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chore(tvix/docs): move [ca]store docs to tvix/docs

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# BlobStore: Chunking & Verified Streaming
`tvix-castore`'s BlobStore is a content-addressed storage system, using [blake3]
as hash function.
Returned data is fetched by using the digest as lookup key, and can be verified
to be correct by feeding the received data through the hash function and
ensuring it matches the digest initially used for the lookup.
This means, data can be downloaded by any untrusted third-party as well, as the
received data is validated to match the digest it was originally requested with.
However, for larger blobs of data, having to download the entire blob at once is
wasteful, if we only care about a part of the blob. Think about mounting a
seekable data structure, like loop-mounting an .iso file, or doing partial reads
in a large Parquet file, a column-oriented data format.
> We want to have the possibility to *seek* into a larger file.
This however shouldn't compromise on data integrity properties - we should not
need to trust a peer we're downloading from to be "honest" about the partial
data we're reading. We should be able to verify smaller reads.
Especially when substituting from an untrusted third-party, we want to be able
to detect quickly if that third-party is sending us wrong data, and terminate
the connection early.
## Chunking
In content-addressed systems, this problem has historically been solved by
breaking larger blobs into smaller chunks, which can be fetched individually,
and making a hash of *this listing* the blob digest/identifier.
- BitTorrent for example breaks files up into smaller chunks, and maintains
a list of sha1 digests for each of these chunks. Magnet links contain a
digest over this listing as an identifier. (See [bittorrent-v2][here for
more details]).
With the identifier, a client can fetch the entire list, and then recursively
"unpack the graph" of nodes, until it ends up with a list of individual small
chunks, which can be fetched individually.
- Similarly, IPFS with its IPLD model builds up a Merkle DAG, and uses the
digest of the root node as an identitier.
These approaches solve the problem of being able to fetch smaller chunks in a
trusted fashion. They can also do some deduplication, in case there's the same
leaf nodes same leaf nodes in multiple places.
However, they also have a big disadvantage. The chunking parameters, and the
"topology" of the graph structure itself "bleed" into the root hash of the
entire data structure itself.
Depending on the chunking parameters used, there's different representations for
the same data, causing less data sharing/reuse in the overall system, in terms of how
many chunks need to be downloaded vs. are already available locally, as well as
how compact data is stored on-disk.
This can be workarounded by agreeing on only a single way of chunking, but it's
not pretty and misses a lot of deduplication potential.
### Chunking in Tvix' Blobstore
tvix-castore's BlobStore uses a hybrid approach to eliminate some of the
disadvantages, while still being content-addressed internally, with the
highlighted benefits.
It uses [blake3] as hash function, and the blake3 digest of **the raw data
itself** as an identifier (rather than some application-specific Merkle DAG that
also embeds some chunking information).
BLAKE3 is a tree hash where all left nodes fully populated, contrary to
conventional serial hash functions. To be able to validate the hash of a node,
one only needs the hash of the (2) children [^1], if any.
This means one only needs to the root digest to validate a constructions, and these
constructions can be sent [separately][bao-spec].
This relieves us from the need of having to encode more granular chunking into
our data model / identifier upfront, but can make this mostly a transport/
storage concern.
For some more description on the (remote) protocol, check
`./blobstore-protocol.md`.
#### Logical vs. physical chunking
Due to the properties of the BLAKE3 hash function, we have logical blocks of
1KiB, but this doesn't necessarily imply we need to restrict ourselves to these
chunk sizes w.r.t. what "physical chunks" are sent over the wire between peers,
or are stored on-disk.
The only thing we need to be able to read and verify an arbitrary byte range is
having the covering range of aligned 1K blocks, and a construction from the root
digest to the 1K block.
Note the intermediate hash tree can be further trimmed, [omitting][bao-tree]
lower parts of the tree while still providing verified streaming - at the cost
of having to fetch larger covering ranges of aligned blocks.
Let's pick an example. We identify each KiB by a number here for illustrational
purposes.
Assuming we omit the last two layers of the hash tree, we end up with logical
4KiB leaf chunks (`bao_shift` of `2`).
For a blob of 14 KiB total size, we could fetch logical blocks `[0..=3]`,
`[4..=7]`, `[8..=11]` and `[12..=13]` in an authenticated fashion:
`[ 0 1 2 3 ] [ 4 5 6 7 ] [ 8 9 10 11 ] [ 12 13 ]`
Assuming the server now informs us about the following physical chunking:
```
[ 0 1 ] [ 2 3 4 5 ] [ 6 ] [ 7 8 ] [ 9 10 11 12 13 14 15 ]`
```
If our application now wants to arbitrarily read from 0 until 4 (inclusive):
```
[ 0 1 ] [ 2 3 4 5 ] [ 6 ] [ 7 8 ] [ 9 10 11 12 13 14 15 ]
|-------------|
```
…we need to fetch physical chunks `[ 0 1 ]`, `[ 2 3 4 5 ]` and `[ 6 ] [ 7 8 ]`.
