Elastic Scaling (#5591)

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docs/learn/learn-elastic-scaling.md

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+---
+id: learn-elastic-scaling
+title: Polkadot's Elastic Scaling
+sidebar_label: Elastic Scaling
+description: Enabling parachains to scale on-demand through instantaneous coretime.
+keywords: [elastic scaling, parachains, coretime, blockspace]
+slug: ../learn-elastic-scaling
+---
+
+The path of parablocks from their creation to their inclusion into the relay chain (discussed in the
+[Parachain Protocol Page](./learn-parachains-protocol.md)) spans two domains: the parachain's and
+relay chain's. Scaling the Polkadot protocol involves consideration of how parablocks are produced
+by the parachain and then validated, processed, secured, made available for additional checks, and
+finally included on the relay chain.
+
+[Asynchronous backing](./learn-async-backing.md) is the optimization implemented on the relay chain
+that allows parachains to produce blocks faster and allows relay chain to process them seamlessly.
+Asynchronous backing also improves the parachain side with unincluded segments and augmented info
+that allows collators to produce multiple parablocks even if the previous blocks are not yet
+included. This upgrade allows parachains to utilize up to 2 seconds execution time per parablock,
+and the relay chain will be able to include a parablock every 6 seconds.
+
+With elastic scaling, parachains can use multiple cores to include multiple parablocks within the
+same relay chain block.
+
+The relay chain receives a sequence of parachain blocks on multiple cores, which are validated and
+checked if all their state roots line up during their inclusion, but assume they’re unrelated
+parachain blocks during backing, availability, and approvals. With elastic scaling implemented, a
+parachain's throughput depends upon its collator infrastructure.
+
+The [elastic scaling implementation](https://github.com/paritytech/polkadot-sdk/issues/1829) will be
+rolled out in multiple phases. In the first phase, elastic scaling is set to work on parachains with
+a trusted/permissioned collator set. With this restriction, it is possible to launch elastic scaling
+without changing the candidate receipt. After successfully implementing the first phase, changes can
+be made to the candidate receipt so the collator set can be untrusted/permissionless again. The
+final phase will feature full integration with the Cumulus framework, enabling parachains to be
+configured to access multiple cores continuously.
+
+Take, for example, a parachain that wants to submit four parablocks to the relay chain. Without
+elastic scaling, it will take 24 seconds to include all of them through one core. Remember that a
+core is occupied after backing and before inclusion, i.e., for the whole data availability process.
+A block cannot enter a core before the previous block has been declared available.
+
+```
+              R1 <----- R2 <----- R3 <----- R4 <----- R5
+
+C1    |P1     B          I
+      |P2                B         I
+      |P3                          B         I
+      |P4                                    B         I
+
+```
+
+The diagram above shows how the backing and inclusion of parablocks (P) happen within the same relay
+chain block (R). With one core (C1), a parablock is included every 6 seconds. Note how P4 is
+included after 30 seconds (not 24 seconds) because when P1 was pushed to the relay chain for being
+backed, there was no previous parablock.
+
+With elastic scaling, it will take just 12 seconds (3-second block time) to include all four
+parablocks using two cores.
+
+```
+              R1 <----- R2 <----- R3
+
+C1    |P1     B          I
+      |P2                B         I
+C2    |P3     B          I
+      |P4                B         I
+
+```
+
+The diagram above shows how four parablocks are backed and included in the relay chain using two
+cores (C1 and C2). Note how P2 and P4 are included after 18 seconds (not 12 seconds) because when P1
+and P3 were pushed to the relay chain for being backed, there were no other parablocks before them.
+
+## Technical Considerations
+
+If the pace per core on the relay chain will not change (backing and inclusion every 6 seconds per
+core), on the parachain side, collators will need to increase the parablock production rate to push
+P1 and P2 to the two relay chain cores.
+
+Assuming a constant number of cores, from the relay chain side, elastic scaling will not see major
+upgrades as a parachain will use multiple existing cores instead of just one. However, from the
+parachain side, collators must produce more parablocks per unit of time, implying that technical
+specifications for collators will likely increase.
+
+For more advanced technical challenges, see the
+[Elastic Scaling GitHub PR](https://github.com/paritytech/polkadot-sdk/issues/1829).

kusama-guide/sidebars.js

@@ -593,6 +593,7 @@ module.exports = {
             'learn/learn-spree',
             "learn/learn-parathreads",
             "learn/learn-async-backing",
+            "learn/learn-elastic-scaling",
             "learn/learn-sassafras",
             "learn/learn-hyperbridge",
           ],

polkadot-wiki/sidebars.js

@@ -625,6 +625,7 @@ module.exports = {
             'learn/learn-spree',
             "learn/learn-parathreads",
             "learn/learn-async-backing",
+            "learn/learn-elastic-scaling",
             "learn/learn-sassafras",
             "learn/learn-hyperbridge",
           ],