How to fix NoNodeAvailableException: None of the configured nodes are available in Elasticsearch

# Check if Elasticsearch service is running
sudo systemctl status elasticsearch

# Or for Docker
docker ps | grep elasticsearch

# Attempt to connect to the configured nodes directly
curl -X GET "http://node1.example.com:9200/"
curl -X GET "http://192.168.1.100:9200/"

# Test from the client machine itself
curl -X GET "http://ELASTICSEARCH_HOST:9200/"

# For systemd
sudo systemctl start elasticsearch

# For Docker
docker start elasticsearch

# Check logs for startup errors
sudo journalctl -u elasticsearch -f
# or
docker logs -f elasticsearch

// Java Transport Client example
TransportClient client = new PreBuiltTransportClient(settings)
    .addTransportAddress(new TransportAddress(InetAddress.getByName("node1.example.com"), 9300))
    .addTransportAddress(new TransportAddress(InetAddress.getByName("node2.example.com"), 9300));

// Node.js Elasticsearch client
const { Client } = require('@elastic/elasticsearch');
const client = new Client({
  nodes: [
    'http://node1.example.com:9200',
    'http://node2.example.com:9200',
    'http://node3.example.com:9200'
  ]
});

# Python Elasticsearch client
from elasticsearch import Elasticsearch

es = Elasticsearch([
    {'host': 'node1.example.com', 'port': 9200},
    {'host': 'node2.example.com', 'port': 9200},
    {'host': 'node3.example.com', 'port': 9200}
])

# Test DNS resolution
nslookup elasticsearch.example.com
dig elasticsearch.example.com

# Or with getent
getent hosts elasticsearch.example.com

# Test connectivity with telnet or nc
telnet node1.example.com 9200
nc -zv node1.example.com 9200

# Test from inside Docker if applicable
docker exec app-container nslookup elasticsearch
docker exec app-container curl http://elasticsearch:9200/

# Check what ports Elasticsearch is listening on
sudo netstat -tulpn | grep java
# or
sudo ss -tulpn | grep 9200
sudo ss -tulpn | grep 9300

# Expected output for port 9200 (HTTP):
# tcp    0    0 0.0.0.0:9200    0.0.0.0:*    LISTEN    12345/java

# Expected output for port 9300 (Transport):
# tcp    0    0 0.0.0.0:9300    0.0.0.0:*    LISTEN    12345/java

# /etc/elasticsearch/elasticsearch.yml
http.port: 9200
transport.port: 9300

# Verify network.host setting (should not bind only to localhost for multi-node)
network.host: 0.0.0.0
# or specify the actual IP
network.host: 192.168.1.100

sudo systemctl restart elasticsearch
sudo systemctl status elasticsearch

# Wait a few seconds for it to start
sleep 5

# Verify ports are now listening
sudo netstat -tulpn | grep java

# Check UFW (Ubuntu/Debian)
sudo ufw status numbered
# Allow ports if needed
sudo ufw allow 9200/tcp
sudo ufw allow 9300/tcp

# Check firewalld (CentOS/RHEL)
sudo firewall-cmd --list-all
sudo firewall-cmd --permanent --add-port=9200/tcp
sudo firewall-cmd --permanent --add-port=9300/tcp
sudo firewall-cmd --reload

# Check iptables directly
sudo iptables -L -n | grep -E "9200|9300"

# Test specific connectivity
telnet elasticsearch-host 9200
# Or using nc
nc -zv elasticsearch-host 9200

# From the client machine, test if you can reach the port
nc -zv elasticsearch-server.com 9200
# Output should be: "succeeded"

// WRONG: Using Transport Client to connect to HTTP port
TransportClient client = new PreBuiltTransportClient(settings)
    .addTransportAddress(new TransportAddress(InetAddress.getByName("localhost"), 9200));
    // ERROR: port 9200 is HTTP, not Transport protocol!

// CORRECT: Transport Client uses port 9300
TransportClient client = new PreBuiltTransportClient(settings)
    .addTransportAddress(new TransportAddress(InetAddress.getByName("localhost"), 9300));

// Use REST client for all modern versions
const { Client } = require('@elastic/elasticsearch');
const client = new Client({
  node: 'http://elasticsearch:9200'  // HTTP port 9200
});

// Use RestHighLevelClient (ES 7.x) or newer Java API client
RestHighLevelClient client = new RestHighLevelClient(
    RestClient.builder(
        new HttpHost("localhost", 9200, "http")
    ).build()
);

// Java Transport Client - increase timeout
Settings settings = Settings.builder()
    .put("client.transport.ping_timeout", "60s")     // Default 5s
    .put("client.transport.nodes_sampler_interval", "60s")  // Default 5s
    .put("cluster.name", "my-cluster")
    .build();

TransportClient client = new PreBuiltTransportClient(settings)
    .addTransportAddress(new TransportAddress(InetAddress.getByName("localhost"), 9300));

// Node.js - increase timeout
const { Client } = require('@elastic/elasticsearch');
const client = new Client({
  node: 'http://elasticsearch:9200',
  requestTimeout: 30000,  // milliseconds
  sniffOnStart: false,    // Don't sniff on startup if nodes unreachable
  sniffOnConnectionFault: false
});

# Python - increase timeout
from elasticsearch import Elasticsearch

es = Elasticsearch(
    ['http://elasticsearch:9200'],
    timeout=30
)

const client = new Client({
  node: 'http://elasticsearch:9200',
  sniffOnStart: false,         // Prevents auto-discovery of nodes
  sniffOnConnectionFault: false
});