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<ol class="chapter"><li class="chapter-item expanded affix "><a href="../introduction.html">Introduction</a></li><li class="spacer"></li><li class="chapter-item expanded affix "><li class="part-title">Newly Proposed</li><li class="chapter-item expanded "><a href="../new/0135-compressed-blob-prefixes.html">RFC-0135: Compressed Blob Prefixes</a></li><li class="spacer"></li><li class="chapter-item expanded affix "><li class="part-title">Proposed</li><li class="chapter-item expanded "><a href="../proposed/0004-remove-unnecessary-allocator-usage.html">RFC-0004: Remove the host-side runtime memory allocator</a></li><li class="chapter-item expanded "><a href="../proposed/0123-pending-code-as-storage-location-for-runtime-upgrades.html">RFC-0123: Introduce :pending_code as intermediate storage key for the runtime code</a></li><li class="chapter-item expanded "><a href="../proposed/0125-xcm-asset-metadata.html">RFC-0125: XCM Asset Metadata</a></li><li class="chapter-item expanded "><a href="../proposed/0126-introduce-xcq.html">RFC-0126: Introduce XCQ(Cross Consensus Query)</a></li><li class="spacer"></li><li class="chapter-item expanded affix "><li class="part-title">Approved</li><li class="chapter-item expanded "><a href="../approved/0001-agile-coretime.html">RFC-1: Agile Coretime</a></li><li class="chapter-item expanded "><a href="../approved/0005-coretime-interface.html">RFC-5: Coretime Interface</a></li><li class="chapter-item expanded "><a href="../approved/0007-system-collator-selection.html">RFC-0007: System Collator Selection</a></li><li class="chapter-item expanded "><a href="../approved/0008-parachain-bootnodes-dht.html">RFC-0008: Store parachain bootnodes in relay chain DHT</a></li><li class="chapter-item expanded "><a href="../approved/0009-improved-net-light-client-requests.html">RFC-0009: Improved light client requests networking protocol</a></li><li class="chapter-item expanded "><a href="../approved/0010-burn-coretime-revenue.html">RFC-0010: Burn Coretime Revenue</a></li><li class="chapter-item expanded "><a href="../approved/0012-process-for-adding-new-collectives.html">RFC-0012: Process for Adding New System Collectives</a></li><li class="chapter-item expanded "><a href="../approved/0013-prepare-blockbuilder-and-core-runtime-apis-for-mbms.html">RFC-0013: Prepare Core runtime API for MBMs</a></li><li class="chapter-item expanded "><a href="../approved/0014-improve-locking-mechanism-for-parachains.html">RFC-0014: Improve locking mechanism for parachains</a></li><li class="chapter-item expanded "><a href="../approved/0022-adopt-encointer-runtime.html">RFC-0022: Adopt Encointer Runtime</a></li><li class="chapter-item expanded "><a href="../approved/0026-sassafras-consensus.html">RFC-0026: Sassafras Consensus Protocol</a></li><li class="chapter-item expanded "><a href="../approved/0032-minimal-relay.html">RFC-0032: Minimal Relay</a></li><li class="chapter-item expanded "><a href="../approved/0042-extrinsics-state-version.html">RFC-0042: Add System version that replaces StateVersion on RuntimeVersion</a></li><li class="chapter-item expanded "><a href="../approved/0043-storage-proof-size-hostfunction.html">RFC-0043: Introduce storage_proof_size Host Function for Improved Parachain Block Utilization</a></li><li class="chapter-item expanded "><a href="../approved/0045-nft-deposits-asset-hub.html">RFC-0045: Lowering NFT Deposits on Asset Hub</a></li><li class="chapter-item expanded "><a href="../approved/0047-assignment-of-availability-chunks.html">RFC-0047: Assignment of availability chunks to validators</a></li><li class="chapter-item expanded "><a href="../approved/0048-session-keys-runtime-api.html">RFC-0048: Generate ownership proof for SessionKeys</a></li><li class="chapter-item expanded "><a href="../approved/0050-fellowship-salaries.html">RFC-0050: Fellowship Salaries</a></li><li class="chapter-item expanded "><a href="../approved/0056-one-transaction-per-notification.html">RFC-0056: Enforce only one transaction per notification</a></li><li class="chapter-item expanded "><a href="../approved/0059-nodes-capabilities-discovery.html">RFC-0059: Add a discovery mechanism for nodes based on their capabilities</a></li><li class="chapter-item expanded "><a href="../approved/0078-merkleized-metadata.html">RFC-0078: Merkleized Metadata</a></li><li class="chapter-item expanded "><a href="../approved/0084-general-transaction-extrinsic-format.html">RFC-0084: