Asynchronous Backing (Deep)

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--- title: Deep Dive, Asynchronous Backing description: Decoupling Backing and Inclusion Through Advance Work Based on Happy Path Assumptions duration: 1 hour ---

Deep Dive, Asynchronous Backing


I'll be presenting the second of 3 lectures providing a window into Polkadot core, a slice of where we're at and where we're headed.

This lecture covers asynchronous backing, the new feature with potential to deliver shorter parachain block times and an order of magnitude increase in quantity of Polkadot blockspace.

Lets get to it


  • Async Backing Motivation
  • Laying the Groundwork, Contextual Execution of Parablocks
  • Prospective Parachains, Storing Products of the Backing Process
  • Supporting Changes
  • Async Backing Advantages, Current and Future

Async Backing Motivation

Terminology: Backable vs Backed

  • Backable candidate:
    • Output of the off-chain backing process
    • Received a quorum of "valid" votes from its backing group
  • Backed candidate:
    • A backable candidate that has been placed on-chain
    • Also termed "pending availability"


We avoid backing any candidate on the relay chain unless we know there is room for that candidate in the availability process. To do otherwise risks wasted on-chain work.

When a candidate is backed on-chain it immediately occupies an availability core and enters the availability, or erasure coding, process.

Synchronous Backing


Can anyone spot a problem with synchronous model?

  • Problem 1

    • Can only start work on new parablock when prior is included
    • One relay block for backing, one for inclusion
    • Minimum block time of 12 seconds
  • Problem 2

    • Minimal time to submit collation for 12 second total block time
    • About .5 seconds
    • Not enough to fill block fully

Asynchronous Backing


  • Point out the two independent processes and the "stopping points between them"
  • Walk through, starting with unincluded segment

The Async Backing Reasonable Collator Assumptions

  1. "The best existing parablock I'm aware of will eventually be included in the relay chain."
  2. "There won't be a chain reversion impacting that best parablock."

The Stakes Are Low


Best is determined by a process similar to the BABE fork choice rule. Brief BABE fork choice rule review

Contextual Execution of Parablocks

Async Backing Execution Context

  • From relay chain
    • Base constraints
    • Relay parent
  • From unincluded segment
    • Constraint modifications
    • Required parent


  • How it was before:
    • Required parent included in relay parent
    • No need for constraint modifications
  • Relay parent vs required parent
  • Base constraints vs modifications

Constraints and Modifications

fn main() {
pub struct Constraints {
	/// The minimum relay-parent number accepted under these constraints.
	pub min_relay_parent_number: BlockNumber,
	/// The maximum Proof-of-Validity size allowed, in bytes.
	pub max_pov_size: usize,
	/// The maximum new validation code size allowed, in bytes.
	pub max_code_size: usize,
	/// The amount of UMP messages remaining.
	pub ump_remaining: usize,
	/// The amount of UMP bytes remaining.
	pub ump_remaining_bytes: usize,
	/// The maximum number of UMP messages allowed per candidate.
	pub max_ump_num_per_candidate: usize,
	/// Remaining DMP queue. Only includes sent-at block numbers.
	pub dmp_remaining_messages: Vec<BlockNumber>,
	/// The limitations of all registered inbound HRMP channels.
	pub hrmp_inbound: InboundHrmpLimitations,
	/// The limitations of all registered outbound HRMP channels.
	pub hrmp_channels_out: HashMap<ParaId, OutboundHrmpChannelLimitations>,
	/// The maximum number of HRMP messages allowed per candidate.
	pub max_hrmp_num_per_candidate: usize,
	/// The required parent head-data of the parachain.
	pub required_parent: HeadData,
	/// The expected validation-code-hash of this parachain.
	pub validation_code_hash: ValidationCodeHash,
	/// The code upgrade restriction signal as-of this parachain.
	pub upgrade_restriction: Option<UpgradeRestriction>,
	/// The future validation code hash, if any, and at what relay-parent
	/// number the upgrade would be minimally applied.
	pub future_validation_code: Option<(BlockNumber, ValidationCodeHash)>,

