIds()) {
          worker::ForComponent(MyComponents{}, component_id, copy_func);
        }
        

This functionality can also be used to implement a View with custom semantics, if desired.

Using the View

worker::View is subclass of worker::Dispatcher defined in a separate header file, <improbable/view.h>, which adds the functionality of automatically maintaining the current state of the worker’s view of the simulated world. It has a field called Entities, which is a map from entity ID to worker::Entity objects. When a View processes an OpList, it automatically updates the state of this map as appropriate. Any user-defined callbacks are also invoked, as per the Dispatcher contract.

Note that the View can be implemented from scratch using the worker::ForEachComponent functionality, and is intended mostly as an example.

Dispatcher invocation order

When processing a worker::OpList, callbacks for most operations are invoked in the order they were registered with the worker::Dispatcher. However, some operations naturally come in pairs of “start” and “end” operations, and the callbacks for the “end” operation are invoked in the reverse of the order they were registered. This means that pairs of callbacks registered for the “start” and “end” operations can correctly depend on resources managed by pairs of callbacks registered previously, similar to the usual C++ construction and destruction order.

For example, AddEntityOp and AddComponentOp callbacks are invoked in the order they were registered with the worker::Dispatcher, while RemoveEntityOp and RemoveComponentOp callbacks are invoked in reverse order. Similarly, AuthorityChangeOp callbacks are invoked in the order they were registered when authority is granted, but in reverse order when authority is revoked (or authority loss is imminent). CriticalSectionOps are invoked in usual order when a critical section is entered but reverse order when a critical section is left. The FlagUpdateOp callbacks are invoked in the order they were registered when a worker flag is created or its value is changed, but in reverse order when a worker flag is deleted.

In particular, this invocation order has the side-effect that the final state of a component removed in a RemoveComponentOp callback can still be inspected in the worker::View.

Critical sections

The protocol supports the concept of critical sections. A critical section is a block of operations that should be processed atomically by the worker. For example, a set of entities representing a stack of cubes should be added to the scene in its entirety before any physics simulation is done.

The CriticalSectionOp callback is invoked with InCriticalSection == true just before the first operation of the critical section is delivered, and is invoked again with InCriticalSection == false just after the last operation of the critical section is delivered.

Worker configuration

A worker is assigned some static configuration settings when it connects to the SpatialOS runtime:

• a worker ID which is a unique identifier string for this particular worker.
• a list of worker attributes that are used when to determine to which worker the authority over a component can be delegated, either automatically or via by the developer through ACLs.

The values of these configuration settings can be retrieved via the GetWorkerId and GetWorkerAttributes methods of a Connection:

std::string GetWorkerId() const;
        worker::List<std::string> GetWorkerAttributes() const;
        

Worker flags

To access the value of a worker flag, use the GetWorkerFlag method of a Connection:

Option<std::string> GetWorkerFlag(const std::string& flag_name) const;
        

For example:

void SetWorkSpeedFlag(worker::Connection& connection) {
          auto work_speed_flag = connection.GetWorkerFlag("mycompany_theproject_work_speed");
          if (work_speed_flag) {
            SetWorkSpeed(*work_speed_flag);
          } else {
            SetWorkSpeed("default_work_speed");
          }
        }

Worker flags are updated before the registered callbacks are invoked, so it is recommended to store the worker flag values in user code when using callbacks and only use GetWorkerFlag for checking the initial flag values.

Sending data to SpatialOS

A worker can send data such as logging, metrics, and component updates to SpatialOS using the worker::Connection.

Sending logs

You can send log messages by calling Connection::SendLogMessage. Log messages can be viewed on deployment dashboards.

Sending and receiving metrics

You can optionally send metrics by calling Connection::SendMetrics. Metrics can be viewed on deployment dashboards and in the Inspector.

There are two typical use cases for sending metrics:

1. Reporting your own custom metrics. Both time series and histogram metrics are supported.
2. Updating a worker’s load when the worker is using dynamic load balancing. The load of a worker is a floating-point value, with 0 indicating an unloaded worker and values above 1 corresponding to an overloaded worker. The reported values direct SpatialOS’s load balancing strategy.

