Envelope¶

The Envelope is the single message type that flows between every Courier node. It lives in src/envelope.rs and is a small, deliberately generic shape:

Envelope
├── meta: Meta
│   ├── key: Option<String>
│   ├── source_id: String
│   ├── timestamp_ms: i64
│   └── headers: HashMap<String, String>
└── payload: serde_json::Value

Keeping a single type at the boundary lets the runtime stay generic-free at the channel layer. The trade-off is that strongly-typed payloads are opt-in — a transform that wants typed data deserializes the payload into a struct, operates on it, and re-serializes on the way out.

Field roles¶

Field	Role
`meta.key`	Logical partitioning key — used by sinks like Kafka to choose the destination partition. The built-in `set_key` transform sets this from a payload field.
`meta.source_id`	Hierarchical id of the producing node, e.g. `pipeline-name/src`. Useful for observability and for transforms that want to behave differently per source.
`meta.timestamp_ms`	Producer-assigned timestamp. Sources stamp it; transforms can overwrite.
`meta.headers`	Free-form string headers. Conventionally used for routing hints and metadata that should not live in the payload.
`payload`	Arbitrary JSON. Most transforms operate here.

Scripted access¶

Script transforms expose the same shape under whichever runtime is running. See Scripting for the full table of binding shapes per runtime.