2 files changed, 357 insertions, 0 deletions
diff --git a/shard/lib/app/blockstore.ex b/shard/lib/app/blockstore.ex
new file mode 100644
index 0000000..8e4fddc
--- /dev/null
+++ b/shard/lib/app/blockstore.ex
@@ -0,0 +1,149 @@
+defmodule SApp.BlockStore do
+  @moduledoc """
+  A module that implements a content-adressable storage (blocks, or pages,
+  identified by the hash of their contents).
+
+  This is not a shard, it is a side process that a shard may use to store its data.
+
+  TODO: WIP
+  """
+
+  use GenServer
+
+  @enforce_keys [:pid]
+  defstruct [:pid, :prefer_ask]
+
+
+  defmodule State do
+    defstruct [:shard_id, :path, :store, :reqs, :retries]
+  end
+
+
+  def start_link(shard_id, path) do
+    GenServer.start_link(__MODULE__, [shard_id, path])
+  end
+
+  def init([shard_id, path]) do
+    Shard.Manager.dispatch_to(shard_id, path, self())
+
+    {:ok, %State{shard_id: shard_id, path: path, store: %{}, reqs: %{}, retries: %{}}}
+  end
+
+  def handle_call({:get, key, prefer_ask}, from, state) do
+    case state.store[key] do
+      nil ->
+        case prefer_ask do
+          [_ | _] ->
+            for peer <- prefer_ask do
+              Shard.Manager.send(peer, {state.shard_id, state.path, {:get, key}})
+            end
+          _ ->
+            ask_random_peers(state, key)
+        end
+        reqs_key = case state.reqs[key] do
+          nil ->
+            MapSet.put(MapSet.new(), from)
+          ms ->
+            MapSet.put(ms, from)
+        end
+        state = put_in(state.reqs[key], reqs_key)
+        {:noreply, state}
+      v ->
+        {:reply, v, state}
+    end
+  end
+
+  def handle_call({:put, val}, _from, state) do
+    hash = SData.term_hash val
+    state = %{state | store: Map.put(state.store, hash, val)}
+    {:reply, hash, state}
+  end
+
+  def handle_cast({:msg, peer_id, _shard_id, _path, msg}, state) do
+    state = case msg do
+      {:get, key} ->
+        case state.store[key] do
+          nil ->
+            Shard.Manager.send(peer_id, {state.shard_id, state.path, {:not_found, key}})
+          v ->
+            Shard.Manager.send(peer_id, {state.shard_id, state.path, {:info, key, v}})
+        end
+        state
+      {:info, hash, value} ->
+        if SData.term_hash value == hash do
+          reqs = case state.reqs[hash] do
+            nil -> state.reqs
+            pids ->
+              for pid <- pids do
+                GenServer.reply(pid, value)
+              end
+              Map.delete(state.reqs, hash)
+          end
+          state = %{state | retries: Map.delete(state.retries, hash)}
+          %{state | store: Map.put(state.store, hash, value), reqs: reqs}
+        else
+          state
+        end
+      {:not_found, key} ->
+        if state.reqs[key] != nil and state.store[key] == nil do
+          nretry = case state.retries[key] do
+            nil -> 1
+            n -> n+1
+          end
+          if nretry < 3 do
+            ask_random_peers(state, key)
+            %{state | retries: Map.put(state.retries, key, nretry)}
+          else
+            for pid <- state.reqs[key] do
+              GenServer.reply(pid, nil)
+            end
+            state = %{state | reqs: Map.delete(state.reqs, key)}
+            state = %{state | retries: Map.delete(state.retries, key)}
+            state
+          end
+        else
+          state
+        end
+    end
+    {:noreply, state}
+  end
+
+  def ask_random_peers(state, key) do
+    peers = :ets.lookup(:shard_peer_db, state.shard_id)
+      |> Enum.shuffle
+      |> Enum.take(3)
+    for {_, peer} <- peers do
+      Shard.Manager.send(peer, {state.shard_id, state.path, {:get, key}})
+    end
+  end
+
+
+  defimpl SData.PageStore do
+    def put(store, page) do
+      hash = GenServer.call(store.pid, {:put, page})
+      { hash, store }
+    end
+
+    def get(store, hash) do
+      try do
+        GenServer.call(store.pid, {:get, hash, store.prefer_ask})
+      catch
+        :exit, {:timeout, _} -> nil
+      end
+    end
+
+    def copy(store, other_store, hash) do
+      page = SData.PageStore.get(other_store, hash)
+      refs = SData.Page.refs(page)
+      for ref <- refs do
+        copy(store, other_store, ref)
+      end
+      GenServer.call(store.pid, {:put, page})
+      store
+    end
+
+    def free(store, _hash) do
+      store ## DO SOMETHING???
