2018年10月29日

Phoenix_8_Channels

Phoenix channel 是一個 conversation,channel 會發送, 接收 messages,並保存 state,這些 messages 稱為 events,state 會存放在稱為 socket 的 struct 內。

conversation 就是 topic,就像是 chat room, local map , a game, or a video。超過一個人在同一時間對相同的主題有興趣,Channels 可用在多個 users 之間互通訊息。因為這是用 erlang isolated, dedicated process 實作的。

網頁的 request/response 是 stateless,但 conversation 是長時間運作的的 process,是 stateful 的。

Phoenix Clients with ES6

利用 ECMAScript 6 JavaScript 功能實作 client,ES6的code 可以 transpile 為 ES5,實作 client 對 video 新增 annotations,並發送給所有 users。

每一個 Phonenix conversation 是一個 Topic,因此要先確認,要以什麼為 Topic,目前是以 video 為 topic。

新增 /web/static/js/video.js

import Player from "./player"

let Video = {

  init(socket, element){ if(!element){ return }
    let playerId = element.getAttribute("data-player-id")
    let videoId  = element.getAttribute("data-id")
    socket.connect()
    Player.init(element.id, playerId, () => { 
      this.onReady(videoId, socket)
    })
  },

  onReady(videoId, socket){
    let msgContainer = document.getElementById("msg-container")
    let msgInput     = document.getElementById("msg-input")
    let postButton   = document.getElementById("msg-submit")
    let vidChannel   = socket.channel("videos:" + videoId)
    // TODO join the vidChannel
  }
}
export default Video

socket.connect() 可產生一個 websocket

注意 let vidChannel = socket.channel("videos:" + videoId) ,這是 ES6 Client 連接 Phoenix VideoChannel 的 channel。

Topic 需要一個 identifier,我們選用 "videos:" + videoId 這個格式,我們需要在 topic 內對其他相同 topic 的 users 發送 events。


修改 /web/static/js/app.js

import "phoenix_html"

import socket from "./socket"
import Video from "./video"

Video.init(socket, document.getElementById("video"))

如果瀏覽 http://localhost:4000/watch/2-elixir 時,js console 會出現這樣的錯誤訊息。

Unable to join – {reason: "unmatched topic"}

Preparing Our Server for the Channel

傳統 web request/response 每一次都會產生一個 connection,也就是 Plug.Conn,每個新的 request 都會有新的 conn,接下來用 pipeline 處理,最後 die。

channel 的流程跟上面的不同,client 會用 socket 建立 connection,在建立連線後,socket 會在整個 connection 的過程中持續被 transformed。socket 就是 client/server 之間持續運作的 conversation。

首先要決定是否能建立 connection,然後要產生 initial socket,包含所有 custom application setup。

修改 /web/static/js/socket.js

import {Socket} from "phoenix"

let socket = new Socket("/socket", {
  params: {token: window.userToken},
  logger: (kind, msg, data) => { console.log(`${kind}: ${msg}`, data) }
})

export default socket

let socket = new Socket("/socket".... 會讓 Phoenix 建立新的 socket。

查看 /lib/rumbl/endpoint.ex,有一個 /socket 的定義,UserSocket 就是處理 socket connection 的 module。

socket "/socket", Rumbl.UserSocket

先看一下 /web/channels/user_socket.ex 的內容

defmodule Rumbl.UserSocket do
  use Phoenix.Socket

  ## Transports
  transport :websocket, Phoenix.Transports.WebSocket
  # transport :longpoll, Phoenix.Transports.LongPoll

  def connect(_params, socket) do
    {:ok, socket}
  end

  def id(_socket), do: nil
end

UserSocket 使用 connection 處理所有 channel processes。 Phoenix 支援兩種 Transport protocols: websocket 或是 longpoll,也可以自訂一個。除了 transport 不同之外,其他的部分都是一樣的。

使用 shared socket abstraction,然後讓 Phoenix 處理其他的工作。

UserSocket 有兩個 functions: connect 及 id。id 是用來識別 socket,以便儲存不同的 state。目前 id 為 nil,connect 基本上就是接受所有連線。

接下來要利用 rumbl.Auth 增加 socket authentication。

如果再一次瀏覽網址 http://localhost:4000/watch/2-elixir,在 js console 就可看到此 debug message,表示已經連上 server。

transport: connected to ws://localhost:4000/socket/websocket?token=undefined&vsn=1.0.0