General transactions in extrinsic format</a></li><li class="chapter-item expanded "><a href="../approved/0091-dht-record-creation-time.html">RFC-0091: DHT Authority discovery record creation time</a></li><li class="chapter-item expanded "><a href="../approved/0097-unbonding_queue.html">RFC-0097: Unbonding Queue</a></li><li class="chapter-item expanded "><a href="../approved/0099-transaction-extension-version.html">RFC-0099: Introduce a transaction extension version</a></li><li class="chapter-item expanded "><a href="../approved/0100-xcm-multi-type-asset-transfer.html">RFC-0100: New XCM instruction: InitiateAssetsTransfer</a></li><li class="chapter-item expanded "><a href="../approved/0101-xcm-transact-remove-max-weight-param.html">RFC-0101: XCM Transact remove require_weight_at_most parameter</a></li><li class="chapter-item expanded "><a href="../approved/0103-introduce-core-index-commitment.html">RFC-0103: Introduce a CoreIndex commitment and a SessionIndex field in candidate receipts</a></li><li class="chapter-item expanded "><a href="../approved/0105-xcm-improved-fee-mechanism.html">RFC-0105: XCM improved fee mechanism</a></li><li class="chapter-item expanded "><a href="../approved/0107-xcm-execution-hints.html">RFC-0107: XCM Execution hints</a></li><li class="chapter-item expanded "><a href="../approved/0108-xcm-remove-testnet-ids.html">RFC-0108: Remove XCM testnet NetworkIds</a></li><li class="chapter-item expanded "><a href="../approved/0122-alias-origin-on-asset-transfers.html">RFC-0122: Asset transfers can alias XCM origin on destination to original origin</a></li><li class="spacer"></li><li class="chapter-item expanded affix "><li class="part-title">Stale</li><li class="chapter-item expanded "><a href="../stale/0000-rewards.html">RFC-0000: Validator Rewards</a></li><li class="chapter-item expanded "><a href="../stale/0006-dynamic-pricing-for-bulk-coretime-sales.html">RFC-0006: Dynamic Pricing for Bulk Coretime Sales</a></li><li class="chapter-item expanded "><a href="../stale/0015-market-design-revisit.html">RFC-0015: Market Design Revisit</a></li><li class="chapter-item expanded "><a href="../stale/0034-xcm-absolute-location-account-derivation.html">RFC-34: XCM Absolute Location Account Derivation</a></li><li class="chapter-item expanded "><a href="../stale/0035-conviction-voting-delegation-modifications.html"> RFC-0035: Conviction Voting Delegation Modifications</a></li><li class="chapter-item expanded "><a href="../stale/0044-rent-based-registration.html">RFC-0044: Rent based registration model</a></li><li class="chapter-item expanded "><a href="../stale/0054-remove-heap-pages.html">RFC-0054: Remove the concept of "heap pages" from the client</a></li><li class="chapter-item expanded "><a href="../stale/0070-x-track-kusamanetwork.html">RFC-0070: X Track for @kusamanetwork</a></li><li class="chapter-item expanded "><a href="../stale/0073-referedum-deposit-track.html">RFC-0073: Decision Deposit Referendum Track</a></li><li class="chapter-item expanded "><a href="../stale/0074-stateful-multisig-pallet.html">RFC-0074: Stateful Multisig Pallet</a></li><li class="chapter-item expanded "><a href="../stale/0077-increase-max-length-of-identity-pgp-fingerprint-value.html">RFC-0077: Increase maximum length of identity PGP fingerprint values from 20 bytes</a></li><li class="chapter-item expanded "><a href="../stale/0088-broker-pallet-slashable-deposit-purchaser-reputation-reserved-cores.html">RFC-0088: Add slashable locked deposit, purchaser reputation, and reserved cores for on-chain identities to broker pallet</a></li><li class="chapter-item expanded "><a href="../stale/00xx-secondary-marketplace-for-regions.html">RFC-0001: Secondary Market for Regions</a></li><li class="chapter-item expanded "><a href="../stale/00xx-smart-contracts-coretime-chain.html">RFC-0002: Smart Contracts on the Coretime Chain</a></li><li class="chapter-item expanded "><a href="../stale/0102-offchain-parachain-runtime-upgrades.html" class="active">RFC-0000: Feature Name Here</a></li><li class="chapter-item expanded "><a href="../stale/0106-xcm-remove-fees-mode.html">RFC-0106: Remove XCM fees mode</a></li><li class="chapter-item expanded "><a href="../stale/0111-pure-proxy-replication.html">RFC-0111: Pure Proxy Replication</a></li><li class="chapter-item expanded "><a href="../stale/0112-compress-state-response-message-in-state-sync.html">RFC-0112: Compress the State Response Message in State Sync</a></li><li class="chapter-item expanded "><a href="../stale/0114-secp256r1-hostfunction.html">RFC-0114: Introduce secp256r1_ecdsa_verify_prehashed Host Function to verify NIST-P256 elliptic curve signatures</a></li><li class="chapter-item expanded "><a href="../stale/0117-unbrick-collective.html">RFC-0117: The Unbrick Collective</a></li><li class="chapter-item expanded "><a href="../stale/0120-referenda-confirmation-by-candle-mechanism.html">RFC-0120: Referenda Confirmation by Candle Mechanism</a></li><li class="chapter-item expanded "><a href="../stale/0121-iterable-referenda-tracks.html">RFC-0121: Iterable Referenda Tracks</a></li><li class="chapter-item expanded "><a href="../stale/0124-extrinsic-version-5.html">RFC-0124: Extrinsic version 5</a></li><li class="chapter-item expanded "><a href="../stale/RFC-114 Adjust Tipper Track Confirmation Periods.html">RFC-114: Adjust Tipper Track Confirmation Periods</a></li><li class="chapter-item expanded "><a href="../stale/TODO-stale-nomination-reward-curve.html">RFC-TODO: Stale Nomination Reward Curve</a></li></ol>
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<h1 class="menu-title">Polkadot Fellowship RFCs</h1>
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<main>
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<p><a href="https://github.com/polkadot-fellows/RFCs/pull/102">(source)</a></p>
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<p><strong>Table of Contents</strong></p>
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<ul>
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<li><a href="#rfc-0000-feature-name-here">RFC-0000: Feature Name Here</a>
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<ul>
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<li><a href="#summary">Summary</a></li>
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<li><a href="#motivation">Motivation</a></li>
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<li><a href="#stakeholders">Stakeholders</a></li>
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<li><a href="#explanation">Explanation</a>
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<ul>
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<li><a href="#introduce-a-new-ump-message-type-requestcodeupgrade">Introduce a new UMP message type <code>RequestCodeUpgrade</code></a></li>
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<li><a href="#handle-requestcodeupgrade-on-backers">Handle <code>RequestCodeUpgrade</code> on backers</a></li>
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<li><a href="#get-the-new-code-to-all-validators">Get the new code to all validators</a></li>
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<li><a href="#on-chain-code-upgrade-process">On-chain code upgrade process</a></li>
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<li><a href="#handling-new-validators">Handling new validators</a></li>
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<li><a href="#how-do-other-parties-get-hold-of-the-pvf">How do other parties get hold of the PVF?</a></li>
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<li><a href="#pruning">Pruning</a></li>
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</ul>
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</li>
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<li><a href="#drawbacks">Drawbacks</a></li>
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<li><a href="#testing-security-and-privacy">Testing, Security, and Privacy</a></li>
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<li><a href="#performance-ergonomics-and-compatibility">Performance, Ergonomics, and Compatibility</a>
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<ul>
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<li><a href="#performance">Performance</a></li>
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<li><a href="#ergonomics">Ergonomics</a></li>
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<li><a href="#compatibility">Compatibility</a></li>
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</ul>
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</li>
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<li><a href="#prior-art-and-references">Prior Art and References</a></li>
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<li><a href="#unresolved-questions">Unresolved Questions</a></li>
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<li><a href="#future-directions-and-related-material">Future Directions and Related Material</a>
