[RPC-Spec-V2] Storage: Add support for storage closest merkle descendant

[lexnv]

Fetch the Merkel value of the provided key when using the closest-descendant-merkle-value param.
When the key is not present in the trie, the Merkle value of the closest descendant must be returned.

From spec: https://github.com/paritytech/json-rpc-interface-spec/blob/main/src/api/chainHead_unstable_storage.md

If the type of an item is closest-descendant-merkle-value, then the so-called trie Merkle value of the key can be found in the result. If key doesn't exist in the trie, then the Merkle value of the closest descendant of key (including branch nodes) is provided. If key doesn't have any descendant in the trie, then the result will not contain any relevant item.

// @paritytech/subxt-team

[rphmeier]

(merkle as in Ralph Merkle, not Angela Merkel)

[lexnv]

We could first check if the provided key exists in the trie-db:

substrate/primitives/state-machine/src/backend.rs

Lines 208 to 220 in 25d9dd1

	/// true if a key exists in storage.
	fn exists_storage(&self, key: &[u8]) -> Result<bool, Self::Error> {
	Ok(self.storage_hash(key)?.is_some())
	}
	/// true if a key exists in child storage.
	fn exists_child_storage(
	&self,
	child_info: &ChildInfo,
	key: &[u8],
	) -> Result<bool, Self::Error> {
	Ok(self.child_storage_hash(child_info, key)?.is_some())
	}

If the key does not exist in storage, we'd need to iterate over the next key available:

substrate/primitives/state-machine/src/backend.rs

Lines 222 to 230 in 25d9dd1

	/// Return the next key in storage in lexicographic order or `None` if there is no value.
	fn next_storage_key(&self, key: &[u8]) -> Result<Option<StorageKey>, Self::Error>;
	/// Return the next key in child storage in lexicographic order or `None` if there is no value.
	fn next_child_storage_key(
	&self,
	child_info: &ChildInfo,
	key: &[u8],
	) -> Result<Option<StorageKey>, Self::Error>;

And finally, we can fetch the proof by providing the key found above

substrate/client/api/src/proof_provider.rs

Lines 27 to 41 in 25d9dd1

	/// Reads storage value at a given block + key, returning read proof.
	fn read_proof(
	&self,
	hash: Block::Hash,
	keys: &mut dyn Iterator<Item = &[u8]>,
	) -> sp_blockchain::Result<StorageProof>;
	/// Reads child storage value at a given block + storage_key + key, returning
	/// read proof.
	fn read_child_proof(
	&self,
	hash: Block::Hash,
	child_info: &ChildInfo,
	keys: &mut dyn Iterator<Item = &[u8]>,
	) -> sp_blockchain::Result<StorageProof>;

I do have a few questions about this approach since I don't have much context wrt our trie-db implementation:

• Is the provided StorageProof the merkle value we wish to return?
• Would iterating over the keys via next_storage_key produce the closest descendant?
• Could we eliminate the calls to exists_storage and rely only on read_proof?
  I believe using the read_proof directly with our key would have a similar effect here.
  The exists_storage method uses storage_hash under the hood to check if the database returns some value.
  While the read_proof uses storage, I do wonder if there is a performance hit on using the read_proof directly?

Would love to get some feedback on this 🙏 (@cheme @arkpar @bkchr)

[cheme]

I got no idea what this closest-descendant-merkle-value is meant for. If it is to do query for a given node (not all trie storage design can do that), it would also require the actual depth of the merkle value, otherwhise we cannot get correct key for content bellow.
Really not sure why it is needed (kind of mix technical content with storage content).

But to reply:

Is the provided StorageProof the merkle value we wish to return?

The merkle value is the hash of a merkle node (branch, or leaf), no idea if it can include inline nodes though: probably can return the sized encoded node in this case, but depending on the purpose, maybe it is the parent hash that we want???

So the storage proof does include this info, but this is not really how one should access it: producing the storage proof is costy and the info is just something we query while accessing the trie.
So this needs to be implemented in the trie crate.
Note that without changing the trie crate, one can use triedbnodeiterator to get it: the iterator return all trie nodes so we can get the hash of the node from the parent node after doing a seek operation. The issue is that if we seek the key and the hash is at the key then we can only return the children node (and the hash is from the parent), so I guess it may be easier to do it at trie level.
But since I don't know why it is usefull, cannot say if it is worth implementing.

Would iterating over the keys via next_storage_key produce the closest descendant?

Next storage only access storage content, since it is a storage rpc, it seems appropriate, but merkle value is an info that we fetch at trie level.

Note that in practice, using triedb iterator on the block state is more efficient than using next_storage to access content.

