Identity Map

About 3 min

Intent

The Identity Map design pattern aims to ensure that each object gets loaded only once by keeping every loaded object in a map. It looks up objects using the map when referring to them, thus avoiding duplicate objects in memory.

Explanation

Real world example

Imagine you are organizing a conference and have a registration desk where every attendee must check in. To avoid unnecessary delays and confusion, each attendee's details are entered into a computer system the first time they check in. If the same attendee comes to the desk again, the system quickly retrieves their details without requiring them to re-submit the same information. This ensures each attendee's information is handled efficiently and consistently, similar to how the Identity Map pattern ensures that an object is loaded only once and reused throughout the application.

In plain words

The Identity Map design pattern ensures that each unique object is loaded only once and reused from a central registry, preventing duplicate objects in an application's memory.

Wikipedia says

In the design of DBMS, the identity map pattern is a database access design pattern used to improve performance by providing a context-specific, in-memory cache to prevent duplicate retrieval of the same object data from the database.

Programmatic Example

For the purpose of this demonstration assume we have already created a database instance db.
Let's first look at the implementation of a person entity, and it's fields:


@EqualsAndHashCode(onlyExplicitlyIncluded = true)
@Getter
@Setter
@AllArgsConstructor
public final class Person implements Serializable {

    private static final long serialVersionUID = 1L;

    @EqualsAndHashCode.Include
    private int personNationalId;
    private String name;
    private long phoneNum;

    @Override
    public String toString() {

        return "Person ID is : " + personNationalId + " ; Person Name is : " + name + " ; Phone Number is :" + phoneNum;

    }

}

The following is the implementation of the personFinder which is the entity that the user will utilize in order
to search for a record in our database. It has the relevant DB attached to it. It also maintains an IdentityMap
to store the recently read records.


@Slf4j
@Getter
@Setter
public class PersonFinder {
    private static final long serialVersionUID = 1L;
    //  Access to the Identity Map
    private IdentityMap identityMap = new IdentityMap();
    private PersonDbSimulatorImplementation db = new PersonDbSimulatorImplementation();

    /**
     * get person corresponding to input ID.
     *
     * @param key : personNationalId to look for.
     */
    public Person getPerson(int key) {
        // Try to find person in the identity map
        Person person = this.identityMap.getPerson(key);
        if (person != null) {
            LOGGER.info("Person found in the Map");
            return person;
        } else {
            // Try to find person in the database
            person = this.db.find(key);
            if (person != null) {
                this.identityMap.addPerson(person);
                LOGGER.info("Person found in DB.");
                return person;
            }
            LOGGER.info("Person with this ID does not exist.");
            return null;
        }
    }
}

The identity map field in the above class is simply an abstraction of a hashMap with personNationalId
as the keys and the corresponding person object as the value. Here is its implementation:


@Slf4j
@Getter
public class IdentityMap {
    private Map<Integer, Person> personMap = new HashMap<>();

    /**
     * Add person to the map.
     */
    public void addPerson(Person person) {
        if (!personMap.containsKey(person.getPersonNationalId())) {
            personMap.put(person.getPersonNationalId(), person);
        } else { // Ensure that addPerson does not update a record. This situation will never arise in our implementation. Added only for testing purposes.
            LOGGER.info("Key already in Map");
        }
    }

    /**
     * Get Person with given id.
     *
     * @param id : personNationalId as requested by user.
     */
    public Person getPerson(int id) {
        Person person = personMap.get(id);
        if (person == null) {
            LOGGER.info("ID not in Map.");
        }
        return person;
    }

    /**
     * Get the size of the map.
     */
    public int size() {
        if (personMap == null) {
            return 0;
        }
        return personMap.size();
    }

}

Now we should construct a dummy person for demonstration purposes and put that person in our database.

Person person1 = new Person(1,"John",27304159);
db.insert(person1);

Now let's create a person finder object and look for person with personNationalId = 1(assume that the personFinder
object already has the db and an IdentityMap attached to it.):

PersonFinder finder = new PersonFinder();
finder.getPerson(1);

At this stage this record will be loaded from the database and the output would be:

ID not in Map.
Person ID is:1;Person Name is:John;Phone Number is:27304159
Person found in DB.

However, the next we search for this record again we will find it in the map hence we will not need to go
to the database.

Person ID is:1;Person Name is:John;Phone Number is:27304159
Person found in Map.

If the corresponding record is not in the DB at all then an Exception is thrown. Here is its implementation.

public class IdNotFoundException extends RuntimeException {
    public IdNotFoundException(final String message) {
        super(message);
    }
}

Class diagram

Applicability

This pattern is used in scenarios where multiple accesses to the same data occur within a single session or transaction, especially in complex systems where object identity needs to be preserved across transactions or requests in a Java application.

Known Uses

ORM (Object-Relational Mapping) frameworks often implement Identity Maps to handle database interactions more efficiently.
Enterprise applications to maintain consistent data states across different business processes.

Consequences

Benefits:

Reduces memory usage by ensuring that only one copy of each object resides in memory.
Prevents inconsistencies during updates, as all parts of the application refer to the same instance.
Improves performance by avoiding repeated database reads for the same data.

Trade-offs:

Increases complexity in object management and persistence logic.
Can lead to stale data if not managed correctly, especially in concurrent environments.

Unit of Workopen in new window: Coordinates the actions of multiple objects by keeping track of changes and handling transactional consistency. Identity Map is used within the Unit of Work to track the objects being affected by a transaction.
Data Mapperopen in new window: Separates persistence logic from domain logic. Identity Map can be used by a Data Mapper to ensure that each object is loaded only once, enhancing performance and data consistency.