Creating the Channel

channel 就是 a conversation on a topic,topic 的 id 為 videos:video_id,我們希望 user 能取得某個 topic 的所有 events,也就是 video 的所有 annotations。

topic id 的一般形式就是 topic:subtopic,topic 為 resource name,subtopic 為 ID

因為 URL 就有參數,可以識別 conversation,也就是 :id


Joining a Channel

在 /web/channels/user_socket.ex 增加一行

channel "videos:*", Rumbl.VideoChannel

videos:* conversation 以 resource name 及 ID 作為 topic 的分類方式


Building the Channel Module

新增 /web/channels/video_channel.ex

defmodule Rumbl.VideoChannel do
  use Rumbl.Web, :channel

  def join("videos:" <> video_id, _params, socket) do
    {:ok, assign(socket, :video_id, String.to_integer(video_id))}
  end
end

channel 的第一個 callback 就是 join,clients 可 join topics on a channel,如果成功就回傳 {:ok, socket},拒絕連線就回傳 {:error, socket}

現在先讓所有 socket 可任意 join video topics,並新增 video ID (由 topic 取得) 到 socket.assigns。socket 會在 socket.assigns 儲存某個 conversation 的所有狀態 state。

socket 會被 transformed 為 loop,而不是一連串的 pipelines。當 events 進出 channel 時,可同時存取 socket state。

修改 /web/static/js/video.js

import Player from "./player"

let Video = {

    init(socket, element){ if(!element){ return }
        let playerId = element.getAttribute("data-player-id")
        let videoId  = element.getAttribute("data-id")
        socket.connect()
        Player.init(element.id, playerId, () => {
            this.onReady(videoId, socket)
    })
    },

    onReady(videoId, socket){
        let msgContainer = document.getElementById("msg-container")
        let msgInput     = document.getElementById("msg-input")
        let postButton   = document.getElementById("msg-submit")
        
        // 以 "videos:" videoId 產生新的 channel object
        let vidChannel   = socket.channel("videos:" + videoId)

        vidChannel.join()
            .receive("ok", resp => console.log("joined the video channel", resp) )
    .receive("error", reason => console.log("join failed", reason) )
    }
}
export default Video

如果再一次瀏覽網址 http://localhost:4000/watch/2-elixir,在 js console 就可看到

[Log] push: videos:2 phx_join (1) – {} (app.js, line 1586)
[Log] receive: ok videos:2 phx_reply (1) – {status: "ok", response: {}} (app.js, line 1586)
[Log] joined the video channel – {}

server 的 console log 為

[info] JOIN videos:2 to Rumbl.VideoChannel
  Transport:  Phoenix.Transports.WebSocket
  Parameters: %{}
[info] Replied videos:2 :ok

Sending and Receiving Event

在 channel 收到的訊息為 event name + payload + arbitrary data

channel 有三種接收訊息的方式:

  1. handle_in

    receives direct channel events

  2. handle_out

    intercepts broadcast events

  3. handle_info

    receives OTP messages


Taking Channels for a Trial run

目前是讓 join function 每 5 seconds 就發送一次 :ping message 到 channel

修改 /web/channels/video_channel.ex

defmodule Rumbl.VideoChannel do
  use Rumbl.Web, :channel

  def join("videos:" <> video_id, _params, socket) do
    :timer.send_interval(5_000, :ping)
    {:ok, socket}
  end

  # 當 elixir message 到達 channel 就會呼叫 handle_info
  # 目前每收到一次就將 :count +1
  def handle_info(:ping, socket) do
    count = socket.assigns[:count] || 1
    push socket, "ping", %{count: count}

     # :noreply 代表不發送 reply,並將 transformed 後的 socket 回傳回去
    {:noreply, assign(socket, :count, count + 1)}
  end
end

client 要對應修改 video.js,增加 vidChannel.on("ping", ({count}) => console.log("PING", count) )

    let vidChannel   = socket.channel("videos:" + videoId)

    vidChannel.on("ping", ({count}) => console.log("PING", count) )

    vidChannel.join()
      .receive("ok", resp => console.log("joined the video channel", resp) )
      .receive("error", reason => console.log("join failed", reason) )

