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# A Paxos Implementation

Andre Henriques, Seoeun Lee

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# Basis of the implentation


![Structure](./structure.png) ![Steps](./steps.png)

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# Steps

```elixir
receive do
  {:leader_elector, proc} -> ...
  {:propose, inst, value, t, pid_to_inform} -> ...
  {:prepare, proc, inst, ballot} -> ...
  {:nack, inst, ballot} -> ...
  {:prepared, inst, ballot, accepted_ballot, accepted_value} -> ...
  {:accept, inst, ballot, value} -> ...
  {:accepted, inst, ballot} -> ...
  {:decide, inst, value} -> ...
end
```

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### Step 1 - "Requirements"

![50% center](./first-step.png)

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### Step 1 - "Requirements" non-trivial aspects

```elixir
receive do
  {:propose, inst, value, t, pid_to_inform} ->
    ...
    broadcast({:other_propose, inst, value})
    ...
  {:other_propose, inst, value} -> ...
end
```

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### Step 2 - "Prepare"

![w:600 center](./second-step.png)

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### Step 2 - "Prepare" non-trivial aspects

```elixir
receive do
  {:nack, inst, ballot, new_ballot} ->
    ...
    broadcast({:abort, inst, ballot})
    ...
  {:abort, inst, ballot} -> ...
end
```

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### Step 3 - "Accept"

![w:600 center](./third-step.png)

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### Step 3 - "Accept" non-trivial aspects

```elixir
receive do
  {:accepted, inst, ballot} ->
    ...
    broadcast({:decide, inst, value})
    ...
  {:decide, inst, value} -> ...
end
```

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### Step 4 - "Leader Crash"

![w:600 center](./fourth-step.png)

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# Application

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### Atomas

![w:300 center top-m](./atomas.png)


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## Safety
 - If a game state exists, then it was created by a user
 - The game state can not divert.

## Liveness
 - Eventually all users will have the same game state

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## Interface

```elixir
receive do
  {:start_game, paticipants} -> {:new_game, game, game_state}
  {:make_move, game, participant, move} -> {:update_game, game, game_state} or {:game_finished, game}
  {:get_game_state, game} -> {:game_state, game, game_state}
end
```

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#### Questions?