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<ul>
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<li><a href="#further-hardening">Further Hardening</a></li>
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<li><a href="#generalize-this-off-chain-storage-mechanism">Generalize this off-chain storage mechanism?</a></li>
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</ul>
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</li>
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</ul>
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</li>
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</ul>
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<h1 id="rfc-0000-feature-name-here"><a class="header" href="#rfc-0000-feature-name-here">RFC-0000: Feature Name Here</a></h1>
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<div class="table-wrapper"><table><thead><tr><th></th><th></th></tr></thead><tbody>
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<tr><td><strong>Start Date</strong></td><td>13 July 2024</td></tr>
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<tr><td><strong>Description</strong></td><td>Implement off-chain parachain runtime upgrades</td></tr>
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<tr><td><strong>Authors</strong></td><td>eskimor</td></tr>
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</tbody></table>
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</div>
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<h2 id="summary"><a class="header" href="#summary">Summary</a></h2>
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<p>Change the upgrade process of a parachain runtime upgrade to become an off-chain
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process with regards to the relay chain. Upgrades are still contained in
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parachain blocks, but will no longer need to end up in relay chain blocks nor in
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relay chain state.</p>
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<h2 id="motivation"><a class="header" href="#motivation">Motivation</a></h2>
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<p>Having parachain runtime upgrades go through the relay chain has always been
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seen as a scalability concern. Due to optimizations in statement
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distribution and asynchronous backing it became less crucial and got
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de-prioritized, the original issue can be found
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<a href="https://github.com/paritytech/polkadot-sdk/issues/971">here</a>.</p>
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<p>With the introduction of Agile Coretime and in general our efforts to reduce
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barrier to entry more for Polkadot more, the issue becomes more relevant again:
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We would like to reduce the required storage deposit for PVF registration, with
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the aim to not only make it cheaper to run a parachain (bulk + on-demand
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coretime), but also reduce the amount of capital required for the deposit. With
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this we would hope for far more parachains to get registered, thousands
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potentially even ten thousands. With so many PVFs registered, updates are
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expected to become more frequent and even attacks on service quality for other
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parachains would become a higher risk.</p>
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<h2 id="stakeholders"><a class="header" href="#stakeholders">Stakeholders</a></h2>
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<ul>
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<li>Parachain Teams</li>
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<li>Relay Chain Node implementation teams</li>
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<li>Relay Chain runtime developers</li>
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</ul>
