BBC iPlayer Content production: The Evolution of an XML Tool-Chain

Relational Database a Handicap

First attempts at implementing iPlayer took for granted that a relational database would be used to store programme data. Project requirements, however, manifested themselves as amendments to the underlying data model. Each of these amendments would, in turn, require an update to the database schema and corresponding modification to the assumptions of the client code. This became cumbersome and confusing. Additionally, serialisation to and from the database store introduced latency to the publication pipeline. Huge quantities of data that quickly became of only historical interest were being stored, requiring tuning of the database server just to make it perform acceptably, using software developer resource when it could better be spent elsewhere.

Rationalisation and Streamlining

It was recognised that the evolving data model could be expressed in terms of a RelaxNG schema. A great deal of work went into getting this right and outputting documents adhering to it in a single transformation on input data. This gave us a readable point of reference and a handy way to determine the feasibility of new requirements: if they can be expressed in terms of a transformation on our so-called ‘Content Package’ they are possible. Business Rules Database removal and input rationalisation allowed us to impose order upon domain-specific business rules. Requirements were no longer implemented in the software but both specified and implemented in terms of a transform on our input data. Separation also enabled more effective testing and tracking of ownership and history of rules.

Content Publication

Output content is destined for both human and machine consumption. All publishing is divided into two stages: firstly, the production of an initial XML representation of an artefact and, secondly, publication of the output itself. Two-pass publication allowed us to validate XML representations against their corresponding schemas for quality assurance. Config-driven implementation means that adding a new output format is just a matter of dropping in a schema that describes it.

Did It Work?

Solution initially conceived as a mechanism to publish a subset of web content has expanded without effort to make two other internal projects redundant and produce all non-dynamic web content as well as inter-component messaging.

BBC iPlayer