This page is an overview on the state of smart contracts in Substrate.
The Substrate Runtime Module Library (SRML) provides two smart contract virtual machines which can be added to your Substrate runtime.
The SRML Contracts module provides functionality for the runtime to deploy and execute WebAssembly smart-contracts. It is designed to iterate on the design of modern smart contract platforms.
The SRML EVM module provides an EVM execution environment that allows unmodified EVM code to be executed in a Substrate-based blockchain. It is designed to most closely emulate the functionality of executing contracts on the Ethereum mainnet within the Substrate runtime.
Substrate is not a platform where you are limited with what comes out of the box.
We encourage further development of alternative smart contract platforms on top of the Substrate runtime. Use these pre-built modules to inform how you might design your own system or port over existing systems to work on a Substrate based chain.
Smart Contract Languages
Smart Contracts vs Runtime Modules
Substrate runtime modules and Substrate smart contracts are two different approaches to building "decentralized applications" using the Substrate framework.
A traditional smart contract platform allows users to publish additional logic on top of some core blockchain logic. Since smart contract logic can be published by anyone, including malicious actors and inexperienced developers, there are a number of intentional safe guards built around these public smart contract platform.
Some examples are:
- Fees: Ensuring that contract developers are charged for the computation and storage they force on the computers running their contract, and not allowed to abuse the block creators.
- Sandbox: A contract is not able to modify core blockchain storage or the storage of other contracts directly. It's power is limited to only modifying it's own state, and the ability to make outside calls to other contracts or runtime functions.
- State Rent: A contract takes up space on the blockchain, and thus should be charged for simply existing. This ensures that people don't take advantage of "free, unlimited storage".
- Reversion: A contract can be prone to have situations which lead to logical errors. The expectations of a contract developer are low, so extra overhead is added to support reverting transactions when they fail so no state is updated when things go wrong.
These different overheads makes running contracts slower and more costly, but again, the "target audience" for contract development is different than runtime developers.
Contracts can allow your community to extend and develop on top of your runtime logic without needing to go through all the craziness of proposals, runtime upgrades, etc... It may even be used as a testing grounds for future runtime changes, but done in a way that isolates your network from any of the growing pains or errors which may occur.
In summary, Substrate Smart Contracts:
- Are inherently safer to the network.
- Have built in economic incentives against abuse.
- Have computational overhead to support graceful failures in logic.
- Have a lower bar to entry for development.
- Enable fast pace community interaction through a playground to write new logic.
Runtime modules on the other hand afford none of these protections or safe guards that Smart Contracts give you. As a runtime developer, the bar to entry on the code you produce jumps way up.
You have full control of the underlying logic that each node on your network will run. You have full access to each and every storage item across all of your modules, which you can modify and control. You can even brick your chain with incorrect logic or poor error handling.
Substrate runtime module development has the intention of producing lean, performant, and fast nodes. It provides none of the protections or overhead of transaction reverting, and does not implicitly introduce any fee system to the computation which nodes on your chain run. This mean while you are developing runtime functions, it is up to you to correctly assess and apply fees to the different parts of your runtime logic such that it will not be abused by bad actors and hurt your network.
In summary, Substrate Runtime Modules:
- Provide low level access to your entire blockchain.
- Have removed the overhead of built-in safety for performance.
- Have a high bar to entry for developers.
- Not necessarily to write working code, but to avoid writing broken code.
- Has no inherent economic incentives to repel bad actors.
The Right Tool For You
Substrate runtime modules and Substrate smart contracts are tools made available to you to solve problems.
There is likely some amount of overlap in the kinds of problems each one can solve, but there is also a clear set of problems suited for only one of the two. To give just one example in each category:
- Runtime Module: Building a privacy layer on top of transactions in your blockchain.
- Shared: Building a gaming dApp which may need to build up a community of users (leaning toward Smart Contract), or may need to scale to millions of transactions a day (leaning toward Runtime Module).
- Smart Contract: Introducing multi-signature wallets over the currency of your blockchain.
In addition to everything written above, you also need to take into account the costs to set up a DApp using a certain tool. Deploying a contract is a relatively simple and easy process since you take advantage of the existing network. The only costs to you are those fees which you pay to deploy and maintain your contract.
Setting up your own blockchain on the other hand has the cost of building a community who find value in your service or establishing a private network with the overhead of cloud computing system and general network maintenance.
It is hard to provide direct guidance on every possible scenario, but use the table below to help influence your decision on the kinds of situations you may want to use these different tools:
|Runtime Module||Either||Smart Contract|
- See how Substrate is iterating on smart contract blockchains with the SRML Contracts module.
- Investigate the SRML EVM module to see if it is what you need for your next project.
- Read about the ink! smart contract language.
- Follow a tutorial to add a runtime module to your Substrate runtime.
- Learn how to start developing with the SRML Contracts module and ink!.