ballerina/tcp Ballerina library



This module provides APIs for sending/receiving messages to/from another application process (local or remote) over the connection-oriented TCP protocol.


The tcp:Client is used to connect to a socket server and interact with it. The client can send the data to the server and retrieve the data from the server.

A client can be defined by providing the remoteHost and the remotePort. A simple client code is as follows.

import ballerina/tcp;

public function main() returns error? {
    tcp:Client socketClient = check new("localhost", 3000);

    string msg = "Hello Ballerina";
    byte[] msgByteArray = msg.toBytes();
    check  socketClient->writeBytes(msgByteArray);

    readonly & byte[] receivedData = check socketClient->readBytes();

    check socketClient->close();


The tcp:Listener is used to listen to the incoming socket request. The onConnect(tcp:Caller) remote method gets invoked when a new client is connected. The new client is represented using the tcp:Caller. The onConnect(tcp:Caller) method may return tcp:ConnectionService|tcp:Error.

The tcp:ConnectionService can have the following remote methods

onBytes(readonly & byte[] data) - This remote method is invoked once the content is received from the client.

onError(readonly & tcp:Error err) - This remote method is invoked in an error situation.

onClose() - This remote method is invoked when the connection is closed.

A tcp:Listener can be defined as follows:

import ballerina/tcp;
import ballerina/io;
import ballerina/log;

service on new tcp:Listener(3000) {
    remote function onConnect(tcp:Caller caller) returns tcp:ConnectionService {
        io:println("Client connected to echoServer: ", caller.remotePort);
        return new EchoService();

service class EchoService {

    remote function onBytes(readonly & byte[] data) returns byte[]|tcp:Error? {
        // echo back the data to the client
        return data;

    remote function onError(tcp:Error err) returns tcp:Error? {
        log:printError("An error occurred", 'error = err);

    remote function onClose() returns tcp:Error? {
         io:println("Client left");

Using the TLS protocol

The Ballerina TCP module allows the use of TLS in communication. This expects a secure socket to be set in the connection configuration as shown below.

Configuring TLS in server side
tcp:ListenerSecureSocket listenerSecureSocket = {
    key: {
        certFile: "../resource/path/to/public.crt",
        keyFile: "../resource/path/to/private.key"

service on new tcp:Listener(9002, secureSocket = listenerSecureSocket) {
    isolated remote function onConnect(tcp:Caller caller) returns tcp:ConnectionService {
        return new EchoService();
Configuring TLS in client side
tcp:Client socketClient = check new ("localhost", 9002, secureSocket = {
    cert: "../resource/path/to/public.crt",


import ballerina/tcp;Copy


Released date: about 1 year ago

Version: 1.7.0

License: Apache-2.0


Platform: java11

Ballerina version: 2201.5.0

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Current verison: 14

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