Generate an orchestra of subsystems from a single struct

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#60 in Rust patterns

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The orchestra pattern is a partial actor pattern, with a global orchestrator regarding relevant work items.


The proc macro provides a convenience generator with a builder pattern, where at it's core it creates and spawns a set of subsystems, which are purely declarative.

    #[orchestra(signal=SigSigSig, event=Event, gen=AllMessages, error=OrchestraError)]
    pub struct Opera {
        #[subsystem(MsgA, sends: [MsgB])]
        sub_a: AwesomeSubSysA,

        #[cfg(any(feature = "feature1", feature = "feature2"))]
        #[subsystem(MsgB, sends: [MsgA])]
        sub_b: AwesomeSubSysB,
  • Each subsystem is annotated with #[subsystem(_)] where MsgA respectively MsgB are the messages being consumed by that particular subsystem. Each of those subsystems is required to implement the subsystem trait with the correct trait bounds. Commonly this is achieved by using #[subsystem] and #[contextbounds] macro.
    • #[contextbounds(Foo, error=Yikes, prefix=wherethetraitsat)] can applied to impl-blocks and fn-blocks. It will add additional trait bounds for the generic Context with Context: FooContextTrait for <Context as FooContextTrait>::Sender: FooSenderTrait besides a few more. Note that Foo here references the name of the subsystem as declared in #[orchestra(..)] macro.
    • #[subsystem(Foo, error=Yikes, prefix=wherethetraitsat)] is a extension to the above, implementing trait Subsystem<Context, Yikes>.
  • error= tells the orchestra to use the user provided error type, if not provided a builtin one is used. Note that this is the one error type used throughout all calls, so make sure it does impl From<E> for all other error types E that are relevant to your application.
  • event= declares an external event type, that injects certain events into the orchestra, without participating in the subsystem pattern.
  • signal= defines a signal type to be used for the orchestra. This is a shared "tick" or "clock" for all subsystems.
  • gen= defines a wrapping enum type that is used to wrap all messages that can be consumed by any subsystem.
  • Features can be feature gated by #[cfg(feature = "feature")] attribute macro expressions. Currently supported are any, all, not and feature.
    /// Execution context, always required.
    pub struct DummyCtx;

    /// Task spawner, always required
    /// and must implement `trait orchestra::Spawner`.
    pub struct DummySpawner;

    fn main() {
        let _orchestra = Opera::builder()

In the shown main, the orchestra is created by means of a generated, compile time erroring builder pattern.

The builder requires all subsystems, baggage fields (additional struct data) and spawner to be set via the according setter method before build method could even be called. Failure to do such an initialization will lead to a compile error. This is implemented by encoding each builder field in a set of so called state generics, meaning that each field can be either Init<T> or Missing<T>, so each setter translates a state from Missing to Init state for the specific struct field. Therefore, if you see a compile time error that blames about Missing where Init is expected it usually means that some subsystems or baggage fields were not set prior to the build call.

To exclude subsystems from such a check, one can set wip attribute on some subsystem that is not ready to be included in the orchestra:

    #[orchestra(signal=SigSigSig, event=Event, gen=AllMessages, error=OrchestraError)]
    pub struct Opera {
        #[subsystem(MsgA, sends: MsgB)]
        sub_a: AwesomeSubSysA,

        #[subsystem(MsgB, sends: MsgA), wip]
        sub_b: AwesomeSubSysB, // This subsystem will not be required nor allowed to be set

Baggage fields can be initialized more than one time, however, it is not true for subsystems: subsystems must be initialized only once (another compile time check) or be replaced by a special setter like method replace_<subsystem>.

A task spawner and subsystem context are required to be defined with Spawner and respectively SubsystemContext implemented.


As always, debugging is notoriously annoying with bugged proc-macros, see feature "expand".


feature "expand"

expander is employed to yield better error messages. Enable with --features=orchestra/expand.

feature "dotgraph"

Generate a directed graph which shows the connectivity according to the declared messages to be send and consumed. Enable with --features=orchestra/dotgraph. The path to the generated file will be displayed and is of the form ${OUT_DIR}/${orchestra|lowercase}-subsystem-messaging.dot. Use dot -Tpng ${OUT_DIR}/${orchestra|lowercase}-subsystem-messaging.dot > connectivity.dot to convert to i.e. a png image or use your favorite dot file viewer. It also creates a .svg alongside the .dot graph, derived from the .dot graph for direct usage.


No tool is without caveats, and orchestra is no exception.

Large Message Types

It is not recommended to have large messages that are sent via channels, just like for other implementations of channels. If you need to transfer data that is larger than a few dozend bytes, use Box<_> around it or use a global identifier to access persisted state such as a database, depending on the use case.

Response Channels

It seems very appealing to have response channels as part of messages, and for many cases, these are a very convenient way of maintaining a strucutured data flow, yet they are ready to shoot you in the foot when not used diligently. The diligence required is regarding three topics:

  1. Circular message dependencies leading to a dead-lock for single threaded subsystems
  2. Too deep message dependencies across many subsystems
  3. Delays due to response channels

Each of them has a variety of solutions, such as local caching to remedy frequent lookups of the same information or splitting up subsystem into multiple to avoid circular dependencies or merging tiny topologically closely related to one subsystem in some exceptional cases, but strongly depend on the individual context in which orchestra is used.

To find these, the feature dotgraph is providing a visualization of all interactions of the subsystem to subsystem level (not on the message level, yet) to investigate cycles. Keep an eye on warnings during the generation phase.


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