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<h2 id="explanation"><a class="header" href="#explanation">Explanation</a></h2>
|
|
<p>The issues with on-chain runtime upgrades are:</p>
|
|
<ol>
|
|
<li>Needlessly costly.</li>
|
|
<li>A single runtime upgrade more or less occupies an entire relay chain block, thus it
|
|
might affect also other parachains, especially if their candidates are also
|
|
not negligible due to messages for example or they want to uprade their
|
|
runtime at the same time.</li>
|
|
<li>The signalling of the parachain to notify the relay chain of an upcoming
|
|
runtime upgrade already contains the upgrade. Therefore the only way to rate
|
|
limit upgrades is to drop an already distributed update in the size of
|
|
megabytes: With the result that the parachain missed a block and more
|
|
importantly it will try again with the very next block, until it finally
|
|
succeeds. If we imagine to reduce capacity of runtime upgrades to let's say 1
|
|
every 100 relay chain blocks, this results in lot's of wasted effort and lost
|
|
blocks.</li>
|
|
</ol>
|
|
<p>We discussed introducing a separate signalling before submitting the actual
|
|
runtime, but I think we should just go one step further and make upgrades fully
|
|
off-chain. Which also helps bringing down deposit costs in a secure way, as we
|
|
are also actually reducing costs for the network.</p>
|
|
<h3 id="introduce-a-new-ump-message-type-requestcodeupgrade"><a class="header" href="#introduce-a-new-ump-message-type-requestcodeupgrade">Introduce a new UMP message type <code>RequestCodeUpgrade</code></a></h3>
|
|
<p>As part of elastic scaling we are already planning to increase flexibility of <a href="https://github.com/polkadot-fellows/RFCs/issues/92#issuecomment-2144538974">UMP
|
|
messages</a>, we can now use this to our advantage and introduce another UMP message:</p>
|
|
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>enum UMPSignal {
|
|
// For elastic scaling
|
|
OnCore(CoreIndex),
|
|
// For off-chain upgrades
|
|
RequestCodeUpgrade(Hash),
|
|
}
|
|
<span class="boring">}</span></code></pre></pre>
|
|
<p>We could also make that new message a regular XCM, calling an extrinsic on the
|
|
relay chain, but we will want to look into that message right after validation
|
|
on the backers on the node side, making a straight forward semantic message more
|
|
apt for the purpose.</p>
|
|
<h3 id="handle-requestcodeupgrade-on-backers"><a class="header" href="#handle-requestcodeupgrade-on-backers">Handle <code>RequestCodeUpgrade</code> on backers</a></h3>
|
|
<p>We will introduce a new request/response protocol for both collators and
|
|
validators, with the following request/response:</p>
|
|
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>struct RequestBlob {
|
|
blob_hash: Hash,
|
|
}
|
|
|
|
struct BlobResponse {
|
|
blob: Vec<u8>
|
|
}
|
|
<span class="boring">}</span></code></pre></pre>
|
|
<p>This protocol will be used by backers to request the PVF from collators in the
|
|
following conditions:</p>
|
|
<ol>
|
|
<li>They received a collation sending <code>RequestCodeUpgrade</code>.</li>
|
|
<li>They received a collation, but they don't yet have the code that was
|
|
previously registered on the relaychain. (E.g. disk pruned, new validator)</li>
|
|
</ol>
|
|
<p>In case they received the collation via PoV distribution instead of from the
|
|
collator itself, they will use the exact same message to fetch from the valiator
|
|
they got the PoV from.</p>
|
|
<h3 id="get-the-new-code-to-all-validators"><a class="header" href="#get-the-new-code-to-all-validators">Get the new code to all validators</a></h3>
|
|
<p>Once the candidate issuing <code>RequestCodeUpgrade</code> got backed on chain, validators
|
|
will start fetching the code from the backers as part of availability
|
|
distribution.</p>
|
|
<p>To mitigate attack vectors we should make sure that serving requests for code
|
|
can be treated as low priority requests. Thus I am suggesting the following
|
|
scheme:</p>
|
|
<p>Validators will notice via a runtime API (TODO: Define) that a new code has been requested, the
|
|
API will return the <code>Hash</code> and a counter, which starts at some configurable
|
|
value e.g. 10. The validators are now aware of the new hash and start fetching,
|
|
but they don't have to wait for the fetch to succeed to sign their bitfield.</p>
|
|
<p>Then on each further candidate from that chain that counter gets decremented.