+    end
+  end
+end
diff --git a/shard/lib/app/chat.ex b/shard/lib/app/chat.ex
new file mode 100644
index 0000000..051fab6
--- /dev/null
+++ b/shard/lib/app/chat.ex
@@ -0,0 +1,208 @@
+defmodule SApp.Chat do
+  @moduledoc """
+  Shard application for a replicated chat room with full history.
+
+  Chat rooms are globally identified by their channel name.
+  A chat room manifest is of the form:
+
+      {:chat, channel_name}
+
+  Future improvements:
+  - message signing
+  - storage of the chatroom messages to disk
+  - storage of the known peers that have this channel to disk
+  - use a DHT to find peers that are interested in this channel
+  - epidemic broadcast (carefull not to be too costly,
+    maybe by limiting the number of peers we talk to)
+  - partial synchronization only == data distributed over peers
+  """
+
+  use GenServer
+
+  require Logger
+  alias SData.MerkleSearchTree, as: MST
+
+  @doc """
+  Start a process that connects to a given channel
+  """
+  def start_link(channel) do
+    GenServer.start_link(__MODULE__, channel)
+  end
+
+  @doc """
+  Initialize channel process.
+  """
+  def init(channel) do
+    manifest = {:chat, channel}
+    id = SData.term_hash manifest
+
+    case Shard.Manager.register(id, manifest, self()) do
+      :ok ->
+        Shard.Manager.dispatch_to(id, nil, self())
+        {:ok, block_store} = SApp.BlockStore.start_link(id, :block_store)
+        mst = %MST{store: %SApp.BlockStore{pid: block_store},
+                   cmp: &msg_cmp/2}
+        GenServer.cast(self(), :init_pull)
+        {:ok,
+          %{channel: channel,
+            id: id,
+            manifest: manifest,
+            block_store: block_store,
+            mst: mst,
+            subs: MapSet.new,
+          }
+        }
+      :redundant ->
+        exit(:redundant)
+    end
+  end
+
+  @doc """
+  Implementation of the :manifest call that returns the chat room's manifest
+  """
+  def handle_call(:manifest, _from, state) do
+    {:reply, state.manifest, state}
+  end
+
+  def handle_call({:read_history, top_bound, num}, _from, state) do
+    ret = MST.last(state.mst, top_bound, num)
+    {:reply, ret, state}
+  end
+
+  @doc """
+  Implementation of the :init_pull handler, which is called when the
+  process starts. It contacts all currently connected peers and asks them to
+  send data for this channel if they have some.
+  """
+  def handle_cast(:init_pull, state) do
+    for {_, pid, _, _} <- :ets.tab2list(:peer_db) do
+      GenServer.cast(pid, {:send_msg, {:interested, [state.id]}})
+    end
+    {:noreply, state}
+  end
+
+  @doc """
+  Implementation of the :chat_send handler. This is the main handler that is used
+  to send a message to the chat room. Puts the message in the store and syncs
+  with all connected peers.
