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Polling app

Application screenshot

We will be creating a simple poll application, in which a user can sign in, create polls, cast votes and see voting results. Creating a poll will cost 200 tokens, voting is free, and both actions will be available only for signed in users.

Requirements

For this tutorial we will be using Starport v0.14.0, an easy to use tool for building blockchains. To install starport into /usr/local/bin, run the following command:

curl https://get.starport.network/starport@v0.14.0! | bash

You can also use Starport v0.14.0 on the web in a browser-based IDE. Learn more about other ways to install Starport.

Creating a blockchain

Run the following command to create a voter project:

starport app github.com/alice/voter --sdk-version launchpad

Starport app command will scaffold a project structure for your application in a voter directory. Make sure to replace alice with your GitHub username.

Inside the voter directory we can see several files and directories:

cd voter
  • app contains files that connect all of the moving parts of your application.
  • cmd is responsible for voterd and votercli programs, which respectively allow you to start your application and interact with it.
  • vue contains a web user interface for your app, reponsible for everything you see on the screenshot above.
  • x contains the main building blocks of you app: modules. Right now we have only one: voter.

Our project's directory contains all the code required to build and launch a blockchain-based app. Let's try launching our app by running starport serve inside our project:

starport serve

You should be able to see the following output - as well as any errors that might show up in your application.

📦 Installing dependencies...
🚧 Building the application...
💫 Initializing the chain...
🙂 Created an account. Password (mnemonic): truth tooth front fabric sing divert zone milk fatigue urban hundred certain sorry merge milk treat foam coral absent run stand invest monkey aspect
🙂 Created an account. Password (mnemonic): famous faculty genre finger build fantasy squirrel icon carbon absent sleep weather fold piece thank earth pioneer excite trim cupboard grow pumpkin bundle auction
🌍 Running a Cosmos 'voter' app with Tendermint.

🚀 Get started: http://localhost:12345/

Note: use starport serve --verbose to visualize detailed operations happening in the background

Congratulations! You now have a blockchain application running on your machine in just two commands. It doesn't do anything yet, so let's work on that.

Our voting applications has two types of entities: polls and votes. A poll is a type that has a title and a list of options.

Adding polls

Open a new terminal in voter directory and run the following:

starport type poll title options

🎉 Created a type `poll`

This command generated code that handles the creation of poll items. If we now run starport serve and visit http://localhost:8080 we will see a form for creating polls. It may take a short while to rebuild the app, so give it a couple of seconds.

Application screenshot

Sign in with one of the passwords printed in the console and try creating a poll. You should see a new object created and displayed above the form. You have successfully created an object and stored it on the blockchain!

This, however, does not look and work exactly like we need. We should be able to add option fields (and store them as an array) and they should be displayed as interactive buttons.

Let's take a look at some of the files modified by the starport type command.

x/voter/types/TypePoll.go

This file contains definition of the Poll type. We can see that a poll has two fields (creator and ID), which will be created automatically, and two fields (title and options) defined by us. Since we want Options to be a list of strings, replace string with []string

package types

import (
	sdk "github.com/cosmos/cosmos-sdk/types"
)

type Poll struct {
	Creator sdk.AccAddress `json:"creator" yaml:"creator"`
	ID      string         `json:"id" yaml:"id"`
	Title   string         `json:"title" yaml:"title"`
	Options []string       `json:"options" yaml:"options"`
}

x/voter/types/MsgCreatePoll.go

This file defines a message that creates a poll.

We need to make options to be stored as a list instead of a string. Replace Options string with Options []string in MsgCreatePoll struct and options string with options []string in the arguments of NewMsgCreatePoll function.

package types

import (
	sdk "github.com/cosmos/cosmos-sdk/types"
	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
)

var _ sdk.Msg = &MsgCreatePoll{}

type MsgCreatePoll struct {
  Creator sdk.AccAddress `json:"creator" yaml:"creator"`
  Title string `json:"title" yaml:"title"`
  Options []string `json:"options" yaml:"options"`
}

func NewMsgCreatePoll(creator sdk.AccAddress, title string, options []string) MsgCreatePoll {
  return MsgCreatePoll{
		Creator: creator,
    Title: title,
    Options: options,
	}
}

...

