Amazon QLDB driver for Java – Cookbook reference

This reference guide shows common use cases of the Amazon QLDB driver for Java. It provides Java code examples that demonstrate how to use the driver to run basic create, read, update, and delete (CRUD) operations. It also includes code examples for processing Amazon Ion data. In addition, this guide highlights best practices for making transactions idempotent and implementing uniqueness constraints.

Importing the driver

The following code example imports the driver, the QLDB session client, Amazon Ion packages, and other related dependencies.

Instantiating the driver

The following code example creates a driver instance that connects to a specified ledger name, and uses specified retry logic with a custom retry limit.

CRUD operations

QLDB runs create, read, update, and delete (CRUD) operations as part of a transaction.

Making transactions idempotent

We recommend that you make write transactions idempotent to avoid any unexpected side effects in the case of retries. A transaction is idempotent if it can run multiple times and produce identical results each time.

For example, consider a transaction that inserts a document into a table named Person. The transaction should first check whether or not the document already exists in the table. Without this check, the table might end up with duplicate documents.

Suppose that QLDB successfully commits the transaction on the server side, but the client times out while waiting for a response. If the transaction isn't idempotent, the same document could be inserted more than once in the case of a retry.

Using indexes to avoid full table scans

We also recommend that you run statements with a WHERE predicate clause using an equality operator on an indexed field or a document ID; for example, WHERE indexedField = 123 or WHERE indexedField IN (456, 789). Without this indexed lookup, QLDB needs to do a table scan, which can lead to transaction timeouts or optimistic concurrency control (OCC) conflicts.

For more information about OCC, see Amazon QLDB concurrency model.

Implicitly created transactions

The QldbDriver.execute method accepts a lambda function that receives an instance of Executor, which you can use to run statements. The Executor instance wraps an implicitly created transaction.

You can run statements within the lambda function by using the Executor.execute method. The driver implicitly commits the transaction when the lambda function returns.

The following sections show how to run basic CRUD operations, specify custom retry logic, and implement uniqueness constraints.

Creating tables

qldbDriver.execute(txn -> {
    txn.execute("CREATE TABLE Person");
});

Creating indexes

qldbDriver.execute(txn -> {
    txn.execute("CREATE INDEX ON Person(GovId)");
});

Reading documents

// Assumes that Person table has documents as follows:
// { GovId: "TOYENC486FH", FirstName: "Brent" }

qldbDriver.execute(txn -> {
    Result result = txn.execute("SELECT * FROM Person WHERE GovId = 'TOYENC486FH'");
    IonStruct person = (IonStruct) result.iterator().next();
    System.out.println(person.get("GovId")); // prints TOYENC486FH
    System.out.println(person.get("FirstName")); // prints Brent
});

Using query parameters

The following code example uses an Ion type query parameter.

qldbDriver.execute(txn -> {
    Result result = txn.execute("SELECT * FROM Person WHERE GovId = ?",
            SYSTEM.newString("TOYENC486FH"));
    IonStruct person = (IonStruct) result.iterator().next();
    System.out.println(person.get("GovId")); // prints TOYENC486FH
    System.out.println(person.get("FirstName")); // prints Brent
});

The following code example uses multiple query parameters.

qldbDriver.execute(txn -> {
    Result result = txn.execute("SELECT * FROM Person WHERE GovId = ? AND FirstName = ?",
            SYSTEM.newString("TOYENC486FH"),
            SYSTEM.newString("Brent"));
    IonStruct person = (IonStruct) result.iterator().next();
    System.out.println(person.get("GovId")); // prints TOYENC486FH
    System.out.println(person.get("FirstName")); // prints Brent
});

The following code example uses a list of query parameters.

qldbDriver.execute(txn -> {
    final List<IonValue> parameters = new ArrayList<>();
    parameters.add(SYSTEM.newString("TOYENC486FH"));
    parameters.add(SYSTEM.newString("ROEE1"));
    parameters.add(SYSTEM.newString("YH844"));
    Result result = txn.execute("SELECT * FROM Person WHERE GovId IN (?,?,?)", parameters);
    IonStruct person = (IonStruct) result.iterator().next();
    System.out.println(person.get("GovId")); // prints TOYENC486FH
    System.out.println(person.get("FirstName")); // prints Brent
});

