3.5 KiB
MVDS Metadata Field
Version: 0.1.0 (Draft)
Authors: Oskar Thorén oskar@status.im, Dean Eigenmann dean@status.im
Table of Contents
Abstract
In this specification, we describe a method to construct message history that will aid the consistency guarantees of MVDS. Additionally we explain how data sync can be used for more lightweight messages that do not require full synchronization.
Motivation
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.
Format
We introduce the metadata message which is used to convey information about a message and how it SHOULD be handled.
package vac.mvds;
message Metadata {
bytes parents = 1;
bool ack_required = 2 [default = true];
}
Nodes MAY transmit a Metadata message by extending the MVDS message with a metadata field.
message Message {
bytes group_id = 1;
int64 timestamp = 2;
bytes body = 3;
+ Metadata metadata = 4;
}
Fields
| Name | Description |
|---|---|
parents |
list of parent message identifiers for the specific message. |
ack_required |
indicates whether a message needs to be acknowledged or not. |
Usage
parents
This field contains a list of parent message identifiers for the specific message. It MUST NOT contain any messages as parent whose ack flag was set to false. This creates a directed acyclic graph (DAG)1 of persistent messages.
Nodes MAY buffer messages until dependencies are satisfied for causal consistency2, they MAY also pass the messages straight away for eventual consistency3.
A parent is any message before a new message that a node is aware of that has no children.
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).
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.
ack_required
When the ack_required flag is set to true, a node MUST acknowledge when they have received and processed a message. If it is set to false, it SHOULD NOT send any acknowledgement.
Messages that are not required to be acknowledged can be considered ephemeral, meaning nodes MAY decide to not persist them and they MUST NOT be shared as part of the message history.
Nodes SHOULD send ephemeral messages in batch mode. As their delivery is not needed to be guaranteed.
Footnotes
- https://en.wikipedia.org/wiki/Directed_acyclic_graph
- https://jepsen.io/consistency/models/causal
- https://en.wikipedia.org/wiki/Eventual_consistency
Acknowledgements
- Andrea Maria Piana