IO , which first establishes a long-polling connection, then tries to upgrade to better transports that are "tested" on the side, like WebSocket. Please see the Goals section for more information. Unless instructed otherwise a disconnected client will try to reconnect forever, until the server is available again.
Please see the available reconnection options here. A heartbeat mechanism is implemented at the Engine. IO level, allowing both the server and the client to know when the other one is not responding anymore.
That functionality is achieved with timers set on both the server and the client, with timeout values the pingInterval and pingTimeout parameters shared during the connection handshake. Those timers require any subsequent client calls to be directed to the same server, hence the sticky-session requirement when using multiples nodes.
In order to create separation of concerns within your application for example per module, or based on permissions , Socket. IO allows you to create several Namespaces , which will act as separate communication channels but will share the same underlying connection. Within each Namespace , you can define arbitrary channels, called Rooms , that sockets can join and leave.
You can then broadcast to any given room, reaching every socket that has joined it. This is a useful feature to send notifications to a group of users, or to a given user connected on several devices for example. Note: Socket. IO is not a WebSocket implementation. Although Socket. IO indeed uses WebSocket as a transport when possible, it adds some metadata to each packet: the packet type, the namespace and the ack id when a message acknowledgement is needed.
That is why a WebSocket client will not be able to successfully connect to a Socket. IO server, and a Socket. Please see the protocol specification here. The following example attaches socket. Starting with 3. You need to pass the Server to socket. Also make sure to call. Like Express. JS, Koa works by exposing an application as a request handler function, but only by calling the callback method.
To integrate Socket. Broadcasting means sending a message to all connected clients. Broadcasting can be done at multiple levels. We can send the message to all the connected clients, to clients on a namespace and clients in a particular room. To broadcast an event to all the clients, we can use the io. In this example, we will broadcast the number of connected clients to all the users. Update the app. This was to send an event to everyone. Now, if we want to send an event to everyone, but the client that caused it in the previous example, it was caused by new clients on connecting , we can use the socket.
So, in your app. Now, the newest client gets a welcome message and others get how many clients are connected currently to the server.
IO allows you to "namespace" your sockets, which essentially means assigning different endpoints or paths. This is a useful feature to minimize the number of resources TCP connections and at the same time separate concerns within your application by introducing separation between communication channels.
Multiple namespaces actually share the same WebSockets connection thus saving us socket ports on the server. Namespaces are created on the server side. However, they are joined by clients by sending a request to the server.
All connections to the server using the socket-object client side are made to the default namespace. This will connect the client to the default namespace. All events on this namespace connection will be handled by the io object on the server. All the previous examples were utilizing default namespaces to communicate with the server and back. We can create our own custom namespaces.
Now, to connect a client to this namespace, you need to provide the namespace as an argument to the io constructor call to create a connection and a socket object on client side.
Every time someone connects to this namespace, they will receive a 'hi' event displaying the message "Hello everyone! Within each namespace, you can also define arbitrary channels that sockets can join and leave. These channels are called rooms. Rooms are used to further-separate concerns. Rooms also share the same socket connection like namespaces. One thing to keep in mind while using rooms is that they can only be joined on the server side. As soon as this room is full, create another room and join clients there.
You can also implement this in custom namespaces in the same fashion. To leave a room, you need to call the leave function just as you called the join function on the socket.
We have worked on local servers until now, which will almost never give us errors related to connections, timeouts, etc. However, in real life production environments, handling such errors are of utmost importance. Therefore, we will now discuss how we can handle connection errors on the client side. To handle errors, we can handle these events using the out-socket object that we created on our client.
IO uses a very famous debugging module developed by ExpresJS's main author, called debug. Earlier Socket. IO used to log everything to the console making it quite difficult to debug the problem.
After the v1. This will colorize and output everything that happens to your server console. For example, we can consider the following screenshot. Paste this to console, click enter and refresh your page. This will again output everything related to Socket. You can limit the output to get the debug info with incoming data from the socket using the following command. There is a very good blog post related to socket.
In this chapter, we will discuss regarding Fallbacks, Connection using Socket. IO, Events and Messages. IO has a lot of underlying transport mechanisms, which deal with various constraints arising due to cross browser issues, WebSocket implementations, firewalls, port blocking, etc. IO provides us with fallback mechanisms, which can deal with such issues. If we develop apps using the native API, we have to implement the fallbacks ourselves.
The Socket. IO connection begins with the handshake. This makes the handshake a special part of the protocol. Apart from the handshake, all the other events and messages in the protocol are transferred over the socket. IO is intended for use with web applications, and therefore it is assumed that these applications will always be able to use HTTP. It is because of this reasoning that the Socket. WebSocket native API only sends messages across. IO provides an addition layer over these messages, which allows us to create events and again helps us develop apps easily by separating the different types of messages sent.
The native API sends messages only in plain text. This is also taken care of by Socket. It handles the serialization and deserialization of data for us. We have an official client API for the web. For other clients such as native mobile phones, other application clients also we can use Socket. IO using the following steps. This format enables Socket. IO to determine the type of the message and the data sent in the message and some metadata useful for operation.
Data is the associated data to be delivered to the socket. In case of messages, it is treated as plain text, for other events, it is treated as JSON. Now that we are well acquainted with Socket. IO, let us write a chat application, which we can use to chat on different chat rooms. We will allow users to choose a username and allow them to chat using them.
Now that we have set up our HTML to request for a username, let us create the server to accept connections from the client.
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