pezkuwi-subxt/polkadot/node/overseer/overseer-gen/adr/01-adr.md

Limit outgoing messages

Status

Accepted + implemented.

Context

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.

Decision

Annotate the #[overlord] inner #[subsystem(..)] annotation with an aditional set of outgoing messages and enforce this via more fine grained trait bounds on the Sender and <Context>::Sender bounds.

Consequences

• A graph will be spawn for every compilation under the OUT_DIR of the crate where #[overlord] is specified.
• 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.
• ${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.
• Job-based subsystems, being on their way out, are patched, but they now are generic over the Sender type, leaking that type.