To write anything to a blockchain or perform any other state transition a client (web app in our case) makes an HTTP POST request with a title and options to http://localhost:1317/voter/poll endpoint handler for which is defined in x/voter/client/rest/txPoll.go. The handler creates an unsigned transaction which contains an array of messages. The client then signs the transaction and sends it to http://localhost:1317/txs. The application then processes the transaction by sending each message to a corresponding handler, in our case x/voter/handlerMessageCreatePoll.go. A handler then calls a CreatePoll function defined in x/voter/keeper/poll.go which writes the poll data into the store.

x/voter/types/MsgSetPoll.go

Also in the MsgSetPoll we need to modify our Options string to []string

package types

import (
	sdk "github.com/cosmos/cosmos-sdk/types"
	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
)

var _ sdk.Msg = &MsgSetPoll{}

type MsgSetPoll struct {
	ID      string         `json:"id" yaml:"id"`
	Creator sdk.AccAddress `json:"creator" yaml:"creator"`
	Title   string         `json:"title" yaml:"title"`
	Options []string       `json:"options" yaml:"options"`
}

func NewMsgSetPoll(creator sdk.AccAddress, id string, title string, options []string) MsgSetPoll {
	return MsgSetPoll{
		ID:      id,
		Creator: creator,
		Title:   title,
		Options: options,
	}
}

...

x/voter/client/rest/txPoll.go

Replace Options string with Options []string in createPollRequest struct.

type createPollRequest struct {
	BaseReq rest.BaseReq `json:"base_req"`
	Creator string `json:"creator"`
	Title string `json:"title"`
	Options []string `json:"options"`
}

Also further below in the setPollRequest struct.

type setPollRequest struct {
	BaseReq rest.BaseReq `json:"base_req"`
	ID      string       `json:"id"`
	Creator string       `json:"creator"`
	Title   string       `json:"title"`
	Options []string     `json:"options"`
}

x/voter/client/cli/txPoll.go

A user will also be able to interact with our application through a command line interface.

votercli tx voter create-poll "Text editors" "Emacs" "Vim" --from user1

This command will generate a transaction with "create poll" message, sign it using a private key of user1 (one of two users created by default) and broadcast it to the blockchain.

The modification we need to make is to change a line that reads arguments from the console.

In the function GetCmdCreatePoll

replace

Args:  cobra.ExactArgs(2),

with

Args:  cobra.MinimumNArgs(2),
argsOptions := args[1:len(args)]

The variable msg is defined to read a string of argOptions, delete the stringification

msg := types.NewMsgCreatePoll(cliCtx.GetFromAddress(), string(argsTitle), argsOptions)

We end up with the following function

func GetCmdCreatePoll(cdc *codec.Codec) *cobra.Command {
	return &cobra.Command{
		Use:   "create-poll [title] [options]",
		Short: "Creates a new poll",
		Args:  cobra.MinimumNArgs(2),
		RunE: func(cmd *cobra.Command, args []string) error {
			argsTitle := string(args[0])
			argsOptions := args[1:len(args)]

			cliCtx := context.NewCLIContext().WithCodec(cdc)
			inBuf := bufio.NewReader(cmd.InOrStdin())
			txBldr := auth.NewTxBuilderFromCLI(inBuf).WithTxEncoder(utils.GetTxEncoder(cdc))
			msg := types.NewMsgCreatePoll(cliCtx.GetFromAddress(), string(argsTitle), argsOptions)
			err := msg.ValidateBasic()
			if err != nil {
				return err
			}
			return utils.GenerateOrBroadcastMsgs(cliCtx, txBldr, []sdk.Msg{msg})
		},
	}
}

The same changes will need to be done for the function GetCmdSetPoll

And in the GetCmdSetPoll we set

Args:  cobra.ExactArgs(3),

to

Args:  cobra.MinimumNArgs(3),
argsOptions := args[2:len(args)]
msg := types.NewMsgSetPoll(cliCtx.GetFromAddress(), id, string(argsTitle), argsOptions)

This will assume that all arguments after the first one represent a list of options.

