Avoiding the Thundering Herd

When a server goes down, there is a possible anti-pattern called the Thundering Herd where all of the clients try to reconnect immediately, thus creating a denial of service attack. In order to prevent this, most NATS client libraries randomize the servers they attempt to connect to. This setting has no effect if only a single server is used, but in the case of a cluster, randomization, or shuffling, will ensure that no one server bears the brunt of the client reconnect attempts.

However, if you want to disable the randomization process for connect and re-connect, so that servers are always checked in the same order, you can do that in most libraries with a connection option:

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, ","), nats.DontRandomize())
if err != nil {
    log.Fatal(err)
}
defer nc.Close()

// Do something with the connection

Java

Options options = new Options.Builder().
                            server("nats://demo.nats.io:4222").
                            noRandomize(). // Disable reconnect shuffle
                            build();
Connection nc = Nats.connect(options);

// Do something with the connection

nc.close();

JavaScript

let nc = NATS.connect({
    noRandomize: true,
    servers: ["nats://127.0.0.1:4443",
        "nats://demo.nats.io:4222"
    ]
});

Python

nc = NATS()
await nc.connect(
   servers=[
      "nats://demo.nats.io:1222",
      "nats://demo.nats.io:1223",
      "nats://demo.nats.io:1224"
      ],
   dont_randomize=True,
   )

# Do something with the connection

await nc.close()

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"], dont_randomize_servers: true) do |nc|
   # Do something with the connection

   # Close the connection
   nc.close
end

TypeScript

// will throw an exception if connection fails
let nc = await connect({
    noRandomize: true,
    servers: ["nats://127.0.0.1:4443",
        "nats://demo.nats.io:4222"
    ]
});
nc.close();

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 = natsOptions_SetNoRandomize(opts, true);
if (s == NATS_OK)
    s = natsConnection_Connect(&conn, opts);

(...)

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