|
|
Validators which have not yet succeeded fetching will now try again. This game
|
|
continues until the counter reached <code>0</code>. Now it is mandatory to have to code in
|
|
order to sign a <code>1</code> in the bitfield.</p>
|
|
<p>PVF pre-checking will happen after the candidate which brought the counter to
|
|
<code>0</code> has been successfully included and thus is also able to assume that 2/3 of
|
|
the validators have the code.</p>
|
|
<p>This scheme serves two purposes:</p>
|
|
<ol>
|
|
<li>Fetching can happen over a longer period of time with low priority. E.g. if
|
|
we waited for the PVF at the very first avaialbility distribution, this might
|
|
actually affect liveness of other chains on the same core. Distributing
|
|
megabytes of data to a thousand validators, might take a bit. Thus this helps
|
|
isolating parachains from each other.</li>
|
|
<li>By configuring the initial counter value we can affect how much an upgrade
|
|
costs. E.g. forcing the parachain to produce 10 blocks, means 10x the cost
|
|
for issuing an update. If too frequent upgrades ever become a problem for the
|
|
system, we have a knob to make them more costly.</li>
|
|
</ol>
|
|
<h3 id="on-chain-code-upgrade-process"><a class="header" href="#on-chain-code-upgrade-process">On-chain code upgrade process</a></h3>
|
|
<p>First when a candidate is backed we need to make the new hash available
|
|
(together with a counter) via a
|
|
runtime API so validators in availability distribution can check for it and
|
|
fetch it if changed (see previous section). For performance reasons, I think we
|
|
should not do an additional call, but replace the <a href="https://github.com/paritytech/polkadot-sdk/blob/d2fd53645654d3b8e12cbf735b67b93078d70113/polkadot/node/subsystem-util/src/runtime/mod.rs#L355">existing one</a> with one containing the new additional information (Option<(Hash, Counter)>).</p>
|
|
<p>Once the candidate gets included (counter 0), the hash is given to pre-checking
|
|
and only after pre-checking succeeded (and a full session passed) it is finally
|
|
enacted and the parachain can switch to the new code. (Same process as it used
|
|
to be.)</p>
|
|
<h3 id="handling-new-validators"><a class="header" href="#handling-new-validators">Handling new validators</a></h3>
|
|
<h4 id="backers"><a class="header" href="#backers">Backers</a></h4>
|
|
<p>If a backer receives a collation for a parachain it does not yet have the code
|
|
as enacted on chain (see "On-chain code upgrade process"), it will use above
|
|
request/response protocol to fetch it from whom it received the collation.</p>
|
|
<h4 id="availablity-distribution"><a class="header" href="#availablity-distribution">Availablity Distribution</a></h4>
|
|
<p>Validators in availability distribution will be changed to only sign a <code>1</code> in
|
|
the bitfield of a candidate if they not only have the chunk, but also the
|
|
currently active PVF. They will fetch it from backers in case they don't have it
|
|
yet.</p>
|
|
<h3 id="how-do-other-parties-get-hold-of-the-pvf"><a class="header" href="#how-do-other-parties-get-hold-of-the-pvf">How do other parties get hold of the PVF?</a></h3>
|
|
<p>Two ways:</p>
|
|
<ol>
|
|
<li>Discover collators via <a href="https://github.com/polkadot-fellows/RFCs/pull/8">relay chain DHT</a> and request from them: Preferred way,
|
|
as it is less load on validators.</li>
|
|
<li>Request from validators, which will serve on a best effort basis.</li>
|
|
</ol>
|
|
<h3 id="pruning"><a class="header" href="#pruning">Pruning</a></h3>
|
|
<p>We covered how validators get hold of new code, but when can they prune old ones?
|
|
In principle it is not an issue, if some validors prune code, because:</p>
|
|
<ol>
|
|
<li>We changed it so that a candidate is not deemed available if validators were
|
|
not able to fetch the PVF.</li>
|
|
<li>Backers can always fetch the PVF from collators as part of the collation
|
|
fetching.</li>
|
|
</ol>
|
|
<p>But the majority of validators should always keep the latest code of any
|
|
parachain and only prune the previous one, once the first candidate using the
|
|
new code got finalized. This ensures that disputes will always be able to
|
|
resolve.</p>
|
|
<h2 id="drawbacks"><a class="header" href="#drawbacks">Drawbacks</a></h2>
|
|
<p>The major drawback of this solution is the same as any solution the moves work
|
|
off-chain, it adds complexity to the node. E.g. nodes needing the PVF, need to
|
|
store them separately, together with their own pruning strategy as well.</p>
|
|
<h2 id="testing-security-and-privacy"><a class="header" href="#testing-security-and-privacy">Testing, Security, and Privacy</a></h2>
|
|
<p>Implementations adhering to this RFC, will respond to PVF requests with the
|
|
