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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="spacer"></li><li class="chapter-item expanded affix "><li class="part-title">Proposed</li><li class="chapter-item expanded "><a href="../proposed/0150-voting-while-delegating.html">RFC-150: Allow Voting While Delegating</a></li><li class="chapter-item expanded "><a href="../proposed/0154-multi-slot-aura.html">RFC-0154: AURA Multi-Slot Collation </a></li><li class="chapter-item expanded "><a href="../proposed/0155-pUSD.html">RFC-0155: pUSD (Polkadot USD over-collateralised debt token)</a></li><li class="chapter-item expanded "><a href="../proposed/0156-bls-signatures.html">RFC-0156: Add BLS12-381 Host Functions</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/0017-coretime-market-redesign.html">RFC-0017: Coretime Market Redesign</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="chapter-item expanded "><a href="../approved/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="../approved/0125-xcm-asset-metadata.html">RFC-0125: XCM Asset Metadata</a></li><li class="chapter-item expanded "><a href="../approved/0126-introduce-pvq.html">RFC-0126: Introduce PVQ (PolkaVM Query)</a></li><li class="chapter-item expanded "><a href="../approved/0135-compressed-blob-prefixes.html" class="active">RFC-0135: Compressed Blob Prefixes</a></li><li class="chapter-item expanded "><a href="../approved/0139-faster-erasure-coding.html">RFC-0139: Faster Erasure Coding</a></li><li class="chapter-item expanded "><a href="../approved/0146-deflationary-fee-proposal.html">RFC-0146: Deflationary Transaction Fee Model for the Relay Chain and its System Parachains</a></li><li class="chapter-item expanded "><a href="../approved/0149-rfc-1-renewal-adjustment.html">RFC-0149: Renewal Adjustment</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-pre-elves_soft.html">RFC-0000: Pre-ELVES soft concensus</a></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/0004-remove-unnecessary-allocator-usage.html">RFC-0004: Remove the host-side runtime memory allocator</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/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">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/0124-extrinsic-version-5.html">RFC-0124: Extrinsic version 5</a></li><li class="chapter-item expanded "><a href="../stale/0138-invulnerable-collator-election.html">RFC-0138: Election mechanism for invulnerable collators on system chains</a></li><li class="chapter-item expanded "><a href="../stale/0152-decentralized-convex-preference-coretime-market-for-polkadot.html">RFC-0152: Decentralized Convex-Preference Coretime Market for Polkadot</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><li class="chapter-item expanded "><a href="../stale/xxxx-improve-the-security-of-proof-of-possession.html">RFC-XXXX: Adding customized mandatory context to proof of possession statement</a></li></ol>
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<p><a href="https://github.com/polkadot-fellows/RFCs/blob/main/text/0135-compressed-blob-prefixes.md">(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-0135-compressed-blob-prefixes">RFC-0135: Compressed Blob Prefixes</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="#overview">Overview</a></li>
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<li><a href="#timeline">Timeline</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></li>
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</ul>
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</li>
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</ul>
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<h1 id="rfc-0135-compressed-blob-prefixes"><a class="header" href="#rfc-0135-compressed-blob-prefixes">RFC-0135: Compressed Blob Prefixes</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>2025-01-06</td></tr>
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<tr><td><strong>Description</strong></td><td>Standardize compressed blob prefixes</td></tr>
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<tr><td><strong>Authors</strong></td><td>s0me0ne-unkn0wn (13WGadgNgqSjiGQvfhimw9pX26mvGdYQ6XgrjPANSEDRoGMt)</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>This RFC proposes a change that makes it possible to identify types of compressed blobs stored on-chain, as well as used off-chain, without the need for decompression.</p>
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<h2 id="motivation"><a class="header" href="#motivation">Motivation</a></h2>
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<p>Currently, a compressed blob does not give any idea of what's inside because the only thing that can be inside, according to the spec, is Wasm. In reality, other blob types are already being used, and more are to come. Apart from being error-prone by itself, the current approach does not allow to properly route the blob through the execution paths before its decompression, which will result in suboptimal implementations when more blob types are used. Thus, it is necessary to introduce a mechanism allowing to identify the blob type without decompressing it.</p>
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<p>This proposal is intended to support future work enabling Polkadot to execute PolkaVM and, more generally, other-than-Wasm parachain runtimes, and allow developers to introduce arbitrary compression methods seamlessly in the future.</p>
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<h2 id="stakeholders"><a class="header" href="#stakeholders">Stakeholders</a></h2>
|
||
<p>Node developers are the main stakeholders for this proposal. It also creates a foundation on which parachain runtime developers will build.</p>
|
||
<h2 id="explanation"><a class="header" href="#explanation">Explanation</a></h2>
|
||
<h3 id="overview"><a class="header" href="#overview">Overview</a></h3>
|
||
