Databases Reference
In-Depth Information
The retrieve
command searches for a stream having the identifier
StreamID
within the cluster and returns the stream to the host indicated by
Host ID
. If the
stream cannot be found within the cluster, it returns false as a return value.
The
insert
command inserts the
Stream
into a place corresponding to the
StreamID
.
When the insertion succeeds, true is returned to the host
HostID
. The
delete
command searches for the stream of
StreamID
then deletes it, and returns the
status to the host
HostIS
.
An EIS command is transformed into combinations of IS commands by pre-
defined ECA rules together with the conditions of the system. The following are
examples of IS commands:
• send(Destination,Stream)
• traverse_directory(StreamID)
• insert_local(Location,StreamID,Stream)
• delete_local(Location,StreamID)
• mapping(DiskID,Type)
• compose(Substreams)
• decompose(Stream)
The IS commands are not limited to these. If new functions are required, new IS
commands can be introduced. Basically, these commands are pre-defined by the
system in some programming language, including the IDA commands. Occasionally,
an IS command may be implemented by a combination of other IS commands and
the ECA rules.
To deal with heterogeneity, implementation of the IS commands can be varied
for each disk. Note that the IS commands should not affect the basic disk control
function of protecting the disk from illegal use of the commands.
The send IS command sends a stream to a destination. Because the disk identifier
is known at the IS command level, the destination is either
HostID or DiskID
.
There is no difference between disk-host and disk-disk communication at this level.
The commands
insert_local and delete_local
execute insert and
delete operations on the stream at the required location in the disk. For fault
tolerance, data should be replicated on several disks. Because a tightly synchronous
update of backups has a large overhead, it is better to update the replicas
asynchronously. Sequential log files can be used to implement the asynchronous
update of backups. For these purposes, meta-information is required to map the
disks, keeping primary, backup and logs. The command
mapping
returns a copy
of the mapping information kept in each disk.
A stream can be fragmented into multiple sub-streams stored in different disks
to enhance disk throughput. The command
decompose
fragments a stream into
substreams, and compose regenerates the original stream from the fragmented
substreams.
