pezkuwi-subxt/polkadot/roadmap/implementers-guide/src/runtime/dmp.md

DMP Module

A module responsible for Downward Message Processing (DMP). See Messaging Overview for more details.

Storage

Storage layout required for implementation of DMP.

/// The downward messages addressed for a certain para.
DownwardMessageQueues: map ParaId => Vec<InboundDownwardMessage>;
/// A mapping that stores the downward message queue MQC head for each para.
///
/// Each link in this chain has a form:
/// `(prev_head, B, H(M))`, where
/// - `prev_head`: is the previous head hash or zero if none.
/// - `B`: is the relay-chain block number in which a message was appended.
/// - `H(M)`: is the hash of the message being appended.
DownwardMessageQueueHeads: map ParaId => Hash;

Initialization

No initialization routine runs for this module.

Routines

Candidate Acceptance Function:

• check_processed_downward_messages(P: ParaId, processed_downward_messages: u32):
  1. Checks that DownwardMessageQueues for P is at least processed_downward_messages long.
  2. Checks that processed_downward_messages is at least 1 if DownwardMessageQueues for P is not empty.

Candidate Enactment:

• prune_dmq(P: ParaId, processed_downward_messages: u32):
  1. Remove the first processed_downward_messages from the DownwardMessageQueues of P.

Utility routines.

queue_downward_message(P: ParaId, M: DownwardMessage): 1. Check if the size of M exceeds the config.max_downward_message_size. If so, return an error. 1. Wrap M into InboundDownwardMessage using the current block number for sent_at. 1. Obtain a new MQC link for the resulting InboundDownwardMessage and replace DownwardMessageQueueHeads for P with the resulting hash. 1. Add the resulting InboundDownwardMessage into DownwardMessageQueues for P.

Session Change

1. For each P in outgoing_paras (generated by Paras::on_new_session):
   1. Remove all DownwardMessageQueues of P.
   2. Remove DownwardMessageQueueHeads for P.