<p>The current approach to compressing binary blobs involves using <code>zstd</code> compression, and the resulting compressed blob is prefixed with a unique 64-bit magic value specified in that subsection. The same procedure is used to compress both Wasm code blobs and proofs-of-validity. Currently, having solely a compressed blob, it's impossible to tell what's inside it without decompression, a Wasm blob, or a PoV. That doesn't cause problems in the current protocol, as Wasm blobs and PoV blobs take completely different execution paths in the code.</p>
|
||
<p>The changes proposed below are intended to define the means for distinguishing compressed blob types in a backward-compatible and future-proof way.</p>
|
||
<p>It is proposed to introduce an open list of 64-bit prefixes, each representing a compressed blob of a specific type compressed with a specific compression method. The currently used prefix becomes deprecated and will be removed or reused when it is no longer in use.</p>
|
||
<p>The proposed list of prefixes to support the current as well as currently known future work follows:</p>
|
||
<div class="table-wrapper"><table><thead><tr><th>Prefix name</th><th>Prefix bytes</th><th>Description</th></tr></thead><tbody>
|
||
<tr><td><code>CBLOB_ZSTD_LEGACY</code></td><td>82, 188, 83, 118, 70, 219, 142, 5</td><td>Wasm code blob or PoV, zstd-compressed</td></tr>
|
||
<tr><td><code>CBLOB_ZSTD_POV</code></td><td>82, 188, 83, 118, 70, 219, 142, 6</td><td>Proof-of-validity, zstd-compressed</td></tr>
|
||
<tr><td><code>CBLOB_ZSTD_WASM_CODE</code></td><td>82, 188, 83, 118, 70, 219, 142, 7</td><td>Wasm code blob, zstd-compressed</td></tr>
|
||
<tr><td><code>CBLOB_ZSTD_PVM_CODE</code></td><td>82, 188, 83, 118, 70, 219, 142, 8</td><td>PolkaVM code blob, zstd-compressed</td></tr>
|
||
</tbody></table>
|
||
</div>
|
||
<p>No runtime code changes should be needed to imnplement this proposal. Node-side changes are trivial; a PoC already implemented as a part of <a href="https://github.com/paritytech/polkadot-sdk/pull/6704">SDK PR#6704</a> may be used as an example.</p>
|
||
<h3 id="timeline"><a class="header" href="#timeline">Timeline</a></h3>
|
||
<ol>
|
||
<li>The proposed prefix changes are implemented and released. No logic changes yet;</li>
|
||
<li>After the supermajority of production networks' nodes upgrades, one more change is released that adds <code>CBLOB_ZSTD_WASM_CODE</code> prefix instead of <code>CBLOB_ZSTD_LEGACY</code> when compiling and compressing Wasm parachain runtimes, and <code>CBLOB_ZSTD_POV</code> instead of <code>CBLOB_ZSTD_LEGACY</code> when compressing PoVs;</li>
|
||
<li>Conservatively, wait until no more PVFs prefixed with <code>CBLOB_ZSTD_LEGACY</code> remain on-chain. That may take quite some time. Alternatively, create a migration that alters prefixes of existing blobs;</li>
|
||
<li>Removing <code>CBLOB_ZSTD_LEGACY</code> prefix will be possible after all the nodes in all the networks cease using the prefix which is a long process, and additional incentives should be offered to the community to make people upgrade.</li>
|
||
</ol>
|
||
<h2 id="drawbacks"><a class="header" href="#drawbacks">Drawbacks</a></h2>
|
||
<p>Currently, the only requirement for a compressed blob prefix is not to coincide with Wasm magic bytes (as stated in code comments). Changes proposed here increase prefix collision risk, given that arbitrary data may be compressed in the future. However, it must be taken into account that:</p>
|
||
<ul>
|
||
<li>Collision probability per arbitrary blob is ≈5,4×10⁻²⁰ for a single random 64-bit prefix (current situation) and ≈2,17×10⁻¹⁹ for the proposed set of four 64-bit prefixes (proposed situation), which is still low enough;</li>
|
||
<li>The current de facto protocol uses the current compression implementation to compress PoVs, which are arbitrary binary data, so the collision risk already exists and is not introduced by changes proposed here.</li>
|
||
</ul>
|
||
<h2 id="testing-security-and-privacy"><a class="header" href="#testing-security-and-privacy">Testing, Security, and Privacy</a></h2>
|
||
<p>As the change increases granularity, it will positively affect both testing possibilities and security, allowing developers to check what's inside a given compressed blob precisely. Testing the change itself is trivial. Privacy is not affected by this change.</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>The current implementation's performance is not affected by this change. Future implementations allowing for the execution of other-than-Wasm parachain runtimes will benefit from this change performance-wise.</p>
|
||
<h3 id="ergonomics"><a class="header" href="#ergonomics">Ergonomics</a></h3>
|
||
<p>The end-user ergonomics is not affected. The ergonomics for developers will benefit from this change as it enables exact checks and less guessing.</p>
|
||
<h3 id="compatibility"><a class="header" href="#compatibility">Compatibility</a></h3>
|
||
<p>The change is designed to be backward-compatible. </p>
|
||
<h2 id="prior-art-and-references"><a class="header" href="#prior-art-and-references">Prior Art and References</a></h2>
|
||
<p><a href="https://github.com/paritytech/polkadot-sdk/pull/6704">SDK PR#6704</a> (WIP) introduces a mechanism similar to that described in this proposal and proves the necessity of such a change.</p>
|
||
<h2 id="unresolved-questions"><a class="header" href="#unresolved-questions">Unresolved Questions</a></h2>
|
||
<p>None</p>
|
||
<h2 id="future-directions-and-related-material"><a class="header" href="#future-directions-and-related-material">Future Directions and Related Material</a></h2>
|
||
<p>This proposal creates a foundation for two future work directions:</p>
|
||
<ul>
|
||
<li>Proposing to introduce other-than-Wasm code executors, including PolkaVM, allowing parachain runtime authors to seamlessly change execution platform using the existing mechanism of runtime upgrades;</li>
|
||
<li>Proposing to use arbitrary compression methods in cases when they make sense from the point of view of efficiency and/or performance, including blob-type-specific compression approaches, streamlining the adoption of such methods and deprecation of the old ones.</li>
|
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