customallocator.java

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package org.garret.perst;

/**
 * Custom allocator interface. Custom allocator can be used for more efficiently 
 * allocate space using application specific semantic of the object. For example,
 * application can place all BLOBs (images, texts, video,...) in separate file, located at separate disk
 * and keep file with the rest of the data (metadata describing this BLOBs) relatively small, improving
 * speed of search operations. Such separation of BLOBs and their descriptors can be achieved
 * using custom allocator in conjunction with multifile. First segment is used for allocation of normal
 * (non-BLOB) objects. It's size can be set practically unlimited: 0x1000 0000 0000 0000.
 * And second segment should be used by custom allocator to allocate BLOBs. So BLOBs offsets are started
 * from 0x1000000000000000 and BLOB content will be stored in separate file which in turn can be located 
 * at separate disk.
 */
public interface CustomAllocator extends IPersistent { 
    /**
     * Allocate object
     * @param size allocated object size
     * @return position of the object in daatbase file,
     * It should not overlap with space covered by main database allocation bitmap
     */
    long allocate(long size);

    /**
     * Reallocate object previously allocated by this allocator.
     * This method should try to extend or shrink this object in its current location
     * and if it is not possible, allocate new space for the object and free its old location.
     * @param pos old position of the object
     * @param oldSize old size of the object
     * @param newSize new size of the object
     * @return new position of the object (it can be equal to old position)
     */
    long reallocate(long pos, long oldSize, long newSize);

    /**
     * Deallocate object previously allocated by this allocator.
     * Space used by this object can not be reused until transaction commit (when commit method is called for this
     * allocator)
     * @param pos position of the object
     * @param size size of allocated object
     */
    void free(long pos, long size);

    /**
     * Make it possible to reused space of all previously deallocated shadow objects. 
     */
    void commit();
}