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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" class="active">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">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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<h1 class="menu-title">Polkadot Fellowship RFCs</h1>
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<p><a href="https://github.com/polkadot-fellows/RFCs/blob/main/text/0007-system-collator-selection.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-0007-system-collator-selection">RFC-0007: System Collator Selection</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>
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<ul>
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<li><a href="#requirements">Requirements</a></li>
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</ul>
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</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="#set-size">Set Size</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>
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<ul>
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<li><a href="#written-discussions">Written Discussions</a></li>
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<li><a href="#prior-feedback-and-input-from">Prior Feedback and Input From</a></li>
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</ul>
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</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-0007-system-collator-selection"><a class="header" href="#rfc-0007-system-collator-selection">RFC-0007: System Collator Selection</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>07 July 2023</td></tr>
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<tr><td><strong>Description</strong></td><td>Mechanism for selecting collators of system chains.</td></tr>
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<tr><td><strong>Authors</strong></td><td>Joe Petrowski</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>As core functionality moves from the Relay Chain into system chains, so increases the reliance on
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the liveness of these chains for the use of the network. It is not economically scalable, nor
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necessary from a game-theoretic perspective, to pay collators large rewards. This RFC proposes a
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mechanism -- part technical and part social -- for ensuring reliable collator sets that are
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resilient to attemps to stop any subsytem of the Polkadot protocol.</p>
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<h2 id="motivation"><a class="header" href="#motivation">Motivation</a></h2>
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<p>In order to guarantee access to Polkadot's system, the collators on its system chains must propose
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blocks (provide liveness) and allow all transactions to eventually be included. That is, some
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collators may censor transactions, but there must exist one collator in the set who will include a
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given transaction. In fact, all collators may censor varying subsets of transactions, but as long
|
|
as no transaction is in the intersection of every subset, it will eventually be included. The
|
|
objective of this RFC is to propose a mechanism to select such a set on each system chain.</p>
|
|
<p>While the network as a whole uses staking (and inflationary rewards) to attract validators,
|
|
collators face different challenges in scale and have lower security assumptions than validators.
|
|
Regarding scale, there exist many system chains, and it is economically expensive to pay collators
|
|
a premium. Likewise, any staked DOT for collation is <em>not</em> staked for validation. Since collator
|
|
sets do not need to meet Byzantine Fault Tolerance criteria, staking as the primary mechanism for
|
|
collator selection would remove stake that is securing BFT assumptions, making the network less
|
|
secure.</p>
|
|
<p>Another problem with economic scalability relates to the increasing number of system chains, and
|
|
corresponding increase in need for collators (i.e., increase in collator slots). "Good" (highly
|
|
available, non-censoring) collators will not want to compete in elections on many chains when they
|
|
could use their resources to compete in the more profitable validator election. Such dilution
|
|
decreases the required bond on each chain, leaving them vulnerable to takeover by hostile
|
|
collator groups.</p>
|
|
<p>This RFC proposes a system whereby collation is primarily an infrastructure service, with the
|
|
on-chain Treasury reimbursing costs of semi-trusted node operators, referred to as "Invulnerables".
|
|
The system need not trust the individual operators, only that as a <em>set</em> they would be resilient to
|
|
coordinated attempts to stop a single chain from halting or to censor a particular subset of
|
|
transactions.</p>
|
|
<p>In the case that users do not trust this set, this RFC also proposes that each chain always have
|
|
available collator positions that can be acquired by anyone by placing a bond.</p>
|
|
<h3 id="requirements"><a class="header" href="#requirements">Requirements</a></h3>
|
|
<ul>
|
|
<li>System MUST have at least one valid collator for every chain.</li>
|
|
<li>System MUST allow anyone to become a collator, provided they <code>reserve</code>/<code>hold</code> enough DOT.</li>
|
|
<li>System SHOULD select a set of collators with reasonable expectation that the set will not collude
|
|
to censor any subset of transactions.</li>
|
|
<li>Collators selected by governance SHOULD have a reasonable expectation that the Treasury will
|
|
reimburse their operating costs.</li>
|
|
</ul>
|
|
<h2 id="stakeholders"><a class="header" href="#stakeholders">Stakeholders</a></h2>
|
|
<ul>
|
|
<li>Infrastructure providers (people who run validator/collator nodes)</li>
|
|
<li>Polkadot Treasury</li>
|
|
</ul>
|
|
<h2 id="explanation"><a class="header" href="#explanation">Explanation</a></h2>
|
|
<p>This protocol builds on the existing
|
|
<a href="https://github.com/paritytech/cumulus/tree/b15da70/pallets/collator-selection">Collator Selection pallet</a>
|
|
and its notion of Invulnerables. Invulnerables are collators (identified by their <code>AccountId</code>s) who
|
|
will be selected as part of the collator set every session. Operations relating to the management
|
|
of the Invulnerables are done through privileged, governance origins. The implementation should
|
|
maintain an API for adding and removing Invulnerable collators.</p>
|
|
<p>In addition to Invulnerables, there are also open slots for "Candidates". Anyone can register as a
|
|
Candidate by placing a fixed bond. However, with a fixed bond and fixed number of slots, there is
|
|
an obvious selection problem: The slots fill up without any logic to replace their occupants.</p>
|
|
<p>This RFC proposes that the collator selection protocol allow Candidates to increase (and decrease)
|
|
their individual bonds, sort the Candidates according to bond, and select the top <code>N</code> Candidates.
