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|
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
alias SData.MerkleList, as: ML
@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
store = ML.new(&msg_cmp/2)
manifest = {:chat, channel}
id = SData.term_hash manifest
GenServer.cast(Shard.Manager, {:register, id, manifest, self()})
GenServer.cast(self(), :init_pull)
{:ok,
%{channel: channel,
id: id,
manifest: manifest,
store: store,
peers: MapSet.new,
subs: MapSet.new,
}
}
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, start, num}, _from, state) do
ret = ML.read(state.store, start, num)
{:reply, ret, state}
end
@doc """
Implementation of the :redundant handler: if another process is already
synchronizing this channel then we exit.
"""
def handle_cast({:redundant, _}, _state) do
exit :normal
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}
new_state = %{state | store: ML.insert(state.store, msgitem)}
for pid <- state.subs do
if Process.alive?(pid) do
send(pid, {:chat_send, state.channel, msgitem})
end
end
for peer <- state.peers do
push_messages(new_state, peer, nil, 5)
end
{:noreply, new_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
push_messages(state, peer_id, nil, 10)
new_peers = MapSet.put(state.peers, peer_id)
{:noreply, %{ state | peers: new_peers }}
end
def handle_cast({:subscribe, pid}, state) do
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, msg}, state) do
case msg do
{:get_manifest} ->
Shard.Manager.send(peer_id, {state.id, {:manifest, state.manifest}})
{:get, start} -> push_messages(state, peer_id, start, 20)
{:info, _start, list, rest} ->
if rest != nil and not ML.has(state.store, rest) do
Shard.Manager.send(peer_id, {state.id, {:get, rest}})
end
who = self()
spawn_link(fn ->
Process.sleep 1000
GenServer.cast(who, {:deferred_insert, list})
end)
_ -> nil
end
if MapSet.member?(state.peers, peer_id) do
{:noreply, state}
else
handle_cast({:interested, peer_id}, state)
end
end
def handle_cast({:deferred_insert, list}, state) do
new_store = ML.insert_many(state.store, list, (fn msg -> msg_callback(state, msg) end))
{:noreply, %{state | store: new_store}}
end
defp push_messages(state, to, start, num) do
case ML.read(state.store, start, num) do
{:ok, list, rest} ->
Shard.Manager.send(to, {state.id, {:info, start, list, rest}})
_ -> nil
end
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
SData.MerkleList.cmp_ts_str({ts1, nick1<>"|"<>msg1},
{ts2, nick2<>"|"<>msg2})
end
end
|
