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#51 in WebAssembly

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CosmWasm

CircleCI Docs crates.io

Web Assembly Smart Contracts for the Cosmos SDK

This repo provides a useful functionality to build smart contracts that are compatible with Cosmos SDK runtime, currently being developed.

Overview

This crate provides the bindings and all imports needed to build a smart contract. However, to get that contract to interact with a system needs many moving parts. To get oriented, here is a list of the various components of the CosmWasm ecosystem:

  • cosmwasm - This crate. All needed functionality and no more - to build a small, efficient wasm smart contract.

Building contracts:

  • cosmwasm-template - A starter-pack to get you quickly building your custom contract compatible with the cosmwasm system.
  • cosmwasm-examples - Some sample contracts (build with cosmwasm-template) for use and inspiration. Please submit your contract via PR.
  • cosmwasm-opt - A docker image and scripts to take your rust code and produce the smallest possible wasm output. Deterministically This is designed both for preparing contracts for deployment as well as validating that a given deployed contract is based on some given rust code., allow a similar contract verification algorithm as etherscan.
  • serde-json-wasm - A custom json library, forked from serde-json-core. This provides an interface similar to serde-json, but without ay floating-point instructions (non-deterministic) and producing builds around 40% of the code size.

Executing contracts:

  • cosmwasm-vm - A sub-crate. Uses the wasmer engine to execute a given smart contract. Also contains code for gas metering, storing, and caching wasm artifacts. Read more here.
  • go-cosmwasm - High-level go bindings to all the power inside cosmwasm-vm. Easily allows you to upload, instantiate and execute contracts, making use of all the optimizations and caching available inside cosmwasm-vm.
  • Cosmos SDK - Currently an WIP fork targeting cosmos/modules to provide an wasm module you can easily plug into any Cosmos-SDK based application.

Ongoing work is currently tracked on this project board for all of the internals, and on this project board for the Cosmos-SDK integration work.

Creating a Smart Contract

You can see some examples of contracts under the contracts directory, which you can look at.

If you want to get started building you own, the simplest way is to go to the cosmwasm-template repository and follow the instructions. This will give you a simple contract along with tests, and a properly configured build environment. From there you can edit the code to add your desired logic and publish it as an independent repo.

If you want to understand a bit more, you can read some instructions on how we configure a library for wasm

API entry points

Web Assembly contracts are basically black boxes. The have no default entry points, and no access to the outside world by default. To make them useful, we need to add a few elements.

If you haven't worked with Web Assembly before, please read an overview on how to create imports and exports in general.

The actual exports provided by the cosmwasm smart contract are:

pub extern "C" fn init(params_ptr: *mut c_void, msg_ptr: *mut c_void) -> *mut c_void;
pub extern "C" fn handle(params_ptr: *mut c_void, msg_ptr: *mut c_void) -> *mut c_void;

pub extern "C" fn allocate(size: usize) -> *mut c_void;
pub extern "C" fn deallocate(pointer: *mut c_void);

(init and handle must be defined by your contract. De-allocate can simply be re-exported exports.rs)

And the imports provided to give you contract access to the environment are:

extern "C" {
    fn c_read(key: *const c_void, value: *mut c_void) -> i32;
    fn c_write(key: *const c_void, value: *mut c_void);
}

(from imports.rs)

You could actually implement a Web Assembly module in any language, and as long as you implement these 6 functions, it will be interoperable, given the JSON data passed around is the proper format.

Note that these *c_void pointers refers to a Slice pointer, containing the offset and length of some Wasm memory, to allow for safe access between the caller and the contract:

/// Slice refers to some heap allocated data in wasm.
/// A pointer to this can be returned over ffi boundaries.
#[repr(C)]
pub struct Slice {
    pub offset: u32,
    pub len: u32,
}

(from memory.rs)

Implementing the Smart Contract

If you followed the instructions above, you should have a runable smart contract. You may notice that all of the Wasm exports are taken care of by lib.rs, which should shouldn't need to modify. What you need to do is simply look in contract.rs and implement init and handle functions, defining your custom InitMsg and HandleMsg structs for parsing your custom message types (as json):

pub fn init<T: Storage>(store: &mut T, params: Params, msg: Vec<u8> -> 
  Result<Vec<CosmosMsg>, Error> { }

pub fn handle<T: Storage>(store: &mut T, params: Params, msg: Vec<u8> -> 
  Result<Vec<CosmosMsg>, Error> { }

The low-level c_read and c_write imports are nicely wrapped for you by a Storage implementation (which can be swapped out between real Wasm code and test code). This gives you a simple way to read and write data to a custom sub-database that this contract can safely write as it wants. It's up to you to determine which data you want to store here:

pub trait Storage {
    fn get(&self, key: &[u8]) -> Option<Vec<u8>>;
    fn set(&mut self, key: &[u8], value: &[u8]);
}

Testing the Smart Contract (rust)

For quick unit tests and useful error messages, it is often helpful to compile the code using native build system and then test all code except for the extern "C" functions (which should just be small wrappers around the real logic).

If you have non-trivial logic in the contract, please write tests using rust's standard tooling. If you run cargo test, it will compile into native code using the debug profile, and you get the normal test environment you know and love. Notably, you can add plenty of requirements to [dev-dependencies] in Cargo.toml and they will be available for your testing joy. As long as they are only used in #[cfg(test)] blocks, they will never make it into the (release) Wasm builds and have no overhead on the production artifact.

Note that for tests, you can use the MockStorage implementation which gives a generic in-memory hashtable in order to quickly test your logic. You can see a simple example how to write a test in our sample contract.

Testing the Smart Contract (wasm)

You may also want to ensure the compiled contract interacts with the environment properly. To do so, you will want to create a canonical release build of the <contract>.wasm file and then write tests in with the same VM tooling we will use in production. This is a bit more complicated but we added some tools to help in cosmwasm-vm which can be added as a dev-dependency.

You will need to first compile the contract using cargo wasm, then load this file in the integration tests. Take a look at the sample tests to see how to do this... it is often quite easy to port a unit test to an integration test.

Production Builds

The above build process (cargo wasm) works well to produce wasm output for testing. However, it is quite large, around 1.5 MB likely, and not suitable for posting to the blockchain. Furthermore, it is very helpful if we have reproducible build step so others can prove the on-chain wasm code was generated from the published rust code.

For that, we have a separate repo, cosmwasm-opt that provides a docker image for building. For more info, look at cosmwasm-opt README, but the quickstart guide is:

export CODE=/path/to/your/wasm/script
docker run --rm -u $(id -u):$(id -g) -v "${CODE}":/code confio/cosmwasm-opt:1.38

It will output a highly size-optimized build as contract.wasm in $CODE. With our example contract, the size went down to 126kB (from 1.6MB from cargo wasm). If we didn't use serde-json, this would be much smaller still...

Benchmarking

You may want to compare how long the contract takes to run inside the Wasm VM compared to in native rust code, especially for computationally intensive code, like hashing or signature verification.

TODO add instructions

Dependencies

~1.3–1.9MB
~43K SLoC