js console 會持續看到

[Log] receive:  videos:2 ping  – {count: 1} (app.js, line 1586)
[Log] receive:  videos:2 ping  – {count: 2} (app.js, line 1586)
[Log] receive:  videos:2 ping  – {count: 3} (app.js, line 1586)
[Log] receive:  videos:2 ping  – {count: 4} (app.js, line 1586)

handle_info 就是 loop

client js 的部分是以 vidChannel.on(event, callback) 處理訊息

後面會看到怎麼用 handle_in 處理 synchronous messaging

controller 處理 request 而 channels hold a conversation


Annotating Videos

需要一個 Annotation model 儲存 user annotations

修改 /web/static/js/video.js

import Player from "./player"

let Video = {

  init(socket, element){ if(!element){ return }
    let playerId = element.getAttribute("data-player-id")
    let videoId  = element.getAttribute("data-id")
    socket.connect()
    Player.init(element.id, playerId, () => {
      this.onReady(videoId, socket)
    })
  },

  onReady(videoId, socket){
    let msgContainer = document.getElementById("msg-container")
    let msgInput     = document.getElementById("msg-input")
    let postButton   = document.getElementById("msg-submit")
    let vidChannel   = socket.channel("videos:" + videoId)

     // 處理 post 按鈕的 click event
     // 利用 vidChannel.push 發送 new_annotation
    postButton.addEventListener("click", e => {
      let payload = {body: msgInput.value, at: Player.getCurrentTime()}
      vidChannel.push("new_annotation", payload)        
                .receive("error", e => console.log(e) ) 
      msgInput.value = ""
    })

    // 接收 server 發送的 new_annotation,把 annotation 顯示在畫面 msgContainer 上
    vidChannel.on("new_annotation", (resp) => {         
      this.renderAnnotation(msgContainer, resp)
    })

    vidChannel.join()
      .receive("ok", resp => console.log("joined the video channel", resp) )
      .receive("error", reason => console.log("join failed", reason) )
  },

  // safely escape user input,可避免發生 XSS attack
  esc(str){ 
    let div = document.createElement("div")
    div.appendChild(document.createTextNode(str))
    return div.innerHTML
  },

  renderAnnotation(msgContainer, {user, body, at}){ 
    let template = document.createElement("div")

    template.innerHTML = `
    <a href="#" data-seek="${this.esc(at)}">
      <b>${this.esc(user.username)}</b>: ${this.esc(body)}
    </a>
    `
    msgContainer.appendChild(template)
    msgContainer.scrollTop = msgContainer.scrollHeight
  }
}
export default Video

Adding Annotation on the Server

修改 /web/channels/video_channel.ex

defmodule Rumbl.VideoChannel do
  use Rumbl.Web, :channel

  def join("videos:" <> video_id, _params, socket) do
    {:ok, socket}
  end

  # 處理 new_annotation,並 broadcast! 給目前 topic 的所有 users
  # broadcast! 有三個參數  socket, name of the event, payload (任意的 map)
  def handle_in("new_annotation", params, socket) do
    broadcast! socket, "new_annotation", %{
      user: %{username: "anon"},
      body: params["body"],
      at: params["at"]
    }

    # :reply 有兩種 :ok 或是 :error
    # 不然就是用 :noreply
    {:reply, :ok, socket}
  end
end

注意: 將原始的 message payload 直接轉送給其他人,而沒有 inspection,可能會有 security 問題

如果這樣寫,就有可能有資安問題

broadcast! socket, "new_annotation", Map.put(params, "user", %{
username: "anon"
})

現在打開兩個網頁,就可以互傳訊息

Socket Authentication

因為 channel 是 long-duration connection,利用 Phoenix.Token token authentication,可為每個 user 指定一個 unique token

不使用 session cookie 的原因是,可能會有 cross-domain attack。

因為已經有利用 Rumbl.Auth plug 增加的 current_user,現在要做的是利用 authenticated user 產生 token 並傳給 socket 前端。

首先修改 /web/templates/layout/app.html.eex,將 userToken 由 layout assigns 取出並放在 browser window 中

    </div> <!-- /container -->
    <script>window.userToken = "<%= assigns[:user_token] %>"</script>
    <script src="<%= static_path(@conn, "/js/app.js") %>"></script>