actual PVF, if they have it. Requesters will persist received PVFs on disk for
|
|
as long as they are replaced by a new one. Implementations must not be lazy
|
|
here, if validators only fetched the PVF when needed, they can be prevented from
|
|
participating in disputes.</p>
|
|
<p>Validators should treat incoming requests for PVFs in general with rather low
|
|
priority, but should prefer fetches from other validators over requests from
|
|
random peers.</p>
|
|
<p>Given that we are altering what set bits in the availability bitfields mean (not
|
|
only chunk, but also PVF available), it is important to have enough validators
|
|
upgraded, before we allow collators to make use of the new runtime upgrade
|
|
mechanism. Otherwise we would risk disputes to not being able to succeed.</p>
|
|
<p>This RFC has no impact on privacy.</p>
|
|
<h2 id="performance-ergonomics-and-compatibility"><a class="header" href="#performance-ergonomics-and-compatibility">Performance, Ergonomics, and Compatibility</a></h2>
|
|
<h3 id="performance"><a class="header" href="#performance">Performance</a></h3>
|
|
<p>This proposal lightens the load on the relay chain and is thus in general
|
|
beneficial for the performance of the network, this is achieved by the
|
|
following:</p>
|
|
<ol>
|
|
<li>Code upgrades are still propagated to all validators, but only once, not
|
|
twice (First statements, then via the containing relay chain block).</li>
|
|
<li>Code upgrades are only communicated to validators and other nodes which are
|
|
interested, not any full node as it has been before.</li>
|
|
<li>Relay chain block space is preserved. Previously we could only do one runtime
|
|
upgrade per relay chain block, occupying almost all of the blockspace.</li>
|
|
<li>Signalling an upgrade no longer contains the upgrade, hence if we need to
|
|
push back on an upgrade for whatever reason, no network bandwidth and core
|
|
time gets wasted because of this.</li>
|
|
</ol>
|
|
<h3 id="ergonomics"><a class="header" href="#ergonomics">Ergonomics</a></h3>
|
|
<p>End users are only affected by better performance and more stable block times.
|
|
Parachains will need to implement the introduced request/response protocol and
|
|
adapt to the new signalling mechanism via an <code>UMP</code> message, instead of sending
|
|
the code upgrade directly.</p>
|
|
<p>For parachain operators we should emit events on initiated runtime upgrade and
|
|
each block reporting the current counter and how many blocks to go until the
|
|
upgrade gets passed to pre-checking. This is especially important for on-demand
|
|
chains or bulk users not occupying a full core. Further more that behaviour of
|
|
requiring multiple blocks to fully initiate a runtime upgrade needs to be well
|
|
documented.</p>
|
|
<h3 id="compatibility"><a class="header" href="#compatibility">Compatibility</a></h3>
|
|
<p>We will continue to support the old mechanism for code upgrades for a while, but
|
|
will start to impose stricter limits over time, with the number of registered
|
|
parachains going up. With those limits in place parachains not migrating to the
|
|
new scheme might be having a harder time upgrading and will miss more blocks. I
|
|
guess we can be lenient for a while still, so the upgrade path for
|
|
parachains should be rather smooth.</p>
|
|
<p>In total the protocol changes we need are:</p>
|
|
<p>For validators and collators:</p>
|
|
<ol>
|
|
<li>New request/response protocol for fetching PVF data from collators and
|
|
validators.</li>
|
|
<li>New UMP message type for signalling a runtime upgrade.</li>
|
|
</ol>
|
|
<p>Only for validators:</p>
|
|
<ol>
|
|
<li>New runtime API for determining to be enacted code upgrades.</li>
|
|
<li>Different behaviour of bitfields (only sign a 1 bit, if validator has chunk +
|
|
"hot" PVF).</li>
|
|
<li>Altered behaviour in availability-distribution: Fetch missing PVFS.</li>
|
|
</ol>
|
|
<h2 id="prior-art-and-references"><a class="header" href="#prior-art-and-references">Prior Art and References</a></h2>
|
|
<p>Off-chain runtime upgrades have been discussed before, the architecture
|
|
described here is simpler though as it piggybacks on already existing features,
|
|
namely:</p>
|
|
<ol>
|
|
<li>availability-distribution: No separate <code>I have code</code> messages anymore.</li>
|
|
<li>Existing pre-checking.</li>
|
|
</ol>
|
|
<p>https://github.com/paritytech/polkadot-sdk/issues/971</p>
|
|
<h2 id="unresolved-questions"><a class="header" href="#unresolved-questions">Unresolved Questions</a></h2>
|
|
<ol>
|
|
<li>What about the initial runtime, shall we make that off-chain as well?</li>
|
|
<li>Good news, at least after the first upgrade, no code will be stored on chain
|
|
any more, this means that we also have to redefine the storage deposit now.
