Listen for Connection Events
- Listen for New Servers
- Listen for Errors

Listen for Connection Events

While the connection status is interesting, it is perhaps more interesting to know when the status changes. Most, if not all, of the NATS client libraries provide a way to listen for events related to the connection and its status.

The actual API for these listeners is language dependent, but the following examples show a few of the more common use cases. See the API documentation for the client library you are using for more specific instructions.

Connection events may include the connection being closed, disconnected or reconnected. Reconnecting involves a disconnect and connect, but depending on the library implementation may also include multiple disconnects as the client tries to find a server, or the server is rebooted.

// There is not a single listener for connection events in the NATS Go Client.
// Instead, you can set individual event handlers using:
nc, err := nats.Connect("demo.nats.io",
    nats.DisconnectErrHandler(func(_ *nats.Conn, err error) {
        log.Printf("client disconnected: %v", err)
    }),
    nats.ReconnectHandler(func(_ *nats.Conn) {
        log.Printf("client reconnected")
    }),
    nats.ClosedHandler(func(_ *nats.Conn) {
        log.Printf("client closed")
    }))
if err != nil {
    log.Fatal(err)
}
defer nc.Close()
DisconnectHandler(cb ConnHandler)
ReconnectHandler(cb ConnHandler)
ClosedHandler(cb ConnHandler)
DiscoveredServersHandler(cb ConnHandler)
ErrorHandler(cb ErrHandler)

Java

class MyConnectionListener implements ConnectionListener {
    public void connectionEvent(Connection natsConnection, Events event) {
        System.out.println("Connection event - "+event);
    }
}
public class SetConnectionListener {
    public static void main(String[] args) {
        try {
            Options options = new Options.Builder().
                                        server("nats://demo.nats.io:4222").
                                        connectionListener(new MyConnectionListener()). // Set the listener
                                        build();
            Connection nc = Nats.connect(options);
            // Do something with the connection
            nc.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

JavaScript

let nc = NATS.connect("nats://demo.nats.io:4222");
nc.on('error', (err) => {
    t.log('error', err);
});
nc.on('connect', () => {
    t.log('client connected');
});
nc.on('disconnect', () => {
    t.log('client disconnected');
});
nc.on('reconnecting', () => {
    t.log('client reconnecting');
});
nc.on('reconnect', () => {
    t.log('client reconnected');
});
nc.on('close', () => {
    t.log('client closed');
});
nc.on('permission_error', (err) => {
    t.log('permission_error', err);
});

Python

# Asyncio NATS client can be defined a number of event callbacks
async def disconnected_cb():
    print("Got disconnected!")
async def reconnected_cb():
    # See who we are connected to on reconnect.
    print("Got reconnected to {url}".format(url=nc.connected_url.netloc))
async def error_cb(e):
    print("There was an error: {}".format(e))
async def closed_cb():
    print("Connection is closed")
# Setup callbacks to be notified on disconnects and reconnects
options["disconnected_cb"] = disconnected_cb
options["reconnected_cb"] = reconnected_cb
# Setup callbacks to be notified when there is an error
# or connection is closed.
options["error_cb"] = error_cb
options["closed_cb"] = closed_cb
await nc.connect(**options)

Ruby

# There is not a single listener for connection events in the Ruby NATS Client.
# Instead, you can set individual event handlers using:
NATS.on_disconnect do
end
NATS.on_reconnect do
end
NATS.on_close do
end
NATS.on_error do
end

TypeScript

// connect will happen once - the first connect
nc.on('connect', (nc) => {
    // nc is the connection that connected
    t.log('client connected');
});
nc.on('disconnect', (url) => {
    // nc is the connection that reconnected
    t.log('disconnected from', url);
});
nc.on('reconnecting', (url) => {
    t.log('reconnecting to', url);
});
nc.on('reconnect', (nc, url) => {
    // nc is the connection that reconnected
    t.log('reconnected to', url);
});

static void
disconnectedCB(natsConnection *conn, void *closure)
{
    // Do something
    printf("Connection disconnected\n");
}
static void
reconnectedCB(natsConnection *conn, void *closure)
{
    // Do something
    printf("Connection reconnected\n");
}
static void
closedCB(natsConnection *conn, void *closure)
{
    // Do something
    printf("Connection closed\n");
}
(...)
natsConnection      *conn      = NULL;
natsOptions         *opts      = NULL;
natsStatus          s          = NATS_OK;
s = natsOptions_Create(&opts);
if (s == NATS_OK)
    s = natsOptions_SetDisconnectedCB(opts, disconnectedCB, NULL);
if (s == NATS_OK)
    s = natsOptions_SetReconnectedCB(opts, reconnectedCB, NULL);
if (s == NATS_OK)
    s = natsOptions_SetClosedCB(opts, closedCB, NULL);
if (s == NATS_OK)
    s = natsConnection_Connect(&conn, opts);
(...)
// Destroy objects that were created
natsConnection_Destroy(conn);
natsOptions_Destroy(opts);

Listen for New Servers

When working with a cluster, servers may be added or changed. Some of the clients allow you to listen for this notification:

