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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, so that servers are always checked in the same order, you can do that in most libraries with a connection options:

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


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();

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

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()


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

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

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