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.
## 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.
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)<sup>1</sup> of persistent messages.
Nodes MAY buffer messages until dependencies are satisfied for causal consistency<sup>2</sup>, they MAY also pass the messages straight away for eventual consistency<sup>3</sup>.
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.
When the `ephemeral` flag is set to `false`, a node MUST send an acknowledgement when they have received and processed a message. If it is set to `true`, it SHOULD NOT send any acknowledgement. The flag is `false` by default.
