Writing the Contract
See also
To better understand the building blocks of the smart contract you will build in this tutorial, view the complete contract.
A smart contract can be considered an instance of a singleton object whose internal state is persisted on the blockchain. Users can trigger state changes through sending it JSON messages, and users can also query its state through sending a request formatted as a JSON message. These messages are different than XPLA Chain messages such as MsgSend
and MsgExecuteContract
.
As a smart contract writer, your job is to define 3 functions that define your smart contract’s interface:
instantiate()
: a constructor which is called during contract instantiation to provide initial stateexecute()
: gets called when a user wants to invoke a method on the smart contractquery()
: gets called when a user wants to get data out of a smart contract
In this section, you will define your expected messages alongside their implementation.
Start with a Template
In your working directory, quickly launch your smart contract with the recommended folder structure and build options by running the following commands:
# install cargo-generate
cargo install cargo-generate
# generate template
cargo generate --git https://github.com/CosmWasm/cw-template.git --branch 1.0 --name my-first-contract
cd my-first-contract
This helps get you started by providing the basic boilerplate and structure for a smart contract. You’ll find in the src/lib.rs
file that the standard CosmWasm entrypoints instantiate()
, execute()
, and query()
are properly exposed and hooked up.
Contract State
The starting template has the basic following state:
- a singleton struct
State
containing:- a 32-bit integer
count
- a XPLA Chain address
owner
- a 32-bit integer
// src/state.rs
use schemars::JsonSchema;
use serde::{Deserialize, Serialize};
use cosmwasm_std::Addr;
use cw_storage_plus::Item;
#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema)]
pub struct State {
pub count: i32,
pub owner: Addr,
}
pub const STATE: Item<State> = Item::new("state");
XPLA Chain smart contracts have the ability to keep persistent state through XPLA Chain’s native LevelDB, a bytes-based key-value store. As such, any data you wish to persist should be assigned a unique key at which the data can be indexed and later retrieved. The singleton in the example above is assigned the key config
(in bytes).
Data can only be persisted as raw bytes, so any notion of structure or data type must be expressed as a pair of serializing and deserializing functions. For instance, objects must be stored as bytes, so you must supply both the function that encodes the object into bytes to save it on the blockchain and the function that decodes the bytes back into data types that your contract logic can understand. The choice of byte representation is up to you, so long as it provides a clean, bi-directional mapping.
Fortunately, the CosmWasm team have provided utility crates such as cosmwasm_storage, which provides convenient high-level abstractions for data containers such as a “singleton” and “bucket”, which automatically provide serialization and deserialization for commonly-used types such as structs and Rust numbers.
Notice how the State
struct holds both count
and owner
. In addition, the derive
attribute is applied to auto-implement some useful traits:
Serialize
: provides serializationDeserialize
: provides deserializationClone
: makes the struct copyableDebug
: enables the struct to be printed to stringPartialEq
: provides equality comparisonJsonSchema
: auto-generates a JSON schema
Addr
, refers to a human-readable XPLA Chain address prefixed with xpla...
. Its counterpart is the CanonicalAddr
, which refers to a XPLA Chain address’s native decoded Bech32 form in bytes.
InstantiateMsg
The InstantiateMsg
is provided when a user creates a contract on the blockchain through a MsgInstantiateContract
. This provides the contract with its configuration as well as its initial state.
On the XPLA Chain, the uploading of a contract’s code and the instantiation of a contract are regarded as separate events, unlike on Ethereum. This is to allow a small set of vetted contract archetypes to exist as multiple instances sharing the same base code but configured with different parameters (imagine one canonical ERC20, and multiple tokens that use its code).
Example
For your contract, you will expect a contract creator to supply the initial state in a JSON message:
{
"count": 100
}
Message Definition
// src/msg.rs
use schemars::JsonSchema;
use serde::{Deserialize, Serialize};
#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema)]
pub struct InstantiateMsg {
pub count: i32,
}
Logic
Here you will define your first entry-point, the instantiate()
, or where the contract is instantiated and passed its InstantiateMsg
. Extract the count from the message and set up your initial state, where:
count
is assigned the count from the messageowner
is assigned to the sender of theMsgInstantiateContract
// src/contract.rs
#[cfg_attr(not(feature = "library"), entry_point)]
pub fn instantiate(
deps: DepsMut,
_env: Env,
info: MessageInfo,
msg: InstantiateMsg,
) -> Result<Response, ContractError> {
let state = State {
count: msg.count,
owner: info.sender.clone(),
};
set_contract_version(deps.storage, CONTRACT_NAME, CONTRACT_VERSION)?;
STATE.save(deps.storage, &state)?;
Ok(Response::new()
.add_attribute("method", "instantiate")
.add_attribute("owner", info.sender)
.add_attribute("count", msg.count.to_string()))
}
ExecuteMsg
The ExecuteMsg
is a JSON message passed to the execute()
function through a MsgExecuteContract
. Unlike the InstantiateMsg
, the ExecuteMsg
can exist as several different types of messages, to account for the different types of functions that a smart contract can expose to a user. The execute()
function demultiplexes these different types of messages to its appropriate message handler logic.
Example
Increment
Any user can increment the current count by 1.
{
"increment": {}
}
Reset
Only the owner can reset the count to a specific number.
{
"reset": {
"count": 5
}
}
Message Definition
As for your ExecuteMsg
, you will use an enum
to multiplex over the different types of messages that your contract can understand. The serde
attribute rewrites your attribute keys in snake case and lower case, so you’ll have increment
and reset
instead of Increment
and Reset
when serializing and deserializing across JSON.
