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Building Video Chat with Elixir and Phoenix @anildigital ElixirConfEU 2018 Anil Wadghule

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About me ❤☕ $ ✈ ⽄

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https://skatter.me

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How a Video Chat works?

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WebRTC peer to peer

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Video data send WebRTC peer to peer

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Video data send Video data send Video data send WebRTC peer to peer

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What if fourth user joins video chat?

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Video data send Video data send Video data send WebRTC peer to peer

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Video data send Video data send Video data send Video data send WebRTC peer to peer Video data send

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How to communicate? • Hardcode IP addresses?

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Video data send Video data send Video data send Video data send Video data send WebRTC peer to peer 212.172.11.23 172.11.0.44 192.2.2.55 202.168.1.1

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Need of Signalling Server

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Signalling Server Phoenix as Signalling Server http://192.55.87.99:4000

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Signalling Server Phoenix as Signalling Server http://192.55.87.99:4000

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Signalling Server Phoenix as Signalling Server http://192.55.87.99:4000

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WebRTC Peer to Peer is not scalable

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Not scalable Phoenix as Signalling Server

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Problem? • Uplink: 4 UDP streams • Downlink: 4 UDP streams

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Solution - WebRTC Gateway Server

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WebRTC Gateway Server Phoenix as Signalling Server WebRTC Gateway Server

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Now • Uplink: 1 UDP stream • Downlink: 2 UDP streams

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Now • Uplink: 1 UDP stream • Downlink: X UDP streams

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Why Elixir & Phoenix?

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Why Elixir & Phoenix? • Elixir • OTP features - GenServer, Agent, GenEvent, GenStage, Supervisor • Phoenix • Phoenix for channels (signalling), web app basics • Authentication • Libraries

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Why Elixir & Phoenix? • Actor model • Battle tested OTP abstractions • Fan out • Fault Tolerant • Soft realtime

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Janus WebRTC Gateway Server

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Janus - WebRTC Gateway • Open source • Small footprint (C implementation) • Pluggable modules

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Janus Plugins

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Janus APIs • RESTful (HTTP) • WebSockets • RabbitMQ • MQTT • UnixSockets

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Using Janus RESTful/HTTP APIs • POST /janus (to create Janus Session) • POST Session (attach plugin to session) • POST Plugin (for other requests) • GET Session (for listening for events)

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Janus Video Room Events • slowlink • configured • talking (true/false) • publishers • leaving • unpublished • webrtcup • media (true/false) • hangup •

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Talking with Janus with Elixir

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elixir-janus client • HTTP client • State • Events https://github.com/ndarilek/elixir-janus

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• Session module • Plugin module • Util module elixir-janus client structure

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Phoenix application Session GenEvent Handler Plugin GenEvent Handler Janus WebRTC Gateway Server Elixir Janus Client Session Plugin State & GenEvent Manager State & GenEvent Manager HTTP Util HTTP calls Room create (returns Session Agent PID & Plugin Agent PID) Agent Agent

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import Janus.Util defmodule Janus.Session do @enforce_keys [:id, :base_url, :event_manager] defstruct [ :id, :base_url, :event_manager, handles: %{} ] …

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import Janus.Util defmodule Janus.Plugin do @enforce_keys [:id, :base_url, :event_manager] defstruct [ :id, :base_url, :event_manager ] …

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defmodule Janus.Session do
 … … def start(url) do case post(url, %{janus: :create}) do {:ok, body} -> id = body.data.id {:ok, event_manager} = GenEvent.start_link() session = %Janus.Session{ id: id, base_url: "#{url}/#{id}", event_manager: event_manager } Agent.start(fn -> session end) v -> v end end

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defmodule Janus.Session do
 … … def start(url) do case post(url, %{janus: :create}) do {:ok, body} -> id = body.data.id {:ok, event_manager} = GenEvent.start_link() session = %Janus.Session{ id: id, base_url: "#{url}/#{id}", event_manager: event_manager } Agent.start(fn -> session end) v -> v end end

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defmodule Janus.Session do
 … … def start(url) do case post(url, %{janus: :create}) do {:ok, body} -> id = body.data.id {:ok, event_manager} = GenEvent.start_link() session = %Janus.Session{ id: id, base_url: "#{url}/#{id}", event_manager: event_manager } Agent.start(fn -> session end) v -> v end end

