Databases Reference
In-Depth Information
Figure 9: Workstation cluster
WAKASHI is a server program that runs on each site of a workstation cluster,
which is a set of computers connected by a computer network (see Figure 9).
ShusseUo is a multi-client multi-server system. WAKASHI provides the services of
the database server to INADA. The main service of WAKASHI is a data storage
space named
heap
. WAKASHI manages heap, and has functions of distributed
locking, distributed caching, transaction commit and abort, logging, and recovery.
A client program directly maps a heap provided by WAKASHI onto a part of the
virtual memory space of the client program. Then, the client program reads and
writes data in heap by memory address.
WAKASHI is a page server and does not depend on the data model or query
language. Heap is a global data storage space in a workstation cluster. All of the
client programs on a workstation cluster share the data in the heap. Heap has
fixedlength and an array of pages. WAKASHI receives a page number and lock
mode from client programs for distributed locking and uses this page number for
data transfer between distributed servers. The operations of heap are open, close,
map, lock, commit, and abort. The lock operation is used to obtain a read page
lock or a write page lock. WAKASHI transfers a data page when the page lock is
obtained and writes the data page back to a database file upon transaction commit.
Data stored in a heap is persistent when the heap is persistent. A heap is persistent
when the heap is mapped onto a database file. The mapping is disk-memory
mapping. WAKASHI uses the memory-mapped file OS service for disk-memory
mapping. WAKASHI maintains the before images of the pages written by client
programs and uses these for transaction abort. In summary, heap is an extension
to distributed shared virtual memory, and a heap is persistent when the heap is
mapped onto a database file (see Figure 10).