// src/msg.rs
#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema)]
#[serde(rename_all = "snake_case")]
pub enum ExecuteMsg {
Increment {},
Reset { count: i32 },
}
Logic
// src/contract.rs
#[cfg_attr(not(feature = "library"), entry_point)]
pub fn execute(
deps: DepsMut,
_env: Env,
info: MessageInfo,
msg: ExecuteMsg,
) -> Result<Response, ContractError> {
match msg {
ExecuteMsg::Increment {} => try_increment(deps),
ExecuteMsg::Reset { count } => try_reset(deps, info, count),
}
}
This is your execute()
method, which uses Rust’s pattern matching to route the received ExecuteMsg
to the appropriate handling logic, either dispatching a try_increment()
or a try_reset()
call depending on the message received.
pub fn try_increment(deps: DepsMut) -> Result<Response, ContractError> {
STATE.update(deps.storage, |mut state| -> Result<_, ContractError> {
state.count += 1;
Ok(state)
})?;
Ok(Response::new().add_attribute("method", "try_increment"))
}
It is quite straightforward to follow the logic of try_increment()
. First, it acquires a mutable reference to the storage to update the singleton located at key b"config"
, made accessible through the config
convenience function defined in the src/state.rs
. It then updates the present state’s count by returning an Ok
result with the new state. Finally, it terminates the contract’s execution with an acknowledgement of success by returning an Ok
result with the Response
.
// src/contract.rs
pub fn try_reset(deps: DepsMut, info: MessageInfo, count: i32) -> Result<Response, ContractError> {
STATE.update(deps.storage, |mut state| -> Result<_, ContractError> {
if info.sender != state.owner {
return Err(ContractError::Unauthorized {});
}
state.count = count;
Ok(state)
})?;
Ok(Response::new().add_attribute("method", "reset"))
}
The logic for reset is very similar to increment – except this time, it first checks that the message sender is permitted to invoke the reset function.
QueryMsg
Example
The template contract only supports one type of QueryMsg
:
Balance
The request:
{
"get_count": {}
}
Which should return:
{
"count": 5
}
Message Definition
To support queries against the contract for data, you’ll have to define both a QueryMsg
format (which represents requests), as well as provide the structure of the query’s output – CountResponse
in this case. You must do this because query()
will send back information to the user through JSON in a structure and you must make the shape of your response known.
Add the following to your src/msg.rs
:
// src/msg.rs
#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema)]
#[serde(rename_all = "snake_case")]
pub enum QueryMsg {
// GetCount returns the current count as a json-encoded number
GetCount {},
}
// Define a custom struct for each query response
#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema)]
pub struct CountResponse {
pub count: i32,
}
Logic
The logic for query()
should be similar to that of execute()
, except that, since query()
is called without the end-user making a transaction, the env
argument is omitted as there is no information.
// src/contract.rs
#[cfg_attr(not(feature = "library"), entry_point)]
pub fn query(deps: Deps, _env: Env, msg: QueryMsg) -> StdResult<Binary> {
match msg {
QueryMsg::GetCount {} => to_binary(&query_count(deps)?),
}
}
fn query_count(deps: Deps) -> StdResult<CountResponse> {
let state = STATE.load(deps.storage)?;
Ok(CountResponse { count: state.count })
}
Building the Contract
To build your contract, run the following command. This will check for any preliminary errors before optimizing.
cargo wasm
Optimizing Your Build
Note
You will need Docker installed to run this command.
You will need to make sure the output WASM binary is as small as possible in order to minimize fees and stay under the size limit for the blockchain. Run the following command in the root directory of your Rust smart contract’s project folder.
cargo run-script optimize
If you are on an arm64 machine:
docker run --rm -v "$(pwd)":/code \
--mount type=volume,source="$(basename "$(pwd)")_cache",target=/code/target \
--mount type=volume,source=registry_cache,target=/usr/local/cargo/registry \
cosmwasm/rust-optimizer-arm64:0.12.6
If you are developing with a Windows exposed Docker daemon connected to WSL 1:
docker run --rm -v "$(wslpath -w $(pwd))":/code \
--mount type=volume,source="$(basename "$(pwd)")_cache",target=/code/target \
--mount type=volume,source=registry_cache,target=/usr/local/cargo/registry \
cosmwasm/rust-optimizer:0.12.6
This will result in an optimized build of artifacts/my_first_contract.wasm
or artifacts/my_first_contract-aarch64.wasm
in your working directory.
Note
Please note that rust-optimizer will produce different contracts on Intel and ARM machines. So for reproducible builds you’ll have to stick to one.
Schemas
In order to make use of JSON-schema auto-generation, you should register each of the data structures that you need schemas for.
// examples/schema.rs
use std::env::current_dir;
use std::fs::create_dir_all;
use cosmwasm_schema::{export_schema, remove_schemas, schema_for};
use my_first_contract::msg::{CountResponse, HandleMsg, InitMsg, QueryMsg};
use my_first_contract::state::State;
fn main() {
let mut out_dir = current_dir().unwrap();
out_dir.push("schema");
create_dir_all(&out_dir).unwrap();
remove_schemas(&out_dir).unwrap();
export_schema(&schema_for!(InstantiateMsg), &out_dir);
export_schema(&schema_for!(ExecuteMsg), &out_dir);
export_schema(&schema_for!(QueryMsg), &out_dir);
export_schema(&schema_for!(State), &out_dir);
export_schema(&schema_for!(CountResponse), &out_dir);
}
You can then build the schemas with:
cargo schema
Your newly generated schemas should be visible in your schema/
directory. The following is an example of schema/query_msg.json
.
{
"$schema": "http://json-schema.org/draft-07/schema#",
"title": "QueryMsg",
"anyOf": [
{
"type": "object",
"required": ["get_count"],
"properties": {
"get_count": {
"type": "object"
}
}
}
]
}
You can use an online tool such as JSON Schema Validator to test your input against the generated JSON schema.