# `RaftedValue.Data`
[🔗](https://github.com/skirino/rafted_value/blob/main/lib/rafted_value/data.ex#L1)

A behaviour that defines data to be replicated by `RaftedValue` servers.

It is required to implement a callback module of this behaviour to run a consensus group.
Concrete implementations of all callback functions of this behaviour must be [pure](https://en.wikipedia.org/wiki/Pure_function),
so that all members of a consensus group can reproduce the data by replicating only `command_arg`.
Therefore, for example, if you want to use "current time" in your command implementation,
you must not obtain current time in `command/1` implementation; you have to pass it by embedding in `command_arg`.
This is a design decision made to support arbitrary data structure while keeping Raft logs compact.

# `command_arg`

```elixir
@type command_arg() :: any()
```

# `command_ret`

```elixir
@type command_ret() :: any()
```

# `data`

```elixir
@type data() :: any()
```

# `query_arg`

```elixir
@type query_arg() :: any()
```

# `query_ret`

```elixir
@type query_ret() :: any()
```

# `command`

```elixir
@callback command(data(), command_arg()) :: {command_ret(), data()}
```

Generic read/write operation on the stored value.

This callback function is invoked by `RaftedValue.command/4`.
Commands are replicated across members of the consensus group and executed in all members
in order to reproduce the same history of stored value.
This function should return a pair of "return value to client" and "next version of stored data".

# `new`

```elixir
@callback new() :: data()
```

Creates an initial value to be stored.

Whenever a new consensus group is started by `RaftedValue.start_link/2` (called with `:create_new_consensus_group`),
this function is called to initialize the stored value.

# `query`

```elixir
@callback query(data(), query_arg()) :: query_ret()
```

Read-only operation on the stored value.

This callback function is invoked by `RaftedValue.query/3`.
This function should return "return value to client".

---

*Consult [api-reference.md](api-reference.md) for complete listing*