mirror of https://github.com/vacp2p/rfc.git
103 lines
3.9 KiB
Markdown
103 lines
3.9 KiB
Markdown
---
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title: MVDS Metadata Field
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version: 0.1.0
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status: Draft
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authors: Oskar Thorén <oskar@status.im>, Dean Eigenmann <dean@status.im>
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redirect_from:
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- /mvds-metadata.html
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---
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## Table of Contents
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1. [Abstract](#abstract)
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2. [Motivation](#motivation)
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3. [Format](#format)
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1. [Fields](#fields)
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4. [Usage](#usage)
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1. [`parents`](#parents)
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2. [`ephemeral`](#ephemeral)
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5. [Changelog](#changelog)
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6. [Acknowledgments](#acknowledgments)
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7. [Copyright](#copyright)
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8. [Footnotes](#footnotes)
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## Abstract
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In this specification, we describe a method to construct message history that will aid the consistency guarantees of [MVDS](mvds.md). Additionally, we explain how data sync can be used for more lightweight messages that do not require full synchronization.
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## Motivation
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In order for more efficient synchronization of conversational messages, information should be provided allowing a node to more effectively synchronize the dependencies for any given message.
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## Format
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We introduce the metadata message which is used to convey information about a message and how it SHOULD be handled.
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```protobuf
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package vac.mvds;
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message Metadata {
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repeated bytes parents = 1;
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bool ephemeral = 2;
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}
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```
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Nodes MAY transmit a `Metadata` message by extending the MVDS [message](mvds.md#payloads) with a `metadata` field.
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```diff
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message Message {
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bytes group_id = 1;
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int64 timestamp = 2;
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bytes body = 3;
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+ Metadata metadata = 4;
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}
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```
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### Fields
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| Name | Description |
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| ---------------------- | -------------------------------------------------------------------------------------------------------------------------------- |
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| `parents` | list of parent [`message identifier`s](mvds.md#payloads) for the specific message. |
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| `ephemeral` | indicates whether a message is ephemeral or not. |
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## Usage
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### `parents`
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This field contains a list of parent [`message identifier`s](mvds.md#payloads) for the specific message. It MUST NOT contain any messages as parent whose `ack` flag was set to `false`. This establishes a directed acyclic graph (DAG)[^1] of persistent messages.
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Nodes MAY buffer messages until dependencies are satisfied for causal consistency[^2], they MAY also pass the messages straight away for eventual consistency[^3].
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A parent is any message before a new message that a node is aware of that has no children.
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The number of parents for a given message is bound by [0, N], where N is the number of nodes participating in the conversation, therefore the space requirements for the `parents` field is O(N).
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If a message has no parents it is considered a root. There can be multiple roots, which might be disconnected, giving rise to multiple DAGs.
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### `ephemeral`
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When the `ephemeral` flag is set to `false`, a node MUST send an acknowledgment when they have received and processed a message. If it is set to `true`, it SHOULD NOT send any acknowledgment. The flag is `false` by default.
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Nodes MAY decide to not persist ephemeral messages, however they MUST NOT be shared as part of the message history.
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Nodes SHOULD send ephemeral messages in batch mode. As their delivery is not needed to be guaranteed.
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## Changelog
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| Version | Comment |
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| :-----: | ------- |
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| [0.1.0](https://github.com/vacp2p/specs/blob/53bc8585add58695c28cfaf4382818f4daf8de84/mdf.md) | Initial Release |
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## Acknowledgments
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- Andrea Maria Piana
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## Copyright
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Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).
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## Footnotes
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[^1]: <https://en.wikipedia.org/wiki/Directed_acyclic_graph>
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[^2]: Jepsen. [Causal Consistency](https://jepsen.io/consistency/models/causal). Jepsen, LLC.
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[^3]: <https://en.wikipedia.org/wiki/Eventual_consistency>
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