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The object database: identity and navigation

Why an object database exists: the mismatch between a graph of objects and tables, the object identifier, persistent objects, and navigating from reference to reference instead of joining.

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  • 5 topics
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The outline, topic by topic

Each topic is weighted the way the exam weighs it.

  1. Concepts
  2. Questions
  3. Sort and link
  4. Scenarios
  5. Understand

A few cards from the deck

  • Object database: what is it?

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    A database that stores a program's objects directly, with their identity, references, collections and inheritance, instead of cutting them into table rows.

    The program handles objects linked to one another; the database keeps them as they are, with no translation to another model on every read and write.

  • What is a == b in Java for two objects created like this, with no equals method overridden?

    ```java
    Adresse a = new Adresse("12 rue Pasteur", "Lyon");
    Adresse b = new Adresse("12 rue Pasteur", "Lyon");
    System.out.println(a == b);
    `

    • false
    • true
    • A runtime error
    • A compilation error
    Show the answer

    false

    == compares references, hence identities: a and b are two distinct objects, even with the same values. This is exactly the distinction that the object identifier makes durable in a database.

  • A … object survives the end of the program that created it; a … object vanishes with it.

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    A persistent object survives the end of the program that created it; a transient object vanishes with it.

    Persistence is a property of the object (or of its class), not of the language: the database is what makes the object durable.

  • Object identifier: what is it?

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    An identity the database gives each object, independent of its values, which never changes during the object's life.

    A relational primary key is a value of the row and can be changed; the object identifier is not a value, it is what designates the object even when all its attributes change.

  • A design tool loads assemblies of parts nested ten levels deep and spends its time in joins. What is the cause, and which storage answers it?

    • A deep graph rebuilt table by table; storage that navigates by reference
    • Too few indexes; adding an index on every foreign key is enough to remove the joins
    • Over-normalised tables; they must be brought to second normal form
    • A slow database; a read replica is enough
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    A deep graph rebuilt table by table; storage that navigates by reference

    Ten levels means ten joins to rebuild one assembly. An index speeds up each join without removing it, a replica does not remove the rebuilding, and denormalising does not remove the levels: a part that contains parts is still one join per level.

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