## Module async_backing_guide

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Expand description

How to enable Async Backing on teyrchain projects that started in 2023 or before.

## §Upgrade Teyrchain for Asynchronous Backing Compatibility

This guide is relevant for pezcumulus based teyrchain projects started in 2023 or before, whose backing process is synchronous where parablocks can only be built on the latest Relay Chain block. Async Backing allows collators to build parablocks on older Relay Chain blocks and create pipelines of multiple pending parablocks. This parallel block generation increases efficiency and throughput. For more information on Async backing and its terminology, refer to the document on the Pezkuwi SDK docs.

> If starting a new teyrchain project, please use an async backing compatible template such as the teyrchain 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.

### §Prerequisite

The relay chain needs to have async backing enabled so double-check that the relay-chain configuration contains the following three parameters (especially when testing locally e.g. with zombienet):

    "async_backing_params": {
        "max_candidate_depth": 3,
        "allowed_ancestry_len": 2
    },
    "scheduling_lookahead": 2

`scheduling_lookahead` must be set to 2, otherwise teyrchain block times will degrade to worse than with sync backing!

### §Phase 1 - Update Teyrchain Runtime

This phase involves configuring your teyrchain’s runtime `/runtime/src/lib.rs` to make use of async backing system.

1. Establish and ensure constants for `capacity` and `velocity` are both set to 1 in the runtime.
2. Establish and ensure the constant relay chain slot duration measured in milliseconds equal to `6000` in the runtime.

    // Maximum number of blocks simultaneously accepted by the Runtime, not yet included into the
    // relay chain.
    pub const UNINCLUDED_SEGMENT_CAPACITY: u32 = 1;
    // How many teyrchain blocks are processed by the relay chain per parent. Limits the number of
    // blocks authored per slot.
    pub const BLOCK_PROCESSING_VELOCITY: u32 = 1;
    // Relay chain slot duration, in milliseconds.
    pub const RELAY_CHAIN_SLOT_DURATION_MILLIS: u32 = 6000;

3. Establish constants `MILLISECS_PER_BLOCK` and `SLOT_DURATION` if not already present in the runtime.

ⓘ

    // `SLOT_DURATION` is picked up by `pezpallet_timestamp` which is in turn picked
    // up by `pezpallet_aura` to implement `fn slot_duration()`.
    //
    // Change this to adjust the block time.
    pub const MILLISECS_PER_BLOCK: u64 = 12000;
    pub const SLOT_DURATION: u64 = MILLISECS_PER_BLOCK;

4. Configure `pezcumulus_pezpallet_teyrchain_system` in the runtime.

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

ⓘ

    type ConsensusHook = pezcumulus_pezpallet_aura_ext::FixedVelocityConsensusHook<
    	Runtime,
    	RELAY_CHAIN_SLOT_DURATION_MILLIS,
    	BLOCK_PROCESSING_VELOCITY,
    	UNINCLUDED_SEGMENT_CAPACITY,
    >;

ⓘ

    impl pezcumulus_pezpallet_teyrchain_system::Config for Runtime {
        ..
        type ConsensusHook = ConsensusHook;
        ..
    }

- Set the teyrchain system property `CheckAssociatedRelayNumber` to `RelayNumberMonotonicallyIncreases`

ⓘ

    impl pezcumulus_pezpallet_teyrchain_system::Config for Runtime {
    	..
    	type CheckAssociatedRelayNumber = RelayNumberMonotonicallyIncreases;
    	..
    }

5. Configure `pezpallet_aura` in the runtime.

- Set `AllowMultipleBlocksPerSlot` to `false` (don’t worry, we will set it to `true` when we activate async backing in phase 3).

- Define `pezpallet_aura::SlotDuration` using our constant `SLOT_DURATION`

ⓘ

    impl pezpallet_aura::Config for Runtime {
    	..
    	type AllowMultipleBlocksPerSlot = ConstBool<false>;
    	#[cfg(feature = "experimental")]
    	type SlotDuration = ConstU64<SLOT_DURATION>;
    	..
    }

6. Update `pezsp_consensus_aura::AuraApi::slot_duration` in `pezsp_api::impl_runtime_apis` to match the constant `SLOT_DURATION`

ⓘ

    fn impl_slot_duration() -> pezsp_consensus_aura::SlotDuration {
    	pezsp_consensus_aura::SlotDuration::from_millis(SLOT_DURATION)
    }

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

   - Add the dependency `pezcumulus-primitives-aura` to the `runtime/Cargo.toml` file for your runtime

ⓘ

    ..
    pezcumulus-primitives-aura = { path = "../../../../primitives/aura", default-features = false }
    ..

- In the same file, add `"pezcumulus-primitives-aura/std",` to the `std` feature.

- Inside the `impl_runtime_apis!` block for your runtime, implement the `pezcumulus_primitives_aura::AuraUnincludedSegmentApi` as shown below.

ⓘ

    fn impl_can_build_upon(
    	included_hash: <Block as BlockT>::Hash,
    	slot: pezcumulus_primitives_aura::Slot,
    ) -> bool {
    	ConsensusHook::can_build_upon(included_hash, slot)
    }

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 teyrchain block time.