The following example demonstrates both use cases:

Workers automatically send several built-in, internal metrics at a period defined by the BuiltInMetricsReportPeriodMillis field of worker::ConnectionParameters. You can register a callback to receive these metrics inside a worker::MetricsOp using Dispatcher::OnMetrics.

Sending and receiving component updates

When the worker has authority over a particular component for some entity, it can send component updates to SpatialOS. This is done using the worker::Connection method SendComponentUpdate<T> which takes a reference to a T::Update object. The template parameter T should be component metaclass in the schema-generated code. These are discussed further below.

A component update can modify the value of a property or trigger an event. You can modify multiple properties and trigger multiple events in the same component update.

Component updates sent by the worker will appear in the operation list returned by a subsequent call to worker::Connection::GetOpList. This means that ComponentUpdateOp callbacks will also be invoked for component updates triggered by the worker itself, but not immediately.

There are several (related) reasons that callbacks for sent component updates should not be invoked immediately: this can lead to extremely unintuitive control flow when components are recursively updated inside a callback; and it violates the guarantee that callbacks are only invoked as a result of a call to Process method on the worker::Dispatcher.

To be notified when a worker receives a component update on an entity in the worker’s local view of the simulation, use the worker::Dispatcher method OnComponentUpdate with the same template paramater T as for sending updates. Note that the component updates can be partial, i.e. only update some properties of the component, do not necessarily have to contain data that is different from the workers current view of the component, and could have been sent by SpatialOS rather than a worker for synchronization purposes.

Sending and receiving component events

Sending and receiving events works much in the same way as component updates. For a schema like the following,

package example;
        
        type SwitchToggled {
          int64 time = 1;
        }
        
        component Switch {
          id = 1234;
          bool is_enabled = 1;
          event SwitchToggled toggled;
        }

triggering an event can be done as follows:

void TriggerEvent(worker::Connection& connection, worker::EntityId entity_id) {
          example::Switch::Update update;
          example::SwitchToggled event{1};
          update.add_toggled(event);
          connection.SendComponentUpdate<example::Switch>(entity_id, update);
        }

If you are not authoritative on the component, your event will be silently ignored.

Receiving an event works just like receiving a component update, by registering a callback on the dispatcher:

dispatcher.OnComponentUpdate<example::Switch>(
            [&](const worker::ComponentUpdateOp<example::Switch>& op) {
              // `op.Update.toggled()` contains a list of SwitchToggled events.
              for (auto it : op.Update.toggled()) {
                std::cout << "Switch toggled at " << it.time() << std::endl;
              }
            });

Sending component interests

For each entity in its view, a worker receives updates for a set of the entity’s components. This set is the union of the set of components the worker has authority over, and a set of components it is explicitly interested in.

The initial set of explicitly interested components can be configured in the bridge settings. This is the default set of explicit interest for every entity. At runtime, the worker can override this set on a per-entity, per-component basis, by using the Connection::SendComponentInterest method.

Whenever the set of interested components for an entity changes, the worker will receive the appropriate OnAddComponent and OnRemoveComponent callbacks to reflect this change.

For example, you might be interested in one specific switch, but not every switch in the world:

void ComponentInterest(worker::Connection& connection, worker::EntityId entity_id) {
          connection.SendComponentInterest(
              entity_id,
              {{example::Switch::ComponentId, worker::InterestOverride{/* IsInterested */ true}}});
        }

Note that a worker is always considered to be interested in the components of the entities it is authoritative on, in addition to any others specificed in the bridge settings or using the method described above.

Sending and receiving component commands

To send a command to be executed by the worker currently authoritative over a particular component for a particular entity, use the worker::Connection method SendCommandRequest<T>. In this case the template parameter T should be a command metaclass defined in the schema-generated code. This function takes an entity ID, an optional timeout, a T::Request object and an optional CommandParameters object. The CommandParameters object contains a field called AllowShortCircuiting, which, if set to true, will try to “short-circuit” the command and avoid a round trip to SpatialOS in some cases. See documentation on commands for more information.

