Oracle C++ Call Interface Programmer's Guide

Overview of Navigational Access
Garbage Collection in the Object Cache
The object cache has two important parameters associated with it which are
properties of environment handle:
■ maximum cache size
■ optimal cache size
These parameters refer to levels of cache memory usage and they help to determine
when the cache automatically ages out eligible objects to make memory available.
If the amount of memory occupied by the objects that are currently in the cache
reaches or exceeds the high water mark, then the cache automatically begins to free
unmarked objects that have a pin count of 0. The cache continues freeing such
objects until memory use in the cache is optimized, or until there are no more
objects in the cache that are eligible for freeing.
OCCI provides set and get methods in environments that allow you to set and get
the cache maximum/optimal sizes. The maximum cache size is specified as a
percentage of cache optimal size. The maximum cache size in bytes is computed as
follows:
maximum_cache_size = optimal_size + optimal_size * max_size_percentage/100
The default value for cache maximum size is 10% and the default value for cache
optimal size is 8MB. When a persistent object is created through the overloaded
PObject::new operator, the newly created object is marked dirty and its pin count
is 0.
"Pin Object" on page 3-11 describes how pin count of an object functions as a
reference count and how an unmarked object with a 0 pin count can become eligible
for garbage collection. In the case of a newly created persistent object, the object is
unmarked after the transaction is committed or aborted and if the object has a 0 pin
count, in other words there are no references referring to it. The object then becomes
a candidate for being aged out.
Transactional Consistency of References
As described in the previous section, dereferencing a Ref for the first time
results in the object being loaded into the object cache from the database server.
From then on, the behavior of operator -> on Ref is the same as any C++
pointer and it provides access to the object copy in the cache. But once the
transaction commits or aborts, the object copy in the cache can no longer be valid
because it could be modified by any other client. Therefore, after the transaction
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