|
|
We no longer charge for chain storage, but validator disk storage -> Should
|
|
be cheaper. Solution to this: Not only store the hash on chain, but also the
|
|
size of the data. Then define a price per byte and charge that, but:
|
|
<ul>
|
|
<li>how do we charge - I guess deposit has to be provided via other means,
|
|
runtime upgrade fails if not provided.</li>
|
|
<li>how do we signal to the chain that the code is too large for it to reject
|
|
the upgrade? Easy: Make available and vote nay in pre-checking.</li>
|
|
</ul>
|
|
</li>
|
|
</ol>
|
|
<p>TODO: Fully resolve these questions and incorporate in RFC text.</p>
|
|
<h2 id="future-directions-and-related-material"><a class="header" href="#future-directions-and-related-material">Future Directions and Related Material</a></h2>
|
|
<h3 id="further-hardening"><a class="header" href="#further-hardening">Further Hardening</a></h3>
|
|
<p>By no longer having code upgrade go through the relay chain, occupying a full relay
|
|
chain block, the impact on other parachains is already greatly reduced, if we
|
|
make distribution and PVF pre-checking low-priority processes on validators. The
|
|
only thing attackers might be able to do is delay upgrades of other parachains.</p>
|
|
<p>Which seems like a problem to be solved once we actually see it as a problem in
|
|
the wild (and can already be mitigated by adjusting the counter). The good thing
|
|
is that we have all the ingredients to go further if need be. Signalling no
|
|
longer actually includes the code, hence there is no need to reject the
|
|
candidate: The parachain can make progress even if we choose not to immediately
|
|
act on the request and no relay chain resources are wasted either.</p>
|
|
<p>We could for example introduce another UMP Signalling message
|
|
<code>RequestCodeUpgradeWithPriority</code> which not just requests a code upgrade, but
|
|
also offers some DOT to get ranked up in a queue.</p>
|
|
<h3 id="generalize-this-off-chain-storage-mechanism"><a class="header" href="#generalize-this-off-chain-storage-mechanism">Generalize this off-chain storage mechanism?</a></h3>
|
|
<p>Making this storage mechanism more general purpose is worth thinking about. E.g.
|
|
by resolving above "fee" question, we might also be able to resolve the pruning
|
|
question in a more generic way and thus could indeed open this storage facility
|
|
for other purposes as well. E.g. smart contracts, so the PoV would only need to
|
|
reference contracts by hash and the actual PoV is stored on validators and
|
|
collators and thus no longer needs to be part of the PoV.</p>
|
|
<p>A possible avenue would be to change the response to:</p>
|
|
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
|
|
</span><span class="boring">fn main() {
|
|
</span>enum BlobResponse {
|
|
Blob(Vec<u8>),
|
|
Blobs(MerkleTree),
|
|
}
|
|
<span class="boring">}</span></code></pre></pre>
|
|
<p>With this the hash specified in the request can also be a merkle root and the
|
|
responder will respond with the entire merkle tree (only hashes, no payload).
|
|
Then the requester can traverse the leaf hashes and use the same request
|
|
response protocol to request any locally missing blobs in that tree.</p>
|
|
<p>One leaf would for example be the PVF others could be smart contracts. With a
|
|
properly specified format (e.g. which leaf is the PVF?), what we got here is
|
|
that a parachain can not only update its PVF, but additional data,
|
|
incrementally. E.g. adding another smart contract, does not require resubmitting
|
|
the entire PVF to validators, only the root hash on the relay chain gets
|
|
updated, then validators fetch the merkle tree and only fetch any missing
|
|
leaves. That additional data could be made available to the PVF via a to be
|
|
added host function. The nice thing about this approach is, that while we can
|
|
upgrade incrementally, lifetime is still tied to the PVF and we get all the same
|
|
guarantees. Assuming the validators store blobs by hash, we even get disk
|
|
sharing if multiple parachains use the same data (e.g. same smart contracts).</p>
|
|
|
|
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