MSSA - Multi-Service Storage Architecture
The multi-service storage architecture (MSSA) was designed to meet the needs of existing and emerging applications such as those with multimedia presentation support requirements and those which wish to use multi-file structured documents. The MSSA allows evolution from, and compatibility with, traditional applications.
Naming and location of data in conventional filing systems such as UNIX are inappropriate for new media types such as audio and video. In the MSSA conventional files, audio, video and structured objects are supported within a common architectural framework and composite objects, such as a display representation, may have components of any of these storage types. New storage technology may be introduced without difficulty and single files may be moved between devices.
The MSSA comprises a two-level hierarchy of storage servers with a clean separation between the file abstractions at the logical level and their realisation at the physical level through byte segments. For a multimedia session a number of related ticket-controlled, rate-guaranteed streams are set up and managed. Efficiency gains can be made when network load makes it necessary to reduce QoS.
The MSSA design is open and object orientated. Value adding services may be created either through specialisation of the MSSA servers' operations or through the creation of new file classes. The access control mechanism extends through the VAS hierarchy and can guarantee secure provision of services in an open, distributed environment.
Access Control Lists (ACLs) are used to allow fine grained expression of policy together with capabilities for efficient runtime access after a once-off ACL check. MSSA capabilities are principal-specific and transient and their design ensures that access to objects is via the correct service hierarchy; for example, a directory object may only be manipulated via a directory service. The implementation of this protection is stateless at the servers above the storage service. The scheme also provides a convenient means to delegate rights for an object, temporarily, to an unprivileged server, for example a print-server. The fact that MSSA capabilities are short-lived alleviates the requirement for selective revocation and crash recovery.
One service that has been developed above the MSSA is a platform for the specification and management of applications that include multimedia (IMP). Another supports a persistent extension to C++ (PC++). Also, we have extended the work on IMP and MSSA access control to explore a general event management system for a distributed object environment.
Involved in the project were:
- Jean Bacon
- Ken Moody
- Robert Sultana
- Richard Hayton (PhD 1996)
- Kit Tsin
- Sai Lai Lo (PhD and RA until 1994)
- Zhixue Wu (PhD 1993)
- Sue Thomson (PhD and RA until 1992)
- Tim Wilson (PhD and RA until 1992)