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docs: Update ADR-040 to store hash(value) in SMT leaf #9680

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docs: Update ADR-040 to store hash(value) in SMT leaf #9680

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Update ADR-40 to store hash(value) in SMT leaf
roysc Jul 12, 2021
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Improve wording
roysc Jul 13, 2021
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Specify SMT structure
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"celestia"
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Merge branch 'master' into roysc/adr-040-update-smt-leaf
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Merge branch 'master' into roysc/adr-040-update-smt-leaf
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28 changes: 18 additions & 10 deletions docs/architecture/adr-040-storage-and-smt-state-commitments.md
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Expand Up @@ -10,7 +10,7 @@ DRAFT Not Implemented

## Abstract

Sparse Merke Tree ([SMT](https://osf.io/8mcnh/)) is a version of a Merkle Tree with various storage and performance optimizations. This ADR defines a separation of state commitments from data storage and the SDK transition from IAVL to SMT.
Sparse Merkle Tree ([SMT](https://osf.io/8mcnh/)) is a version of a Merkle Tree with various storage and performance optimizations. This ADR defines a separation of state commitments from data storage and the SDK transition from IAVL to SMT.

## Context

Expand All @@ -30,23 +30,23 @@ Moreover, the IAVL project lacks support and a maintainer and we already see bet

## Decision

We propose to separate the concerns of state commitment (**SC**), needed for consensus, and state storage (**SS**), needed for state machine. Finally we replace IAVL with [LazyLedgers' SMT](https://github.com/lazyledger/smt). LazyLedger SMT is based on Diem (called jellyfish) design [*] - it uses a compute-optimised SMT by replacing subtrees with only default values with a single node (same approach is used by Ethereum2) and implements compact proofs.
We propose to separate the concerns of state commitment (**SC**), needed for consensus, and state storage (**SS**), needed for state machine. Finally we replace IAVL with [Celestia's SMT](https://github.com/lazyledger/smt). Celestia SMT is based on Diem (called jellyfish) design [*] - it uses a compute-optimised SMT by replacing subtrees with only default values with a single node (same approach is used by Ethereum2) and implements compact proofs.

The storage model presented here doesn't deal with data structure nor serialization. It's a Key-Value database, where both key and value are binaries. The storage user is responsible for data serialization.

### Decouple state commitment from storage

Separation of storage and commitment (by the SMT) will allow the optimization of different components according to their usage and access patterns.

`SS` (SMT) is used to commit to a data and compute merkle proofs. `SC` is used to directly access data. To avoid collisions, both `SS` and `SC` will use a separate storage namespace (they could use the same database underneath). `SC` will store each `(key, value)` pair directly (map key -> value).
`SC` (SMT) is used to commit to a data and compute merkle proofs. `SS` is used to directly access data. To avoid collisions, both `SS` and `SC` will use a separate storage namespace (they could use the same database underneath). `SS` will store each `(key, value)` pair directly (map key -> value).

SMT is a merkle tree structure: we don't store keys directly. For every `(key, value)` pair, `hash(key)` is stored in a path (we hash a key to evenly distribute keys in the tree) and `hash(key, value)` in a leaf. Since we don't know a structure of a value (in particular if it contains the key) we hash both the key and the value in the `SC` leaf.
SMT is a merkle tree structure: we don't store keys directly. For every `(key, value)` pair, `hash(key)` is used as leaf path (we hash a key to uniformly distribute leaves in the tree) and `(hash(key), hash(value))` as the leaf contents. The tree structure is specified in more depth [below](#celestia-smt-for-state-commitment).

For data access we propose 2 additional KV buckets (namespaces for the key-value pairs, sometimes called [column family](https://github.com/facebook/rocksdb/wiki/Terminology)):
For data access we propose 2 additional KV buckets (implemented as namespaces for the key-value pairs, sometimes called [column family](https://github.com/facebook/rocksdb/wiki/Terminology)):

1. B1: `key → value`: the principal object storage, used by a state machine, behind the SDK `KVStore` interface: provides direct access by key and allows prefix iteration (KV DB backend must support it).
2. B2: `hash(key, value) → key`: a reverse index to get a key from an SMT path. Recall that SMT will store `(k, v)` as `(hash(k), hash(key, value))`. So, we can get an object value by composing `SMT_path → B2 → B1`.
3. we could use more buckets to optimize the app usage if needed.
2. B2: `hash(key) → key`: a reverse index to get a key from an SMT path. Recall that SMT will store `(k, v)` as `(hash(k), hash(value))`. So, we can get an object value by composing `hash(key) → B2 → B1`.
3. We could use more buckets to optimize the app usage if needed.

