Glossary

Glossary of Technical Terms

Collection: A collection is a logical name given to a set of data objects. All data objects stored in SRB/MCAT are stored in some collection. A collection can have sub-collections, and hence provides a hierarchical structure. As a simple analogy, a collection in SRB/MCAT can be equated to a directory in a Unix file system. But unlike a file system, a collection is not limited to a single device (or partition). A collection is logical but the data objects grouped under a collection can be stored in heterogeneous storage devices. There is one obvious restriction, the name given to a data object in a collection or sub-collection should be unique in that collection.

Compound Resource: A compound resource allows the SRB to function as a complete (although basic) archival storage system (also known as an hierarchical storage system (HSM)). A compound resource may be configured to contain a pool of cache resources and a tape resource. When a user creates a file using a compound resource, the object created becomes a "compound object". The actual data of a "compound object" may reside on cache or tape or both. Unlike the SRB replica, a "compound object" always appears as a single object even though there may be multiple copies of the data. It is a simple hierarchical system where data migrate automatically between cache and tape. Data is always staged on cache automatically whenever it is accessed and migrates to tape by the system administrator when more cache space is needed. The cache and tape resources can be distributed across a WAN.

Container: A Container is a way to put together a lot of small files into one larger file to improve performance. This works very well with resources that include tapes (such as HPSS). The whole container is retrieved from tape, cached on SRB disk, and then multiple files can be quickly read and written on the container copy on disk. The SRB handles the book-keeping for the container. Also see the FAQ container items.

Data object: A data object can usually be thought of as a file, although it can also be a real-time data stream, or database binary large object. Also see the FAQ item

Dublin Core: The Dublin Core is a metadata element set. It includes all DCMI terms (that is, refinements, encoding schemes, and controlled vocabulary terms) intended to facilitate discovery of resources. See Dublin Core.

GUID: A Globally Unique Identifier or GUID is a pseudo-random number used in many software applications. While each generated GUID is not guaranteed to be unique, the total number of unique keys (2128 or 3.4028×1038) is so large that the possibility of the same number being generated twice is very small.

Logical Resource: A logical SRB resource is a SRB Resource that is derived from Physical Resources. A logical SRB resource might be derived with further constraints on a registered physical resource or by combining more than one physical resource as an entity. For example, if a physical resource 'A' is defined using a particular directory in a HPSS, a logical resource A-bar might be defined as a resource that restricts to a further sub-directory in 'A'. A 'logical SRB resource set' is kind of logical SRB resource. It is defined as a set of physical SRB resources. The aim is for this is to give a unique (logical) name to a set of resources and when SRB opens or writes a buffer to the logical resource it opens or writes to every resource in that set. A logical resource containing multiple physical resources can be treated as a 'single' resource when using it.

MCAT: The MCAT is the Metadata Catalog. It is a database (using a DBMS) and contains all the information the SRB needs to function. This includes the location and physical path to each individual Data object (file), user and group information, and system and user defined metadata. Also see the MCAT Section in the FAQ and the MCAT writeup.

Resource: In the terminology of SRB, a resource is a software/hardware system that provides the storage functionalities. For example, HPSS can be a resource, as can a Unix file system. The term is equivalent to "Physical Resource". Logical Resources and Compound Resources are composed of multiple physical resources.

System Metadata: MCAT considers five kinds of entities as primitive objects on which it keeps additional information. These are: data objects, resources, collections, users and methods. The system-level MCAT meta data items are these primitive objects and others derived from these.

User Group: A user group is a uniquely identifiable name given to a set of SRB registered users.

User-Defined Metadata: User-defined metadata is information about SRB data objects that users can establish. In addition to the built-in System Metadata, users can set up and query user-defined metadata as attribute-value-unit triplets (accessible via the Sufmeta Scommand). The Extensible Schema is another way in which users can define and use metadata specific to their application. Also see the FAQ item, What is application level meta data?.