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18: Scalable dapp example


The CanCan sample application is a simplified video-sharing service that demonstrates several features that you can use as models for your own applications. For example, here are a few things you can learn by exploring the CanCan sample application:

  • How to build a scalable application by splitting content into fragments for upload and storage then using queries to retrieve and reassemble the fragments for efficient streaming.

  • How to configure interoperability for an application that uses canisters written in different backend languages.

  • How to implement a basic authentication model for storing videos uploaded by different users.

  • How to build a frontend that implements more sophisticated user interface features for desktop or mobile apps.

This dapp is designed with the following architecture and design choices:

  • Splitting uploaded content into multiple canisters: Canisters on ICP are compiled into WebAssembly modules which have certain known limitations. For example, currently WebAssembly modules have a maximum of 4GB for memory and an upper limit on the number of object calls allowed. For a video-sharing sample applications like CanCan, these limitations mean that multiple canisters are required and that data must be broken into smaller chunks for storage and retrieval.

  • Simplifying scalability: Because the Internet Computer protocol relies on replicated state across nodes in a subnet, it provides certain guarantees about fault tolerance and failover natively. The BigMap library provides a simple, plug-in library for building scalable applications using key-value storage on the Internet Computer. By using the BigMap library as a backend service, the CanCan sample application can dynamically chunk, serialize, and distribute data to multiple canisters.

  • Demonstrating interoperability: The BigMap service included in the CanCan sample application repository is written in the Rust programming language. However, the CanCan sample application also demonstrates interoperability between canisters written in different languages. By deploying different parts of the sample application as canisters, the interaction between them provides a seamless user experience.

Although the BigMap service you see in the CanCan repository is written in Rust, the service was actually implemented in both Rust and Motoko programming languages to demonstrate the following:

  • You can run both Motoko code and Rust code deployed as canisters on the Internet Computer.

  • You can switch between backend languages without affecting the operation of the CanCan sample application.

  • Both language implementation work seamlessly because the Candid language provides a common language for describing the BigMap API, independent of JavaScript, Rust, or Motoko.

  • Authentication model: This CanCan sample application uses the public-private key pair, browser-based local storage, and the Principal data type to authenticate users.

  • Implementing frontend features: The CanCan sample application uses the React library in combination with TypeScript to implement frontend user interface.

Data model overview

The application stores information about users and information about videos. To support most browsers, the videos are serialized into byte arrays with video data stored in 500KB segments of bytes that are referred to as Uint8Array objects. When a video is requested, the manifest loads the list of chunks required to play the video and concatenates the chunks before displaying the video in a standard <video> element.

User profiles

User profiles are stored as profiles/{username} and are defined using the following data model:

export interface Profile {
username: string; // maya
following: string[]; // [`alice`, `bob]`
followers: string[]; // ['palice']
uploadedVideos: string[]; // ['profiles/maya/videos/a5b54646-2ea3-4e0e-82d1-da3ab8148df2']
likedVideos: string[]; // ['profiles/bob/videos/b74e4eb0-dea8-4a4a-a1ae-d4593dc86930']
avatar?: string; // ?ImageData (TODO)


Uploaded videos are identified by a unique identifier stored at profiles/{username}/videos/{videoId} and in public/videos (an array of all existing videos in the dapp).

Metadata for videos is stored at profiles/{username}/videos/{videoId}/metadata Individual video fragments are stored at profiles/{username}/videos/{videoId}/chunks/chunk.{0-10}.

Videos are stored as profiles/{username} and are defined using the following data model:

export interface Video {
src: string; // 'profiles/maya/videos/f5a44646-2ea3-4e0f-83d2-da3ab8148df2'
userId: string; // 'maya'
createdAt: string; //
caption: string; // 'cool movie, punk'
tags: string[]; // ['outside', 'grilling', 'beveragino']
likes: string[]; // ['sam', 'kelly']
viewCount: number; // 102
name: string; // 'grilling'


Before getting started, assure you have set up your developer environment according to the instructions in the developer environment guide.

Additionally, the following packages are required for this guide:

Deploying the CanCan dapp

Double-check you have vessel installed at version 0.6.*, then clone this repository and navigate to the cancan directory.

vessel --version
# vessel 0.6.0

git clone
cd cancan

Start a local Internet Computer replica by running the following command:

dfx start --clean

Then, edit the dfx.json file to use the latest version of dfx:

  "dfx": "0.14.1"

Execute the following command in another terminal tab in the same directory:

npm install

Replace the contents of the ./ script with the following:


set -e
echo "Running bootstrap script..."

# Support bootstrapping CanCan from any folder
cd "$(dirname "$0")"


echo "dfx build"
npm run deploy

# echo "Running seed script..."
# echo "\nThis command may prompt you to install node to run.\nPlease accept and continue."
# npm run seed -- 2

URL="http://$(dfx canister id cancan_ui).$host:4943/"

echo "Open at $URL"

case $(uname) in
Linux) xdg-open $URL || true ;;
*) open $URL || true ;;

Next, edit the package-set.dhall file to update the upstream URL:

let upstream =

Then, run the bootstrap script:


This will deploy a local canister called cancan_ui_backend. To open the front-end, get the frontend canister id by running dfx canister id cancan_ui_frontend. Then open your browser, and navigate to http://<cancan_ui-canister-id>.localhost:8000/sign-in.

To run a development server with fast refreshing and hot-reloading, you can use this command in the app's root directory:

npm run start

Your default browser will open (or focus) a tab at localhost:3000, to which you must then append /?canisterId=${cancan_ui_canister_id}, where cancan_ui_canister_id is typically (at current) ryjl3-tyaaa-aaaaa-aaaba-cai.

Now you can make changes to any frontend code and see instant updates, in many cases not even requiring a page refresh, so UI state is preserved between changes. Occasionally adding a CSS rule won't trigger an update, and the user has to manually refresh to see those changes.

Next steps

To complete this guide, check out some additional sample dapps.