Above, we propose to use a KV DB. However, for the state machine, we could use an RDBMS, which we discuss below.

Expand All @@ -66,10 +66,18 @@ State Commitment requirements:
+ tree path should be short
+ pruning (garbage collection)

### LazyLedger SMT for State Commitment
### Celestia SMT for State Commitment

A Sparse Merkle tree is based on the idea of a complete Merkle tree of an intractable size. The assumption here is that as the size of the tree is intractable, there would only be a few leaf nodes with valid data blocks relative to the tree size, rendering a sparse tree.

The full specification can be found at [Celestia](https://github.com/celestiaorg/celestia-specs/blob/ec98170398dfc6394423ee79b00b71038879e211/src/specs/data_structures.md#sparse-merkle-tree). In summary:

* The SMT consists of a binary Merkle tree, constructed in the same fashion as described in [Certificate Transparency (RFC-6962)](https://tools.ietf.org/html/rfc6962), but using as the hashing function SHA-2-256 as defined in [FIPS 180-4](https://doi.org/10.6028/NIST.FIPS.180-4).
* Leaves and internal nodes are hashed differently: the one-byte `0x00` is prepended for leaf nodes while `0x01` is prepended for internal nodes.
* Default values are given to leaf nodes with empty leaves.
* While the above rule is sufficient to pre-compute the values of intermediate nodes that are roots of empty subtrees, a further simplification is to extend this default value to all nodes that are roots of empty subtrees. The 32-byte zero is used as the default value. This rule takes precedence over the above one.
* An internal node that is the root of a subtree that contains exactly one non-empty leaf is replaced by that leaf's leaf node.

### Snapshots for storage sync and state versioning

Below, with simple _snapshot_ we refer to a database snapshot mechanism, not to a _ABCI snapshot sync_. The latter will be referred as _snapshot sync_ (which will directly use DB snapshot as described below).
Expand Down Expand Up @@ -122,7 +130,7 @@ We change the storage layout of the state machine, a storage hard fork and netwo

+ Decoupling state from state commitment introduce better engineering opportunities for further optimizations and better storage patterns.
+ Performance improvements.
+ Joining SMT based camp which has wider and proven adoption than IAVL. Example projects which decided on SMT: Ethereum2, Diem (Libra), Trillan, Tezos, LazyLedger.
+ Joining SMT based camp which has wider and proven adoption than IAVL. Example projects which decided on SMT: Ethereum2, Diem (Libra), Trillan, Tezos, Celestia.

### Negative

Expand Down Expand Up @@ -158,7 +166,7 @@ We were discussing use case where modules can use a support database, which is n
+ [IAVL What's Next?](https://github.com/cosmos/cosmos-sdk/issues/7100)
+ [IAVL overview](https://docs.google.com/document/d/16Z_hW2rSAmoyMENO-RlAhQjAG3mSNKsQueMnKpmcBv0/edit#heading=h.yd2th7x3o1iv) of it's state v0.15
+ [State commitments and storage report](https://paper.dropbox.com/published/State-commitments-and-storage-review--BDvA1MLwRtOx55KRihJ5xxLbBw-KeEB7eOd11pNrZvVtqUgL3h)
+ [LazyLedger SMT](https://github.com/lazyledger/smt)
+ [Celestia SMT](https://github.com/lazyledger/smt)
+ Facebook Diem (Libra) SMT [design](https://developers.diem.com/papers/jellyfish-merkle-tree/2021-01-14.pdf)
+ [Trillian Revocation Transparency](https://github.com/google/trillian/blob/master/docs/papers/RevocationTransparency.pdf), [Trillian Verifiable Data Structures](https://github.com/google/trillian/blob/master/docs/papers/VerifiableDataStructures.pdf).
+ Design and implementation [discussion](https://github.com/cosmos/cosmos-sdk/discussions/8297).