insert_typed

HeaderMap::insert_typed writes a header contract into a header map, the way a publish transform stamps every message of a slot.

A field becomes one header named after it, its value written as a string. A None field writes nothing, and a header of the same name is overwritten. A field that holds a nested struct, a sequence or a map is an error. The example's transform is a publish transform on an Out slot, so every message the slot sends carries the contract.

Example

Macros

examples/typed_headers.rs

/// Where the service runs, as a contract stamped on everything the slot sends.
#[derive(Clone, Serialize)]
struct Origin {
    region: &'static str,
    canary: bool,
}

struct Stamp(Origin);

impl<Options> PublishTransform<ForSlot, Options> for Stamp {
    type Destination = Reads;

    fn apply(
        &self,
        out: &mut Outgoing<'_>,
        _options: &mut Option<Options>,
        _cx: &SlotContext<'_>,
    ) {
        if let Err(error) = out.headers_mut().insert_typed(&self.0) {
            tracing::warn!(%error, "origin headers left out");
        }
    }
}

fn app() -> impl App {
    let origin = Origin {
        region: "eu-west",
        canary: false,
    };
    RustStream::new(AppInfo::new("transcoder", "0.1.0")).with_broker(MemoryBroker::new(), |b| {
        b.include(convert)
            .out(Events, Publish)
            .transform(Stamp(origin))
            .build();
    })
}

Manual

examples/manual/typed_headers.rs

/// Where the service runs, as a contract stamped on everything the slot sends.
#[derive(Clone, Serialize)]
struct Origin {
    region: &'static str,
    canary: bool,
}

struct Stamp(Origin);

impl<Options> PublishTransform<ForSlot, Options> for Stamp {
    type Destination = Reads;

    fn apply(
        &self,
        out: &mut Outgoing<'_>,
        _options: &mut Option<Options>,
        _cx: &SlotContext<'_>,
    ) {
        if let Err(error) = out.headers_mut().insert_typed(&self.0) {
            tracing::warn!(%error, "origin headers left out");
        }
    }
}

fn app() -> impl App {
    let origin = Origin {
        region: "eu-west",
        canary: false,
    };
    RustStream::new(AppInfo::new("transcoder", "0.1.0")).with_broker(MemoryBroker::new(), |b| {
        b.include(subscriber("chunks", Convert).build())
            .out(Events, Publish)
            .transform(Stamp(origin))
            .build();
    })
}