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defmodule Janus.Session do
 … def attach_plugin(pid, id) do base_url = Agent.get(pid, &(&1.base_url)) v = case post(base_url, %{janus: :attach, plugin: id}) do {:ok, body} -> id = body.data.id {:ok, event_manager} = GenEvent.start_link() plugin = %Janus.Plugin{ id: id, base_url: "#{base_url}/#{id}", event_manager: event_manager } {:ok, plugin_pid} = Agent.start(fn -> plugin end) Agent.update pid, fn(session) -> new_handles = Map.put(session.handles, id, plugin_pid) %{ session | handles: new_handles} end

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defmodule Janus.Session do
 … def attach_plugin(pid, id) do base_url = Agent.get(pid, &(&1.base_url)) v = case post(base_url, %{janus: :attach, plugin: id}) do {:ok, body} -> id = body.data.id {:ok, event_manager} = GenEvent.start_link() plugin = %Janus.Plugin{ id: id, base_url: "#{base_url}/#{id}", event_manager: event_manager } {:ok, plugin_pid} = Agent.start(fn -> plugin end) Agent.update pid, fn(session) -> new_handles = Map.put(session.handles, id, plugin_pid) %{ session | handles: new_handles} end

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defmodule Janus.Session do
 … def attach_plugin(pid, id) do base_url = Agent.get(pid, &(&1.base_url)) v = case post(base_url, %{janus: :attach, plugin: id}) do {:ok, body} -> id = body.data.id {:ok, event_manager} = GenEvent.start_link() plugin = %Janus.Plugin{ id: id, base_url: "#{base_url}/#{id}", event_manager: event_manager } {:ok, plugin_pid} = Agent.start(fn -> plugin end) Agent.update pid, fn(session) -> new_handles = Map.put(session.handles, id, plugin_pid) %{ session | handles: new_handles} end

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defmodule Janus.Session do
 … def attach_plugin(pid, id) do base_url = Agent.get(pid, &(&1.base_url)) v = case post(base_url, %{janus: :attach, plugin: id}) do {:ok, body} -> id = body.data.id {:ok, event_manager} = GenEvent.start_link() plugin = %Janus.Plugin{ id: id, base_url: "#{base_url}/#{id}", event_manager: event_manager } {:ok, plugin_pid} = Agent.start(fn -> plugin end) Agent.update pid, fn(session) -> new_handles = Map.put(session.handles, id, plugin_pid) %{ session | handles: new_handles} end

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defmodule Janus.Session do
 … def add_handler(session, handler, args) do Agent.get session, &(GenEvent.add_handler(&1.event_manager, handler, args)) end

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defp poll(pid) do session = Agent.get pid, &(&1) spawn fn -> case get(session.base_url) do {:ok, data} -> event_manager = session.event_manager case data do %{janus: "keepalive"} -> GenEvent.notify(event_manager, {:keepalive, pid}) %{sender: sender} -> plugin_pid = session.handles[sender] if plugin_pid do case data do %{janus: "event", plugindata: plugindata} -> jsep = data[:jsep] Agent.get plugin_pid, &(GenEvent.notify(&1.event_manager, {:event, pid, plugin_pid, plugindata.data, jsep}))

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defp poll(pid) do session = Agent.get pid, &(&1) spawn fn -> case get(session.base_url) do {:ok, data} -> event_manager = session.event_manager case data do %{janus: "keepalive"} -> GenEvent.notify(event_manager, {:keepalive, pid}) %{sender: sender} -> plugin_pid = session.handles[sender] if plugin_pid do case data do %{janus: "event", plugindata: plugindata} -> jsep = data[:jsep] Agent.get plugin_pid, &(GenEvent.notify(&1.event_manager, {:event, pid, plugin_pid, plugindata.data, jsep}))

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defp poll(pid) do session = Agent.get pid, &(&1) spawn fn -> case get(session.base_url) do {:ok, data} -> event_manager = session.event_manager case data do %{janus: "keepalive"} -> GenEvent.notify(event_manager, {:keepalive, pid}) %{sender: sender} -> plugin_pid = session.handles[sender] if plugin_pid do case data do %{janus: "event", plugindata: plugindata} -> jsep = data[:jsep] Agent.get plugin_pid, &(GenEvent.notify(&1.event_manager, {:event, pid, plugin_pid, plugindata.data, jsep}))