Before sending the command, a callback to handle the response should be registered with the worker::Dispatcher with OnCommandResponse<T>. The worker::RequestId<worker::OutgoingCommandRequest<T>> returned by SendCommandRequest can be matched up with the one in the worker::CommandResponseOp to identify the request that is being responded to.

Note that commands may fail, so the worker::StatusCode field in the worker::CommandResponseOp should be checked, and the command can be retried as necessary. The caller will always get a response callback, but it can be one of several failure cases, including:

• ApplicationError (rejected by the target worker or by SpatialOS)
• AuthorityLost (target worker lost authority, or no worker had authority)
• NotFound (target entity, or target component on the entity, didn’t exist)
• PermissionDenied (sending worker didn’t have permission to send request)
• Timeout
• InternalError (most likely indicates a bug in SpatialOS, should be reported)

To handle commands issued by another worker, the opposite flow is used. Register a callback with the worker::Dispatcher with OnCommandRequest<T>; when the callback is executed, the worker should make sure to call the worker::Connection method SendCommandResponse<T>, supplying the worker::RequestId<worker::IncomingCommandRequest<T>> provided by the worker::CommandRequestOp and an appropriate T::Response response object. Alternatively, the worker can call SendCommandFailure<T> to fail the command instead.

Note that a RequestId only uniquely identifies a single command invocation from the perspective of a single worker; that is, the RequestId used by the calling worker and the RequestId used by the caller worker for handling that same command are unrelated.

Entity queries

A worker can run remote entity queries against the simulation by using the worker::Connection method SendEntityQueryRequest. This takes a worker::query::EntityQuery object and an optional timeout.

The query object is made of an worker::query::Constraint and a worker::query::ResultType. The constraint determines which entities are matched by the query, and the result type determines what data is returned for matched entities. Available constraints and result types are described below.

Constraint	Description
EntityIdConstraint	Matches a specific entity ID.
ComponentConstraint	Matches entities with a particular component.
SphereConstraint	Matches entities contained in the given sphere.
AndConstraint	Matches entities that match all of the given subconstraints.
OrConstraint	Matches entities that match any of the given subconstraints.
NotConstraint	Matches entities that do not match the given subconstraint.

Result type	Description
CountResultType	Returns the number of entities that matched the query.
SnapshotResultType	Returns a snapshot of component data for entities that matched the query. To select all components, use worker::query::SnapshotResultType snapshotResultType{{}};. To select every component whose ID is contained in the given set, use worker::query::SnapshotResultType snapshotResultType{{componentIdSet}}; (thus, pass an empty set to get no components but entity IDs only).

Like other request methods, this returns a worker::RequestId, which can be used to match a request with its response. The response is received via a callback registered with the worker::Dispatcher using the OnEntityQueryResponse method.

The EntityQueryResponseOp contains a std::size_t ResultCount field (for CountResultType requests) and a Map<EntityId, Entity> (for SnapshotResultType) requests. Again, success or failure of the request is indicated by the StatusCode field of the response object, but in the failure case the result may still contain some data: the count or snapshot map might still contain the data for some entities that matched the query, but won’t necessarily contain all matching entities. This is because the worker might still be able to do something useful with a partial result.

Creating and deleting entities

A worker can request SpatialOS to reserve one entity ID, reserve a range of entity IDs, create an entity, or delete an entity, by using the worker::Connection methods SendReserveEntityIdRequest, SendReserveEntityIdsRequest, SendCreateEntityRequest, and SendDeleteEntityRequest, respectively.

These methods return a worker::RequestId, which can be used to match a request with its response. The response is received via a callback registered with the worker::Dispatcher using OnReserveEntityIdResponse, OnReserveEntityIdsResponse, OnCreateEntityResponse, and OnDeleteEntityResponse, respectively.

Note that these operations may fail, so the StatusCode field in the ReserveEntityIdResponseOp, ReserveEntityIdsResponseOp, CreateEntityResponseOp, or DeleteEntityResponseOp argument of the respective callback should be checked, and the command can be retried as necessary. The caller will always get a response callback, but it might be a timeout.