// Assumes that Person table has documents as follows:
// {GovId: "TOYENC486FH", FirstName: "Brent" }

qldbDriver.execute(txn -> {
    try {
        Result result = txn.execute("SELECT * FROM Person WHERE GovId = 'TOYENC486FH'");
        Person person = MAPPER.readValue(result.iterator().next(), Person.class);
        System.out.println(person.getFirstName()); // prints Brent
        System.out.println(person.getGovId()); // prints TOYENC486FH
    } catch (IOException e) {
        e.printStackTrace();
    }
});

qldbDriver.execute(txn -> {
    try {
        Result result = txn.execute("SELECT * FROM Person WHERE GovId = ?",
                MAPPER.writeValueAsIonValue("TOYENC486FH"));
        Person person = MAPPER.readValue(result.iterator().next(), Person.class);
        System.out.println(person.getFirstName()); // prints Brent
        System.out.println(person.getGovId()); // prints TOYENC486FH
    } catch (IOException e) {
        e.printStackTrace();
    }
});

qldbDriver.execute(txn -> {
    try {
        Result result = txn.execute("SELECT * FROM Person WHERE GovId = ? AND FirstName = ?",
                MAPPER.writeValueAsIonValue("TOYENC486FH"),
                MAPPER.writeValueAsIonValue("Brent"));
        Person person = MAPPER.readValue(result.iterator().next(), Person.class);
        System.out.println(person.getFirstName()); // prints Brent
        System.out.println(person.getGovId()); // prints TOYENC486FH
    } catch (IOException e) {
        e.printStackTrace();
    }
});

qldbDriver.execute(txn -> {
    try {
        final List<IonValue> parameters = new ArrayList<>();
        parameters.add(MAPPER.writeValueAsIonValue("TOYENC486FH"));
        parameters.add(MAPPER.writeValueAsIonValue("ROEE1"));
        parameters.add(MAPPER.writeValueAsIonValue("YH844"));
        Result result = txn.execute("SELECT * FROM Person WHERE GovId IN (?,?,?)", parameters);
        Person person = MAPPER.readValue(result.iterator().next(), Person.class);
        System.out.println(person.getFirstName()); // prints Brent
        System.out.println(person.getGovId()); // prints TOYENC486FH
    } catch (IOException e) {
        e.printStackTrace();
    }
});

Inserting documents

The following code example inserts Ion data types.

qldbDriver.execute(txn -> {
    // Check if a document with GovId:TOYENC486FH exists
    // This is critical to make this transaction idempotent
    Result result = txn.execute("SELECT * FROM Person WHERE GovId = ?",
            SYSTEM.newString("TOYENC486FH"));
    // Check if there is a result
    if (!result.iterator().hasNext()) {
        IonStruct person = SYSTEM.newEmptyStruct();
        person.put("GovId").newString("TOYENC486FH");
        person.put("FirstName").newString("Brent");
        // Insert the document
        txn.execute("INSERT INTO Person ?", person);
    }
});

qldbDriver.execute(txn -> {
    try {
        // Check if a document with GovId:TOYENC486FH exists
        // This is critical to make this transaction idempotent
        Result result = txn.execute("SELECT * FROM Person WHERE GovId = ?",
                MAPPER.writeValueAsIonValue("TOYENC486FH"));
        // Check if there is a result
        if (!result.iterator().hasNext()) {
            // Insert the document
            txn.execute("INSERT INTO Person ?",
                    MAPPER.writeValueAsIonValue(new Person("Brent", "TOYENC486FH")));
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
});

This transaction inserts a document into the Person table. Before inserting, it first checks if the document already exists in the table. This check makes the transaction idempotent in nature. Even if you run this transaction multiple times, it won't cause any unintended side effects.

Inserting multiple documents in one statement

To insert multiple documents by using a single INSERT statement, you can pass a parameter of type IonList (explicitly cast as an IonValue) to the statement as follows.

// people is an IonList explicitly cast as an IonValue
txn.execute("INSERT INTO People ?", (IonValue) people);

You don't enclose the variable placeholder (?) in double angle brackets ( <<...>> ) when passing an IonList. In manual PartiQL statements, double angle brackets denote an unordered collection known as a bag.

Updating documents

qldbDriver.execute(txn -> {
    final List<IonValue> parameters = new ArrayList<>();
    parameters.add(SYSTEM.newString("John"));
    parameters.add(SYSTEM.newString("TOYENC486FH"));
    txn.execute("UPDATE Person SET FirstName = ? WHERE GovId = ?", parameters);
});

qldbDriver.execute(txn -> {
    try {
        final List<IonValue> parameters = new ArrayList<>();
        parameters.add(MAPPER.writeValueAsIonValue("John"));
        parameters.add(MAPPER.writeValueAsIonValue("TOYENC486FH"));
        txn.execute("UPDATE Person SET FirstName = ? WHERE GovId = ?", parameters);
    } catch (IOException e) {
        e.printStackTrace();
    }
});

Deleting documents

qldbDriver.execute(txn -> {
    txn.execute("DELETE FROM Person WHERE GovId = ?",
            SYSTEM.newString("TOYENC486FH"));
});

qldbDriver.execute(txn -> {
    try {
        txn.execute("DELETE FROM Person WHERE GovId = ?",
                MAPPER.writeValueAsIonValue("TOYENC486FH"));
    } catch (IOException e) {
        e.printStackTrace();
    }
});

