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
- 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
defmodule Manifest do
defstruct [:channel]
end
defimpl Shard.Manifest, for: Manifest do
def start(m) do
SApp.Chat.start_link(m.channel)
end
end
@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 = %Manifest{channel: channel}
id = SData.term_hash manifest
case Shard.Manager.register(id, manifest, self()) do
:ok ->
Shard.Manager.dispatch_to(id, nil, self())
{:ok, page_store} = SApp.PageStore.start_link(id, :page_store)
mst = %MST{store: %SApp.PageStore{pid: page_store},
cmp: &msg_cmp/2}
GenServer.cast(self(), :init_pull)
{:ok,
%{channel: channel,
id: id,
manifest: manifest,
page_store: page_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, _, _} <- Shard.Manager.list_peers 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 <- Shard.Manager.get_shard_peers(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}
GenServer.cast(state.page_store, {:set_roots, [mst2.root]})
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
prev_last = for {x, true} <- MST.last(state.mst, nil, 100), into: MapSet.new, do: x
mgmst = %{state.mst | root: new_root}
mgmst = put_in(mgmst.store.prefer_ask, [source_peer])
mst = MST.merge(state.mst, mgmst)
for {x, true} <- MST.last(mst, nil, 100) do
if not MapSet.member? prev_last, x do
msg_callback(state, x)
end
end
GenServer.cast(state.page_store, {:set_roots, [mst.root]})
%{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
