10b: Building Views

We're going to set up a kappa architecture and a view for sorting messages by timestamp.

There are some pre-made general purpose views built on kappa-core, like a key-value store (kappa-view-kv) and a sorted list (kappa-view-list).

Here is an example of a sorted list view that sorts a message's timestamp field alphabetically. kappa-view-list sorts lexigraphically by default.

// kappa-chat.js

const kappa = require('kappa-core')
const memdb = require('memdb')
const list = require('kappa-view-list')

const timestampView = list(memdb(), function (msg, next) {
  if (msg.value.timestamp && typeof msg.value.timestamp === 'string') {
    // sort on the 'timestamp' field
    next(null, [msg.value.timestamp])
  } else {
    next()
  }
})

const core = kappa('./multichat', {valueEncoding: 'json'})
core.use('chats', timestampView)

Running core.use(title, view) installs the list view API under core.api.chats.

Exercise

Look at the API for kappa-view-list and write code that will print out every name in the view, in alphabetical order.

Tips

• core.api.chats.read() will return a Readable stream of entries in the list.
• You can always append test data using a local feed:

core.writer('local', function (err, feed) {
  feed.append({type: 'chat-message', timestamp: new Date().toISOString() })
})

Once you solve this exercise continue to exercise 11