pezkuwi-subxt/polkadot/roadmap/implementors-guide/src/runtime/router.md

Router Module

The Router module is responsible for storing and dispatching Upward and Downward messages from and to parachains respectively. It is intended to later handle the XCMP logic as well.

For each enacted block the queue_upward_messages entry-point is called.

Storage

Storage layout:

/// Messages ready to be dispatched onto the relay chain.
/// This is subject to `max_upward_queue_count` and
/// `watermark_queue_size` from `HostConfiguration`.
RelayDispatchQueues: map ParaId => Vec<UpwardMessage>;
/// Size of the dispatch queues. Caches sizes of the queues in `RelayDispatchQueue`.
/// First item in the tuple is the count of messages and second
/// is the total length (in bytes) of the message payloads.
RelayDispatchQueueSize: map ParaId => (u32, u32);
/// The ordered list of `ParaId`s that have a `RelayDispatchQueue` entry.
NeedsDispatch: Vec<ParaId>;

Initialization

No initialization routine runs for this module.

Routines

• queue_upward_messages(ParaId, Vec<UpwardMessage>):
  1. Updates NeedsDispatch, and enqueues upward messages into RelayDispatchQueue and modifies the respective entry in RelayDispatchQueueSize.

Finalization

1. Dispatch queued upward messages from RelayDispatchQueues in a FIFO order applying the config.watermark_upward_queue_size and config.max_upward_queue_count limits.