We end up with the following function

func GetCmdSetPoll(cdc *codec.Codec) *cobra.Command {
	return &cobra.Command{
		Use:   "set-poll [id]  [title] [options]",
		Short: "Set a new poll",
		Args:  cobra.MinimumNArgs(3),
		RunE: func(cmd *cobra.Command, args []string) error {
			id := args[0]
			argsTitle := string(args[1])
			argsOptions := args[2:len(args)]

			cliCtx := context.NewCLIContext().WithCodec(cdc)
			inBuf := bufio.NewReader(cmd.InOrStdin())
			txBldr := auth.NewTxBuilderFromCLI(inBuf).WithTxEncoder(utils.GetTxEncoder(cdc))
			msg := types.NewMsgSetPoll(cliCtx.GetFromAddress(), id, string(argsTitle), argsOptions)
			err := msg.ValidateBasic()
			if err != nil {
				return err
			}
			return utils.GenerateOrBroadcastMsgs(cliCtx, txBldr, []sdk.Msg{msg})
		},
	}
}

Now that we have made all the necessary changes to our app, let's take a look at the client-side application.

starport serve

Front-end application

Starport has generated a basic front-end for our application. For convenience Vue.js framework is used with Vuex for state management, but since all features of our application are exposed through an HTTP API, clients can be built using any language or framework.

We'll be mostly interested in vue/src/views directory, which contains page templates of our app, vue/src/store/index.js handles sending transactions and receiving data from our blockchain and @tendermint/vue directory, which contains components, like buttons and forms.

Inside vue/src/store/index.js we import @tendermint/vue/src/store/cosmos which uses CosmJS, a library for handling wallets, creating, signing and broadcasting transactions and define a Vuex store. We'll use entitySubmit function for sending data to our blockchain (like a JSON representing a newly created poll), entityFetch for requesting a list of polls and accountUpdate to fetch information about our token balance.

vue/src/views/Index.vue

Since we don't need the default form component replace

<sp-type-form type="poll" :fields="['title', 'options', ]" module="voter" />

inside of vue/src/views/Index.vue with a new component and a Title

<SpH3>
 Polls
</SpH3>
<poll-form />

In the <script></script> tags below, we import our component like this

import * as sp from "@tendermint/vue";
import PollForm from "../components/PollForm";

export default {
  components: { PollForm, ...sp },
};

For our PollForm we need to create a new directory components in our vue/src/ path. In this directory we create a new file PollForm.vue and fill our first component with life.

vue/src/components/PollForm.vue

We create our PollForm component. Which will have a title and two buttons.

<template>
  <div>
    <sp-input placeholder="Title" v-model="title" />
    <div v-for="option in options">
      <sp-input placeholder="Option" v-model="option.title" />
    </div>
    <sp-button @click.native="add">+ Add option</sp-button>
    <sp-button @click.native="submit">Create poll</sp-button>
  </div>
</template>

in between <script></script> tags below this:

import * as sp from "@tendermint/vue";
export default {
  components: { ...sp },
  data() {
    return {
      title: "",
      options: []
    };
  },
  methods: {
    add() {
      this.options = [...this.options, { title: "" }];
    },
    async submit() {
      const payload = {
        type: "poll",
        module: "voter",
        body: {
          title: this.title,
          options: this.options.map(o => o.title)
        }
      };
      await this.$store.dispatch("cosmos/entitySubmit", payload);
        await this.$store.dispatch("cosmos/entityFetch", payload);
    }
  }
};

We also need to setup our Vue store.

vue/src/store/index.js

import Vue from "vue";
import Vuex from "vuex";
import cosmos from "@tendermint/vue/src/store/cosmos.js";

Vue.use(Vuex);

export default new Vuex.Store({
  modules: { cosmos },
});

In our main App.vue file, we make sure to initialize the Cosmos Wallet functions that we created as follows:

vue/src/App.vue

In the <script></script> tag at the end of the file, we dispatch to initialize our own app and the cosmos Vue framework

export default {
  created() {
    this.$store.dispatch("cosmos/init");
    this.$store.dispatch("cosmos/entityFetch", {type: "poll", module: "voter"});
  },
};

Refresh the page, sign in with a password and create a new poll. It takes a couple of seconds to process a transaction. Now, if you visit http://localhost:1317/voter/poll you should see a list of polls (this endpoint is defined in x/voter/rest/queryPoll.go):

{
  "height": "0",
  "result": [
    {
      "creator": "cosmos19qqa7j73735w4pcx9mkkaxr00af7p432n62tv6",
      "id": "826477ab-0005-4e68-8031-19758d331681",
      "title": "A poll title",
      "options": ["First option", "The second option"]
    }
  ]
}

Adding votes

A vote type contains poll ID and a value (string representation of the selected option).

starport type vote pollID value

vue/src/views/Index.vue

Delete the just bootstrapped for us <sp-type-form type="vote" :fields="['pollID', 'value', ]" module="voter" />. Add <poll-list /> into the vue/src/view/Index.vue file after the poll form component we just created.