SendReserveEntityIdRequest takes an optional timeout. If the operation succeeds, the response contains an entity ID, which is guaranteed to be unused in the current deployment.

SendReserveEntityIdsRequest takes an optional timeout. If the operation succeeds, the response contains an entity ID, which is the first in a contiguous range guaranteed to be unused in the current deployment, and the corresponding number of entity IDs in the reserved range. If the operation fails, the entity ID is not set and the number of entity IDs in the reserved range is 0.

SendCreateEntityRequest takes a worker::Entity representing the initial state of the entity, an optional entity ID (which, if provided, must have been obtained by a previous call to SendReserveEntityIdRequest or SendReserveEntityIdsRequest), and an optional timeout. If the operation succeeds, the response contains the ID of the newly created entity.

SendDeleteEntityRequest takes an entity ID and an optional timeout. The response contains no additional data.

Example

Here’s an example of reserving an entity ID, creating an entity with that ID and some initial state, and finally deleting it:

void CreateDeleteEntity(worker::Connection& connection, worker::Dispatcher& dispatcher) {
          constexpr uint32_t kTimeoutMillis = 500;
        
          // Reserve an entity ID.
          worker::RequestId<worker::ReserveEntityIdRequest> entity_id_reservation_request_id =
              connection.SendReserveEntityIdRequest(kTimeoutMillis);
        
          // When the reservation succeeds, create an entity with the reserved ID.
          worker::RequestId<worker::CreateEntityRequest> entity_creation_request_id;
          dispatcher.OnReserveEntityIdResponse([&](const worker::ReserveEntityIdResponseOp& op) {
            if (op.RequestId == entity_id_reservation_request_id &&
                op.StatusCode == worker::StatusCode::kSuccess) {
              // ID reservation was successful - create an entity with the reserved ID.
              worker::Entity entity;
              entity.Add<improbable::Position>({{1, 2, 3}});
              entity_creation_request_id =
                  connection.SendCreateEntityRequest(entity, op.EntityId, kTimeoutMillis);
            }
          });
        
          // When the creation succeeds, delete the entity.
          worker::RequestId<worker::DeleteEntityRequest> entity_deletion_request_id;
          dispatcher.OnCreateEntityResponse([&](const worker::CreateEntityResponseOp& op) {
            if (op.RequestId == entity_creation_request_id &&
                op.StatusCode == worker::StatusCode::kSuccess) {
              entity_deletion_request_id = connection.SendDeleteEntityRequest(*op.EntityId, kTimeoutMillis);
            }
          });
        
          // When the deletion succeeds, we're done.
          dispatcher.OnDeleteEntityResponse([&](const worker::DeleteEntityResponseOp& op) {
            if (op.RequestId == entity_deletion_request_id &&
                op.StatusCode == worker::StatusCode::kSuccess) {
              // Test successful!
            }
          });
        }

Entity ACLs

Entity ACLs are exposed to the worker as a component, and can be manipulated as any other component. ACLs can be set at entity creation time, or modified dynamically at runtime by the worker that has authority over the EntityAcl component.

This example adds an EntityAcl to an Entity given a CommandRequestOp, which is currently the only way to make a specific worker (as opposed to, potentially, a set of workers) to qualify for authority of a component. Specifically:

• The entity will be visible to workers that have the “client” or “physics” worker attribute.
• Any “physics” worker (i.e. a worker with “physics” as one of its attributes) can be authoritative over the entity’s Position and EntityAcl components.
• The worker, which issued the CommandRequestOp, is the only worker that can be authoritative over the PlayerControls component.

