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clarify message ID per topic, and default function

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protolambda
2020-09-24 19:58:48 +02:00
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pubsub/README.md
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@@ -149,13 +149,17 @@ and
## Message Identification
To uniquely identify a message in a set of topics, a `message_id` is computed based on the message.
This can be configured on the application layer, as `message_id_fn(*Message) => message_id`,
which generally fits in two flavors:
- **origin-stamped** messaging: the concatenation of the `seqno` and `from` fields
This can be configured on the application layer, as `message_id_fn(*Message) => message_id`.
A `message_id_fn` may conditionally call different `message_id_fn` implementations per topic (or group thereof).
The message ID approach generally fits in two flavors:
- **origin-stamped** messaging: the combination of the `seqno` and `from` fields
uniquely identifies a message based on the *author*.
- **content-stamped** messaging: a message ID derived from the `data` field
uniquely identifies a message based on the *data*.
The default `message_id_fn` is origin-stamped, and defined as the string concatenation of `from` and `seqno`.
If fabricated collisions are not a concern, or difficult enough within the window the message is relevant in,
a `message_id` based on a short digest of inputs may benefit performance.