But generally our subscription are running over the change of storage value seen after each block processing. These changes are pure storage change (no trie level info).

Getting change in merkle value for subscription (done efficiently, I mean not querying at each block) would mean having it plugged on the trie root processing which is quite not easy to do (we can read the delta of all trie node change but these do not have structural information so no way to say which is the right merkle value unless rebuilding the trie and accessing it which is not as bad as doing query on state but still an unneeded overhead).

So I don't have a good solution (costy or breaking badly the architecture by leaking rpc subscription into block processsing).
Ok correct solution would be to attach the key (prefix that we have currently in storage but that I wish we could remove) to the trie delta and pass the trie delta to the rpc subscription layer, as we currrently do with the storage delta).

• link to the storage change currently accessible from the rpc subscription: https://github.com/paritytech/substrate/blob/74ae05142849bdf2a816df38e80f419741b10dc6/client/db/src/lib.rs#L822C19-L822C36
• the one with the trie content:

  substrate/client/db/src/lib.rs

  Line 821 in 74ae051

  db_updates: PrefixedMemoryDB<HashFor<Block>>,

  
  Then in the memorydb the key matching can be extract by knowing the memorydb key is "key node" ++ "MerkleValue".
  But as I mention, we want to get rid of this construction (needed by rocksdb but not by paritydb).
  And generally passing trie change to rpc subscription really need to have a meaning (it is a clone).

Could we eliminate the calls to exists_storage and rely only on read_proof?

if it is just for query, I would say yes, but I am not sure if a read_proof is needed here, and if IIUC the target is subscription, so there should be no call to exists_storage (just compare subscribed item against each block storage changes).

The exists_storage method uses storage_hash under the hood to check if the database returns some value.
While the read_proof uses storage, I do wonder if there is a performance hit on using the read_proof directly?

Using the read proof is better (we access trie content in memory when storage_hash may hit the db), but producing the read proof is more costy in the first place.

[cheme]

Thinking of closest-descendant-merkle-value, it can be use to know if any change happens bellow a given prefix key: if hash did change (and without returning all the changes).
If it is the primary use, it would be easier to just return a boolean indicating something did change and directly doable with current subscription code.

(but maybe there is the intention to directly query content from the return merkle value, but this is something that limit how to store the trie (currently we can somehow do it but some future design would break this possibility, but would just mean accessing the key from root which may not be that bad (but then just a boolean is enough)).

[bkchr]

I got no idea what this closest-descendant-merkle-value is meant for.

paritytech/json-rpc-interface-spec#37 (comment) maybe this helps

[cheme]

Thanks, got my replies:
paritytech/json-rpc-interface-spec#37 (comment) for inline value consideration and more importantly (from issue description):

The closest-ancestor-merkle-value query type makes it possible to know when the content of a map has changed. It can be seen as similar to hash, except that hash is only the hash of the value while closest-ancestor-merkle-value is the hash of the value plus the hash of all the descendants in the trie.
Basically, in order to watch a map, a JSON-RPC client would periodically query the closest-ancestor-merkle-value of the map, then redownload the list of keys if this Merkle value changes

So if the intention is to check for a change, then I think we should just return event with key indicating a change did occur at or under the key.
This would make things easier to implement (in the subscription from substrate we just check if one of the changed value from the block starts with this key) and simplier (returning a hash at a different depth than the one queried should be attached with its depth to be potentially used).

[jsdw]

@tomaka, just pinging you as this conversation is relevant re the closest-descendant-merkle-value impl/shape ^

[tomaka]

The closest-descendant-merkle-value isn't meant to be interpreted by the JSON-RPC client (after all, it's a hash most of the time).
JSON-RPC clients are supposed to treat it as an opaque value.

I can't just say "an opaque value" in the specification, as the JSON-RPC client might want to disconnect and reconnect and still have the same value. In other words, the meaning of this opaque value had to be defined so that all server implementations return the same.

[lexnv]

Considering the above it would be simpler from the substrate point of view to have an event-based notification regarding storage changes (similar what we have today).

One downside of this would be indeed that clients may disconnect and reconnect. In this window of time, the storage might change without the client knowing it.
That could be mitigated by the user fetching-refetching the hash of the value from storage on a fresh boot/disconnection.

I'm not sure how expensive that might be from the light-client perspective.
@tomaka would love to hear your thoughts about the event-based storage changes 🙏

[tomaka]

If you're talking about changes to the JSON-RPC API, please let's not discuss this in the Substrate repo. The reason why there's a JSON-RPC API repo is specifically so that everyone can follow discussions, rather than look around for random bits of discussions in Substrate issues like happens often.