修改 /web/controllers/auth.ex

  def call(conn, repo) do
    user_id = get_session(conn, :user_id)

    cond do
      user = conn.assigns[:current_user] ->
        put_current_user(conn, user) 
      user = user_id && repo.get(Rumbl.User, user_id) ->
        put_current_user(conn, user)
      true ->
        assign(conn, :current_user, nil)
    end
  end

  def login(conn, user) do
    conn
    |> put_current_user(user) 
    |> put_session(:user_id, user.id)
    |> configure_session(renew: true)
  end

  # 將 current_user 及 user_token 放到 conn.assigns
  defp put_current_user(conn, user) do
    token = Phoenix.Token.sign(conn, "user socket", user.id)

    conn
    |> assign(:current_user, user)
    |> assign(:user_token, token)
  end

修改 /web/static/js/socket.js ,將 user token 傳入 socket.connect,並在 UserSocket.connect callback 中驗證 token。

import {Socket} from "phoenix"

let socket = new Socket("/socket", {
  // :params 會出現在 UserSocket.connect 的第一個參數
  params: {token: window.userToken},
  logger: (kind, msg, data) => { console.log(`${kind}: ${msg}`, data) }
})

export default socket

修改 /web/channles/user_socket.ex

defmodule Rumbl.UserSocket do
  use Phoenix.Socket

  ## Channels
  channel "videos:*", Rumbl.VideoChannel

  ## Transports
  transport :websocket, Phoenix.Transports.WebSocket
  # transport :longpoll, Phoenix.Transports.LongPoll

  # 2 weeks
  @max_age 2 * 7 * 24 * 60 * 60

  def connect(%{"token" => token}, socket) do
    case Phoenix.Token.verify(socket, "user socket", token, max_age: @max_age) do
      {:ok, user_id} ->
        {:ok, assign(socket, :user_id, user_id)}
      {:error, _reason} ->
        :error
    end
  end
  def connect(_params, _socket), do: :error

  def id(socket), do: "users_socket:#{socket.assigns.user_id}"
end

利用 Phoenix.Token.verify 檢查 token,可設定 max_age

如果 token 正確,就會收到 user_id 並存在 socket.assigns,回傳 {:ok, socket} 用以建立 connection。token 錯誤,就 return :error

refresh your page,application 還是能正常運作,但已經有了 user authentication

Persisting Annotations

建立 Annotation model,create annotations on videos,每個 annotation 會 belong to a user and a video

$ mix phoenix.gen.model Annotation annotations body:text at:integer user_id:references:users video_id:references:videos
* creating web/models/annotation.ex
* creating test/models/annotation_test.exs
* creating priv/repo/migrations/20170906163920_create_annotation.exs

Remember to update your repository by running migrations:

    $ mix ecto.migrate


$ mix ecto.migrate
Compiling 1 file (.ex)
Generated rumbl app
[info] == Running Rumbl.Repo.Migrations.CreateAnnotation.change/0 forward
[info] create table annotations
[info] create index annotations_user_id_index
[info] create index annotations_video_id_index
[info] == Migrated in 0.0s

還要處理 User, Video 的 relationships

修改 /web/models/user.ex 及 /web/models/video.ex,增加 has_many

has_many :annotations, Rumbl.Annotation

回到 /web/channels/video_channel.ex

defmodule Rumbl.VideoChannel do
  use Rumbl.Web, :channel

  def join("videos:" <> video_id, _params, socket) do
    {:ok, assign(socket, :video_id, String.to_integer(video_id))}
  end

  # 確保所有 events 都會有 current_user,然後再呼叫其他 handle_in
  def handle_in(event, params, socket) do 
    user = Repo.get(Rumbl.User, socket.assigns.user_id)
    handle_in(event, params, user, socket)
  end

  def handle_in("new_annotation", params, user, socket) do 
    # 以 new_annotation 產生 changeset 並透過 Repo 存到 DB
    changeset =
      user
      |> build_assoc(:annotations, video_id: socket.assigns.video_id)
      |> Rumbl.Annotation.changeset(params)

    case Repo.insert(changeset) do
      # insert 成功,才 broadcast 給所有 subscribers
      # 也可以用  {:noreply, socket} 不送 reply
      {:ok, annotation} ->
        broadcast! socket, "new_annotation", %{
          id: annotation.id,
          user: Rumbl.UserView.render("user.json", %{user: user}), 
          body: annotation.body,
          at: annotation.at
        }
        {:reply, :ok, socket}

