The fundamental elements of media workflows
Mar 1, 2009 12:00 PM, By Al Kovalick
Designing for reliability
Figure 4. High availability techniques
Click to enlarge
Let's consider the aspect of designing for reliability. Figure 4 on page 51 outlines 14 strategies to achieve high availability (HA) designs. For the ultimate doomsday, bulletproof system, all 14 could be applied at once. This is hardly practical, but some real-world, mission-critical systems come close. Normally, a few methods are applied, as determined by business needs.
Every design should have a service availability goal as a percentage of uptime. For example, 99.9999 percent uptime or ~32 seconds per year of downtime could be a goal. This value is achievable and allows for one or more serious failures with 32 seconds available, or less, to route around the failed element. Another approach is to decide what length of time a system can afford to be down and design from that value. Given the acceptable downtime, a designer can select options from Figure 4 to meet this goal.
Important concepts in HA designs are a single point of failure (SPOF) and no single point of failure (NSPOF). An SPOF device is not designed with redundant components. If any critical component fails — such as power supply, controller or storage — then the unit often fails. SPOF devices are used when NSPOF is not necessary or affordable. NSPOF, on the other hand, is used when the element is a critical link in a workflow. For example, most large video server storage arrays are designed with NSPOF thinking. In this case, any single element can fail, and the unit keeps plugging along with no apparent downtime. NSPOF devices have dual power supplies, multiple fans, duplicate network I/O, dual controllers, passive backplanes and more.
Some themes in Figure 4 are dual elements and I/O, dual pathing, NSPOF, mirrors, self-healing, caching, and failover to a spare. Each of these helps to resolve a particular device, link, I/O port, store or control point failure. Auto failover is vital when coupled with other methods. When a standalone element fails, such as a link, it is necessary to reroute the data traffic to an alternate link automatically. So, early fault detection is also paramount in an auto failover scenario. The total time it takes to detect and correct, or bypass, a fault is important.
Standards ubiquity
No practical system should be constructed without applying standards from a broad range of sources. Gone are the days when SMPTE standards were the only glue needed to create a video system. Today, in addition, we need the standards from the Internet Engineering Task Force (IETF), W3C and the Institute of Electrical and Electronics Engineers (IEEE) among others. User groups such as the Advanced Media Workflow Association have a mission to create best practices and recommended technologies for implementing workflows in networked media environments. They are currently developing specifications for the Advanced Authoring Format (AAF) and Material eXchange Format (MXF) component inventory method (AS-02) and a constrained MXF version for program delivery (AS-03) to broadcasters and other specifications.
Workflow documentation methods
Finally, under the design banner, consider documentation. For many years, facility documentation was a collection of diagrams showing equipment layout and racking with detailed wiring and inventory diagrams. This level of documentation is still necessary but not sufficient to describe the workflow component of an installation. While not every installation will require a workflow diagram, many will. If the workflow is complex, with disparate processes and time-related dependencies, the following methods should be of value.
Workflow stakeholders include the analysts who create and refine the processes, the technical developers responsible for implementing the processes, and the technical managers who monitor and manage the processes. Two diagramming methods are gaining acceptance to define process flow. One is based on Unified Modeling Language (UML) and another on Business Process Modeling Notation (BPMN).
UML offers activity, sequence, communication and timing diagrams. Which one should be used to describe a media workflow? Well, each has a special purpose, so depending on what aspects are to be communicated, the choice is flexible. The activity diagram offers the most flexible layout for media workflow, but all will find usage.
| Want to use this article? Click here for options! |  |
