MUMOC Metadata Browser

The module offers functions for metadata querying, inserting, and deleting. In addition, it encapsulates dynamically modifiable strategies to verify VoD caching convenience for a certain node. MUMOC currently employs a locality-based strategy that considers the already reached replication degree of the VoD flow in the locality.

The Metadata Browser module (MB), given a multimedia title, description, and eventually hints about the desired VoD quality, searches and returns the required multimedia presentation, i.e., its corresponding metadata.

Any MB locally hosts a partial replica of XML-based metadata for the available VoD contents and can coordinate with other distributed metadata browser components to retrieve the needed VoD description information. In particular, local MUMOC metadata repositories extend MUM repositories with new functions to store metadata according to standard, open, and extensible formats.

The distributed MB exploits MUM location abstractions. An MB component runs on each tree node and may communicate with other metadata browsers on parent and children nodes, as shown in Figure 1. Each node maintains metadata for all the VoD presentations stored in its sub-tree. For instance, A1 metadata maintain all meta-data stored in nodes from B1 to Bn. As a consequence, the root node caches metadata for all the presentations stored in the system. Let us observe that this does not generate an excessive load for the root node because:

VoD metadata have limited size (several magnitude orders lower than VoD content);
VoD metadata are replicated and disseminated all over the distributed system.

Client nodes, instead, exploit a lightweight client, called Metadata Browser Client (MBC), simply to access disseminated VoD metadata, with no possibility of locally hosting a metadata repository, in order to minimize the utilization of the usually limited client storage.

Figure 1: The MUMOC Metadata Browser

Three protocols were employed for metadata search, delete and addition, which are detailed in the following.

A client looking for a specific VoD title initiates a metadata query. If the VoD title is not present at the metadata repository on the wired node where the client is currently attached, the query is propagated recursively up until one metadata browser can respond. The protocol design strives to reduce both network traffic and response time and contributes to fast playback startup by following the locality principle.
When a VoD flow elimination occurs, all corresponding VoD metadata must be discarded over the distributed implementation of the metadata browser. The removal protocol proceeds as follows: first, VoD metadata are discarded from the node where the VoD flow was stored; then, the delete request is forwarded up to the root. Some critical races are possible: during the phase of recursive metadata elimination, MUMOC clients may get metadata for VoD flows that have already been discarded. In this case, the MUMOC middleware then ascertains that the required VoD flow is no longer available and notifies this unavailability to the client.
In the case of insertion of new VoD metadata, MUMOC immediately propagates the metadata from the node where the VoD flow has been added to the root node. This increases VoD metadata availability, by making the new metadata rapidly visible in all the distributed system.

References

P. Bellavista, A. Corradi, L. Foschini, "MUMOC: an Active Infrastructure for Open Video Caching", Proceedings of the 1st International Conference on Distributed Frameworks for Multimedia Applications (DFMA'05), Besançon, France, February 6-9, 2005, IEEE Computer Society Press.