Module websocket




This module provides APIs for connecting and interacting with WebSocket endpoints.

This module facilitates two types of network entry points as the Client and Listener.


The websocket:Client can be used to read/write text/binary messages synchronously.

A simple client code to handle text messages as follows.


Similar to the above, writeMessage and readMessage can be used to handle binary messages as well. This module also has some low-level APIs to handle text and binary messages such as writeTextMessage, readTextMessage, writeBinaryMessage and readBinaryMessage functions. A callback service with the two onPing and onPong remote functions can be registered at the initialization of the client to receive the ping/pong control frames.



On the listener-side, an initial WebSocket upgrade service can be attached to the websocket:Listener to handle upgrade requests. It has a single get resource, which takes in an http:Request optionally. The get resource returns a websocket:Service to which incoming messages get dispatched after a successful WebSocket connection upgrade. This resource can be used to intercept the initial HTTP upgrade with custom headers or to cancel the WebSocket upgrade by returning an error. The returning websocket:Service has a fixed set of remote methods.


Remote methods associated with websocket:Service

onOpen: As soon as the WebSocket handshake is completed and the connection is established, the onOpen remote method is dispatched.

onMessage: This remote function accepts both types of text and binary messages. This function accepts anydata as the function parameter as this remote function supports data binding.

onTextMessage: The received text messages are dispatched to this remote method. Users are not allowed to have this remote functions along with the onMessage remote function.

onBinaryMessage: The received binary messages are dispatched to this remote method. Users are not allowed to have this remote functions along with the onMessage remote function.

onPing and onPong: The received ping and pong messages are dispatched to these remote methods respectively.

onIdleTimeout: This remote method is dispatched when the idle timeout is reached. The idleTimeout has to be configured either in the WebSocket service or the client configuration.

onClose: This remote method is dispatched when a close frame with a statusCode and a reason is received.

onError: This remote method is dispatched when an error occurs in the WebSocket connection. This will always be preceded by a connection closure with an appropriate close frame.

Control messages

A WebSocket contains three types of control messages: close, ping, and pong. A WebSocket server or a client can send a ping message and the opposite side should respond with a corresponding pong message by returning the same payload sent with the ping message. These ping/pong sequences are used as a heartbeat mechanism to check if the connection is healthy.

You do not need to explicitly control these messages as they are handled automatically by the services and clients. However, if required, you can override the default implementations of the ping/pong messages by registering a websocket:PingPongService in the client side as given in the above client code sample and by including the onPing and onPong remote functions in the websocket:Service in the server side.


A WebSocket server or a client can close the WebSocket connection by calling the close function. In the event of a connection closure, the service will be notified by invoking the onClose remote function. Also, on the client side, you will get a connection closure error if you try to read/write messages.


WebSocket compression

Per message compression extensions are supported by the Ballerina websocket module and this is enabled by default for both the WebSocket client and the server. Compression can be enabled or disabled by setting the webSocketCompressionEnabled to true or false in the ClientConfiguration and ListenerConfiguration. Once the compression is successfully negotiated, receiving compressed messages will be automatically decompressed when reading.

Origin considerations

The Origin header can be used to differentiate between WebSocket connections from different hosts or between those made from a browser and some other kind of network client. It is recommended to validate this Origin header before accepting the WebSocket upgrade.


Using the TLS protocol to secure WebSocket communication

It is strongly recommended to use the wss:// protocol to protect against man-in-the-middle attacks. The Ballerina websocket module allows the use of TLS in communication to do this. This expects a secure socket to be set in the connection configuration as shown below.

Configuring TLS in server side


Configuring TLS in client side


Other versions

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