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* ADR-060 accepted -> proposed Signed-off-by: Thane Thomson <connect@thanethomson.com> * ADR-061 accepted -> proposed Signed-off-by: Thane Thomson <connect@thanethomson.com> * ADR-062 implemented -> proposed Signed-off-by: Thane Thomson <connect@thanethomson.com> * ADR-063 implemented -> accepted Signed-off-by: Thane Thomson <connect@thanethomson.com> * ADR-077: Update title and status to reflect current reality Signed-off-by: Thane Thomson <connect@thanethomson.com> * ADR-078: Update status to reflect current reality Signed-off-by: Thane Thomson <connect@thanethomson.com> * ADR-079: accepted Signed-off-by: Thane Thomson <connect@thanethomson.com> * ADR-068 and ADR-080: proposed Signed-off-by: Thane Thomson <connect@thanethomson.com> * ADR-065: implemented Signed-off-by: Thane Thomson <connect@thanethomson.com> * ADR-076: implemented Signed-off-by: Thane Thomson <connect@thanethomson.com> Signed-off-by: Thane Thomson <connect@thanethomson.com>
83 lines
3.5 KiB
Markdown
83 lines
3.5 KiB
Markdown
# ADR 078: Non-Zero Genesis
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## Changelog
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- 2020-07-26: Initial draft (@erikgrinaker)
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- 2020-07-28: Use weak chain linking, i.e. `predecessor` field (@erikgrinaker)
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- 2020-07-31: Drop chain linking (@erikgrinaker)
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- 2020-08-03: Add `State.InitialHeight` (@erikgrinaker)
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- 2021-02-11: Migrate to tendermint repo (Originally [RFC 002](https://github.com/tendermint/spec/pull/119))
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## Author(s)
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- Erik Grinaker (@erikgrinaker)
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## Context
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The recommended upgrade path for block protocol-breaking upgrades is currently to hard fork the
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chain (see e.g. [`cosmoshub-3` upgrade](https://blog.cosmos.network/cosmos-hub-3-upgrade-announcement-39c9da941aee).
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This is done by halting all validators at a predetermined height, exporting the application
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state via application-specific tooling, and creating an entirely new chain using the exported
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application state.
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As far as Tendermint is concerned, the upgraded chain is a completely separate chain, with e.g.
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a new chain ID and genesis file. Notably, the new chain starts at height 1, and has none of the
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old chain's block history. This causes problems for integrators, e.g. coin exchanges and
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wallets, that assume a monotonically increasing height for a given blockchain. Users also find
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it confusing that a given height can now refer to distinct states depending on the chain
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version.
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An ideal solution would be to always retain block backwards compatibility in such a way that chain
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history is never lost on upgrades. However, this may require a significant amount of engineering
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work that is not viable for the planned Stargate release (Tendermint 0.34), and may prove too
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restrictive for future development.
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As a first step, allowing the new chain to start from an initial height specified in the genesis
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file would at least provide monotonically increasing heights. There was a proposal to include the
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last block header of the previous chain as well, but since the genesis file is not verified and
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hashed (only specific fields are) this would not be trustworthy.
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External tooling will be required to map historical heights onto e.g. archive nodes that contain
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blocks from previous chain version. Tendermint will not include any such functionality.
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## Proposal
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Tendermint will allow chains to start from an arbitrary initial height:
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- A new field `initial_height` is added to the genesis file, defaulting to `1`. It can be set to any
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non-negative integer, and `0` is considered equivalent to `1`.
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- A new field `InitialHeight` is added to the ABCI `RequestInitChain` message, with the same value
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and semantics as the genesis field.
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- A new field `InitialHeight` is added to the `state.State` struct, where `0` is considered invalid.
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Including the field here simplifies implementation, since the genesis value does not have to be
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propagated throughout the code base separately, but it is not strictly necessary.
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ABCI applications may have to be updated to handle arbitrary initial heights, otherwise the initial
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block may fail.
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## Status
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Implemented
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## Consequences
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### Positive
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- Heights can be unique throughout the history of a "logical" chain, across hard fork upgrades.
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### Negative
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- Upgrades still cause loss of block history.
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- Integrators will have to map height ranges to specific archive nodes/networks to query history.
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### Neutral
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- There is no explicit link to the last block of the previous chain.
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## References
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- [#2543: Allow genesis file to start from non-zero height w/ prev block header](https://github.com/tendermint/tendermint/issues/2543)
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