Skip to content

Hello Cloud

Hello Cloud is a simple dapp designed to help you learn the fundamentals of shardus. The function of the dapp is simple: allow the user to set some small state into the network (as a number) and then read that state back.

If you went through the quickstart guide, you will already have been introduced to the Hello Cloud dapp.


This app is a typescript version of the app you've seen on quickstart section


First, clone and install the repo:

git clone [email protected]:shardus/applications/hello-cloud.git
cd hello-cloud
npm install


Head on into the repo and open up index.ts. This is where you'll find all the real juicy goods. This repo has been made to be self explanatory - you'll find detailed explanation of all the ins and outs right there within the code.


Below you'll find more high level explanations. Once you finish reading through both the code and the below, you should have enough understanding of shardus to start developing your own dapps.

The Types

The most important type you'll find is the Account type. This type represents the way we store our data. This type will change the most from dapp to dapp.

type Account = {
  id: HexString
  timestamp: TimeStamp // represents last update time
  state: number

The id and timestamp fields will be universal across dapps, however the state field is totally up to us here in Hello Cloud. This is where we keep the number we're saving into the cloud. If we were developing a dapp that, say, tracked Koala Bear movement (for no particular reason at all), maybe instead of a state key, we'd have something more like { id, timestamp, name: string, age: number, lastSeenLocation: GeoPoint } or something like that.

We save our accounts into an object keyed with the accoundId. This is a convenient way to hold on to accounts when using an in memory representation like this and we'll find it across many shardus dapps. If we were saving into a database instead, we could add an index on accountId for fast lookups.

type Accounts = {
  [accountId: HexString]: Account

Every node in a shard has the same collection of Accounts, which it saves locally. Nodes in other shards will have a different collection of Accounts.

The Setup

Most of the real application logic goes into the shardus.setup({...}) call. Pay particular attention to the apply(...) and the updateAccountFull(...) functions.


apply(tx: Transaction, dataAccounts: WrappedAccounts) {
  dataAccounts[tx.accountId].data.state = tx.state
  const txId = hashTx(tx)
  return shardus.createApplyResponse(txId, tx.timestamp)

The network has agreed upon some state via its internal consensus protocols. The job of apply is to mutate that state based on an incoming transaction. Every node in a shard will compute this mutation and return it to the other nodes. If a majority of nodes' applys return the same new state, that state will become the new state in the network.

Take a look at this first line: dataAccounts[tx.accountId].data.state = tx.state. We're mutating dataAccounts, which is the the original state the network has before this transaction came in. This is how we tell the network the result of the the transaction.


updateAccountFull(wrappedState, localCache: Account, applyResponse) {
  const { accountId, accountCreated } = wrappedState
  const updatedAccount = as Account

  const hashBefore = accounts[accountId] ? hashAccount(accounts[accountId]) : '' // Can't pass in undefined
  const hashAfter  = hashAccount(updatedAccount)

  // Update our local state
  accounts[accountId] = updatedAccount

  // See more:

Once the nodes in the network have agreed upon what the new state should be by reaching consensus on their apply results, each node needs to update its internal representation of the state. It's in this function we do that, particularly in the line accounts[accountId] = updatedAccount.


We've covered some of the most important bits in here, but there is more going on in the code. We highly encourage you to read through the code in the Hello Cloud dapp, and in the next apps you'll find here in the examples guide, until you reach the level of grokking you desire.