kappa-view-trie v1.0.0

kappa-view-trie

Base for creating kappa-core views over LevelDB for view state and Hypertrie for data.

Materialized Views

A materialized view does two things:

1. A map function, that maps a list of log entries to modifications to a view. In this case, a LevelDB database for view state and Hypertrie for view data.
2. An api object, which are functions and variables that the view exposes in order to retrieve data from the Hypertrie that map writes to.

This module handles view lifecycle logic for you: normally a kappa view needs to manage storing and fetching the state of the indexer ("up to what log sequence numbers have I processed?"), as well as purging the view's LevelDB database when the version of the view gets bumped.

Example

This is a view for a very simple single-value key-value store, that maps log entries like { key: 'foo', value: 'bar' } to an API that allows count(prefix) queries.

var kappa = require('kappa-core')
var View = require('.')
var ram = require('random-access-memory')
var memdb = require('memdb')
const hypertrie = require('hypertrie')
const trie = hypertrie(ram, {valueEncoding: 'json'})

var core = kappa(ram, { valueEncoding: 'json' })
var state = memdb()

var view = View(state, trie, function (data) {
  return {
    map: function (entries, next) {
      const batch = entries.map(({value: {key, value}}) => {
        return { 
          type: 'put',
          key,
          value
        }
      })
      data.batch(batch, (err) => {
        if (err) return next(err)
        return next()
      })
    },
    
    api: {
      count: function (core, prefix, cb) {
        core.ready(function () {  // wait for all views to catch up
          data.list(prefix, (err, ret) => {
            if (err) return cb(err);
            const count = ret ? ret.length : 0;
            cb(null, count);
          })
        })
      }
    }
  };
})

core.use('mapper', view)

core.writer('default', function (err, feed) {
  feed.append({key: 'foo', value: 'bar'})
  feed.append({key: 'bax', value: 'baz'})
  feed.append({key: 'foo/bar', value: 'nix'})
  core.ready('mapper', function () {
    core.api.mapper.count('foo', console.log)
  })
})

outputs

null 2

API

var makeView = require('kappa-view')

var view = makeView(level, hypertrie, opts, setupFunction)

Create a new view, backed by LevelDB.

Expects a LevelUP or LevelDOWN instance level.

Expects a Hypertrie instance hypertrie.

setupFunction is a function that is given parameters db (LevelDB instance) and core (kappa-core instance). It is called exactly once. A kappa view must be returned, which is an object with the keys

• map: function (entries, next): receives an array of log entries. Once you've persisted whatever changes you'd like to db, call next() to signal the view is ready for the next batch of log entries.
• api: {}: an object that defines API functions that the view exposes. These can have whatever names you want, be sync or async, and return whatever you'd like.

Install

With npm installed, run

$ npm install kappa-view-trie

License

ISC