      {:error, changeset} ->
        {:reply, {:error, %{errors: changeset}}, socket}
    end
  end
end

因為也需要 notify subscribers 該 user 的資訊,在 UserView 新增 user.json template

defmodule Rumbl.UserView do
  use Rumbl.Web, :view
  alias Rumbl.User

  def first_name(%User{name: name}) do
    name
    |> String.split(" ")
    |> Enum.at(0)
  end

  def render("user.json", %{user: user}) do
    %{id: user.id, username: user.username}
  end
end

現在新增 annotation, server log 就會出現

INSERT INTO `annotations` (`at`,`body`,`user_id`,`video_id`,`inserted_at`,`updated_at`) VALUES (?,?,?,?,?,?) [0, "test", 1, 2, {{2017, 9, 6}, {16, 49, 10, 444088}}, {{2017, 9, 6}, {16, 49, 10, 447017}}]
[debug] QUERY OK db=0.6ms

在 refresh page 後,annotations 就會消失,所以在 user join channel 時,要把 messages 送給 client

修改 /web/channels/video_channel.ex,改寫 join,取得 video's annotations

defmodule Rumbl.VideoChannel do
  use Rumbl.Web, :channel
  alias Rumbl.AnnotationView

  def join("videos:" <> video_id, _params, socket) do
    video_id = String.to_integer(video_id)
    video = Repo.get!(Rumbl.Video, video_id)

    # 要 preload user
    annotations = Repo.all(
      from a in assoc(video, :annotations),
        order_by: [asc: a.at, asc: a.id],
        limit: 200,
        preload: [:user]
    )

    resp = %{annotations: Phoenix.View.render_many(annotations, AnnotationView,
                                                   "annotation.json")}
    {:ok, resp, assign(socket, :video_id, video_id)}
  end

  def handle_in("new_annotation", params, socket) do
    user = Rumbl.Repo.get(Rumbl.User, socket.assigns.user_id)

    changeset =
      user
      |> build_assoc(:annotations, video_id: socket.assigns.video_id)
      |> Rumbl.Annotation.changeset(params)

    case Repo.insert(changeset) do
      {:ok, annotation} ->
        broadcast! socket, "new_annotation", %{
          id: annotation.id,
          user: Rumbl.UserView.render("user.json", %{user: user}),
          body: annotation.body,
          at: annotation.at
        }
        {:reply, :ok, socket}

      {:error, changeset} ->
        {:reply, {:error, %{errors: changeset}}, socket}
    end
  end
end

Phoenix.View.render_many 能夠 collects the render results for all elements in the enumerable passed to it

新增 /web/views/annotation_view.ex

defmodule Rumbl.AnnotationView do
  use Rumbl.Web, :view

  def render("annotation.json", %{annotation: ann}) do
    %{
      id: ann.id,
      body: ann.body,
      at: ann.at,
      user: render_one(ann.user, Rumbl.UserView, "user.json")
    }
  end
end

注意 annotaion's user 的 render_one,可處理 nil results

更新 vidChannle.join() 以便 render list of annotations on join

    vidChannel.join()
      .receive("ok", ({annotations}) => {
        annotations.forEach( ann => this.renderAnnotation(msgContainer, ann) )
      })
      .receive("error", reason => console.log("join failed", reason) )

現在 reload 頁面就能看到所有 annotations


現在我們需要 schedule the annotations to appear synced up with the video playback

更新 /web/static/js/video.js


import Player from "./player"

let Video = {

    init(socket, element){ if(!element){ return }
        let playerId = element.getAttribute("data-player-id")
        let videoId  = element.getAttribute("data-id")
        socket.connect()
        Player.init(element.id, playerId, () => {
            this.onReady(videoId, socket)
    })
    },

    onReady(videoId, socket){
        let msgContainer = document.getElementById("msg-container")
        let msgInput     = document.getElementById("msg-input")
        let postButton   = document.getElementById("msg-submit")
        let vidChannel   = socket.channel("videos:" + videoId)

        postButton.addEventListener("click", e => {
            let payload = {body: msgInput.value, at: Player.getCurrentTime()}
            vidChannel.push("new_annotation", payload)
            .receive("error", e => console.log(e) )
        msgInput.value = ""
    })

        msgContainer.addEventListener("click", e => {
            e.preventDefault()
        let seconds = e.target.getAttribute("data-seek") ||
            e.target.parentNode.getAttribute("data-seek")
        if(!seconds){ return }

