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变更日志

  • 2019年11月26日：第一版开始
  • 2019年12月2日：第一版最终草案

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背景

为了让 Cosmos SDK 实现 IBC 规范，Cosmos SDK 内的模块必须能够检查最近的共识状态（验证者集合和承诺根），因为在握手过程中需要校验这些值在其他链上的证明。

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决策

应用程序必须在持久化存储中保存最近的 n 个区块头。最初，这个存储可以是当前的 Merklised 存储。之后也可以使用非 Merklised 存储，因为不需要证明。 应用程序必须在处理 abci.RequestBeginBlock 时立即存储新区块头，以保存这些信息： 
func BeginBlock(ctx sdk.Context, keeper HistoricalHeaderKeeper, req abci.RequestBeginBlock)

abci.ResponseBeginBlock {
    info := HistoricalInfo{
    Header: ctx.BlockHeader(),
    ValSet: keeper.StakingKeeper.GetAllValidators(ctx), // note that this must be stored in a canonical order
}

keeper.SetHistoricalInfo(ctx, ctx.BlockHeight(), info)
    n := keeper.GetParamRecentHeadersToStore()

keeper.PruneHistoricalInfo(ctx, ctx.BlockHeight() - n)
  // continue handling request
}
或者，应用程序也可以只存储验证者集合的哈希。 应用程序必须通过 Keeper 的 GetHistoricalInfo 函数，向 Cosmos SDK 模块提供对过去这 n 个已提交区块头的查询能力。这个能力可以实现为一个新模块，也可以集成到现有模块中（很可能是 x/staking 或 x/ibc）。 n 可以配置为参数存储中的一个参数，这样就可以通过 ParameterChangeProposal 进行修改。不过，如果增大 n，已存储的信息需要经过若干个区块后才能补齐。

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状态

提议中。

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影响

实现此 ADR 需要对 Cosmos SDK 进行修改，但不需要对 Tendermint 进行修改。

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正面影响

  • Cosmos SDK 中任意位置的模块都可以轻松获取最近历史高度的区块头和状态根。
  • 无需对 Tendermint 发起 RPC 调用。
  • 无需修改 ABCI。

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负面影响

  • 在 Tendermint 和应用程序中重复存储了 n 份区块头数据（会增加磁盘使用量）。从长期来看，可能更适合采用类似这里的方法。

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中性影响

（暂无已知）

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参考资料

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Changelog

  • 26 November 2019: Start of first version
  • 2 December 2019: Final draft of first version

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Context

In order for the Cosmos SDK to implement the IBC specification, modules within the Cosmos SDK must have the ability to introspect recent consensus states (validator sets & commitment roots) as proofs of these values on other chains must be checked during the handshakes.

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Decision

The application MUST store the most recent n headers in a persistent store. At first, this store MAY be the current Merklised store. A non-Merklised store MAY be used later as no proofs are necessary. The application MUST store this information by storing new headers immediately when handling abci.RequestBeginBlock: 
func BeginBlock(ctx sdk.Context, keeper HistoricalHeaderKeeper, req abci.RequestBeginBlock)

abci.ResponseBeginBlock {
    info := HistoricalInfo{
    Header: ctx.BlockHeader(),
    ValSet: keeper.StakingKeeper.GetAllValidators(ctx), // note that this must be stored in a canonical order
}

keeper.SetHistoricalInfo(ctx, ctx.BlockHeight(), info)
    n := keeper.GetParamRecentHeadersToStore()

keeper.PruneHistoricalInfo(ctx, ctx.BlockHeight() - n)
  // continue handling request
}
Alternatively, the application MAY store only the hash of the validator set. The application MUST make these past n committed headers available for querying by Cosmos SDK modules through the Keeper’s GetHistoricalInfo function. This MAY be implemented in a new module, or it MAY also be integrated into an existing one (likely x/staking or x/ibc). n MAY be configured as a parameter store parameter, in which case it could be changed by ParameterChangeProposals, although it will take some blocks for the stored information to catch up if n is increased.

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Status

Proposed.

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Consequences

Implementation of this ADR will require changes to the Cosmos SDK. It will not require changes to Tendermint.

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Positive

  • Easy retrieval of headers & state roots for recent past heights by modules anywhere in the Cosmos SDK.
  • No RPC calls to Tendermint required.
  • No ABCI alterations required.

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Negative

  • Duplicates n headers data in Tendermint & the application (additional disk usage) - in the long term, an approach such as this might be preferable.

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Neutral

(none known)

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References