|
|
The selection and changeover should be coordinated by the session manager.</p>
|
|
<p>A FRAME pallet already exists for sorting ("bagging") "top N" groups, the
|
|
<a href="https://github.com/paritytech/substrate/blob/5032b8d/frame/bags-list/src/lib.rs">Bags List pallet</a>.
|
|
This pallet's <code>SortedListProvider</code> should be integrated into the session manager of the Collator
|
|
Selection pallet.</p>
|
|
<p>Despite the lack of apparent economic incentives (i.e., inflation), several reasons exist why one
|
|
may want to bond funds to participate in the Candidates election, for example:</p>
|
|
<ul>
|
|
<li>They want to build credibility to be selected as Invulnerable;</li>
|
|
<li>They want to ensure availability of an application, e.g. a stablecoin issuer might run a collator
|
|
on Asset Hub to ensure transactions in its asset are included in blocks;</li>
|
|
<li>They fear censorship themselves, e.g. a voter might think their votes are being censored from
|
|
governance, so they run a collator on the governance chain to include their votes.</li>
|
|
</ul>
|
|
<p>Unlike the fixed-bond mechanism that fills up its Candidates, the election mechanism ensures that
|
|
anyone can join the collator set by placing the <code>Nth</code> highest bond.</p>
|
|
<h3 id="set-size"><a class="header" href="#set-size">Set Size</a></h3>
|
|
<p>In order to achieve the requirements listed under <em>Motivation</em>, it is reasonable to have
|
|
approximately:</p>
|
|
<ul>
|
|
<li>20 collators per system chain,</li>
|
|
<li>of which 15 are Invulnerable, and</li>
|
|
<li>five are elected by bond.</li>
|
|
</ul>
|
|
<h2 id="drawbacks"><a class="header" href="#drawbacks">Drawbacks</a></h2>
|
|
<p>The primary drawback is a reliance on governance for continued treasury funding of infrastructure
|
|
costs for Invulnerable collators.</p>
|
|
<h2 id="testing-security-and-privacy"><a class="header" href="#testing-security-and-privacy">Testing, Security, and Privacy</a></h2>
|
|
<p>The vast majority of cases can be covered by unit testing. Integration test should ensure that the
|
|
Collator Selection <code>UpdateOrigin</code>, which has permission to modify the Invulnerables and desired
|
|
number of Candidates, can handle updates over XCM from the system's governance location.</p>
|
|
<h2 id="performance-ergonomics-and-compatibility"><a class="header" href="#performance-ergonomics-and-compatibility">Performance, Ergonomics, and Compatibility</a></h2>
|
|
<p>This proposal has very little impact on most users of Polkadot, and should improve the performance
|
|
of system chains by reducing the number of missed blocks.</p>
|
|
<h3 id="performance"><a class="header" href="#performance">Performance</a></h3>
|
|
<p>As chains have strict PoV size limits, care must be taken in the PoV impact of the session manager.
|
|
Appropriate benchmarking and tests should ensure that conservative limits are placed on the number
|
|
of Invulnerables and Candidates.</p>
|
|
<h3 id="ergonomics"><a class="header" href="#ergonomics">Ergonomics</a></h3>
|
|
<p>The primary group affected is Candidate collators, who, after implementation of this RFC, will need
|
|
to compete in a bond-based election rather than a race to claim a Candidate spot.</p>
|
|
<h3 id="compatibility"><a class="header" href="#compatibility">Compatibility</a></h3>
|
|
<p>This RFC is compatible with the existing implementation and can be handled via upgrades and
|
|
migration.</p>
|
|
<h2 id="prior-art-and-references"><a class="header" href="#prior-art-and-references">Prior Art and References</a></h2>
|
|
<h3 id="written-discussions"><a class="header" href="#written-discussions">Written Discussions</a></h3>
|
|
<ul>
|
|
<li><a href="https://github.com/paritytech/roadmap/issues/34">GitHub: Collator Selection Roadmap</a></li>
|
|
<li><a href="https://github.com/paritytech/cumulus/issues/1159">GitHub: Revisit Collator Selection Mechanism</a></li>
|
|
<li><a href="https://forum.polkadot.network/t/economic-model-for-system-para-collators/1010">Polkadot Forum: Economic Model for System Para Collators</a></li>
|
|
</ul>
|
|
<h3 id="prior-feedback-and-input-from"><a class="header" href="#prior-feedback-and-input-from">Prior Feedback and Input From</a></h3>
|
|
<ul>
|
|
<li>Kian Paimani</li>
|
|
<li>Jeff Burdges</li>
|
|
<li>Rob Habermeier</li>
|
|
<li>SR Labs Auditors</li>
|
|
<li>Current collators including Paranodes, Stake Plus, Turboflakes, Peter Mensik, SIK, and many more.</li>
|
|
</ul>
|
|
<h2 id="unresolved-questions"><a class="header" href="#unresolved-questions">Unresolved Questions</a></h2>
|
|
<p>None at this time.</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>There may exist in the future system chains for which this model of collator selection is not
|
|
appropriate. These chains should be evaluated on a case-by-case basis.</p>
|
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