#Data access

The module lowlevel.data_access provides tools for acessing the data in a running application.

ObjectPool

• ObjectPool is a singleton holding a weak reference to every object created. It is used for immediate access strategy.

• instance_hook is a hook to be set when using ObjectPool. It adds objects to the pool when they are created. Setting this hook requires using our custom version of Pypy. When instantiating the ObjectPool this hook is set up automatically.

HeapWalker

The HeapWalker class is to be extended to define a walker specifically adapted to the application to be updated. It walks the heap of the application to access the data and transform it.

When extending the HeapWalker class, one has to define a walk_<type> method that will be called to walk data of <type>.

traverseHeap(walker,module_names) walks the heap using walker on each module whose name are in the list module_names.

DataAccessor

DataAccessor is a class that can be used to access the data. Its constructor takes the target class whose instance we want to access and a string indicating the strategy we want to use (immediate or progressive).

DataAccessor is an iterator, meaning that we can use a for loop to iterate over every object it accesses. When iterating over a data accessor using progressive startegy, it will hold each iteration until a new object has been accessed.

⚠ Warning : The immediate strategy uses the ObjectPool to access objects. Remember to create the ObjectPool before the objects you want to access are created.

Getter and Setter routers

GetItemRouter and SetItemRouter are used by DataAccessor when using the progressive access strategy, to overload the __setattr__ and __getattribute__ methods of the target class. When called (i.e. when accessing an attribute of objects of the target class), these routers will add a reference to their caller to the data accessor.

These routers are placed using the add_getter_router and add_setter_router functions.

Eager and Lazy updates

• setLazyUpdate(class,transformer) starts a lazy update of objects of class class, applies transformer to them when accessed. This combines progressive access startegy and instant update startegy.

• startEagerUpdate(class,transformer) start an eager update of ovjects of class class, applies transformer to them when accessed. This combines immediate access strategy and instant update strategy.