        Player.seekTo(seconds)
    })

        vidChannel.on("new_annotation", (resp) => {
            this.renderAnnotation(msgContainer, resp)
    })

        vidChannel.join()
            .receive("ok", resp => {
            this.scheduleMessages(msgContainer, resp.annotations)
    })
    .receive("error", reason => console.log("join failed", reason) )
    },

    renderAnnotation(msgContainer, {user, body, at}){
        let template = document.createElement("div")
        template.innerHTML = `
    <a href="#" data-seek="${this.esc(at)}">
      [${this.formatTime(at)}]
      <b>${this.esc(user.username)}</b>: ${this.esc(body)}
    </a>
    `
        msgContainer.appendChild(template)
        msgContainer.scrollTop = msgContainer.scrollHeight
    },

    scheduleMessages(msgContainer, annotations){
        setTimeout(() => {
            let ctime = Player.getCurrentTime()
            let remaining = this.renderAtTime(annotations, ctime, msgContainer)
            this.scheduleMessages(msgContainer, remaining)
    }, 1000)
    },

    renderAtTime(annotations, seconds, msgContainer){
        return annotations.filter( ann => {
                if(ann.at > seconds){
            return true
        } else {
            this.renderAnnotation(msgContainer, ann)
            return false
        }
    })
    },

    formatTime(at){
        let date = new Date(null)
        date.setSeconds(at / 1000)
        return date.toISOString().substr(14, 5)
    },

    esc(str){
        let div = document.createElement("div")
        div.appendChild(document.createTextNode(str))
        return div.innerHTML
    }
}
export default Video

根據 current player time 去 render annotations

scheduleMessages 每秒都會執行一次,每次都呼叫 renderAtTime

renderAtTime 會 filter 要 render 的 messages

現在再次 reload 頁面,就可看到 annotation 的時間


增加讓 annotation 可以點擊的功能,就可以直接跳躍影片到該 annotaion 產生的時間

修改 /web/static/js/video.js

    msgContainer.addEventListener("click", e => {
      e.preventDefault()
      let seconds = e.target.getAttribute("data-seek") ||
                    e.target.parentNode.getAttribute("data-seek")
      if(!seconds){ return }

      Player.seekTo(seconds)
    })

Handling Disconnects

JS client 可斷線再 reconnect,Server 可能會 restart,或是網路可能發生問題,這些問題都會造成斷線。

如果發送一個 annotation,然後馬上把 server 關掉,client 會以 exponential back-off 的方式進行 reconnect。重新啟動 server,會發現 server 會認為是新的連線,然後發送所有的 annotations,client 會出現重複的 annotations。client 必須偵測 duplicate annotations 並忽略處理。

我們可以在 client 追蹤 lastseenid,並在每次收到新的 annotation 時更新這個值。

當 client 重連時,可將 lastseenid 發送給 server,server 就只需要發送未收到的訊息。

修改 /web/static/js/video.js,增加 vidChannel.params.lastseenid

        vidChannel.on("new_annotation", (resp) => {
            vidChannel.params.last_seen_id = resp.id
        this.renderAnnotation(msgContainer, resp)
    })

        vidChannel.join()
            .receive("ok", resp => {
            let ids = resp.annotations.map(ann => ann.id)
        if(ids.length > 0){ vidChannel.params.last_seen_id = Math.max(...ids) }
        this.scheduleMessages(msgContainer, resp.annotations)
    })
    .receive("error", reason => console.log("join failed", reason) )

client 的 channel 會儲存 params 物件,並在每次 join 時,發送給 server。在 join 也要更新這個參數。

修改 /web/channels/video_channel.ex 的 join

  def join("videos:" <> video_id, params, socket) do
    last_seen_id = params["last_seen_id"] || 0
    video_id = String.to_integer(video_id)
    video = Repo.get!(Rumbl.Video, video_id)

    annotations = Repo.all(
      from a in assoc(video, :annotations),
      where: a.id > ^last_seen_id,
      order_by: [asc: a.at, asc: a.id],
      limit: 200,
      preload: [:user]
    )

    resp = %{annotations: Phoenix.View.render_many(annotations, AnnotationView,
      "annotation.json")}
    {:ok, resp, assign(socket, :video_id, video_id)}
  end

References

Programming Phoenix