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Bernhard Schuster
22 lines
2.0 KiB
Markdown
22 lines
2.0 KiB
Markdown
# Limit outgoing messages
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## Status
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Accepted + implemented.
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## Context
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Previously, there was no way to limit and hence reason about a subset of subsystems, and if they form a cycle. Limiting the outgoing message types is a first step to create respective graphs and use classic graph algorithms to detect those and leave it to the user to resolve these.
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## Decision
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Annotate the `#[overlord]` inner `#[subsystem(..)]` annotation
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with an aditional set of outgoing messages and enforce this via more fine grained trait bounds on the `Sender` and `<Context>::Sender` bounds.
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## Consequences
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* A graph will be spawn for every compilation under the `OUT_DIR` of the crate where `#[overlord]` is specified.
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* Each subsystem has a consuming message which is often referred to as generic `M` (no change on that, is as before), but now we have trait `AssociateOutgoing { type OutgoingMessages = ..; }` which defines an outgoing helper `enum` that is generated with an ident constructed as `${Subsystem}OutgoingMessages` where `${Subsystem}` is the subsystem identifier as used in the overseer declaration. `${Subsystem}OutgoingMessages` is used throughout everywhere to constrain the outgoing messages (commonly referred to as `OutgoingMessage` generic bounded by `${Subsystem}OutgoingMessages: From<OutgoingMessage>` or `::OutgoingMessages: From`. It's what allows the construction of the graph and compile time verification.
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* `${Subsystem}SenderTrait` and `${Subsystem}ContextTrait` are accumulation traits or wrapper traits, that combine over all annotated M or `OutgoingMessages` from the overseer declaration or their respective outgoing types. It is usage convenience and assures consistency within a subsystem while also maintaining a single source of truth for which messages can be sent by a particular subsystem. Note that this is sidestepped for the test subsystem, which may consume `gen=AllMessages`, the global message wrapper type.
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* `Job`-based subsystems, being on their way out, are patched, but they now are generic over the `Sender` type, leaking that type.
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