# cosmwasm-storage

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CosmWasm library with useful helpers for Storage patterns. You can use `Storage`
implementations in `cosmwasm-std`, or rely on these to remove some common
boilterplate.

## Contents

- [PrefixedStorage](#prefixed-storage)
- [TypedStorage](#typed-storage)
- [Bucket](#bucket)
- [Singleton](#singleton)

### Prefixed Storage

One common technique in smart contracts, especially when multiple types of data
are being stored, is to create separate sub-stores with unique prefixes. Thus
instead of directly dealing with storage, we wrap it and put all `Foo` in a
Storage with key `"foo" + id`, and all `Bar` in a Storage with key `"bar" + id`.
This lets us add multiple types of objects without too much cognitive overhead.
Similar separation like Mongo collections or SQL tables.

Since we have different types for `Storage` and `ReadonlyStorage`, we use two
different constructors:

```rust
use cosmwasm_std::testing::MockStorage;
use cosmwasm_storage::{prefixed, prefixed_read};

let mut store = MockStorage::new();

let mut foos = prefixed(b"foo", &mut store);
foos.set(b"one", b"foo");

let mut bars = prefixed(b"bar", &mut store);
bars.set(b"one", b"bar");

let read_foo = prefixed_read(b"foo", &store);
assert_eq!(b"foo".to_vec(), read_foo.get(b"one").unwrap());

let read_bar = prefixed_read(b"bar", &store);
assert_eq!(b"bar".to_vec(), read_bar.get(b"one").unwrap());
```

Please note that only one mutable reference to the underlying store may be valid
at one point. The compiler sees we do not ever use `foos` after constructing
`bars`, so this example is valid. However, if we did use `foos` again at the
bottom, it would properly complain about violating unique mutable reference.

The takeaway is to create the `PrefixedStorage` objects when needed and not to
hang around to them too long.

### Typed Storage

As we divide our storage space into different subspaces or "buckets", we will
quickly notice that each "bucket" works on a unique type. This leads to a lot of
repeated serialization and deserialization boilerplate that can be removed. We
do this by wrapping a `Storage` with a type-aware `TypedStorage` struct that
provides us a higher-level access to the data.

Note that `TypedStorage` itself does not implement the `Storage` interface, so
when combining with `PrefixStorage`, make sure to wrap the prefix first.

```rust
use cosmwasm_std::testing::MockStorage;
use cosmwasm_storage::{prefixed, typed};

let mut store = MockStorage::new();
let mut space = prefixed(b"data", &mut store);
let mut bucket = typed::<_, Data>(&mut space);

// save data
let data = Data {
    name: "Maria".to_string(),
    age: 42,
};
bucket.save(b"maria", &data).unwrap();

// load it properly
let loaded = bucket.load(b"maria").unwrap();
assert_eq!(data, loaded);

// loading empty can return Ok(None) or Err depending on the chosen method:
assert!(bucket.load(b"john").is_err());
assert_eq!(bucket.may_load(b"john"), Ok(None));
```

Beyond the basic `save`, `load`, and `may_load`, there is a higher-level API
exposed, `update`. `Update` will load the data, apply an operation and save it
again (if the operation was successful). It will also return any error that
occurred, or the final state that was written if successful.

```rust
let on_birthday = |mut m: Option<Data>| match m {
    Some(mut d) => {
        d.age += 1;
        Ok(d)
    },
    None => NotFound { kind: "Data" }.fail(),
};
let output = bucket.update(b"maria", &on_birthday).unwrap();
let expected = Data {
    name: "Maria".to_string(),
    age: 43,
};
assert_eq!(output, expected);
```

### Bucket

Since the above idiom (a subspace for a class of items) is so common and useful,
and there is no easy way to return this from a function (bucket holds a
reference to space, and cannot live longer than the local variable), the two are
often combined into a `Bucket`. A Bucket works just like the example above,
except the creation can be in another function:

```rust
use cosmwasm_std::StdResult;
use cosmwasm_std::testing::MockStorage;
use cosmwasm_storage::{bucket, Bucket};

fn people<'a, S: Storage>(storage: &'a mut S) -> Bucket<'a, S, Data> {
    bucket(b"people", storage)
}

fn do_stuff() -> StdResult<()> {
    let mut store = MockStorage::new();
    people(&mut store).save(b"john", &Data{
        name: "John",
        age: 314,
    })?;
    OK(())
}
```

### Singleton

Singleton is another wrapper around the `TypedStorage` API. There are cases when
we don't need a whole subspace to hold arbitrary key-value lookup for typed
data, but rather a single storage key. The simplest example is some
_configuration_ information for a contract. For example, in the
[name service example](https://github.com/CosmWasm/cosmwasm-examples/tree/master/nameservice),
there is a `Bucket` to look up name to name data, but we also have a `Singleton`
to store global configuration - namely the price of buying a name.

Please note that in this context, the term "singleton" does not refer to
[the singleton pattern](https://en.wikipedia.org/wiki/Singleton_pattern) but a
container for a single element.

```rust
use cosmwasm_std::{Coin, coin, StdResult};
use cosmwasm_std::testing::MockStorage;

use cosmwasm_storage::{singleton};

#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema)]
pub struct Config {
    pub purchase_price: Option<Coin>,
    pub transfer_price: Option<Coin>,
}

fn initialize() -> StdResult<()> {
    let mut store = MockStorage::new();
    let config = singleton(&mut store, b"config");
    config.save(&Config{
        purchase_price: Some(coin("5", "FEE")),
        transfer_price: None,
    })?;
    config.update(|mut cfg| {
        cfg.transfer_price = Some(coin(2, "FEE"));
        Ok(cfg)
    })?;
    let loaded = config.load()?;
    OK(())
}
```

`Singleton` works just like `Bucket`, except the `save`, `load`, `update`
methods don't take a key, and `update` requires the object to already exist, so
the closure takes type `T`, rather than `Option<T>`. (Use `save` to create the
object the first time). For `Buckets`, we often don't know which keys exist, but
`Singleton`s should be initialized when the contract is instantiated.

Since the heart of much of the smart contract code is simply transformations
upon some stored state, we may be able to just code the state transitions and
let the `TypedStorage` APIs take care of all the boilerplate.

## License

This package is part of the cosmwasm repository, licensed under the Apache
License 2.0 (see
[NOTICE](https://github.com/CosmWasm/cosmwasm/blob/master/NOTICE) and
[LICENSE](https://github.com/CosmWasm/cosmwasm/blob/master/LICENSE)).