+  """
+  def handle_cast({:chat_send, msg}, state) do
+    msgitem = {(System.os_time :seconds),
+               Shard.Identity.get_nickname(),
+               msg}
+    prev_root = state.mst.root
+    mst = MST.insert(state.mst, msgitem)
+    state = %{state | mst: mst}
+
+    for pid <- state.subs do
+      if Process.alive?(pid) do
+        send(pid, {:chat_send, state.channel, msgitem})
+      end
+    end
+
+    notif = {:append, prev_root, msgitem, mst.root}
+    for {_, peer_id} <- :ets.lookup(:shard_peer_db, state.id) do
+      Shard.Manager.send(peer_id, {state.id, nil, notif})
+    end
+
+    {:noreply, state}
+  end
+
+  @doc """
+  Implementation of the :interested handler, this is called when a peer we are
+  connected to asks to recieve data for this channel.
+  """
+  def handle_cast({:interested, peer_id}, state) do
+    Shard.Manager.send(peer_id, {state.id, nil, {:root, state.mst.root}})
+    {:noreply, state}
+  end
+
+  def handle_cast({:subscribe, pid}, state) do
+    Process.monitor(pid)
+    new_subs = MapSet.put(state.subs, pid)
+    {:noreply, %{ state | subs: new_subs }}
+  end
+
+  @doc """
+  Implementation of the :msg handler, which is the main handler for messages
+  comming from other peers concerning this chat room.
+
+  Messages are:
+  - `{:get, start}`: get some messages starting at a given Merkle hash
+  - `{:info, start, list, rest}`: put some messages and informs of the
+    Merkle hash of the store of older messages.
+  """
+  def handle_cast({:msg, peer_id, _shard_id, nil, msg}, state) do
+    state = case msg do
+      {:get_manifest} ->
+        Shard.Manager.send(peer_id, {state.id, nil, {:manifest, state.manifest}})
+        state
+      {:append, prev_root, msgitem, new_root} ->
+        # Append message: one single mesage has arrived
+        if new_root == state.mst.root do
+          # We already have the message, do nothing
+          state
+        else
+          # Try adding the message
+          if prev_root == state.mst.root do
+            mst2 = MST.insert(state.mst, msgitem)
+            if mst2.root == new_root do
+              # This was the only message missing, we are happy!
+              state = %{state | mst: mst2}
+              msg_callback(state, msgitem)
+              state
+            else
+              # More messages are missing, start a full merge
+              init_merge(state, new_root, peer_id)
+            end
+          else
+            init_merge(state, new_root, peer_id)
+          end
+        end
+      {:root, new_root} ->
+        if new_root == state.mst.root do
+          # already up to date, ignore
+          state
+        else
+          init_merge(state, new_root, peer_id)
+        end
+      x ->
+        Logger.info("Unhandled message: #{inspect x}")
+        state
+    end
+    {:noreply, state}
+  end
+
+  defp init_merge(state, new_root, source_peer) do
+    # TODO: make the merge asynchronous
+    mgmst = %{state.mst | root: new_root}
+    mgmst = put_in(mgmst.store.prefer_ask, [source_peer])
+    mst = MST.merge(state.mst, mgmst, fn msgitem, true -> msg_callback(state, msgitem) end)
+    %{state | mst: mst}
+  end
+
+  def handle_info({:DOWN, _ref, :process, pid, _reason}, state) do
+    new_subs = MapSet.delete(state.subs, pid)
+    {:noreply, %{ state | subs: new_subs }}
+  end
+
+  defp msg_callback(state, {ts, nick, msg}) do
+    for pid <- state.subs do
+      if Process.alive?(pid) do
+        send(pid, {:chat_recv, state.channel, {ts, nick, msg}})
+      end
+    end
+  end
+
+  defp msg_cmp({ts1, nick1, msg1}, {ts2, nick2, msg2}) do
+    cond do
+      ts1 > ts2 -> :after
+      ts1 < ts2 -> :before
+      nick1 > nick2 -> :after
+      nick1 < nick2 -> :before
+      msg1 > msg2 -> :after
+      msg1 < msg2 -> :before
+      true -> :duplicate
+    end
+  end 
+end