Detail

Towards the dependable cloud: Building the foundations for tomorrow’s dependable cloud computing

Cloud computing is being increasingly adopted by individuals, organizations, and governments. However,
as the computations that are offloaded to the cloud expand to societal-critical services, the dependability
requirements of cloud services become much higher, and we need to ensure that the infrastructure
that supports these services is ready to meet these requirements. In particular, this proposal tackles the
challenges that arise from two distinctive characteristic of the cloud infrastructure.
The first is that non-crash faults, despite being considered highly unlikely by the designers of traditional
systems, become commonplace at the scale and complexity of the cloud infrastructure. We point out that the
current ad-hoc methods for handling these faults are insufficient, and that the existing principled approach
of assuming Byzantine faults is too pessimistic. Therefore, we call for a new systematic approach to
tolerating non-crash, non-adversarial faults. This requires the definition of a new fault model, and the
construction of a series of building blocks and key protocol elements that enable the construction of faulttolerant
cloud services.
The second issue is that, in order to meet their scalability and reliability requirements, cloud services
replicate state across multiple sites, and direct users to the lowest latency one. This raises the issue that not
all operations can be executed optimistically, without being aware of concurrent operations over the same
data, and thus multiple levels of consistency must coexist. However, this puts the onus of reasoning about
which behaviors are allowed under such a hybrid consistency model on the programmer of the service. We
call for a systematic solution to this problem, which will include a novel consistency model that allows
for developing highly scalable services that are fast when possible and consistent when necessary, and a
labeling methodology to guide the programmer in deciding which operations can run at each consistency
level.