Athena Developer Guide

Athena Chapter 8 StoreGate - the event data access model Version/Issue: 2.0.0
HENP systems such as BaBar, CDF, CLEO, D0, we identified some requirements that would not be
readily satisfied using the existing Gaudi Event Data Model:
• Identify (collections of) DataObjects based on their type.
• Identify (collections of) DataObjects based on the identifier of the Algorithm which added
them to the Transient Data Store (TDS).
• A scheme that allows developers optionally to define key classes tailored to their DataObjects
and to use the keys to store and retrieve the objects from the TDS.
• A well-defined and supported access control policy: as will be discussed later on, objects
stored into the TDS will be “almost read-only” with the adoption of a lock mechanism.
• A mechanism to hide as much as possible the details of the TDS access from the algorithmic
code, using the standard C++ iterator and/or pointer syntax.
• The same mechanism should be used to express associations among Data Objects.
• A mechanism to group related DataObjects into a developer-defined view that provides a
high-level alternative access scheme to the store.
• A mechanism to define a flexible “cache-fault policy” (that is to say a way to create an object
requested by the user and not yet in the TDS). This should include the functionality to
reconstruct a DataObject on demand.
8.2.1 System Features and Entities
page 56
A StoreGate will allow algorithmic code to interact with the TDS. When an Algorithm invokes the
StoreGate retrieve method, it is returned a DataHandle which points to the desired DataObject in the
Transient Data Store. The DataHandle defines the interface to access the DataObjects retrieved. From
the user perspective a DataHandle behaves as a C++ pointer/iterator, and its first implementation
pointer is indeed a C++ const pointer. It will have to evolve into a more complex object to satisfy some
of the requirements mentioned above. A complete implementation of the DataHandle should include:
• Begin/end methods providing iterators over contained objects,
• Enforcement of an almost-const access policy, whereby the DataHandle checks, upon
dereference, if the client is authorized to modify a DataObject in the TDS.
• An update method used (presumably by the TDS) to mark the current Data Object the
DataHandle refers to as “obsolete” when, for example, a new event is read in.
• Support for persistable object associations (e.g. track associatedHits),
• The ability for the client to view the data in different ways.
• A user-defined Key can be used while recording/retrieving the DataObject. In the prototype,
the key object can be a simple string passed to the StoreGate record method or a more
complex object that defines the necessary operators to return a string. Behind the scenes, the
string is added to the Event Data Service object “pathname” thus providing the backward
Gaudi compatibility.