78 lines
2.4 KiB
Markdown
78 lines
2.4 KiB
Markdown
# ADR 020: Limiting txs size inside a block
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## Changelog
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13-08-2018: Initial Draft
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15-08-2018: Second version after Dev's comments
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28-08-2018: Third version after Ethan's comments
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30-08-2018: AminoOverheadForBlock => MaxAminoOverheadForBlock
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31-08-2018: Bounding evidence and chain ID
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## Context
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We currently use MaxTxs to reap txs from the mempool when proposing a block,
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but enforce MaxBytes when unmarshalling a block, so we could easily propose a
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block thats too large to be valid.
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We should just remove MaxTxs all together and stick with MaxBytes, and have a
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`mempool.ReapMaxBytes`.
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But we can't just reap BlockSize.MaxBytes, since MaxBytes is for the entire block,
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not for the txs inside the block. There's extra amino overhead + the actual
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headers on top of the actual transactions + evidence + last commit.
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## Proposed solution
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Therefore, we should
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1) Get rid of MaxTxs.
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2) Rename MaxTxsBytes to MaxBytes.
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When we need to ReapMaxBytes from the mempool, we calculate the upper bound as follows:
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```
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ExactLastCommitBytes = {number of validators currently enabled} * {MaxVoteBytes}
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MaxEvidenceBytesPerBlock = MaxBytes / 10
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ExactEvidenceBytes = cs.evpool.PendingEvidence(MaxEvidenceBytesPerBlock) * MaxEvidenceBytes
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mempool.ReapMaxBytes(MaxBytes - MaxAminoOverheadForBlock - ExactLastCommitBytes - ExactEvidenceBytes - MaxHeaderBytes)
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```
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where MaxVoteBytes, MaxEvidenceBytes, MaxHeaderBytes and MaxAminoOverheadForBlock
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are constants defined inside the `types` package:
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- MaxVoteBytes - 170 bytes
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- MaxEvidenceBytes - 364 bytes
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- MaxHeaderBytes - 476 bytes (~276 bytes hashes + 200 bytes - 50 UTF-8 encoded
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symbols of chain ID 4 bytes each in the worst case + amino overhead)
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- MaxAminoOverheadForBlock - 8 bytes (assuming MaxHeaderBytes includes amino
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overhead for encoding header, MaxVoteBytes - for encoding vote, etc.)
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ChainID needs to bound to 50 symbols max.
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When reaping evidence, we use MaxBytes to calculate the upper bound (e.g. 1/10)
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to save some space for transactions.
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NOTE while reaping the `max int` bytes in mempool, we should account that every
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transaction will take `len(tx)+aminoOverhead`, where aminoOverhead=1-4 bytes.
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We should write a test that fails if the underlying structs got changed, but
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MaxXXX stayed the same.
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## Status
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Proposed.
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## Consequences
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### Positive
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* one way to limit the size of a block
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* less variables to configure
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### Negative
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* constants that need to be adjusted if the underlying structs got changed
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### Neutral
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