/// Modifications to constraints as a result of prospective candidates.
#[derive(Debug, Clone, PartialEq)]
pub struct ConstraintModifications {
	/// The required parent head to build upon.
	pub required_parent: Option<HeadData>,
	/// The new HRMP watermark
	pub hrmp_watermark: Option<HrmpWatermarkUpdate>,
	/// Outbound HRMP channel modifications.
	pub outbound_hrmp: HashMap<ParaId, OutboundHrmpChannelModification>,
	/// The amount of UMP messages sent.
	pub ump_messages_sent: usize,
	/// The amount of UMP bytes sent.
	pub ump_bytes_sent: usize,
	/// The amount of DMP messages processed.
	pub dmp_messages_processed: usize,
	/// Whether a pending code upgrade has been applied.
	pub code_upgrade_applied: bool,


Constraints to Highlight:

  • required_parent: Fragment would place its corresponding candidate here for children
  • min_relay_parent_number: Monotonically increasing rule, max_ancestry_len
  • ump_messages_sent mods ump_remaining
  • code_upgrade_applied: Only one in the unincluded segment at a time!

Prospective Parachains

Storing Products of the Backing Process

Prospective Parachains Snapshot


  • Fragment trees only built for active leaves
  • Fragment trees built per scheduled parachain at each leaf
  • Fragment trees may have 0 or more fragments representing potential parablocks making up possible futures for a parachain's state.
  • Collation generation, passing, and seconding work has already been completed for each fragment.

Anatomy of A Fragment Tree


In this order

  • Scope
  • Root node: corresponds to most recently included candidate
  • Child nodes: Mention required parent rule
  • FragmentNode contents
  • CandidateStorage
  • GetBackableCandidate

Fragment Tree Inclusion Checklist

When and where can a candidate be included in a fragment tree?

  • Required parent is in tree
    • Included as child of required parent, if at all
  • Fragment::validate_against_constraints() passes
  • Relay parent in scope

Relay Parent Limitations for Fragments

What does it mean for a relay parent to be in scope?

When is a relay parent allowed to be out of scope?


In Scope:

  • On same fork of the relay chain
  • Within allowed_ancestry_len

Out of scope:

  • Candidates pending availability have been seen on-chain and need to be accounted for even if they go out of scope. The most likely outcome for candidates pending availability is that they will become available, so we need those blocks to be in the FragmentTree to accept their children.
  • Relay parent can't move backwards relative to that of the required parent

Assembling Base Constraints

Excerpt from backing_state() in runtime/parachains/src/runtime_api_impl/

fn main() {
let (ump_msg_count, ump_total_bytes) = <ump::Pallet<T>>::relay_dispatch_queue_size(para_id);
let ump_remaining = config.max_upward_queue_count - ump_msg_count;

let constraints = Constraints {
		max_pov_size: config.max_pov_size,
		max_code_size: config.max_code_size,
		max_ump_num_per_candidate: config.max_upward_message_num_per_candidate,
		max_hrmp_num_per_candidate: config.hrmp_max_message_num_per_candidate,

Applying Constraint Modifications

Excerpt from Constraints::apply_modifications()

fn main() {
if modifications.dmp_messages_processed > new.dmp_remaining_messages.len() {
	return Err(ModificationError::DmpMessagesUnderflow {
		messages_remaining: new.dmp_remaining_messages.len(),
		messages_processed: modifications.dmp_messages_processed,
} else {
	new.dmp_remaining_messages =

Validating Against Constraints

Excerpt from Fragment::validate_against_constraints()

fn main() {
if relay_parent.number < constraints.min_relay_parent_number {
	return Err(FragmentValidityError::RelayParentTooOld(


fn main() {
pub struct AsyncBackingParams {
	/// The maximum number of para blocks between the para head in a relay parent
	/// and a new candidate. Restricts nodes from building arbitrary long chains
	/// and spamming other validators.
	/// When async backing is disabled, the only valid value is 0.
	pub max_candidate_depth: u32,
	/// How many ancestors of a relay parent are allowed to build candidates on top
	/// of.
	/// When async backing is disabled, the only valid value is 0.
	pub allowed_ancestry_len: u32,

Numbers in use for testing Prospective Parachains:

  • max_candidate_depth = 4
  • allowed_ancestry_len = 3

Supporting Changes

Statement Distribution Changes


Why do we need the refactor?

