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Upgrade Parachain for Asynchronous Backing Compatibility

Who is this guide for?

This guide is relevant for cumulus based parachain projects started in 2023 or before. Later projects should already be async backing compatible. If starting a new parachain project, please use an async backing compatible template such as cumulus/parachain-template.

The rollout process for Async Backing has three phases. Phases 1 and 2 below put new infrastructure in place. Then we can simply turn on async backing in phase 3. But first, some pre-reqs and context to set the stage.

Async Backing Prerequisites


For more contextual information about asynchronous backing, see this page.

Pull the latest version of Cumulus for use with your parachain. It contains necessary changes for async backing compatibility. Latest on master branch of Polkadot-SDK is currently sufficient. Any 2024 release will work as well.

Async Backing Terminology and Parameters

Time for a bit of context before we get started. The following concepts will aid in demystifying the collator side of Async Backing and establish a basic understanding of the changes being made:

  • Unincluded segment - From the perspective of a parachain block under construction, the unincluded segment describes a chain of recent block ancestors which have yet to be included on the relay chain. The ability to build new blocks on top of the unincluded segment rather than on top of blocks freshly included in the relay chain is the core of asynchronous backing.
  • Capacity - The maximum size of the unincluded segment. The longer this is, the farther ahead a parachain can work, producing new candidates before the ancestors of those candidates have been seen as included on-chain. Practically, a capacity of 2-3 is sufficient to realize the full benefits of asynchronous backing, at least until the release of elastic scaling.
  • Velocity - The base rate at which a parachain should produce blocks. A velocity of 1 indicates that 1 parachain block should be produced per relay chain block. In order to fill the unincluded segment with candidates, collators may build up to Velocity + 1 candidates per aura slot while there is remaining capacity. When elastic scaling has been released velocities greater than 1 will be supported.
  • AllowMultipleBlocksPerSlot - If this is true, Aura will allow slots to stay the same across sequential parablocks. Otherwise the slot number must increase with each block. To fill the unincluded segment as described above we need this to be true.
  • FixedVelocityConsensusHook - This is a variety of ConsensusHook intended to be passed to parachain-system as part of its Config. It is triggered on initialization of a new runtime. An instance of FixedVelocityConsensusHook is defined with both a fixed capacity and velocity. It aborts the runtime early if either capacity or velocity is exceeded, as the collator shouldn’t be creating additional blocks in that case.
  • AsyncBackingParams.max_candidate_depth - This parameter determines the maximum unincluded segment depth the relay chain will support. Candidates sent to validators which exceed max_candidate_depth will be ignored. Capacity, as mentioned above, should not exceed max_candidate_depth.
  • AsyncBackingParams.allowed_ancestry_len - Each parachain block candidate has a relay_parent from which its execution and validation context is derived. Before async backing the relay_parent for a candidate not yet backed was required to be the fresh head of a fork. With async backing we can relax this requirement. Instead we set a conservative maximum age in blocks for the relay_parents of candidates in the unincluded segment. This age, allowed_ancestry_len lives on the relay chain and is queried by parachains when deciding which block to build on top of.
  • Lookahead Collator - A collator for Aura that looks ahead of the most recently included parachain block when determining what to build upon. This collator also builds additional blocks when the maximum backlog is not saturated. The size of the backlog is determined by invoking the AuraUnincludedSegmentApi. If that runtime API is not supported, this assumes a maximum backlog size of 1.

Phase 1 - Update Parachain Runtime

This phase involves configuring your parachain’s runtime to make use of async backing system.

  1. Establish constants for capacity and velocity and set both of them to 1 in /runtime/src/

  2. Establish a constant relay chain slot duration measured in milliseconds equal to 6000 in /runtime/src/


  1. Establish constants MILLISECS_PER_BLOCK and SLOT_DURATION if not already present in /runtime/src/


  1. Configure cumulus_pallet_parachain_system in runtime/src/

    • Define a FixedVelocityConsensusHook using our capacity, velocity, and relay slot duration constants. Use this to set the parachain system ConsensusHook property.


    • Set the parachain system property CheckAssociatedRelayNumber to RelayNumberMonotonicallyIncreases


  2. Configure pallet_aura in runtime/src/

    • Set AllowMultipleBlocksPerSlot to false
    • Define pallet_aura::SlotDuration using our constant SLOT_DURATION


  3. Update aura_api::SlotDuration() to match the constant SLOT_DURATION


  4. Implement the AuraUnincludedSegmentApi, which allows the collator client to query its runtime to determine whether it should author a block.

    • Add the dependency cumulus-primitives-aura to the runtime/Cargo.toml file for your runtime


    • Inside the impl_runtime_apis! block for your runtime, implement the AuraUnincludedSegmentApi as shown below.


      Important note: With a capacity of 1 we have an effective velocity of ½ even when velocity is configured to some larger value. This is because capacity will be filled after a single block is produced and will only be freed up after that block is included on the relay chain, which takes 2 relay blocks to accomplish. Thus with capacity 1 and velocity 1 we get the customary 12 second parachain block time.

  5. If your runtime/src/ provides a CheckInherents type to register_validate_block, remove it. FixedVelocityConsensusHook makes it unnecessary. The following example shows how register_validate_block should look after removing CheckInherents.


Phase 2 - Update Parachain Nodes

This phase consists of plugging in the new lookahead collator node.

  1. Import cumulus_primitives_core::ValidationCode to node/src/


  1. In node/src/, modify sc_service::spawn_tasks to use a clone of Backend rather than the original


  1. Add backend as a parameter to start_consensus() in node/src/



  1. In start_consensus() import the lookahead collator rather than the basic collator


  1. In start_consensus() replace the BasicAuraParams struct with AuraParams
    • Change the struct type from BasicAuraParams to AuraParams
    • In the para_client field, pass in a cloned para client rather than the original
    • Add a para_backend parameter after para_client, passing in our para backend
    • Provide a code_hash_provider closure like that shown below
    • Increase authoring_duration from 500 milliseconds to 1500

Note: Set authoring_duration to whatever you want, taking your own hardware into account. But if the backer who should be slower than you due to reading from disk, times out at two seconds your candidates will be rejected.


  1. In start_consensus() replace basic_aura::run with aura::run


Phase 3 - Activate Async Backing

This phase consists of changes to your parachain’s runtime that activate async backing feature.

  1. Configure pallet_aura, setting AllowMultipleBlocksPerSlot to true in runtime/src/


  1. Increase the maximum unincluded segment capacity in runtime/src/


  1. Either decrease MILLISECS_PER_BLOCK to 6000 or increase BLOCK_PROCESSING_VELOCITY to 2 in runtime/src/
  • Note: For a parachain which measures time in terms of its own block number rather than by relay block number it may be preferable to increase velocity. Changing block time may cause complications, requiring additional changes. See the section “Timing by Block Number”.


  1. Update MAXIMUM_BLOCK_WEIGHT to reflect the increased time available for block production.


  1. Add a feature flagged alternative for MinimumPeriod in pallet_timestamp. The type should be ConstU64<0> with the feature flag experimental, and ConstU64<{SLOT_DURATION / 2}> without.


Timing by Block Number

With asynchronous backing it will be possible for parachains to opt for a block time of 6 seconds rather than 12 seconds. But modifying block duration isn’t so simple for a parachain which was measuring time in terms of its own block number. It could result in expected and actual time not matching up, stalling the parachain.

One strategy to deal with this issue is to instead rely on relay chain block numbers for timing. Relay block number is kept track of by each parachain in pallet-parachain-system with the storage value LastRelayChainBlockNumber. This value can be obtained and used wherever timing based on block number is needed.