Merkle Tree Stream
We covered how Dat's Merkle Trees work in the Merkle Tree Chapter. In this chapter we'll discuss how to implement it, and what to look out for when optimizing.
At its core, the merkle tree stream is a stateful algorithm. It takes data in (leaf nodes), hashes it, and computes as many parent hashes as it can.
At its core this can be expressed as a stateful class that has a
.next() method. Internally it keeps track of three properties:
- An index to keep track of how far along in the sequence we are.
- A vector of hashes.
- The current roots.
To prevent allocations, both the vector of hashes and current roots could be passed in externally, but this is not a requirement.
merkle-tree-stream should operate as a generic structure without tying itself
to any particular hash function. This means that when creating an instance, the
methods have to be passed in. The following methods are generally required:
- leaf: takes data in, and produces a hash of the data.
- parent: takes two hashes, and produces a new hash.
In strictly typed languages, both the input
hash need to be passed
as type parameters into the generic function. In dynamically typed languages,
both can generally be expressed as byte array.
In practice it's perfectly fine to write the stream as a synchronous method.
For most implementations of Dat I/O typically tends to be the bottleneck, but on
a live system that might not always be the case (e.g. contention with other
resources). So to improve the overall performance of
can be useful to schedule work over multiple cores.
The first candidate for parallelization is the hashing of the leaf nodes. Unlike the parent nodes, the hashes in the leaf nodes are just functions of data, so they can be freely scheduled on different cores without any problem.
A bigger challenge is when creating hashes for the parent nodes. Because each parent depends on having two child nodes, it means you can't compute a parent node before its children have been computed. This means that that if we're only adding a single entry to the stream it cannot be parallelized, because each hash depends on the hash that came before it.
Take the following tree:
0──┐ 1──┐ 2──┘ │ 3 4──┐ │ 5──┘ 6──┘
Computing the hashes of
[0 2 4 6] are parallizable. Once those are complete,
[1 5] can be computed. And then
 can be computed.
The exact underlying mechanisms for storing and synchronizing nodes is a topic that could use more research. But starting with a read-write lock around the internal vector of hashes, and creating a task queue seems like the right starting point.