Answer: Cap on simultaneous candidates per backing group ~3x higher


  • Announcement - Acknowledgement
  • Request - Response

Provisioner Changes

Function request_backable_candidates from the Provisioner subsystem

fn main() {
/// Requests backable candidates from Prospective Parachains subsystem
/// based on core states.
/// Should be called when prospective parachains are enabled.
async fn request_backable_candidates(
	availability_cores: &[CoreState],
	bitfields: &[SignedAvailabilityBitfield],
	relay_parent: Hash,
	sender: &mut impl overseer::ProvisionerSenderTrait,
) -> Result<Vec<CandidateHash>, Error> {
	let block_number = get_block_number_under_construction(relay_parent, sender).await?;

	let mut selected_candidates = Vec::with_capacity(availability_cores.len());

	for (core_idx, core) in availability_cores.iter().enumerate() {
		let (para_id, required_path) = match core {
			CoreState::Scheduled(scheduled_core) => {
				// The core is free, pick the first eligible candidate from
				// the fragment tree.
				(scheduled_core.para_id, Vec::new())
			CoreState::Occupied(occupied_core) => {
				if bitfields_indicate_availability(core_idx, bitfields, &occupied_core.availability)
					if let Some(ref scheduled_core) = occupied_core.next_up_on_available {
						// The candidate occupying the core is available, choose its
						// child in the fragment tree.
						(scheduled_core.para_id, vec![occupied_core.candidate_hash])
					} else {
				} else {
					if occupied_core.time_out_at != block_number {
					if let Some(ref scheduled_core) = occupied_core.next_up_on_time_out {
						// Candidate's availability timed out, practically same as scheduled.
						(scheduled_core.para_id, Vec::new())
					} else {
			CoreState::Free => continue,

		let candidate_hash =
			get_backable_candidate(relay_parent, para_id, required_path, sender).await?;

		match candidate_hash {
			Some(hash) => selected_candidates.push(hash),
			None => {
					target: LOG_TARGET,
					leaf_hash = ?relay_parent,
					core = core_idx,
					"No backable candidate returned by prospective parachains",



  • Per core
    • Discuss core states free, scheduled, occupied
    • Discuss core freeing criteria
      • bitfields_indicate_availability
        • next_up_on_available
      • availability time out
        • next_up_on_timeout
    • Explain what required path is
    • Why is required path left empty?

Cumulus Changes

  • Consensus driven block authoring
  • Parachain consensus refactor
    • Aura rewrite
    • Custom sequencing consensus:
      • Tendermint
      • Hotshot consensus

Async Backing Advantages, Current and Future

Advantages of Asynchronous Backing

  1. 3-5x more extrinsics per block
  2. Shorter parachain block times 6s vs 12s
  3. Resulting 6-10x boost in quantity of blockspace
  4. Fewer wasted parachain blocks


  1. Collators have more time to fill each block
  2. Advance work ensures backable candidates for each parachain are present to be backed on the relay chain every 6 seconds
  3. Self explanatory
  4. Allow parachain blocks to be β€˜reused’ when they don’t make it onto the relay chain in the first attempt

Async Backing and Exotic Core Scheduling

  • What is exotic core scheduling?
    • Multiple cores per parachain
    • Overlapping leases of many lengths
    • Lease + On-demand
  • How does asynchronous backing help?


  • The unincluded segment is necessary to build 2 or more parablocks in a single relay block

Shorter Block Times?

  • Async backing gives us unincluded block queuing
  • What else we need for useful shorter times:
    • Soft finality
    • Inclusion dependencies (comes with elastic scaling)


  • Soft finality means that the collators will submit as many new blocks with the same extrinsics as needed to retain the same ordering if a parablock candidate is dropped.
  • Inclusion dependencies: Take two parablocks a and b, where a is built on top of b. Then if a and b are being made available on two different cores during the same block we need to ensure that b waits for inclusion until a is also included.


  1. Polkadot Async Backing PR
  2. Implementers Guide: Prospective Parachains