Send message to specific client with socket io and node js

Question

I m working with socket io and node js and until now it seems pretty good  but I don t know how to send a message from the server to an specific client  something like this   client send message  receiverSessionId    But neither the  send   nor the  broadcast   methods seem to supply my need   What I have found as a possible solution  is that the  broadcast   method accepts as a second parameter an array of SessionIds to which not send the message  so I could pass an array with all the SessionIds connected at that moment to the server  except the one I wish send the message  but I feel there must be a better solution   Any ideas

User · Answer

Socket IO allows you to    namespace    your sockets  which essentially means assigning different endpoints or paths   This might help  http   socket io docs rooms-and-namespaces

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You can do this   On server   global io require  socket io   server    io on  connection  function client       console log  client is   client id         This is handle by current connected client      client emit  messages   hello  world          This is handle by every client     io sockets emit  data   data  This is handle by every client        app1 saveSession client id       client on  disconnect  function            app1 deleteSession client id          console log  client disconnected  client id                     And this is handle by particular client      var socketId req query id     if io sockets connected socketId   null            io sockets connected socketId  emit  particular User    data   Event response by particular user              And on client  it is very easy to handle    var socket io connect  http   localhost 8080        socket on  messages  function data           console log  message is   data             alert data             socket on  data  function data           console log  data is   data             alert data              socket on  particular User  function data           console log  data from server   data             alert data

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You can use    send message only to sender-client  socket emit  message    check this       or you can send to all listeners including the sender  io emit  message    check this       send to all listeners except the sender  socket broadcast emit  message    this is a message       or you can send it to a room  socket broadcast to  chatroom   emit  message    this is the message to all

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Also you can keep clients refferences  But this makes your memmory busy   Create an empty object and set your clients into it   const myClientList          server on  connection    socket    gt        console info  Client connected  id   socket id           myClientList socket id    socket             socket on  disconnect    socket    gt        delete myClientList socket id           then call your specific client by id from the object  myClientList specificId  emit  blabla   somedata

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Well you have to grab the client for that  surprise   you can either go the simple way   var io   io listen server   io clients sessionID  send     Which may break  I doubt it  but it s always a possibility that io clients might get changed  so use the above with caution  Or you keep track of the clients yourself  therefore you add them to your own clients object in the connection listener and remove them in the disconnect listener   I would use the latter one  since depending on your application you might want to have more state on the clients anyway  so something like clients id     conn  clientConnect  data         might do the job

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In 1 0 you should use   io sockets connected socketid  emit

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Whatever version we are using if we just console log   the  io  object that we use in our server side nodejs code   e g  io on  connection   function socket            we can see that  io  is just an json object and there are many child objects where the socket id and socket objects are stored   I am using socket io version 1 3 5  btw   If we look in the io object  it contains    sockets      name           server   Circular       sockets     Object    Object         connected         B5AC9w0sYmOGWe4fAAAA   Object           hWzf97fmU-TIwwzWAAAB    Object       here we can see the socketids  B5AC9w0sYmOGWe4fAAAA  etc  So  we can do   io sockets connected socketid  emit      Again  on further inspection we can see segments like    eio      clients         B5AC9w0sYmOGWe4fAAAA   Object           hWzf97fmU-TIwwzWAAAB    Object       So  we can retrieve a socket from here by doing  io eio clients socketid  emit      Also  under engine we have   engine     clients        B5AC9w0sYmOGWe4fAAAA   Object          hWzf97fmU-TIwwzWAAAB    Object       So  we can also write   io engine clients socketid  emit      So  I guess we can achieve our goal in any of the 3 ways I listed above    io sockets connected socketid  emit     OR io eio clients socketid  emit     OR io engine clients socketid  emit

User · Answer

each socket joins a room with a socket id for a name  so you can just   io to socket id  emit  hey       docs  http   socket io docs rooms-and-namespaces  default-room  Cheers

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Ivo Wetzel s answer doesn t seem to be valid in Socket io 0 9 anymore   In short you must now save the socket id and use io sockets socket savedSocketId  emit       to send messages to it   This is how I got this working in clustered Node js server   First you need to set Redis store as the store so that messages can go cross processes   var express   require  express    var redis   require  redis    var sio   require  socket io     var client   redis createClient   var app   express createServer    var io   sio listen app    io set  store   new sio RedisStore        In this example we have one master client socket     that receives messages from others   io sockets on  connection   function socket          Promote this socket as master   socket on  I m the master   function             Save the socket id to Redis so that all processes can access it      client set  mastersocket   socket id  function err          if  err  throw err        console log  Master socket is now    socket id                    socket on  message to master   function msg            Fetch the socket id from Redis     client get  mastersocket   function err  socketId          if  err  throw err        io sockets socket socketId  emit msg                        I omitted the clustering code here  because it makes this more cluttered  but it s trivial to add  Just add everything to the worker code  More docs here http   nodejs org api cluster html

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The simplest  most elegant way verified working with socket io v3 1 1 It s as easy as  client emit  quot your message quot     And that s it  Ok  but how does it work  Minimal working example Here s an example of a simple client-server interaction where each client regularly receives a message containing a sequence number  There is a unique sequence for each client and that s where the  quot I need to send a message to a particular client quot  comes into play  Server server js const      Server    require  quot socket io quot        server   new Server 8000    let     sequenceNumberByClient   new Map        event fired every time a new client connects  server on  quot connection quot    socket    gt        console info  Client connected  id   socket id             initialize this client s sequence number     sequenceNumberByClient set socket  1           when socket disconnects  remove it from the list      socket on  quot disconnect quot        gt            sequenceNumberByClient delete socket           console info  Client gone  id   socket id                      sends each client its current sequence number setInterval      gt        for  const  client  sequenceNumber  of sequenceNumberByClient entries              client emit  quot seq-num quot   sequenceNumber           sequenceNumberByClient set client  sequenceNumber   1            1000    The server starts listening on port 8000 for incoming connections  As soon as a new connection is established  that client is added to a map that keeps track of its sequence number  The server also listens for the disconnect event to remove the client from the map when it leaves  Each and every second  a timer is fired  When it does  the server walks through the map and sends a message to every client with their current sequence number  incrementing it right after  That s all that is to it  Easy peasy  Client The client part is even simpler  It just connects to the server and listens for the seq-num message  printing it to the console every time it arrives  client js const     io   require  quot socket io-client quot        ioClient   io connect  quot http   localhost 8000 quot     ioClient on  quot seq-num quot    msg    gt  console info msg     Running the example Install the required libraries  npm install socket io 3 1 1 socket io-client 3 1 1  Run the server  node server  Open other terminal windows and spawn as many clients as you want by running  node client  I have also prepared a gist with the full code here

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As of version 1 4 5  be sure you provide a properly prefixed socketId in io to    I was taking the socketId the Client logged to debug and it was without prefix so I ended up searching forever till I found out  So you might have to do it like this if the Id you have is not prefixed   io to        socketId  emit  myevent    foo   bar

User · Answer

io sockets sockets socket id  emit      worked for me in v0 9

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