gocept.reference 0.2

Motivation

When developing an application we often find the need to reference objects that are managed within the application itself. Those objects are typically master data-like and are managed centrally within the application.

The reference to those objects is typically intrinsic to the application we develop so they should behave like normal Python object references that are under the control of our application.

Within the world of Zope and ZODB there are different ways to achieve this. The various approaches have different semantics and side effects. Our goal is to unify the way of intrinsically referencing objects and to provide an ability to switch between different semantics as needed without rewriting application code and without the need to migrate persistent data structures (at least from the application’s point of view).

Model comparison

Our goal was to determine the advantages and disadvantages of the different approaches. We included three general approaches from the world of Python/Zope/ZODB and also the standard relational approach to normalisation tables.

We used four criteria to describe each solution:

Reference data

What data is stored to describe the reference.

Reference semantic

What meaning does the reference have? How can the meaning change?

Integrity

What might happen to my application if data that is involved in the reference might change or become deleted?

Set/Lookup

What do I (as an application developer) have to do to set a reference or look up a referenced object?

Observations

• Relational: every object (row) has a canonical place that defines a primary key.

  The ZODB (like a filesystem) can have multiple hard links to an object. Objects are deleted when the last hard link to an object is removed. This makes it impossible to use hard links for referencing an object because object deletion will not be noticed and the objects will continue to live. The ZODB itself does not have a notion of a canonical place where an object is defined.

• Relational: When referencing an object we can enforce integrity by declaring a foreign key. This is orthogonal to the data stored.

• Relational: As an application-level key is used for identifying the target of a reference the application can choose to delete a row and re-add a row with the same primary key later. If the integrity is enforced this requires support on the database level to temporarily ignore broken foreign keys.

• Normal Python references embed themselves naturally in the application. Properties allow hiding implementation on how references are looked up/stored.

Conclusions / Requirements for the reference implementation

• Allow configuration of foreign key constraints (none, always, end-of-transaction). This selection must be possible to be changed afterwards and provide an automatic migration path.

• Use application level keys to refer to an object.

• Use a canonical location and a primary key to store objects and to determine whether an object was deleted.

• Distinguish between two use cases when modifying an object’s key:

  1. The application references the right object but it has the wrong key (as the key might have meaning for the application). In this case the object must be updated to receive the new, correct key and the references must be updated to refer to this new key.

  2. The application references the wrong object with the right key. In this case the object with the referenced key must be moved away and the key must be given to the new object.

Implementation notes

• Canonical location is determined by location/containment. The primary key for a reference is the referenced object’s location.

• Constraints are enforced by monitoring containment events.

• The different ways of updating/changing a key’s meaning are supported by an indirection that enumerates all keys and stores a reference id on the referencing object instead of the location. The two use cases for changing the meaning are reflected to either:

  1. associate a new path with an existing reference id

  2. associate a new reference id with an existing path