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defp poll(pid) do session = Agent.get pid, &(&1) spawn fn -> case get(session.base_url) do {:ok, data} -> event_manager = session.event_manager case data do %{janus: "keepalive"} -> GenEvent.notify(event_manager, {:keepalive, pid}) %{sender: sender} -> plugin_pid = session.handles[sender] if plugin_pid do case data do %{janus: "event", plugindata: plugindata} -> jsep = data[:jsep] Agent.get plugin_pid, &(GenEvent.notify(&1.event_manager, {:event, pid, plugin_pid, plugindata.data, jsep}))

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defp poll(pid) do session = Agent.get pid, &(&1) spawn fn -> case get(session.base_url) do {:ok, data} -> event_manager = session.event_manager case data do %{janus: "keepalive"} -> GenEvent.notify(event_manager, {:keepalive, pid}) %{sender: sender} -> plugin_pid = session.handles[sender] if plugin_pid do case data do %{janus: "event", plugindata: plugindata} -> jsep = data[:jsep] Agent.get plugin_pid, &(GenEvent.notify(&1.event_manager, {:event, pid, plugin_pid, plugindata.data, jsep}))

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Phoenix application Session GenEvent Handler Plugin GenEvent Handler Janus WebRTC Gateway Server Elixir Janus Client Session Plugin State & GenEvent Manager State & GenEvent Manager HTTP Util HTTP calls Room create (returns Session Agent PID & Plugin Agent PID) Agent Agent

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Video Room start sequence • Create Janus Room (HTTP Call) • Init Session Agent PID and Plugin Agent PID • Store these Agents PIDs in Cache (Cache is GenServer based) • PIDs to further communicate to Janus

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Video Room start sequence • Create Janus Room (HTTP Call) • Init Session Agent PID and Plugin Agent PID • Store these Agents PIDs in Cache (Cache is GenServer based) • PIDs to further communicate to Janus

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Video Room start sequence • Create Janus Room (HTTP Call) • Init Session Agent PID and Plugin Agent PID • Store these Agents PIDs in Cache (Cache is GenServer based) • PIDs to further communicate to Janus

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Video Room start sequence • Create Janus Room (HTTP Call) • Init Session Agent PID and Plugin Agent PID • Store these Agents PIDs in Cache (Cache is GenServer based) • PIDs to further communicate with Janus

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Problems solved with Elixir

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Abrupt browser/tab close

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Browser 1 Browser 2 Browser 3

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Browser 1 Browser 2 Browser 3 Tab Closed

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Browser 2 Browser 3 Others still see the user present

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Browser 2 Browser 3 Appears there till minute

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Browser 2 Browser 3 After one minute

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How to solve this problem?

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DynamicSupervisor, GenServers & Monitors

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Browser Tab 1 Phoenix channel GenServer Plugin Agent Sesson Agent DynamicSupervisor Tab/Browser Closed

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Browser Tab 1 Phoenix channel GenServer Plugin Agent Sesson Agent DynamicSupervisor

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Browser Tab 1 Phoenix channel GenServer Plugin Agent Sesson Agent DynamicSupervisor

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Browser Tab 1 Phoenix channel GenServer Plugin Agent Sesson Agent DynamicSupervisor

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Browser Tab 1 Phoenix channel GenServer Plugin Agent Sesson Agent DynamicSupervisor

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Browser Tab 1 Phoenix channel GenServer Plugin Agent Sesson Agent DynamicSupervisor

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defmodule Janus.Session.GenServer do use GenServer def init(state) do %{url: _url, session: _session, handle: _handle, channel_pid: channel_pid} = state Process.monitor(channel_pid) send(self(), :setup) {:ok, state} end

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defmodule Janus.Session.GenServer do use GenServer def init(state) do %{url: _url, session: _session, handle: _handle, channel_pid: channel_pid} = state Process.monitor(channel_pid) send(self(), :setup) {:ok, state} end