This can be used as part of creating a new entity in response to a command request.

template <typename C>
        void AddComponentDelegations(const worker::CommandRequestOp<C>& op, worker::Entity& entity) {
          static const std::string physics = "physics";
          static const std::string client = "client";
        
          // This requirement set matches only the command caller, i.e. the worker that issued the command,
          // since attribute set includes the caller's unique attribute.
          auto callerWorkerAttributeSet = improbable::WorkerAttributeSet{op.CallerAttributeSet};
          auto callerWorkerRequirementSet = improbable::WorkerRequirementSet{
              worker::List<improbable::WorkerAttributeSet>{callerWorkerAttributeSet}};
        
          auto physicsWorkerAttributeSet = worker::List<std::string>{physics};
        
          // This requirement set matches any worker with the attribute "physics".
          auto physicsWorkerRequirementSet = improbable::WorkerRequirementSet{
              worker::List<improbable::WorkerAttributeSet>{{physicsWorkerAttributeSet}}};
        
          // This requirement set matches any worker with the attribute "client" or "physics".
          auto clientOrPhysicsRequirementSet =
              improbable::WorkerRequirementSet{worker::List<improbable::WorkerAttributeSet>{
                  improbable::WorkerAttributeSet{worker::List<std::string>{client}},
                  physicsWorkerAttributeSet}};
        
          // Give authority over Position and EntityAcl to any physics worker, and over PlayerControls to
          // the caller worker.
          worker::Map<worker::ComponentId, improbable::WorkerRequirementSet> componentAcl{
              {improbable::Position::ComponentId, physicsWorkerRequirementSet},
              {EntityAcl::ComponentId, physicsWorkerRequirementSet},
              {example::PlayerControls::ComponentId, callerWorkerRequirementSet}};
        
          entity.Add<EntityAcl>(
              EntityAcl::Data{/* read */ clientOrPhysicsRequirementSet, /* write */ componentAcl});
        }

The worker authoritative over the EntityAcl component can later decide to give the authority over position (or any other component) to a different worker, e.g. the client. In order to do this, EntityAclData needs to be modified in order to map Position::ComponentId to callerWorkerRequirementSet (created above). This change can be made using the method below:

// Take a copy of the EntityAclData argument, so that this method doesn't modify the original one.
        improbable::EntityAclData
        DelegateComponent(improbable::EntityAclData acl, worker::ComponentId componentId,
                          const improbable::WorkerRequirementSet& requirementSet) {
          // Set the write ACL for the specified component to the specified attribute set,
          // assuming the componentAcl option is not empty.
          acl.component_write_acl().insert({{componentId, requirementSet}});
          return acl;
        }

For these changes to take effect, the worker authoritative over the EntityAcl component needs send the changes through a component update.

Please read Worker attributes and worker requirements for more information.

Generated code

The C++ schema-generated code consists of two main parts: data classes and component classes. Data classes are used to represent data at rest and correspond to the schemalang type definitions; component classes correspond to schemalang component definitions and contain metadata and classes for sending and receiving component data (updates, command requests and command responses).

A data class will be generated for each type defined in the schema, with fields corresponding to each field in the schema definition.

For each component, we generate:

• a metaclass. These metaclasses are used when referring to specific components when using the API - every template parameter T expects a metaclass argument.
• an Update class nested inside the metaclass. This has an optional field for each field in the component (since it represents a diff).
• command metaclasses nested inside the metaclass, as SomeComponent::Commands::SomeCommand. These work in a similar way to component metaclasses, but for command-related functions.

Data representation

• Strings are represented as UTF-8 encoded std::string members. Make sure to use this same encoding when updating a component.
• list<T> fields are represented as a worker::List<T> of the repeated type. This type is very similar to std::vector<T> and has most of the methods you’d expect.
• map<Key, Value> fields are represented as worker::Map<Key, Value>. This type is very similar to std::unordered_map<Key, Value> and has most of the methods you’d expect. The biggest difference is it has a well-defined iteration order, which is insertion order.
• option<T> fields are represented as worker::Option<T>. First, check if the option contains a value with !option.empty() or if (option) { ... }, then dereference the option to access the contained value. This type is very similar to the std::optional<T> type introduced in C++17.

Example

Consider the following simple schema (in package example):

type StatusEffect {
          string name = 1;
          int32 multiplier = 2;
        }
        
        component Creature {
          id = 12345;
          int32 health = 1;
          list<StatusEffect> effects = 2;
        }

The generated code will contain a data class StatusEffect corresponding to type defined in the schema. For the creature component, a Creature metaclass containing a Creature::Update class will be generated, alongside a CreatureData data class.

