Merge PR cosmos#4998: [ADR-012] - State Accessors

docs/architecture/README.MD

@@ -29,3 +29,4 @@ Please add a entry below in your Pull Request for an ADR.
 - [ADR 006: Secret Store Replacement](./adr-006-secret-store-replacement.md)
 - [ADR 010: Modular AnteHandler](./adr-010-modular-antehandler.md)
 - [ADR 011: Generalize Genesis Accounts](./adr-011-generalize-genesis-accounts.md)
+- [ADR 012: State Accessors](./adr-012-state-accessors.md)

docs/architecture/adr-012-state-accessors.md

@@ -0,0 +1,155 @@
+# ADR 012: State Accessors
+
+## Changelog
+
+- 2019 Sep 04: Initial draft
+
+## Context
+
+SDK modules currently use the `KVStore` interface and `Codec` to access their respective state. While
+this provides a large degree of freedom to module developers, it is hard to modularize and the UX is
+mediocre.
+
+First, each time a module tries to access the state, it has to marshal the value and set or get the
+value and finally unmarshal. Usually this is done by declaring `Keeper.GetXXX` and `Keeper.SetXXX` functions,
+which are repetitive and hard to maintain.
+
+Second, this makes it harder to align with the object capability theorem: the right to access the
+state is defined as a `StoreKey`, which gives full access on the entire Merkle tree, so a module cannot
+send the access right to a specific key-value pair (or a set of key-value pairs) to another module safely.
+
+Finally, because the getter/setter functions are defined as methods of a module's `Keeper`, the reviewers
+have to consider the whole Merkle tree space when they reviewing a function accessing any part of the state.
+There is no static way to know which part of the state that the function is accessing (and which is not).
+
+## Decision
+
+We will define a type named `Value`:
+
+```go
+type Value struct {
+  m   Mapping
+  key []byte
+}
+```
+
+The `Value` works as a reference for a key-value pair in the state, where `Value.m` defines the key-value
+space it will access and `Value.key` defines the exact key for the reference.
+
+We will define a type named `Mapping`:
+
+```go
+type Mapping struct {
+  storeKey sdk.StoreKey
+  cdc      *codec.Codec
+  prefix   []byte
+}
+```
+
+The `Mapping` works as a reference for a key-value space in the state, where `Mapping.storeKey` defines
+the IAVL (sub-)tree and `Mapping.prefix` defines the optional subspace prefix.
+
+We will define the following core methods for the `Value` type:
+
+```go
+// Get and unmarshal stored data, noop if not exists, panic if cannot unmarshal
+func (Value) Get(ctx Context, ptr interface{}) {}
+
+// Get and unmarshal stored data, return error if not exists or cannot unmarshal
+func (Value) GetSafe(ctx Context, ptr interface{}) {}
+
+// Get stored data as raw byte slice
+func (Value) GetRaw(ctx Context) []byte {}
+
+// Marshal and set a raw value
+func (Value) Set(ctx Context, o interface{}) {}
+
+// Check if a raw value exists
+func (Value) Exists(ctx Context) bool {}
+
+// Delete a raw value value
+func (Value) Delete(ctx Context) {}
+```
+
+We will define the following core methods for the `Mapping` type:
+
+```go
+// Constructs key-value pair reference corresponding to the key argument in the Mapping space
+func (Mapping) Value(key []byte) Value {}
+
+// Get and unmarshal stored data, noop if not exists, panic if cannot unmarshal
+func (Mapping) Get(ctx Context, key []byte, ptr interface{}) {}
+
+// Get and unmarshal stored data, return error if not exists or cannot unmarshal
+func (Mapping) GetSafe(ctx Context, key []byte, ptr interface{})
+
+// Get stored data as raw byte slice
+func (Mapping) GetRaw(ctx Context, key []byte) []byte {}
+
+// Marshal and set a raw value
+func (Mapping) Set(ctx Context, key []byte, o interface{}) {}
+
+// Check if a raw value exists
+func (Mapping) Has(ctx Context, key []byte) bool {}
+
+// Delete a raw value value
+func (Mapping) Delete(ctx Context, key []byte) {}
+```
+
+Each method of the `Mapping` type that is passed the arugments `ctx`, `key`, and `args...` will proxy
+the call to `Mapping.Value(key)` with arguments `ctx` and `args...`.
+
+In addition, we will define and provide a common set of types derived from the `Value` type:
+
+```go
+type Boolean struct { Value }
+type Enum struct { Value }
+type Integer struct { Value; enc IntEncoding }
+type String struct { Value }
+// ...
+```
+
+Where the encoding schemes can be different, `o` arguments in core methods are typed, and `ptr` arguments
+in core methods are replaced by explicit return types.
+
+Finally, we will define a family of types derived from the `Mapping` type:
+
+```go
+type Indexer struct {
+  m   Mapping
+  enc IntEncoding
+}
+```
+
+Where the `key` argument in core method is typed.
+
+Some of the properties of the accessor types are:
+
+- State access happens only when a function which takes a `Context` as an argument is invoked
+- Accessor type structs give rights to access the state only that the struct is referring, no other
+- Marshalling/Unmarshalling happens implicitly within the core methods
+
+## Status
+
+Proposed
+
+## Consequences
+
+### Positive
+
+- Serialization will be done automatically
+- Shorter code size, less boilerplate, better UX
+- References to the state can be transferred safely
+- Explicit scope of accessing
+
+### Negative
+
+- Serialization format will be hidden
+- Different architecture from the current, but the use of accessor types can be opt-in
+- Type-specific types (e.g. `Boolean` and `Integer`) have to be defined manually
+
+### Neutral
+
+## References
+
+- [#4554](https://github.com/cosmos/cosmos-sdk/issues/4554)