WebAssembly Smart Contracts for the Cosmos SDK.

The following packages are maintained here:

Crate	Usage	Download	Docs
cosmwasm-crypto	Internal only
cosmwasm-derive	Internal only
cosmwasm-schema	Contract development
cosmwasm-std	Contract development
cosmwasm-storage	Contract development
cosmwasm-vm	Host environments

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:

Standard library:

This code is compiled into Wasm bytecode as part of the smart contract.

• cosmwasm-std - A crate in this workspace. Provides the bindings and all imports needed to build a smart contract.
• cosmwasm-storage - A crate in this workspace. This optional addition to cosmwasm-std includes convenience helpers for interacting with storage.
• cw-storage-plus - A crate in cosmwasm-plus, which fills the same role as cosmwasm-storage, but with much more powerful types supporting composite primary keys, secondary indexes, automatic snapshotting, and more. This is newer and a bit less stable than cosmwasm-storage but used in most modern contracts.

Building contracts:

• cosmwasm-template - A starter-pack to get you quickly building your custom contract compatible with the cosmwasm system.
• cosmwasm-plus - Some sample contracts for use and inspiration. These provide usable primitives and interfaces for many use cases, such as fungible tokens, NFTs, multisigs, governance votes, staking derivatives, and more. Look in packages for docs on the various standard interfaces, and contracts for the implementations. Please submit your contract or interface via PR.
• rust-optimizer - 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 source code, allowing 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 crate in this workspace. Uses the wasmer engine to execute a given smart contract. Also contains code for gas metering, storing, and caching wasm artifacts.
• wasmvm - 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.
• wasmd - A basic Cosmos SDK app to host WebAssembly smart contracts. It can be run as is, or you can import the x/wasm module from it and use it in your blockchain. It is designed to be imported and customized for other blockchains, rather than forked.

You can see some examples of contracts under the contracts directory, which you can look at. They are simple and self-contained, primarily meant for testing purposes, but that also makes them easier to understand.

You can also look at cosmwasm-plus for examples and inspiration on more production-like contracts and also how we call one contract from another. If you are working on DeFi or Tokens, please look at the cw20, cw721 and/or cw1155 packages that define standard interfaces as analogues to some popular ERC designs. (cw20 is also inspired by erc777).

If you want to get started building you own contract, 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.

We also recommend you review our documentation site which contains a few tutorials to guide you in building your first contracts. We also do public workshops on various topics about once a month. You can find past recordings under the "Videos" section, or join our Discord server to ask for help.

See Minimum Supported Rust Version (MSRV).

WebAssembly 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 WebAssembly before, please read an overview on how to create imports and exports in general.

The required exports provided by the cosmwasm smart contract are:

// signal for 0.16 compatibility
extern "C" fn interface_version_7() -> () {}

// copy memory to/from host, so we can pass in/out Vec<u8>
extern "C" fn allocate(size: usize) -> u32;
extern "C" fn deallocate(pointer: u32);

// main contract entry points
extern "C" fn instantiate(env_ptr: u32, info_ptr: u32, msg_ptr: u32) -> u32;
extern "C" fn execute(env_ptr: u32, info_ptr: u32, msg_ptr: u32) -> u32;
extern "C" fn query(env_ptr: u32, msg_ptr: u32) -> u32;

Contracts may also implement one or more of the following to extend their functionality:

// in-place contract migrations
extern "C" fn migrate(env_ptr: u32, info_ptr: u32, msg_ptr: u32) -> u32;

// support submessage callbacks
extern "C" fn reply(env_ptr: u32, msg_ptr: u32) -> u32;

// expose privileged entry points to Cosmos SDK modules, not external accounts
extern "C" fn sudo(env_ptr: u32, msg_ptr: u32) -> u32;

// and to write an IBC application as a contract, implement these:
extern "C" fn ibc_channel_open(env_ptr: u32, msg_ptr: u32) -> u32;
extern "C" fn ibc_channel_connect(env_ptr: u32, msg_ptr: u32) -> u32;
extern "C" fn ibc_channel_close(env_ptr: u32, msg_ptr: u32) -> u32;
extern "C" fn ibc_packet_receive(env_ptr: u32, msg_ptr: u32) -> u32;
extern "C" fn ibc_packet_ack(env_ptr: u32, msg_ptr: u32) -> u32;
extern "C" fn ibc_packet_timeout(env_ptr: u32, msg_ptr: u32) -> u32;

allocate/deallocate allow the host to manage data within the Wasm VM. If you're using Rust, you can implement them by simply re-exporting them from cosmwasm::exports. instantiate, execute and query must be defined by your contract.