`[ 0 1 ]` and `[ 2 3 4 5 ]` are obvious, they contain the data we're
interested in.
We however also need to fetch the physical chunks `[ 6 ]` and `[ 7 8 ]`, so we
can assemble `[ 4 5 6 7 ]` to verify both logical chunks:
```
[ 0 1 ] [ 2 3 4 5 ] [ 6 ] [ 7 8 ] [ 9 10 11 12 13 14 15 ]
^ ^ ^ ^
|----4KiB----|------4KiB-----|
```
Each physical chunk fetched can be validated to have the blake3 digest that was
communicated upfront, and can be stored in a client-side cache/storage, so
subsequent / other requests for the same data will be fast(er).
---
[^1]: and the surrounding context, aka position inside the whole blob, which is available while verifying the tree
[bittorrent-v2]: https://blog.libtorrent.org/2020/09/bittorrent-v2/
[blake3]: https://github.com/BLAKE3-team/BLAKE3
[bao-spec]: https://github.com/oconnor663/bao/blob/master/docs/spec.md
[bao-tree]: https://github.com/n0-computer/bao-tree

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# BlobStore: Protocol / Composition
This documents describes the protocol that BlobStore uses to substitute blobs
other ("remote") BlobStores.
How to come up with the blake3 digest of the blob to fetch is left to another
layer in the stack.
To put this into the context of Tvix as a Nix alternative, a blob represents an
individual file inside a StorePath.
In the Tvix Data Model, this is accomplished by having a `FileNode` (either the
`root_node` in a `PathInfo` message, or a individual file inside a `Directory`
message) encode a BLAKE3 digest.
However, the whole infrastructure can be applied for other usecases requiring
exchange/storage or access into data of which the blake3 digest is known.
## Protocol and Interfaces
As an RPC protocol, BlobStore currently uses gRPC.
On the Rust side of things, every blob service implements the
[`BlobService`](../src/blobservice/mod.rs) async trait, which isn't
gRPC-specific.
This `BlobService` trait provides functionality to check for existence of Blobs,
read from blobs, and write new blobs.
It also provides a method to ask for more granular chunks if they are available.
In addition to some in-memory, on-disk and (soon) object-storage-based
implementations, we also have a `BlobService` implementation that talks to a
gRPC server, as well as a gRPC server wrapper component, which provides a gRPC
service for anything implementing the `BlobService` trait.
This makes it very easy to talk to a remote `BlobService`, which does not even
need to be written in the same language, as long it speaks the same gRPC
protocol.
It also puts very little requirements on someone implementing a new
`BlobService`, and how its internal storage or chunking algorithm looks like.
The gRPC protocol is documented in `../protos/rpc_blobstore.proto`.
Contrary to the `BlobService` trait, it does not have any options for seeking/
ranging, as it's more desirable to provide this through chunking (see also
`./blobstore-chunking.md`).
## Composition
Different `BlobStore` are supposed to be "composed"/"layered" to express
caching, multiple local and remote sources.
The fronting interface can be the same, it'd just be multiple "tiers" that can
respond to requests, depending on where the data resides. [^1]
This makes it very simple for consumers, as they don't need to be aware of the
entire substitutor config.
The flexibility of this doesn't need to be exposed to the user in the default
case; in most cases we should be fine with some form of on-disk storage and a
bunch of substituters with different priorities.
### gRPC Clients
Clients are encouraged to always read blobs in a chunked fashion (asking for a
list of chunks for a blob via `BlobService.Stat()`, then fetching chunks via
`BlobService.Read()` as needed), instead of directly reading the entire blob via
`BlobService.Read()`.
In a composition setting, this provides opportunity for caching, and avoids
downloading some chunks if they're already present locally (for example, because
they were already downloaded by reading from a similar blob earlier).
It also removes the need for seeking to be a part of the gRPC protocol
alltogether, as chunks are supposed to be "reasonably small" [^2].
There's some further optimization potential, a `BlobService.Stat()` request
could tell the server it's happy with very small blobs just being inlined in
an additional additional field in the response, which would allow clients to
populate their local chunk store in a single roundtrip.
## Verified Streaming
As already described in `./docs/blobstore-chunking.md`, the physical chunk
information sent in a `BlobService.Stat()` response is still sufficient to fetch
in an authenticated fashion.
The exact protocol and formats are still a bit in flux, but here's some notes:
- `BlobService.Stat()` request gets a `send_bao` field (bool), signalling a
[BAO][bao-spec] should be sent. Could also be `bao_shift` integer, signalling
how detailed (down to the leaf chunks) it should go.
