# CW2 Spec: Contract Info

Most of the CW\* specs are focused on the _public interfaces_
of the contract – the APIs used for `ExecuteMsg` or `QueryMsg`.
However, when we wish to migrate or inspect smart contract info,
we need some form of smart contract information embedded on state.

This is where CW2 comes in. It specifies a special Item to
be stored on disk by all contracts on `instantiate`.

`ContractInfo` must be stored under the `"contract_info"` key which translates
to `"636F6E74726163745F696E666F"` in hex format.
Since the state is well defined, we do not need to support any "smart queries".
We do provide a helper to construct a "raw query" to read the ContractInfo
of any CW2-compliant contract.

You can query using:

```shell
wasmd query wasm contract-state raw [contract_addr] 636F6E74726163745F696E666F --node $RPC
```

When the `migrate` function is called, then the new contract
can read that data and see if this is an expected contract we can
migrate from. And also contain extra version information if we
support multiple migrate paths.
This crate provides an `ensure_from_older_version` helper that
handles this.

### Data structures

**Required**

All CW2-compliant contracts must store the following data:

- key: `b"contract_info"` (i.e. not using [the length prefixed encoding](https://github.com/CosmWasm/cosmwasm/blob/main/docs/STORAGE_KEYS.md))
- data: JSON-serialized `ContractVersion`

```rust
pub struct ContractVersion {
    /// contract is a globally unique identifier for the contract.
    /// it should build off standard namespacing for whichever language it is in,
    /// and prefix it with the registry we use.
    /// For rust we prefix with `crates.io:`, to give us eg. `crates.io:cw20-base`
    pub contract: String,
    /// version is any string that this implementation knows. It may be simple counter "1", "2".
    /// or semantic version on release tags "v0.7.0", or some custom feature flag list.
    /// the only code that needs to understand the version parsing is code that knows how to
    /// migrate from the given contract (and is tied to it's implementation somehow)
    pub version: String,
}
```

Thus, an serialized example may looks like:

```json
{
  "contract": "crates.io:cw20-base",
  "version": "v0.1.0"
}
```