Update the imports below in the <script></script> tags as follows:

import * as sp from "@tendermint/vue";
import PollForm from "../components/PollForm";
import PollList from "../components/PollList";

export default {
  components: { PollForm, PollList, ...sp },
};

Then make a new component at vue/src/components/PollList.vue and add the following:

vue/src/components/PollList.vue

<template>
  <div>
    
    <div v-for="poll in polls" v-bind:key="poll.id">
      <SpH3>
        Poll {{ poll.title }}
      </SpH3>
      <app-radio-item
        @click.native="submit(poll.id, option)"
        v-for="option in poll.options"
        v-bind:key="option"
        :value="option"
      />
      <app-text type="subtitle">Results: {{ results(poll.id) }}</app-text>
    </div>
  </div>
</template>

in between <script></script> tags below this:

import * as sp from "@tendermint/vue";
import AppRadioItem from "./AppRadioItem";
import AppText from "./AppText";
import {countBy } from "lodash"
export default {
  components: { AppText, AppRadioItem, ...sp },
  data() {
    return {
      selected: ""
    };
  },
  computed: {
    polls() {
      return this.$store.state.cosmos.data["voter/poll"] || [];
    },
    votes() {
      return this.$store.state.cosmos.data["voter/vote"] || [];
    }
  },
  methods: {
    results(id) {
      const results = this.votes.filter(v => v.pollID === id);
      return countBy(results, "value");
    },
    async submit(pollID, value) {
      const type = { type: "vote" };
      const body = { pollID, value };
      await this.$store.dispatch("cosmos/entitySubmit", { ...type, module:"voter", body });
      await this.$store.dispatch("cosmos/entityFetch", {...type, module: "voter"});
    }
  }
};

The PollList component lists for every poll, all the options for that poll, as buttons. Selecting an option triggers a submit method that broadcasts a transaction with a "create vote" message and fetches data back from our application.

Two components are still missing from our App, to make it a bit better looking. Let's add AppRadioItem.vue and AppText.vue.

vue/src/components/AppRadioItem.vue

<template>
  <div>
    <button class="button">{{ value }}</button>
  </div>
</template>

<style scoped>
.button {
  box-shadow: inset 0 0 0 1px rgba(0, 0, 0, 0.1);
  padding: 0.75rem 1rem;
  margin-bottom: 0.5rem;
  border-radius: 6px;
  user-select: none;
  cursor: pointer;
  transition: all 0.1s;
  font-weight: 500;
  outline: none;
  border: none;
  background: rgba(0, 0, 0, 0.01);
  width: 100%;
  font-family: inherit;
  text-align: left;
  font-size: 1rem;
  font-weight: inherit;
}
.button:hover {
  box-shadow: inset 0 0 0 1px rgba(0, 0, 0, 0.1),
    0 1px 5px -1px rgba(0, 0, 0, 0.1);
}
.button:focus {
  box-shadow: inset 0 0 0 1px rgba(0, 0, 0, 0.2),
    0 1px 5px -1px rgba(0, 0, 0, 0.1);
}
.button:active {
  box-shadow: inset 0 0 0 1px rgba(0, 0, 0, 0.07);
  color: rgba(0, 0, 0, 0.7);
  transform: scale(0.998);
}
</style>

In the <script></script> tag below this:

export default {
  props: {
    value: "",
  },
};

vue/src/components/AppText.vue

<template>
  <div>
    <div :class="[`${type}`]">
      <slot />
    </div>
  </div>
</template>

<style scoped>
.h1 {
  font-size: 2rem;
  font-weight: 800;
  letter-spacing: 0.03em;
  margin-bottom: 2rem;
}
.h2 {
  font-weight: 800;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-bottom: 0.5rem;
  margin-top: 2rem;
}
.subtitle {
  color: rgba(0, 0, 0, 0.5);
  font-size: 0.85rem;
}
</style>

