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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