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defmodule Janus.Session.GenServer do use GenServer … def handle_info({:DOWN, ref, :process, other_pid, _reason}, state) do %{url: _url, session: session, handle: handle, channel_pid: _channel_pid} = state cleanup(state) {:noreply, state} end def cleanup(state) do %{url: _url, session: session, handle: handle, channel_pid: _channel_pid} = state VideoroomCall.stop(session, handle) Process.exit(self(), :kill) state end

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defmodule Janus.Session.GenServer do use GenServer … def handle_info({:DOWN, ref, :process, other_pid, _reason}, state) do %{url: _url, session: session, handle: handle, channel_pid: _channel_pid} = state cleanup(state) {:noreply, state} end def cleanup(state) do %{url: _url, session: session, handle: handle, channel_pid: _channel_pid} = state VideoroomCall.stop(session, handle) Process.exit(self(), :kill) state end

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Demo

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What if Agent crashes?

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Agents can crash because • HTTP call fails • Exception in Event Handler code

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Browser 1 Browser 2 Browser 3

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Browser Tab 1 Phoenix channel GenServer Plugin Agent Sesson Agent DynamicSupervisor Crashed

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Browser Tab 1 Phoenix channel GenServer Plugin Agent Sesson Agent DynamicSupervisor

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Browser Tab 1 Phoenix channel GenServer Plugin Agent Sesson Agent DynamicSupervisor

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Browser Tab 1 Phoenix channel GenServer Plugin Agent Sesson Agent Phoenix channel receives DOWN message DynamicSupervisor

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Browser Tab 1 Phoenix channel Send JavaScript ‘Stop Call’ message DynamicSupervisor

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Browser Tab 1 Phoenix channel GenServer Plugin Agent Sesson Agent User starts call again DynamicSupervisor

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defmodule SkatterWeb.SkatterRoomChannel do use SkatterWeb, :channel … defp init_janus_room(jsep, room_id, recording) do room = Rooms.find_by_id(room_id) {:ok, session_server} = DynamicSupervisor.start_child(Janus.Supervisor,
 Janus.Session.GenServer.child_spec([{:channel_pid, self()}]))
 ref = Process.monitor(session_server)
 
 {session, plugin_pid} = Janus.Session.GenServer.start_session(session_server, room_name) …

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defmodule SkatterWeb.SkatterRoomChannel do use SkatterWeb, :channel … defp init_janus_room(jsep, room_id, recording) do room = Rooms.find_by_id(room_id) {:ok, session_server} = DynamicSupervisor.start_child(Janus.Supervisor,
 Janus.Session.GenServer.child_spec([{:channel_pid, self()}]))
 ref = Process.monitor(session_server)
 
 {session, plugin_pid} = Janus.Session.GenServer.start_session(session_server, room_name) …

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defmodule SkatterWeb.SkatterRoomChannel do use SkatterWeb, :channel … defp init_janus_room(jsep, room_id, recording) do room = Rooms.find_by_id(room_id) {:ok, session_server} = DynamicSupervisor.start_child(Janus.Supervisor,
 Janus.Session.GenServer.child_spec([{:channel_pid, self()}]))
 ref = Process.monitor(session_server)
 
 {session, plugin_pid} = Janus.Session.GenServer.start_session(session_server, room_name) …

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defmodule SkatterWeb.SkatterRoomChannel do use SkatterWeb, :channel … def handle_info({:DOWN, ref, :process, _pid, _reason}, socket) do room_name = get_room_name(socket) StandupsWeb.Endpoint.broadcast(room_name, "data", %{ type: "stop_call"}) {:noreply, socket} end …

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defmodule SkatterWeb.SkatterRoomChannel do use SkatterWeb, :channel … def handle_info({:DOWN, ref, :process, _pid, _reason}, socket) do room_name = get_room_name(socket) StandupsWeb.Endpoint.broadcast(room_name, "data", %{ type: "stop_call"}) {:noreply, socket} end …

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defmodule SkatterWeb.SkatterRoomChannel do use SkatterWeb, :channel … def handle_info({:DOWN, ref, :process, _pid, _reason}, socket) do room_name = get_room_name(socket) SkatterWeb.Endpoint.broadcast(room_name, "data", %{ type: "stop_call"}) {:noreply, socket} end …

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Demo

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Benefits of using Elixir • Useful abstractions • Control • Robust • Clarity

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@anildigital Questions? anil@anilwadghule.com Thank you!

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https://skatter.me