The exact format (and request fields) still need to be defined, edef has some
ideas around omitting some intermediate hash nodes over the wire and
recomputing them, reducing size by another ~50% over [bao-tree].
- `BlobService.Stat()` response gets some bao-related fields (`bao_shift`
field, signalling the actual format/shift level the server replies with, the
actual bao, and maybe some format specifier).
It would be nice to also be compatible with the baos used by [iroh], so we
can provide an implementation using it too.
---
[^1]: We might want to have some backchannel, so it becomes possible to provide
feedback to the user that something is downloaded.
[^2]: Something between 512K-4M, TBD.
[bao-spec]: https://github.com/oconnor663/bao/blob/master/docs/spec.md
[bao-tree]: https://github.com/n0-computer/bao-tree
[iroh]: https://github.com/n0-computer/iroh

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# Data model
This provides some more notes on the fields used in castore.proto.
See `//tvix/store/docs/api.md` for the full context.
## Directory message
`Directory` messages use the blake3 hash of their canonical protobuf
serialization as its identifier.
A `Directory` message contains three lists, `directories`, `files` and
`symlinks`, holding `DirectoryNode`, `FileNode` and `SymlinkNode` messages
respectively. They describe all the direct child elements that are contained in
a directory.
All three message types have a `name` field, specifying the (base)name of the
element (which MUST not contain slashes or null bytes, and MUST not be '.' or '..').
For reproducibility reasons, the lists MUST be sorted by that name and the
name MUST be unique across all three lists.
In addition to the `name` field, the various *Node messages have the following
fields:
## DirectoryNode
A `DirectoryNode` message represents a child directory.
It has a `digest` field, which points to the identifier of another `Directory`
message, making a `Directory` a merkle tree (or strictly speaking, a graph, as
two elements pointing to a child directory with the same contents would point
to the same `Directory` message).
There's also a `size` field, containing the (total) number of all child
elements in the referenced `Directory`, which helps for inode calculation.
## FileNode
A `FileNode` message represents a child (regular) file.
Its `digest` field contains the blake3 hash of the file contents. It can be
looked up in the `BlobService`.
The `size` field contains the size of the blob the `digest` field refers to.
The `executable` field specifies whether the file should be marked as
executable or not.
## SymlinkNode
A `SymlinkNode` message represents a child symlink.
In addition to the `name` field, the only additional field is the `target`,
which is a string containing the target of the symlink.

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## Why not git tree objects?
We've been experimenting with (some variations of) the git tree and object
format, and ultimately decided against using it as an internal format, and
instead adapted the one documented in the other documents here.
While the tvix-store API protocol shares some similarities with the format used
in git for trees and objects, the git one has shown some significant
disadvantages:
### The binary encoding itself
#### trees
The git tree object format is a very binary, error-prone and
"made-to-be-read-and-written-from-C" format.
Tree objects are a combination of null-terminated strings, and fields of known
length. References to other tree objects use the literal sha1 hash of another
tree object in this encoding.
Extensions of the format/changes are very hard to do right, because parsers are
not aware they might be parsing something different.
The tvix-store protocol uses a canonical protobuf serialization, and uses
the [blake3][blake3] hash of that serialization to point to other `Directory`
messages.
It's both compact and with a wide range of libraries for encoders and decoders
in many programming languages.
The choice of protobuf makes it easy to add new fields, and make old clients
aware of some unknown fields being detected [^adding-fields].
#### blob
On disk, git blob objects start with a "blob" prefix, then the size of the
payload, and then the data itself. The hash of a blob is the literal sha1sum
over all of this - which makes it something very git specific to request for.
tvix-store simply uses the [blake3][blake3] hash of the literal contents
when referring to a file/blob, which makes it very easy to ask other data
sources for the same data, as no git-specific payload is included in the hash.
This also plays very well together with things like [iroh][iroh-discussion],
which plans to provide a way to substitute (large)blobs by their blake3 hash
over the IPFS network.
In addition to that, [blake3][blake3] makes it possible to do
[verified streaming][bao], as already described in other parts of the
documentation.
The git tree object format uses sha1 both for references to other trees and
hashes of blobs, which isn't really a hash function to fundamentally base
everything on in 2023.
The [migration to sha256][git-sha256] also has been dead for some years now,
and it's unclear what a "blake3" version of this would even look like.
[bao]: https://github.com/oconnor663/bao
[blake3]: https://github.com/BLAKE3-team/BLAKE3
[git-sha256]: https://git-scm.com/docs/hash-function-transition/
[iroh-discussion]: https://github.com/n0-computer/iroh/discussions/707#discussioncomment-5070197
[^adding-fields]: Obviously, adding new fields will change hashes, but it's something that's easy to detect.