Connecting to a Cluster

When connecting to a cluster, there are a few things to think about.

• Passing a URL for each cluster member (semi-optional)

• The connection algorithm

• The reconnect algorithm (discussed later)

• Server provided URLs

When a client library first tries to connect it will use the list of URLs provided to the connection options or function. These URLs are usually checked in random order as to not have every client connect to the same server. The first successful connection is used. Randomization can be explicitly disabled.

After a client connects to the server, the server may provide a list of URLs for additional known servers. This allows a client to connect to one server and still have other servers available during reconnect.

To ensure the initial connection, your code should include a list of reasonable front line or seed servers. Those servers may know about other members of the cluster, and may tell the client about those members. But you don't have to configure the client to pass every valid member of the cluster in the connect method.

By providing the ability to pass multiple connect options, NATS can handle the possibility of a machine going down or being unavailable to a client. By adding the ability of the server to feed clients a list of known servers as part of the client-server protocol the mesh created by a cluster can grow and change organically while the clients are running.

Note, failure behavior is library dependent, please check the documentation for your client library on information about what happens if the connect fails.

Go

servers := []string{"nats://127.0.0.1:1222", "nats://127.0.0.1:1223", "nats://127.0.0.1:1224"}

nc, err := nats.Connect(strings.Join(servers, ","))
if err != nil {
    log.Fatal(err)
}
defer nc.Close()

// Do something with the connection

Java

Options options = new Options.Builder()
    .server("nats://127.0.0.1:1222,nats://127.0.0.1:1223,nats://127.0.0.1:1224")
    .build();
Connection nc = Nats.connect(options);

// Do something with the connection

nc.close();

JavaScript

const nc = await connect({
    servers: [
      "nats://demo.nats.io:4222",
      "nats://localhost:4222",
    ],
});
// Do something with the connection
doSomething();
// When done close it
await nc.close();

Python

nc = NATS()
await nc.connect(servers=[
   "nats://127.0.0.1:1222",
   "nats://127.0.0.1:1223",
   "nats://127.0.0.1:1224"
   ])

# Do something with the connection

await nc.close()

C#

// dotnet add package NATS.Net
using NATS.Net;

await using var client = new NatsClient("nats://127.0.0.1:1222,nats://127.0.0.1:1223,nats://127.0.0.1:1224");

// It's optional to call ConnectAsync()
// as it will be called when needed automatically
await client.ConnectAsync();

Ruby

require 'nats/client'

NATS.start(servers: ["nats://127.0.0.1:1222", "nats://127.0.0.1:1223", "nats://127.0.0.1:1224"]) do |nc|
   # Do something with the connection

   # Close the connection
   nc.close
end

C

natsConnection      *conn      = NULL;
natsOptions         *opts      = NULL;
natsStatus          s          = NATS_OK;
const char          *servers[] = {"nats://127.0.0.1:1222", "nats://127.0.0.1:1223", "nats://127.0.0.1:1224"};

s = natsOptions_Create(&opts);
if (s == NATS_OK)
    s = natsOptions_SetServers(opts, servers, 3);
if (s == NATS_OK)
    s = natsConnection_Connect(&conn, opts);

(...)

// Destroy objects that were created
natsConnection_Destroy(conn);
natsOptions_Destroy(opts);