2.10 Advanced Topics

The tutorial covers the essential topics and concepts for using Elephant. Many people will find that these features are the ones that are most often needed and used in ordinary applications.

More sophisticated uses of Elephant may require additional features that are covered in the user guide. The following is a list of major features in the user guide that were not covered in this tutorial.

• Using Multiple Threads and Processes What constraints must be accommodated to use Elephant data stores in multiple threads? What capabilities are there to share data stores among multiple processes or machines?
• Class Heirarchies and Queries There are some subtle issues to take into account when querying persistent classes. For example, how do you query a base class of type people to get subclass instances such as employee, manager, consultant, etc?
• Derived Class Indices You can create your own indices for classes that are arbitrary lisp functions of the persistent object.
• Dynamic Class Index Management It is possible to add and remove indexes from classes at runtime.
• Class Definition/Database Conflict Resolution When you startup lisp, there are potential conflicts between the class definition and the indexing records in the database. There are some constraints to account for and some facilities to manage how slots, class indices and
• Indexed BTrees Indexed BTrees are just like BTrees, except it is possible to add indexes which are BTrees who's values are primary keys in the parent indexed-btree. This allows for multiple ordering and groupings of the values of a BTree.
• BTree Cursors If you need to do more than iterate over a collection, or you need to delete elements of the collection as you iterate cursors are an important data structure. They implement a variety of operators for moving backward and forward over a btree, including ranged operations and iterating of duplicate or unique values.
• Using the Map Operators Mapping operators can be very efficient if properly utilized.
• Using Multiple Stores Multiple store controllers can be open simultaneously. However it does make the code more complex and you need to be careful about how you use them to avoid crashes and other unpleasant side effects.
• Custom Transaction Architecture You can implement your own version of with-transaction using the underlying controller methods for starting, aborting and committing transactions. You had better know what you are doing, however!
• Handling Errors and Conditions There are a variety of errors that can occur in Elephant that need to be dealt with by applications.
• Deadlock Detection in Berkeley DB Berkeley DB requires an external process to detect deadlock conditions among transactions. The :deadlock-detect keyword argument to open-store for Berkeley DB specs will launch this process on most lisps.

Further, see Design Patterns for information about Elephant design patterns, solutions to common problems and other scenarios with multiple possible solutions.