Order systems with channels

Access conflicts order systems that touch the same components. When two systems touch different components but one must still run after the other — B consumes an effect A produced — express that ordering with a channel.

Channel IDs

A channel is identified by a ChannelID. Model resources own these; they are not created directly. An environment key, a message handle, and a space handle each own a channel, and a model can register dedicated phase keys used only for ordering. Read the id from the resource, for example key.channel_id() or handle.channel_id().

Declaring produce and consume

A system declares channels through its AccessSets: insert a channel id into produces or consumes. Build the access set explicitly and construct the system with FnSystem::new:

let mut access = AccessSets::default();
access.read.set(firm_id);
access.write.set(bank_id);
access.produces.insert(phases.credit_cleared.channel_id());

let system = FnSystem::new(id, "credit_market", access, move |ecs| {
    // ...
    Ok(())
});

A later system that must run after it consumes the same channel:

let mut access = AccessSets::default();
access.read.set(bank_id);
access.consumes.insert(phases.credit_cleared.channel_id());

The scheduler places every consumer of a channel in a stage after all of its producers. Systems that neither produce nor consume the channel are unaffected and may still parallelise with either side.

Ordering whole phases

To sequence phases of a tick — "aggregate, then set targets, then run the labour market" — give each phase a channel: the phase's systems produce it, and the next phase's systems consume it. The macroeconomy example orders its quarterly schedule this way; see examples/macroeconomy/systems.rs.

Deriving component access and adding channels

FnSystem::from_queries derives component access from queries but does not add channels. When a system needs both, build an AccessSets (setting the component reads and writes, or deriving them), insert the channels, and use FnSystem::new.