The imports provided to give the contract access to the environment are:

// This interface will compile into required Wasm imports.
// A complete documentation those functions is available in the VM that provides them:
// https://github.com/CosmWasm/cosmwasm/blob/0.7/lib/vm/src/instance.rs#L43
//
extern "C" {
    fn db_read(key: u32) -> u32;
    fn db_write(key: u32, value: u32);
    fn db_remove(key: u32);

    // scan creates an iterator, which can be read by consecutive next() calls
    #[cfg(feature = "iterator")]
    fn db_scan(start: u32, end: u32, order: i32) -> u32;
    #[cfg(feature = "iterator")]
    fn db_next(iterator_id: u32) -> u32;

    fn addr_validate(source_ptr: u32) -> u32;
    fn addr_canonicalize(source: u32, destination: u32) -> u32;
    fn addr_humanize(source: u32, destination: u32) -> u32;

    /// Verifies message hashes against a signature with a public key, using the
    /// secp256k1 ECDSA parametrization.
    /// Returns 0 on verification success, 1 on verification failure, and values
    /// greater than 1 in case of error.
    fn secp256k1_verify(message_hash_ptr: u32, signature_ptr: u32, public_key_ptr: u32) -> u32;

    /// Verifies a message against a signature with a public key, using the
    /// ed25519 EdDSA scheme.
    /// Returns 0 on verification success, 1 on verification failure, and values
    /// greater than 1 in case of error.
    fn ed25519_verify(message_ptr: u32, signature_ptr: u32, public_key_ptr: u32) -> u32;

    /// Verifies a batch of messages against a batch of signatures and public keys, using the
    /// ed25519 EdDSA scheme.
    /// Returns 0 on verification success, 1 on verification failure, and values
    /// greater than 1 in case of error.
    fn ed25519_batch_verify(messages_ptr: u32, signatures_ptr: u32, public_keys_ptr: u32) -> u32;

    /// Executes a query on the chain (import). Not to be confused with the
    /// query export, which queries the state of the contract.
    fn query_chain(request: u32) -> u32;

    /// Evaporates a specified amount of gas (1 evaporate = 1 sdk gas)
    fn gas_evaporate(evaporate: u32) -> u32;
}

(from imports.rs)

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

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

/// Refers to some heap allocated data in Wasm.
/// A pointer to an instance of this can be returned over FFI boundaries.
///
/// This struct is crate internal since the VM defined the same type independently.
#[repr(C)]
pub struct Region {
    pub offset: u32,
    /// The number of bytes available in this region
    pub capacity: u32,
    /// The number of bytes used in this region
    pub length: u32,
}

(from memory.rs)

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 instantiate and execute functions, defining your custom InstantiateMsg and ExecuteMsg structs for parsing your custom message types (as json):

#[entry_point]
pub fn instantiate(
  deps: DepsMut,
  env: Env,
  info: MessageInfo,
  msg: InstantiateMsg,
) -> Result<Response, ContractError> {}

#[entry_point]
pub fn execute(
  deps: DepsMut,
  env: Env,
  info: MessageInfo,
  msg: ExecuteMsg,
) -> Result<Response, ContractError> {}

#[entry_point]
pub fn query(deps: Deps, env: Env, msg: QueryMsg) -> Result<Binary, ContractError> {}

#[entry_point]
pub fn migrate(deps: DepsMut, env: Env, msg: MigrateMsg) -> Result<Response, ContractError> {}

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]);
  fn remove(&mut self, key: &[u8]);

  // and for iterating over a range of values
  #[cfg(feature = "iterator")]
  fn range<'a>(
    &'a self,
    start: Option<&[u8]>,
    end: Option<&[u8]>,
    order: Order,
  ) -> Box<dyn Iterator<Item = Pair>   'a>;
}

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.

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.

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, rust-optimizer that provides a docker image for building. For more info, look at rust-optimizer README, but the quickstart guide is:

docker run --rm -v "$(pwd)":/code \
  --mount type=volume,source="$(basename "$(pwd)")_cache",target=/code/target \
  --mount type=volume,source=registry_cache,target=/usr/local/cargo/registry \
  cosmwasm/rust-optimizer:0.11.4

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...

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

The ultimate auto-updating guide to building this project is the CI configuration in .circleci/config.yml.

For manually building this repo locally during development, here are a few commands. They assume you use a stable Rust version by default and have a nightly toolchain installed as well.

Workspace

./devtools/check_workspace.sh

Contracts