Build a Resilient Application with MongoDB

<dependencies>
    <dependency>
        <groupId>org.mongodb</groupId>
        <artifactId>mongodb-driver-sync</artifactId>
        <version>4.0.1</version>
    </dependency>
</dependencies>

dependencies {
  compile 'org.mongodb:mongodb-driver-sync:4.0.1'
}

// Latest 'mongodb' version installed with npm
const MongoClient = require('mongodb').MongoClient;

// Create a variable for your connection string
String uri = "mongodb://[<username>:<password>@]hostname0<:port>[,hostname1:<port1>][,hostname2:<port2>][...][,hostnameN:<portN>]";
// Instantiate the MongoDB client with the URI
MongoClient client = MongoClients.create(uri);

// Create a variable for your connection string
const uri = "mongodb://[<username>:<password>@]hostname0<:port>[,hostname1:<port1>][,hostname2:<port2>][...][,hostnameN:<portN>]";
// Instantiate the MongoDB client with the URI
const client = new MongoClient(uri, {
    useNewUrlParser: true,
    useUnifiedTopology: true
});

Unable to insert due to an error: com.mongodb.MongoWriteException:
E11000 duplicate key error collection: <db>.<collection> ...

// Declare a logger instance from java.util.logging.Logger
private static final Logger LOGGER = ...
...
try {
    InsertOneResult result = collection.insertOne(new Document()
        .append("_id", 1)
        .append("body", "I'm a goofball trying to insert a duplicate _id"));
    // Everything is OK
    LOGGER.info("Inserted document id: " + result.getInsertedId());
// Refer to the API documentation for specific exceptions to catch
} catch (MongoException me) {
    // Report the error
    LOGGER.severe("Failed due to an error: " + me);
}

...
collection.insertOne({
    _id: 1,
    body: "I'm a goofball trying to insert a duplicate _id"
})
.then(result => {
    response.sendStatus(200) // send "OK" message to the client
},
err => {
    response.sendStatus(400); // send "Bad Request" message to the client
});

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// File: App.java
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import java.util.Map;
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import java.util.logging.Logger;
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import org.bson.Document;
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import org.json.JSONArray;
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import com.mongodb.MongoException;
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import com.mongodb.client.MongoClient;
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import com.mongodb.client.MongoClients;
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import com.mongodb.client.MongoCollection;
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import com.mongodb.client.MongoDatabase;
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import fi.iki.elonen.NanoHTTPD;
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public class App extends NanoHTTPD {
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    private static final Logger LOGGER = Logger.getLogger(App.class.getName());
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    static int port = 3000;
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    static MongoClient client = null;
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    public App() throws Exception {
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        super(port);
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        // Replace the uri string with your MongoDB deployment's connection string
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        String uri = "mongodb://<username>:<password>@hostname0:27017,hostname1:27017,hostname2:27017/?retryWrites=true&w=majority";
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        client = MongoClients.create(uri);
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        start(NanoHTTPD.SOCKET_READ_TIMEOUT, false);
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        LOGGER.info("\nStarted the server: http://localhost:" + port + "/ \n");
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    }
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    public static void main(String[] args) {
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        try {
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            new App();
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        } catch (Exception e) {
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            LOGGER.severe("Couldn't start server:\n" + e);
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        }
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    }
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    @Override
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    public Response serve(IHTTPSession session) {
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        StringBuilder msg = new StringBuilder();
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        Map<String, String> params = session.getParms();
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        Method reqMethod = session.getMethod();
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        String uri = session.getUri();
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        if (Method.GET == reqMethod) {
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            if (uri.equals("/")) {
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                msg.append("Welcome to my API!");
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            } else if (uri.equals("/users")) {
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                msg.append(listUsers(client));
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            } else {
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                msg.append("Unrecognized URI: ").append(uri);
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            }
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        } else if (Method.POST == reqMethod) {
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            try {
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                String name = params.get("name");
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                if (name == null) {
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                    throw new Exception("Unable to process POST request: 'name' parameter required");
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                } else {
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                    insertUser(client, name);
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                    msg.append("User successfully added!");
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                }
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            } catch (Exception e) {
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                msg.append(e);
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            }
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        }
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        return newFixedLengthResponse(msg.toString());
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    }
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    static String listUsers(MongoClient client) {
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        MongoDatabase database = client.getDatabase("test");
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        MongoCollection<Document> collection = database.getCollection("users");
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        final JSONArray jsonResults = new JSONArray();
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        collection.find().forEach((result) -> jsonResults.put(result.toJson()));
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        return jsonResults.toString();
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    }
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    static String insertUser(MongoClient client, String name) throws MongoException {
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        MongoDatabase database = client.getDatabase("test");
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        MongoCollection<Document> collection = database.getCollection("users");
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        collection.insertOne(new Document().append("name", name));
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        return "Successfully inserted user: " + name;
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    }
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}

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const express = require('express');
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const bodyParser = require('body-parser');
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// Use the latest drivers by installing & importing them
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const MongoClient = require('mongodb').MongoClient;
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const app = express();
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app.use(bodyParser.json());
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app.use(bodyParser.urlencoded({ extended: true }));
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// Use a connection string that lists all hosts
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// with retryable writes & majority write concern
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const uri = "mongodb://<username>:<password>@hostname0:27017,hostname1:27017,hostname2:27017/?retryWrites=true&w=majority";
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const client = new MongoClient(uri, {
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    useNewUrlParser: true,
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    useUnifiedTopology: true
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});
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// ----- API routes ----- //
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app.get('/', (req, res) => res.send('Welcome to my API!'));
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app.get('/users', (req, res) => {
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    const collection = client.db("test").collection("users");
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    collection
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    .find({})
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    // In this example, 'maxTimeMS' throws an error after 5 seconds,
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    // alerting the application to a lasting network outage
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    .maxTimeMS(5000)
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    .toArray((err, data) => {
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        if (err) {
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            // Handle errors in your application
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            // In this example, by sending the client a message
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            res.send("The request has timed out. Please check your connection and try again.");
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        }
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        return res.json(data);
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    });
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});
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app.post('/users', (req, res) => {
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    const collection = client.db("test").collection("users");
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    collection.insertOne({ name: req.body.name })
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    .then(result => {
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        res.send("User successfully added!");
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    }, err => {
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        // Handle errors in your application
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        // In this example, by sending the client a message
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        res.send("An application error has occurred. Please try again.");
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    })
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});
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// ----- End of API routes ----- //
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app.listen(3000, () => {
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    console.log(`Listening on port 3000.`);
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    client.connect(err => {
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        if (err) {
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            console.log("Not connected: ", err);
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            process.exit(0);
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        }
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        console.log('Connected.');
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    });
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});