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chore(docs): update store v2 adr (backport #22794) #22801

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Dec 9, 2024
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chore(docs): update store v2 adr (backport #22794) #22801

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chore(docs): update store v2 adr (#22794)
tac0turtle Dec 9, 2024
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Delete store/v2/README.md
julienrbrt Dec 9, 2024
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Delete store/v2/pruning/README.md
julienrbrt Dec 9, 2024
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72 changes: 7 additions & 65 deletions docs/architecture/adr-065-store-v2.md
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Expand Up @@ -79,13 +79,11 @@ We propose to build upon some of the great ideas introduced in [ADR-040](./adr-0
while being a bit more flexible with the underlying implementations and overall
less intrusive. Specifically, we propose to:

* Separate the concerns of state commitment (**SC**), needed for consensus, and
state storage (**SS**), needed for state machine and clients.
* Reduce layers of abstractions necessary between the RMS and underlying stores.
* Remove unnecessary store types and implementations such as `CacheKVStore`.
* Simplify the branching logic.
* Remove the branching logic from the store package.
* Ensure the `RootStore` interface remains as lightweight as possible.
* Allow application developers to easily swap out SS and SC backends.
* Allow application developers to easily swap out SC backends.

Furthermore, we will keep IAVL as the default [SC](https://cryptography.fandom.com/wiki/Commitment_scheme)
backend for the time being. While we might not fully settle on the use of IAVL in
Expand All @@ -95,18 +93,12 @@ to change the backing commitment store in the future should evidence arise to
warrant a better alternative. However there is promising work being done to IAVL
that should result in significant performance improvement <sup>[1,2]</sup>.

Note, we will provide applications with the ability to use IAVL v1 and IAVL v2 as
Note, we will provide applications with the ability to use IAVL v1, IAVL v2 and MemIAVL as
either SC backend, with the latter showing extremely promising performance improvements
over IAVL v0 and v1, at the cost of a state migration.

### Separating SS and SC

By separating SS and SC, it will allow for us to optimize against primary use cases
and access patterns to state. Specifically, The SS layer will be responsible for
direct access to data in the form of (key, value) pairs, whereas the SC layer (e.g. IAVL)
will be responsible for committing to data and providing Merkle proofs.

#### State Commitment (SC)
### State Commitment (SC)

A foremost design goal is that SC backends should be easily swappable, i.e. not
necessarily IAVL. To this end, the scope of SC has been reduced, it must only:
Expand All @@ -121,45 +113,6 @@ due to the time and space constraints, but since store v2 defines an API for his
proofs there should be at least one configuration of a given SC backend which
supports this.

#### State Storage (SS)

The goal of SS is to provide a modular storage backend, i.e. multiple implementations,
to facilitate storing versioned raw key/value pairs in a fast embedded database.
The responsibility and functions of SS include the following:

* Provided fast and efficient queries for versioned raw key/value pairs
* Provide versioned CRUD operations
* Provide versioned batching functionality
* Provide versioned iteration (forward and reverse) functionality
* Provide pruning functionality

All of the functionality provided by an SS backend should work under a versioned
scheme, i.e. a user should be able to get, store, and iterate over keys for the latest
and historical versions efficiently and a store key, which is used for name-spacing
purposes.

We propose to have three defaulting SS backends for applications to choose from:

* RocksDB
* CGO based
* Usage of User-Defined Timestamps as a built-in versioning mechanism
* PebbleDB
* Native
* Manual implementation of MVCC keys for versioning
* SQLite
* CGO based
* Single table for all state

Since operators might want pruning strategies to differ in SS compared to SC,
e.g. having a very tight pruning strategy in SC while having a looser pruning
strategy for SS, we propose to introduce an additional pruning configuration,
with parameters that are identical to what exists in the SDK today, and allow
operators to control the pruning strategy of the SS layer independently of the
SC layer.

Note, the SC pruning strategy must be congruent with the operator's state sync
configuration. This is so as to allow state sync snapshots to execute successfully,
otherwise, a snapshot could be triggered on a height that is not available in SC.

#### State Sync

Expand All @@ -179,7 +132,7 @@ the primary interface for the application to interact with. The `RootStore` will
be responsible for housing SS and SC backends. Specifically, a `RootStore` will
provide the following functionality:

* Manage commitment of state (both SS and SC)
* Manage commitment of state
* Provide modules access to state
* Query delegation (i.e. get a value for a <key, height> tuple)
* Providing commitment proofs
Expand All @@ -197,12 +150,7 @@ solely provide key prefixing/namespacing functionality for modules.

#### Proofs

Since the SS layer is naturally a storage layer only, without any commitments
to (key, value) pairs, it cannot provide Merkle proofs to clients during queries.

So providing inclusion and exclusion proofs, via a `CommitmentOp` type, will be
the responsibility of the SC backend. Retrieving proofs will be done through the
a `RootStore`, which will internally route the request to the SC backend.
Providing a `CommitmentOp` type, will be the responsibility of the SC backend. Retrieving proofs will be done through the a `RootStore`, which will internally route the request to the SC backend.

#### Commitment

Expand Down Expand Up @@ -231,9 +179,6 @@ and storage backends for further performance, in addition to a reduced amount of
abstraction around KVStores making operations such as caching and state branching
more intuitive.

However, due to the proposed design, there are drawbacks around providing state
proofs for historical queries.

### Backwards Compatibility

This ADR proposes changes to the storage implementation in the Cosmos SDK through
Expand All @@ -243,17 +188,14 @@ be broken or modified.

### Positive

* Improved performance of independent SS and SC layers
* Improved performance of SC layers
* Reduced layers of abstraction making storage primitives easier to understand
* Atomic commitments for SC
* Redesign of storage types and interfaces will allow for greater experimentation
such as different physical storage backends and different commitment schemes
for different application modules

### Negative

* Providing proofs for historical state is challenging

### Neutral

* Removal of OCAP-based store keys in favor of simple strings for state retrieval
Expand Down
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