Cloud computing has recently emerged as a subject of substantial industrial and academic interest, though its meaning and scope is hotly debated. For some researchers, clouds are a natural evolution towards the full commercialisation of grid systems, while others dismiss the term as a mere re-branding of existing pay-per-use technologies. From either perspective, "cloud" is now the label of choice for accountable pay-per-use access to third party applications and computational resources on a massive scale. Clouds support patterns of less predictable resource use for applications and...