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implement candidate selection subsystem (#1645)

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* choose the straightforward candidate selection algorithm for now

* add draft implementation of candidate selection

* fix typo in summary

* more properly report misbehaving collators

* describe how CandidateSelection subsystem becomes aware of candidates

* revise candidate selection / collator protocol interaction pattern

* implement rest of candidate selection per the guide

* review: resolve nits

* start writing test suite, harness

* implement first test

* add second test

* implement third test

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>
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Peter Goodspeed-Niklaus
2020-09-08 11:48:48 +02:00
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commit d1b1c17285
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polkadot/roadmap/implementers-guide/src/node/backing/candidate-selection.md
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@@ -24,16 +24,18 @@ Output:
Overarching network protocol + job for every relay-parent
> TODO The Candidate Selection network protocol is currently intentionally unspecified pending further discussion.
Several approaches have been selected, but all have some issues:
- The most straightforward approach is for this subsystem to simply second the first valid parablock candidate which it sees per relay head. However, that protocol is vulnerable to a single collator which, as an attack or simply through chance, gets its block candidate to the node more often than its fair share of the time.
- It may be possible to do some BABE-like selection algorithm to choose an "Official" collator for the round, but that is tricky because the collator which produces the PoV does not necessarily actually produce the block.
- We could use relay-chain BABE randomness to generate some delay `D` on the order of 1 second, +- 1 second. The collator would then second the first valid parablock which arrives after `D`, or in case none has arrived by `2*D`, the last valid parablock which has arrived. This makes it very hard for a collator to game the system to always get its block nominated, but it reduces the maximum throughput of the system by introducing delay into an already tight schedule.
- A variation of that scheme would be to randomly choose a number `I`, and have a fixed acceptance window `D` for parablock candidates. At the end of the period `D`, count `C`: the number of parablock candidates received. Second the one with index `I % C`. Its drawback is the same: it must wait the full `D` period before seconding any of its received candidates, reducing throughput.
For the moment, the candidate selection algorithm is simply to second the first valid parablock candidate per relay head. See [Future Work](#future-work).
## Candidate Selection Job
- Aware of validator key and assignment
- One job for each relay-parent, which selects up to one collation for the Candidate Backing Subsystem
## Future Work
Several approaches have been discussed, but all have some issues:
- The current approach is very straightforward. However, that protocol is vulnerable to a single collator which, as an attack or simply through chance, gets its block candidate to the node more often than its fair share of the time.
- It may be possible to do some BABE-like selection algorithm to choose an "Official" collator for the round, but that is tricky because the collator which produces the PoV does not necessarily actually produce the block.
- We could use relay-chain BABE randomness to generate some delay `D` on the order of 1 second, +- 1 second. The collator would then second the first valid parablock which arrives after `D`, or in case none has arrived by `2*D`, the last valid parablock which has arrived. This makes it very hard for a collator to game the system to always get its block nominated, but it reduces the maximum throughput of the system by introducing delay into an already tight schedule.
- A variation of that scheme would be to randomly choose a number `I`, and have a fixed acceptance window `D` for parablock candidates. At the end of the period `D`, count `C`: the number of parablock candidates received. Second the one with index `I % C`. Its drawback is the same: it must wait the full `D` period before seconding any of its received candidates, reducing throughput.