Re: [Tarantool-patches] [PATCH vshard 6/7] router: make discovery smoother in a big cluster

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

On 08/05/2020 00:45, Konstantin Osipov wrote:
> * Vladislav Shpilevoy <v.shpilevoy@tarantool.org> [20/05/01 22:16]:
> 
> Why not use merkle trees to only fetch the changed subset?

This is what I found: https://en.wikipedia.org/wiki/Merkle_tree
"Every leaf node is labelled with the cryptographic hash of a data
block, and every non-leaf node is labelled with the cryptographic
hash in the labels of its child nodes. They can help ensure that
data blocks received from other peers in a peer-to-peer network
are received undamaged and unaltered, and even to check that the
other peers do not lie and send fake blocks."

Correct me if I found something wrong.

Firstly, hashes has nothing to do with that. Discovery fetches
bucket ids (in ranges, usually). And I still need to fetch bucket
ids. It can't dehash a value, received from the storage, into a
range of buckets.

Secondly, storage does not depend on router and can't keep a state
of every router on it. If you meant, that the storage should keep
something on it.

Thirdly, there is no in-place change, which would mean you just need
to fetch a new version of a bucket from the same storage. Change means
the bucket was moved to a different replicaset (in 99.99999% cases).
Deleted from one, and added on another. So you need to download it from
the new place.

Otherwise I probably didn't understand what you meant.