Running multiple statements in a transaction

// This code snippet is intentionally trivial. In reality you wouldn't do this because you'd
// set your UPDATE to filter on vin and insured, and check if you updated something or not.
public static boolean InsureCar(QldbDriver qldbDriver, final String vin) {
    final IonSystem ionSystem = IonSystemBuilder.standard().build();
    final IonString ionVin = ionSystem.newString(vin);

    return qldbDriver.execute(txn -> {
        Result result = txn.execute(
                "SELECT insured FROM Vehicles WHERE vin = ? AND insured = FALSE",
                ionVin);
        if (!result.isEmpty()) {
            txn.execute("UPDATE Vehicles SET insured = TRUE WHERE vin = ?", ionVin);
            return true;
        }
        return false;
    });
}

Retry logic

The driver's execute method has a built-in retry mechanism that retries the transaction if a retryable exception occurs (such as timeouts or OCC conflicts).

Implementing uniqueness constraints

QLDB doesn't support unique indexes, but you can implement this behavior in your application.

Suppose that you want to implement a uniqueness constraint on the GovId field in the Person table. To do this, you can write a transaction that does the following:

If a competing transaction concurrently passes the assertion, only one of the transactions will commit successfully. The other transaction will fail with an OCC conflict exception.

The following code example shows how to implement this uniqueness constraint logic.

qldbDriver.execute(txn -> {
    Result result = txn.execute("SELECT * FROM Person WHERE GovId = ?",
            SYSTEM.newString("TOYENC486FH"));
    // Check if there is a result
    if (!result.iterator().hasNext()) {
        IonStruct person = SYSTEM.newEmptyStruct();
        person.put("GovId").newString("TOYENC486FH");
        person.put("FirstName").newString("Brent");
        // Insert the document
        txn.execute("INSERT INTO Person ?", person);
    }
});

Working with Amazon Ion

There are multiple ways to process Amazon Ion data in QLDB. You can use built-in methods from the Ion library to create and modify documents flexibly as needed. Or, you can use FasterXML's Jackson data format module for Ion to map Ion documents to plain old Java object (POJO) models.

The following sections provide code examples of processing Ion data using both techniques.

Importing the Ion packages

Add the artifact ion-java as a dependency in your Java project.

Import the following Ion packages.

import com.amazon.ion.IonStruct;
import com.amazon.ion.IonSystem;
import com.amazon.ion.system.IonSystemBuilder;

import com.amazon.ion.IonReader;
import com.amazon.ion.IonStruct;
import com.amazon.ion.system.IonReaderBuilder;
import com.amazon.ion.system.IonSystemBuilder;

import com.fasterxml.jackson.dataformat.ion.IonObjectMapper;
import com.fasterxml.jackson.dataformat.ion.ionvalue.IonValueMapper;

Initializing Ion

IonSystem SYSTEM = IonSystemBuilder.standard().build();

IonObjectMapper MAPPER = new IonValueMapper(IonSystemBuilder.standard().build());

Creating Ion objects

The following code example creates an Ion object by using the IonStruct interface and its built-in methods.

IonStruct ionStruct = SYSTEM.newEmptyStruct();

ionStruct.put("GovId").newString("TOYENC486FH");
ionStruct.put("FirstName").newString("Brent");

System.out.println(ionStruct.toPrettyString()); // prints a nicely formatted copy of ionStruct

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonProperty;

public static class Person {
    private final String firstName;
    private final String govId;

    @JsonCreator
    public Person(@JsonProperty("FirstName") final String firstName,
                  @JsonProperty("GovId") final String govId) {
        this.firstName = firstName;
        this.govId = govId;
    }

    @JsonProperty("FirstName")
    public String getFirstName() {
        return firstName;
    }

    @JsonProperty("GovId")
    public String getGovId() {
        return govId;
    }
}

IonStruct ionStruct = (IonStruct) MAPPER.writeValueAsIonValue(new Person("Brent", "TOYENC486FH"));

Reading Ion objects

The following code example prints each field of the ionStruct instance.

// ionStruct is an instance of IonStruct
System.out.println(ionStruct.get("GovId")); // prints TOYENC486FH
System.out.println(ionStruct.get("FirstName")); // prints Brent

// ionStruct is an instance of IonStruct
IonReader reader = IonReaderBuilder.standard().build(ionStruct);
Person person = MAPPER.readValue(reader, Person.class);
System.out.println(person.getFirstName()); // prints Brent
System.out.println(person.getGovId()); // prints TOYENC486FH

For more information about working with Ion, see the Amazon Ion documentation on GitHub. For more code examples of working with Ion in QLDB, see Working with Amazon Ion data types in Amazon QLDB.