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Write overview

Beginner
Concept

ICP supports a wide range of application and architecture types. Apps can range from single smart contracts to complex, multi-canister projects that are controlled by a DAO, and everything in between. To begin writing and structuring your smart application, there are a few different workflows you can follow.

  • Standard workflow: The developer writes both the frontend code and the backend code. The frontend code is hosted on-chain in a canister and is served to the browser together with other web assets such as HTML and CSS. When the frontend code runs in the browser, it calls functions of the backend canister.

  • Framework-based workflow:

    • Juno is a community project that is tailored for Web2 developers. It takes care of hosting code and data in canisters such that developers can write Web3 applications using familiar Web2 concepts and patterns. For more details please follow the official Juno documentation.

    • Bitfinity EVM is tailored for Solidity developers. It is a canister that runs an instance of the Ethereum virtual machine and allows developers to upload and execute smart contracts written in Solidity. For more details please follow the official Bitfinity documentation.

This page will focus on the standard workflow of writing applications consisting of the frontend and the backend components.

Choosing the programming language for the backend​

The backend stores the application’s data and contains the core logic. Several languages are supported, such as:

  • Rust: supported by DFINITY. Among ICP languages, Rust is the language with the most production coverage. All system smart contracts, such as the DAO governing ICP, the ICP ledger, the Bitcoin and Ethereum integration smart contracts, are written in Rust. This language gives the developer full control over all aspects of the smart contract starting from performance to memory management. The only disadvantage of Rust is that it is lower-level compared to other languages and requires more expert programming skills to write safe and secure code. Learn more about using Rust.

  • Motoko: supported by DFINITY. Motoko is production ready and was specifically designed to onboard developers onto ICP and leverage the actor-based programming model of ICP. It is a high-level language with a garbage collector and syntax that is similar to TypeScript. Examples of production smart contracts that use Motoko: Sonic DEX, ICDex, Cycles manager. Learn more about using Motoko.

  • TypeScript (beta): supported by Demergent Labs under the name Azle. Currently Azle is in beta, but already allows writing smart contracts that run on ICP. Please check the Azle website for updates on when it will be ready for production.

  • Python (beta): supported by Demergent Labs under the name Kybra. Currently Kybra is in beta, but already allows writing smart contracts that run on ICP. Please check the Kybra website for updates on when it will be ready for production.

Choosing the web framework for the frontend​

The HTTP Gateway protocol of ICP allows browsers to load the web assets such as JS, HTML, CSS from a canister via HTTP. This means that web assets can be stored fully on-chain and developers don’t need traditional centralized web hosting to serve the UI of their application.

The typical development workflow of the frontend is:

  1. The developer writes HTML, JS, CSS code.
  2. The developer uses the default asset canister created by dfx or writes their own custom canister to store the web assets on-chain.
  3. Users open the application in the browser by navigating to the URL https://<canister-id>.ic0.app or a custom domain that the developer has registered for the canister.
  4. The canister serves the web assets to the browser via its http_request endpoint that gets invoked for each HTTP request.
  5. When the JS code runs in the browser, it can call the backend canister endpoints using the agent-js library, which is analogous to web3.js and ethers.js of Ethereum.

Since ICP supports general HTML, JS, and CSS code, any web framework should be compatible with ICP. SvelteKit, React, Vue have been used successfully in production. dfx v0.17.0 and newer can be used to automatically generate project templates using frameworks such as these. Learn more about using these frameworks.

One feature of modern web frameworks that currently doesn’t work in a canister is server-side rendering (SSR). That’s because SSR requires running JS code in the canister. In the future, this might become possible with Azle. Until then, if SSR is required, then one solution is to host the frontend outside of ICP while keeping the core logic in the backend canister.

Having no frontend at all is also a valid option for smart contracts that don’t have a UI and are callable only by other smart contracts.

Architecture considerations​

A common question when developing an application is how and where to store the data. In contrast to traditional platforms, ICP does not provide a database. Instead, ICP automatically persists changes in the canister state including its Wasm and stable memories. This means that developers have a lot of freedom in organizing and storing the data. The recommended practice is to use already existing libraries, such as the Rust stable-structures library, to store data in the stable memory.

Another question that developers should ask is how to structure their application’s canisters. It is possible to build an application consisting of multiple canisters that communicate with each other. A common pitfall for new developers is designing the application for millions of users from the get go without understanding the underlying trade-offs of the system. It is better to start with the simplest possible architecture and iteratively improve it with user growth.

Single canister architecture​

This is the simplest possible architecture and the recommended starting point for most developers. A single canister can host the entire application stack including its web assets, core logic, and data. To write a single canister that hosts frontend assets and backend core logic, you will need to use a library for the assets storage API, such as the ic-certified-assets library for Rust canisters. A few examples of single canister projects include:

Even though this architecture is simple, it can scale to thousands of users and gigabytes of data.

Note that by default, dfx generates a dual canister project where one canister is the backend canister and the other canister is the frontend canister that stores web assets. This is because having a dedicated canister for the frontend allows any language to be used for the backend canister without needing to use a library for the assets storage API.

Canister per subnet architecture​

ICP scales horizontally via subnets, so smart contracts can also scale by utilizing more subnets. One way to achieve this is to have one or multiple canisters per subnet and then shard data over these canisters to distribute the load. This is the most scalable architecture and could, in theory, support millions of users and terabytes of data. Since the application data and logic is distributed over multiple subnets, this requires expert knowledge of distributed programming. In other words, the cost of development and maintenance is much higher compared to the single canister architecture.

Canister per service architecture​

Canisters can be considered as microservices, where each canister is responsible for a specific service of the application such as managing users, storing data, or processing data. Note that all benefits and disadvantages of the traditional microservice architecture apply here as well. The default project structure that dfx generates can be viewed as the simplest microservice architecture with the frontend canister being responsible for serving web assets and the backend canister being responsible for the core logic and of the application.

Canister per user architecture​

This architecture is based on the vision that Web3 users should have full control over their data. The idea is to create a canister per user and make the user the controller of their canister. The main canister of the application would then orchestrate user canisters to implement the application’s functionality. Since users are controllers of their canisters, they can install their own code, decide how to participate in the application and determine what data to share. These user benefits come at large development costs because the main canister needs to be programmed in such a way that it can handle all possible actions of potentially malicious user canisters. This is a new and unprecedented way of programming. There hasn’t been a successful implementation of this vision yet. A couple of projects that opted for this architecture, but only NFID Vaults have given the ownership of canisters to the users. A common misconception is that the canister-per-user architecture is the most scalable, actually canister-per-subnet is more performant because it can utilize multiple subnets without having the overhead of too many canisters.