Shallow Dive, Asynchronous Backing
Hello again everyone
Today I'll be speaking to you about asynchronous backing, the new feature which delivers shorter parachain block times and an order of magnitude increase in quantity of Polkadot blockspace.
Lets get to it
Overview
Synchronous vs asynchronous
Why is asynchronous backing desirable?
High level mechanisms of async backing
The unincluded segment, and prospective parachains
Async backing enabling other roadmap items
Synchronous Backing Simplified
The dividing line between the left and right is when a candidate is backed on chain
Approvals, disputes, and finality don't immediately gate the production of farther candidates.
So we don't need to represent those steps in this model.
Async Backing Simplified
Our cache of parablock candidates allows us to pause just before that dividing line, on-chain backing
The Async Backing Optimistic Collator Assumptions
"The best existing parablock I'm aware of will eventually be included in the relay chain."
"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
Advantages of Asynchronous Backing
3-5x more extrinsics per block
Shorter parachain block times 6s vs 12s
Resulting 6-10x boost in quantity of blockspace
Fewer wasted parachain blocks
Collators have more time to fill each block
Advance work ensures backable candidates for each parachain are present to be backed on the relay chain every 6 seconds
Self explanatory
Allow parachain blocks to be ‘reused’ when they don’t make it onto the relay chain in the first attempt
Parablock Validation Pipelining
Synchronous Backing, Another Look
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Now let's take a closer look at when each step of backing and inclusion takes place both with synchronous and asynchronous backing.
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Whole process is a cycle of duration 12 seconds (2 relay blocks).
No part of this cycle can be started for a second candidate of the same parachain until the first is included.
Async Backing, Another Look
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Candidates stored in prospective parachains (detail on that later)
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Now we see our relay block cycle.
It is 6 seconds rather than 12.
It completes on-chain backing for one candidate and inclusion for another each cycle.
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Collation generation and off-chain backing are outside of the relay block cycle.
Because of this, collators have the freedom to work several blocks in advance. In practice, even working 2-3 blocks in advance gives a collator ample time to fully fill blocks (PoV size 5MiB)
Notice that a part of the collation generation context, the unincluded segment, comes from the collator itself.
The Unincluded Segment
A parachain's record of all parablocks on a particular chain fork produced but not yet included
Used to apply limitations when constructing future blocks
Lives in the parachain runtime
Viewed from the perspective of a new parablock under construction
Limitation example, upward messages remaining before the relay chain would have to drop incoming messages from our parachain
Unincluded Segment
Segment added to as each new block is imported into the parachain runtime
Segment shrinks when one of its ancestor blocks becomes included
Maximum unincluded segment capacity is set both on the parachain and relay chain
Unincluded Segment
UsedBandwidth:
pub ump_msg_count: u32,
pub ump_total_bytes: u32,
pub hrmp_outgoing: BTreeMap<ParaId, HrmpChannelUpdate>,
Prospective Parachains
The relay chain's record of all candidates on all chain forks from all parachains
As if you folded all unincluded segments into one huge structure
Used to store candidates and later provide them to the on-chain backing process
Lives in the relay client (off chain)
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.
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
Resume presentation
Shallow Dive, Asynchronous Backing Hello again everyone Today I'll be speaking to you about asynchronous backing, the new feature which delivers shorter parachain block times and an order of magnitude increase in quantity of Polkadot blockspace. Lets get to it
