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tendermint/docs/architecture/adr-020-block-size.md

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ADR 020: Limiting txs size inside a block

Changelog

13-08-2018: Initial Draft 15-08-2018: Second version after Dev's comments 28-08-2018: Third version after Ethan's comments 30-08-2018: AminoOverheadForBlock => MaxAminoOverheadForBlock 31-08-2018: Bounding evidence and chain ID

Context

We currently use MaxTxs to reap txs from the mempool when proposing a block, but enforce MaxBytes when unmarshalling a block, so we could easily propose a block thats too large to be valid.

We should just remove MaxTxs all together and stick with MaxBytes, and have a mempool.ReapMaxBytes.

But we can't just reap BlockSize.MaxBytes, since MaxBytes is for the entire block, not for the txs inside the block. There's extra amino overhead + the actual headers on top of the actual transactions + evidence + last commit.

Proposed solution

Therefore, we should

  1. Get rid of MaxTxs.
  2. Rename MaxTxsBytes to MaxBytes.

When we need to ReapMaxBytes from the mempool, we calculate the upper bound as follows:

ExactLastCommitBytes = {number of validators currently enabled} * {MaxVoteBytes}
MaxEvidenceBytesPerBlock = MaxBytes / 10
ExactEvidenceBytes = cs.evpool.PendingEvidence(MaxEvidenceBytesPerBlock) * MaxEvidenceBytes

mempool.ReapMaxBytes(MaxBytes - MaxAminoOverheadForBlock - ExactLastCommitBytes - ExactEvidenceBytes - MaxHeaderBytes)

where MaxVoteBytes, MaxEvidenceBytes, MaxHeaderBytes and MaxAminoOverheadForBlock are constants defined inside the types package:

  • MaxVoteBytes - 170 bytes
  • MaxEvidenceBytes - 364 bytes
  • MaxHeaderBytes - 476 bytes (~276 bytes hashes + 200 bytes - 50 UTF-8 encoded symbols of chain ID 4 bytes each in the worst case + amino overhead)
  • MaxAminoOverheadForBlock - 8 bytes (assuming MaxHeaderBytes includes amino overhead for encoding header, MaxVoteBytes - for encoding vote, etc.)

ChainID needs to bound to 50 symbols max.

When reaping evidence, we use MaxBytes to calculate the upper bound (e.g. 1/10) to save some space for transactions.

NOTE while reaping the max int bytes in mempool, we should account that every transaction will take len(tx)+aminoOverhead, where aminoOverhead=1-4 bytes.

We should write a test that fails if the underlying structs got changed, but MaxXXX stayed the same.

Status

Proposed.

Consequences

Positive

  • one way to limit the size of a block
  • less variables to configure

Negative

  • constants that need to be adjusted if the underlying structs got changed

Neutral