multi-dwebtree

Multi-writer dwebtree

This repository owes origins of its design to @RangerMauve's awesome multi-hyperdrive, and of course, the very awesome DWebTree and the whole of Hypercore.

About

A LevelUP compatible leaderless multi-master database with eventual consistency, using dwebtree + CRDT + HLC. Similarly CockroachDB achieves replication on top of RocksDB, but here it is a pure P2P streaming database, with zero central management. LevelDB compatibility allows to use Dynalite on top to achieve DynamoDB compatibility with multiple tables, auto-updated secondary indexes, and fairly complex queries combining those indexes. Work on @mhart's Dynalite is almost completed to remove the HTTP server, to make this combination perfect as an embedded database and for serverless scenarios.

The need

DWebTree is one-of-a-kind steaming database that can change the way we work with databases. But like all other low-level components of ddatabase ecosystem it is a single-writer data structure. Multi-writer is a higher-level abstraction, hence the name multi-dwebtree.

Use cases

• Multi-device support. One or more devices are personal cloud peers.
• Later we will consider a shared DB for a team

Usage

const MultiDTree = require('multi-dwebtree')

const hyperbeeOpts = { keyEncoding: 'utf-8', valueEncoding: 'json' }
const multiHyperbee = new MultiDTree(storage, hyperbeeOpts)

// Each app usually has its own key exchange mechanism with remote peers. So after exchange is completed, 
// we will know the keys of the peer's diff feeds. To receive updates from them, you need to add them here. Repeat for all remote peers.
{
  await multiHyperbee.addPeer(peerDiffFeedKey)
}  

Example

See example here.

API

const db = new MultiDTree(storage, [options], [customMergeHandler])

creates a new MultiDTree with two single-writer hypercores:

• Store - a dwebtree into which we will store objects created/changed locally or received from peers. This dwebtree is not replicated to peers. Multi-dwebtree's main goal is to achieve convergence, that is to keep this store in exactly the same state as store on other peers. This can't happen synchronously as peers are not expected to be only all the time, but eventually.
• Diff - here we store all changes made locally. Other peers replicate this and merge each diff into their own store. Options included:

{
  keyEncoding: 'utf-8' | 'binary' | 'ascii', // or some abstract encoding
  valueEncoding: <same as above>
}

customMergeHandler - CRDT handler to apply changes to the Object. If not using default, it should be implemented in a following way

class MergeHandler {
  constructor(store) {
    this.store = store
  }
  // It'll apply diff to the correct version of an object 
  merge(diff) {  
  }
  // That will generate diff based on the last version of the object in the store. 
  genDiff(oldValue, newValue) {
    return diff
  }
}

await db.put(key, storeValue)

storeValue - should be a JSON object

Put will write two objects at the same time to Store and to Diff dwebtree. Put will add to each of the objects following properties:

to Store object:

• _objectId - which is the same as key
• _timestamp - HLC timestamp
• _prevTimestamp - if the objct is not new

to Diff object

• _timestamp
• _prevTimestamp to the Diff.obj property if the Store object is not new

Diff object will be put with the key key/_timestamp

Diff object can be set as a property of the storeValue or it will be generated by MultiDTree based on the previous version of the resource in the Store (that still needs to ). If Diff is set as a property of the value it should be added to Object as property _diff. It will be deleted from the Store object and won't be a part of the Store object

Check the diff format here

Diff object that is written to diffDWebTree will have a key that is consists of 2 parts:

• storeValue key
• storeValue timestamp

const replicaPeer = await db.addPeer(replicaKey)

Created replica DWebTree using replica key

const replicaPeer = db.removePeer(replicaKey)

removes replica DWebTree

const stream = db.createUnionStream(key)

Use it for writing your own custom merge handler. It creates a union stream from all the replica hyperbees where all the entries are the Diff objects. It runs on each replica dwebtree.

// key has timestamp in it. To limit the search we define the top limit as a key without the timestamp

let lte = key.split('/').slice(0, -1).join('/')
createReadStream({gte: key, lte })

It is used by mergeHandler for applying changes to the Store object when for example:

• Say device was offline for some time,
• User made changes on his other devices.
• When device comes online again it needs to catch up with all other devices

the rest of API is the same as DWebTree

Roadmap

• MH - generate diff for insert/remove to array changes
• batch is not yet supported
• Tighten the non-atomic failure modes when process dies after writing to diff and before writing to store, or after reading from feed and applying to `store'.
• Support multiple bees, tries. We invision that peers will use one replication log to establish multi-writer for any number of shared data structures, that is for data structures local and remote peers can write into simultaneously. Using one replication log can help support atomic changes across multiple data structures.

Extend support to Hyperdrive

In this version we only add multi-writer to DWebTree. But we can extend it to Trie and Drive. Here are our thoughts on how this might work.

Previous version of the design did not have a diff feed and thus followed multi-hyperdrive's design more closely. Multi-hyperdrive does not apply updates to the primary, which we did even in the initial version of multi-dwebtree. Instead on each read it checks on the fly which file is fresher and returns that file (It checks in primary and in all the replicas from peers). It also supports on-the-fly merging of the directory listings, without changing any structures on disk. In this design we deviated even further as we needed to support CRDT merging.

To implement CRDT for Hyperdrive files, we might need to change multi-hyperdrive's design to use the diff feed:

• CRDT diff must apply to the local (primary) hyperdrive. Multi-hyperdrive does not do that, keeping all changed files in the replica.
• file diff in CRDT format is 3x the size of the changed data, so might warrant a second diff feed, to avoid slowing down DB replication. Hyperdrive uses 2 structures, hypertrie for directory and file metadata and ddatabase for data. So changes in hypertrie can be propagated via diff and.

Algorithm

This is a third interation of the design, previous is described and implemented in the release tagged v0.1.

In the prior design we had a primary dwebtree and sparse replicas of other peers' primary hyperbees. In the new design the full object is not replicated to the peers, only its diff (this design eliminated the ping-pong problem of the prior design as the store is not replicated).

In this design we have 2 hyperbees into which we write, store and diff. Store contains a full set of fresh objects (and their older versions, as does any ddatabase). Diff contains only the specially formatted objects (our custom CRDT format) that represent modifications to local objects. Every peer replicates other peers' diff hyperbees (but not the store). Upon diff dwebtree getting an update() event, we apply the received diff object to the store using the algo below:

For the CRDT algorithm to do its magic, we first rewind to the proper object version and then apply local diffs and a newly arrived remote diff:

• remote diff refers to the version of the object that was nodified on remote peer
• we find the same version of the object in store
• we find all diffs that were applied locally since that version
• we sort all local diffs and the remote diff by time, and apply them in that order
• new version of the object is put() into store

This algorithm ensures that all peers have the store in exactly the same state.

Cost and future optimizations

Read performance: equals normal dwebtree performance Write performance: quite expensive:

• Diff coming from replica is written to disk
• Union range query across all diff replicas and primary diff to find diffs since a particualr HLC time
• Reed matching version of the object in store
• Merge in memory and Write new version to store

Failure modes

Done

• HLC clock needs to be restored on restart
  • the HLC clock is restored from the timestamp of the last record in Store dwebtree
• Recovery after restart - persistent storage only
  • Multidwebtree has a manifest that contains the list of peers keys.
  • The key to the manifest is saved in the header block of the MultiDTree Store.
  • Peer keys are used to restore the peers in case of the restart.
• update() event on replica occured and computer died before we applied it to store. (Will it arrive again? - it does not)
  • this needs a test that simulates crash before Diff(s) processed and checks that thei are processed on restart.