Sun Microsystems Sun StorageTek 5800 - Manual (Page 20)

The metadata associated with an object is immutable. There is no operation to modify the
metadata associated with an object after the object has been stored. Instead, the storeMetadata
operation can be used to create a completely new object by associating new user metadata with
the underlying data and system-metadata of an existing object. The storeMetadata operation
does not merge the new metadata in with the metadata from the original OID; instead, the
storeMetadata
operation creates a new metadata record pointing to the same data object. To
accomplish a merge of new field values into existing metadata, the customer application must
manually retrieve the existing metadata from the original object, perform the merge into a
single NameValueRecord on the client side, and then call storeMetadata to create a new object
with the merged metadata.
When creating a new object using storeMetadata, a new system.object_id and new
system.object_ctime
are generated, to indicate that a new object has been created. The
metadata computed from the object data itself (system.object_length,
system.object_hash_alg
, and system.object_hash) does not change. Both the storeObject
and the storeMetadata operations return a SystemRecord value that includes all of the
system-assigned fields.
While retrieving the OID is the most common use of the SystemRecord, the other system fields
can also be helpful. For example, the customer application might use the
system.object_length
, the system.object_hash_alg and the system.object_hash fields to
verify that the data as stored matches the data as present in the customer application. If a hash
independently computed on the client matches the hash stored on the 5800 system, then the
data store has been validated.
The metadata values associated with an object can be retrieved using the retrieveMetadata
operation. The retrieveMetadata operation takes an OID as input, and returns the entire set
of user, system, and system-computed metadata. The retrieved metadata is in the form of a
NameValueRecord
that contains the value of each field as originally stored. The system fields
occur using their field names, for example. the field system.object_ctime contains the object
creation time. There is no operation to retrieve just a single field or a subset of fields by
supplying a list of field names. The retrieveMetadata operation retrieves the values of both
queryable
and non-queryable fields.
The 5800 System Query Model
One of the primary methods for retrieving data is to specify the characteristics of the desired
data and then let the system find it for you. In the 5800 system, a query expression specifies a set
of conditions on metadata field values. The system then returns a list of all the objects whose
metadata values match the query conditions. Each object is considered individually without
reference to any other objects. There are no queries that compare fields in one object with fields
in a different object.
5800 System Overview
Sun StorageTek 5800 System Client API Reference Manual • June 2008 20