// Be notified if a new server joins the cluster.
// Print all the known servers and the only the ones that were discovered.
nc, err := nats.Connect("demo.nats.io",
    nats.DiscoveredServersHandler(func(nc *nats.Conn) {
        log.Printf("Known servers: %v\n", nc.Servers())
        log.Printf("Discovered servers: %v\n", nc.DiscoveredServers())
    }))
if err != nil {
    log.Fatal(err)
}
defer nc.Close()
// Do something with the connection

Java

class ServersAddedListener implements ConnectionListener {
    public void connectionEvent(Connection nc, Events event) {
        if (event == Events.DISCOVERED_SERVERS) {
            for (String server : nc.getServers()) {
                System.out.println("Known server: "+server);
            }
        }
    }
}
public class ListenForNewServers {
    public static void main(String[] args) {
        try {
            Options options = new Options.Builder().
                                        server("nats://demo.nats.io:4222").
                                        connectionListener(new ServersAddedListener()). // Set the listener
                                        build();
            Connection nc = Nats.connect(options);
            // Do something with the connection
            nc.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

JavaScript

let nc = NATS.connect("nats://demo.nats.io:4222");
nc.on('serversDiscovered', (urls) => {
    t.log('serversDiscovered', urls);
});

Python

# Asyncio NATS client does not support discovered servers handler right now

Ruby

# The Ruby NATS client does not support discovered servers handler right now

TypeScript

nc.on('serversChanged', (ce) => {
    t.log('servers changed\n', 'added: ',ce.added, 'removed:', ce.removed);
});

static void
discoveredServersCB(natsConnection *conn, void *closure)
{
    natsStatus  s         = NATS_OK;
    char        **servers = NULL;
    int         count     = 0;
    s = natsConnection_GetDiscoveredServers(conn, &servers, &count);
    if (s == NATS_OK)
    {
        int i;
        // Do something...
        for (i=0; i<count; i++)
            printf("Discovered server: %s\n", servers[i]);
        // Free allocated memory
        for (i=0; i<count; i++)
            free(servers[i]);
        free(servers);
    }
}
(...)
natsConnection      *conn      = NULL;
natsOptions         *opts      = NULL;
natsStatus          s          = NATS_OK;
s = natsOptions_Create(&opts);
if (s == NATS_OK)
    s = natsOptions_SetDiscoveredServersCB(opts, discoveredServersCB, NULL);
if (s == NATS_OK)
    s = natsConnection_Connect(&conn, opts);
(...)
// Destroy objects that were created
natsConnection_Destroy(conn);
natsOptions_Destroy(opts);

Listen for Errors

The client library may separate server-to-client errors from events. Many server events are not handled by application code and result in the connection being closed. Listening for the errors can be very useful for debugging problems.

// Set the callback that will be invoked when an asynchronous error occurs.
nc, err := nats.Connect("demo.nats.io",
    nats.ErrorHandler(func(_ *nats.Conn, _ *nats.Subscription, err error) {
        log.Printf("Error: %v", err)
    }))
if err != nil {
    log.Fatal(err)
}
defer nc.Close()
// Do something with the connection

Java

class MyErrorListener implements ErrorListener {
    public void errorOccurred(Connection conn, String error)
    {
        System.out.println("The server notificed the client with: "+error);
    }
    public void exceptionOccurred(Connection conn, Exception exp) {
        System.out.println("The connection handled an exception: "+exp.getLocalizedMessage());
    }
    public void slowConsumerDetected(Connection conn, Consumer consumer) {
        System.out.println("A slow consumer was detected.");
    }
}
public class SetErrorListener {
    public static void main(String[] args) {
        try {
            Options options = new Options.Builder().
                                        server("nats://demo.nats.io:4222").
                                        errorListener(new MyErrorListener()). // Set the listener
                                        build();
            Connection nc = Nats.connect(options);
            // Do something with the connection
            nc.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

JavaScript

let nc = NATS.connect("nats://demo.nats.io:4222");
// on node you *must* register an error listener. If not registered
// the library emits an 'error' event, the node process will exit.
nc.on('error', (err) => {
    t.log('client got an error:', err);
});

Python

nc = NATS()
async def error_cb(e):
   print("Error: ", e)
await nc.connect(
   servers=["nats://demo.nats.io:4222"],
   reconnect_time_wait=10,
   error_cb=error_cb,
   )
# Do something with the connection.

Ruby

require 'nats/client'
NATS.start(servers:["nats://demo.nats.io:4222"]) do |nc|
   nc.on_error do |e|
    puts "Error: #{e}"
  end
  nc.close
end

TypeScript

// on node you *must* register an error listener. If not registered
// the library emits an 'error' event, the node process will exit.
nc.on('error', (err) => {
    t.log('client got an out of band error:', err);
});

static void
errorCB(natsConnection *conn, natsSubscription *sub, natsStatus s, void *closure)
{
    // Do something
    printf("Error: %d - %s\n", s, natsStatus_GetText(s));
}
(...)
natsConnection      *conn      = NULL;
natsOptions         *opts      = NULL;
natsStatus          s          = NATS_OK;
s = natsOptions_Create(&opts);
if (s == NATS_OK)
    s = natsOptions_SetErrorHandler(opts, errorCB, NULL);
if (s == NATS_OK)
    s = natsConnection_Connect(&conn, opts);
(...)
// Destroy objects that were created
natsConnection_Destroy(conn);
natsOptions_Destroy(opts);