In between <script></script> tags below this

export default {
  props: {
    type: {
      default: "p1",
    },
  },
};

Now in our App.vue we need to update to fetch our votes. In the <script></script> tag we should have the following resulting code:

vue/src/App.vue

export default {
  created() {
    this.$store.dispatch("cosmos/init");
    this.$store.dispatch("cosmos/entityFetch", {type: "poll", module: "voter"});
    this.$store.dispatch("cosmos/entityFetch", {type: "vote", module: "voter"});
  },
};

By now should be able to see the same UI as in the first screenshot. Try creating polls and casting votes. You may notice that it's possible to cast multiple votes for one poll. This is not what we want, so let's fix this behaviour.

Casting votes only once

To fix this issue we first have to understand how data is stored in our application.

We can think of our data storage as a lexicographically ordered key value store. You can loop through the entries, filter by key prefix, add, update and delete entries. It is easier to visualize the store as JSON:

{
  "poll-1a266241-c58d-4cbc-bacf-aaf939c95de1": {
    "creator": "cosmos15c6g4v5yvq0hy3kjllyr9ytlx45r936y0m6dm6",
    "id": "1a266241-c58d-4cbc-bacf-aaf939c95de1",
    "title": "Soft drinks",
    "options": ["Coca-Cola", "Pepsi"]
  },
  "vote-cd63b110-2959-45b0-8ce3-afa2fb7a5652": {
    "creator": "cosmos15c6g4v5yvq0hy3kjllyr9ytlx45r936y0m6dm6",
    "id": "cd63b110-2959-45b0-8ce3-afa2fb7a5652",
    "pollID": "1a266241-c58d-4cbc-bacf-aaf939c95de1",
    "value": "Pepsi"
  }
}

Both poll- and vote- are prefixes. They are added to keys for ease of filtering. By convention, prefixes are defined in x/voter/types/key.go.

x/voter/keeper/vote.go

Whenever a user casts a vote, a new "create vote" message is handled by a handler and is passed to a keeper. Keeper takes a vote- prefix, adds a UUID (unique to every message) and uses this string as a key. x/voter/keeper/vote.go:

key := []byte(types.VotePrefix + vote.ID)

These strings are unique and we get duplicate votes. To fix that we need to make sure a keeper records every vote only once by choosing the right key. In our case, we can use poll ID and creator address to make sure that one user can cast only one vote per poll.

key := []byte(types.VotePrefix + vote.PollID + "-" + string(vote.Creator))

Restart the application and try voting multiple times on a single poll, you'll see you can vote as many times as you want but only your most recent vote is counted.

Introducing a fee for creating polls

Let's make it so that creating a poll costs 200 tokens.

This feature is very easy to add. We already require users to have accounts registered, and each user has tokens on balance. The only thing we need to do is to send coins from user's account to a module account before we create a poll.

x/voter/handlerMsgCreatePoll.go:

package voter

import (
	sdk "github.com/cosmos/cosmos-sdk/types"

	"github.com/alice/voter/x/voter/keeper"
	"github.com/alice/voter/x/voter/types"
	"github.com/tendermint/tendermint/crypto"
)

func handleMsgCreatePoll(ctx sdk.Context, k keeper.Keeper, msg types.MsgCreatePoll) (*sdk.Result, error) {
	var poll = types.Poll{
		Creator: msg.Creator,
		Title:   msg.Title,
		Options: msg.Options,
	}
	moduleAcct := sdk.AccAddress(crypto.AddressHash([]byte(types.ModuleName)))
	payment, _ := sdk.ParseCoins("200token")
	if err := k.CoinKeeper.SendCoins(ctx, poll.Creator, moduleAcct, payment); err != nil {
		return nil, err
	}
	k.CreatePoll(ctx, msg)

	return &sdk.Result{Events: ctx.EventManager().Events()}, nil
}

The fee payment happens before k.CreatePoll(ctx, poll). This way, if a user does not have enough tokens, the application will raise an error and will not proceed to creating a poll. Make sure to have "github.com/tendermint/tendermint/crypto" added to the import statement (if your text editor didn't do that for you).

Now, restart the app and try creating several polls to see how this affects your token balance.

Congratulations, you have built a blockchain voting application.