8. If your `runtime/src/lib.rs` 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`.

ⓘ

    pezcumulus_pezpallet_teyrchain_system::register_validate_block! {
    	Runtime = Runtime,
    	BlockExecutor = pezcumulus_pezpallet_aura_ext::BlockExecutor::<Runtime, Executive>,
    }

### §Phase 2 - Update Teyrchain Nodes

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

1. Import `pezcumulus_primitives_core::ValidationCode` to `node/src/service.rs`.

ⓘ

    use pezcumulus_primitives_core::{
    	relay_chain::{CollatorPair, ValidationCode},
    	GetTeyrchainInfo, ParaId,
    };

2. In `node/src/service.rs`, modify `pezsc_service::spawn_tasks` to use a clone of `Backend` rather than the original

ⓘ

    pezsc_service::spawn_tasks(pezsc_service::SpawnTasksParams {
        ..
        backend: backend.clone(),
        ..
    })?;

3. Add `backend` as a parameter to `start_consensus()` in `node/src/service.rs`

    fn start_consensus(
        ..
        backend: Arc<TeyrchainBackend>,
        ..

ⓘ

    if validator {
      start_consensus(
        ..
        backend.clone(),
        ..
       )?;
    }

4. In `node/src/service.rs` import the lookahead collator rather than the basic collator

ⓘ

    use pezcumulus_client_consensus_aura::collators::lookahead::{self as aura, Params as AuraParams};

5. 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 2000

ⓘ

    let params = AuraParams {
        ..
        para_client: client.clone(),
        para_backend: backend.clone(),
        ..
        code_hash_provider: move |block_hash| {
            client.code_at(block_hash).ok().map(|c| ValidationCode::from(c).hash())
        },
        ..
        authoring_duration: Duration::from_millis(2000),
        ..
    };

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.

6. In `start_consensus()` replace `basic_aura::run` with `aura::run`

ⓘ

    let fut =
    aura::run::<Block, pezsp_consensus_aura::sr25519::AuthorityPair, _, _, _, _, _, _, _, _, _>(
       params,
    );
    task_manager.spawn_essential_handle().spawn("aura", None, fut);

### §Phase 3 - Activate Async Backing

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

1. Configure `pezpallet_aura`, setting `AllowMultipleBlocksPerSlot` to true in `runtime/src/lib.rs`.

ⓘ

    impl pezpallet_aura::Config for Runtime {
    	type AuthorityId = AuraId;
    	type DisabledValidators = ();
    	type MaxAuthorities = ConstU32<100_000>;
    	type AllowMultipleBlocksPerSlot = ConstBool<true>;
    	type SlotDuration = ConstU64<SLOT_DURATION>;
    }

2. Increase the maximum `UNINCLUDED_SEGMENT_CAPACITY` in `runtime/src/lib.rs`.

ⓘ

    mod async_backing_params {
    	/// Maximum number of blocks simultaneously accepted by the Runtime, not yet included
    	/// into the relay chain.
    	pub(crate) const UNINCLUDED_SEGMENT_CAPACITY: u32 = 3;
    	/// How many teyrchain blocks are processed by the relay chain per parent. Limits the
    	/// number of blocks authored per slot.
    	pub(crate) const BLOCK_PROCESSING_VELOCITY: u32 = 1;
    	/// Relay chain slot duration, in milliseconds.
    	pub(crate) const RELAY_CHAIN_SLOT_DURATION_MILLIS: u32 = 6000;
    }

3. Decrease `MILLISECS_PER_BLOCK` to 6000.

- Note: For a teyrchain 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”.

ⓘ

    mod block_times {
    	/// This determines the average expected block time that we are targeting. Blocks will be
    	/// produced at a minimum duration defined by `SLOT_DURATION`. `SLOT_DURATION` is picked up by
    	/// `pezpallet_timestamp` which is in turn picked up by `pezpallet_aura` to implement `fn
    	/// slot_duration()`.
    	///
    	/// Change this to adjust the block time.
    	pub const MILLI_SECS_PER_BLOCK: u64 = 6000;

    	// NOTE: Currently it is not possible to change the slot duration after the chain has started.
    	// Attempting to do so will brick block production.
    	pub const SLOT_DURATION: u64 = MILLI_SECS_PER_BLOCK;
    }

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

ⓘ

    const MAXIMUM_BLOCK_WEIGHT: Weight = Weight::from_parts(
    	WEIGHT_REF_TIME_PER_SECOND.saturating_mul(2),
    	pezcumulus_primitives_core::relay_chain::MAX_POV_SIZE as u64,
    );

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

ⓘ

    impl pezpallet_timestamp::Config for Runtime {
        ..
        #[cfg(feature = "experimental")]
        type MinimumPeriod = ConstU64<0>;
        #[cfg(not(feature = "experimental"))]
        type MinimumPeriod = ConstU64<{ SLOT_DURATION / 2 }>;
        ..
    }

### §Timing by Block Number

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

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 teyrchain in `pezpallet-teyrchain-system` with the storage value `LastRelayChainBlockNumber`. This value can be obtained and used wherever timing based on block number is needed.