The Creature metaclass can be passed as a template parameter to any templated API function, for example to send an update for the creature component. Note that the Creature metaclass is also used with the worker::Entity class (which the View uses to represent entities) to inspect component data “at rest”.

Here are some ways that these classes can be used with the API:

void GeneratedCodeExamples(worker::Connection& connection) {
          worker::Dispatcher dispatcher(MyComponents());
        
          dispatcher.OnAuthorityChange<example::Creature>([&](const worker::AuthorityChangeOp& op) {
            if (op.Authority == worker::Authority::kAuthoritative) {
              // We were granted authority over the creature component. Send an update.
              example::Creature::Update update;
              update.set_health(10);
              connection.SendComponentUpdate<example::Creature>(op.EntityId, update);
            } else {
              // Authority was revoked.
            }
          });
        
          dispatcher.OnComponentUpdate<example::Creature>(
              [&](const worker::ComponentUpdateOp<example::Creature>& op) {
                const example::Creature::Update& update = op.Update;
                if (update.effects()) {
                  auto effects = update.effects().data();
                  for (const auto& effect : *effects) {
                    std::cout << op.EntityId << " has effect " << effect.name() << " with multiplier "
                              << effect.multiplier() << std::endl;
                  }
                }
              });
        }

Snapshots

The SDK provides two classes to manipulate snapshots stored in files, worker::SnapshotInputStream to read snapshot files from disk, one entity at a time, and worker::SnapshotOutputStream to write snapshot files to disk, one entity at a time. These stream classes are the recommended methods of manipulating snapshots as they do not require the entire snapshot to be stored in memory when reading or writing a snapshot.

worker::SnapshotInputStream has a constructor and two public methods:

/** Creates a SnapshotInputStream to read the snapshot file from the given path. */
        explicit SnapshotInputStream(const ComponentRegistry& registry, const std::string& path);
        
        /** Returns true if SnapshotInputStream has not reached EOF. */
        bool HasNext();
        
        /**
         * Loads the next EntityId and Entity pair from the Snapshot. Returns an
         * error message if error occurred.
         */
        Option<std::string> ReadEntity(EntityId& entity_id, Entity& entity);
        

worker::SnapshotOutputStream has a constructor and one public method:

/** Creates a SnapshotOutputStream to read the snapshot file from the given path. */
        explicit SnapshotOutputStream(const ComponentRegistry& registry, const std::string& path);
        
        /**
         * Writes the EntityId and Entity pair to the output stream. Returns an
         * error message if error occurred.
         */
        Option<std::string> WriteEntity(EntityId entity_id, const Entity& entity);
        

Note that when a SnapshotOutputStream is destructed, the end of file is written and the stream’s resources are released.

Note that, unlike the rest of the API described in this document, snapshot manipulation does not require a Connection, making it possible to develop standalone, offline snapshot manipulation tools. However, we recommend using the build infrastructure provided by SpatialOS for workers to build your standalone tools.

Here is an example of loading a snapshot, performing some manipulation on it, and saving it back. It uses the types and components defined in the example above.

void AddLowHealthEffectToEntities(const std::string& snapshot_filename) {
          // Load a snapshot from a file.
          std::unordered_map<worker::EntityId, worker::Entity> entities;
          auto load_error_opt = worker::LoadSnapshot(MyComponents(), snapshot_filename, entities);
          if (load_error_opt) {
            std::cerr << "Error loading snapshot: " << *load_error_opt << std::endl;
            return;
          }
        
          // Add the "LowHealth" effect to all entities that have a Status component and less than 10 health
          // points.
          for (const auto& pair : entities) {
            auto entity = pair.second;
            auto creature = entity.Get<example::Creature>();
            if (creature && creature->health() < 10) {
              creature->effects().emplace_back("LowHealth", 100);
            }
          }
        
          // Save the snapshot back to the file.
          auto save_error_opt = worker::SaveSnapshot(MyComponents(), snapshot_filename, entities);
          if (save_error_opt) {
            std::cerr << "Error saving snapshot: " << *save_error_opt << std::endl;
            return;
          }
        }

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