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Language Support for Loosely Consistent Distributed Programming

  • Author(s): Conway, Neil Robert George
  • Advisor(s): Hellerstein, Joseph M
  • et al.
Abstract

Driven by the widespread adoption of both cloud computing and mobile devices,

distributed computing is increasingly commonplace. As a result, a growing

proportion of developers must tackle the complexity of distributed

programming---that is, they must ensure correct application behavior in the

face of asynchrony, concurrency, and partial failure.

To help address these difficulties, developers have traditionally relied upon

system infrastructure that provides strong consistency guarantees

(e.g., consensus protocols and distributed transactions). These mechanisms

hide much of the complexity of distributed computing---for example, by

allowing programmers to assume that all nodes observe the same set of events

in the same order. Unfortunately, providing such strong guarantees becomes

increasingly expensive as the scale of the system grows, resulting in

availability and latency costs that are unacceptable for many modern

applications.

Hence, many developers have explored building applications that only require

loose consistency guarantees---for example, storage systems that only

guarantee that all replicas eventually converge to the same state, meaning

that a replica might exhibit an arbitrary state at any particular

time. Adopting loose consistency involves making a well-known tradeoff:

developers can avoid paying the latency and availability costs incurred by

mechanisms for achieving strong consistency, but in exchange they must deal

with the full complexity of distributed computing. As a result, achieving

correct application behavior in this environment is very difficult.

This thesis explores how to aid developers of loosely consistent applications

by providing programming language support for the difficulties they

face. The language level is a natural place to tackle this problem: because

developers that use loose consistency have fewer system facilities that they

can depend on, consistency concerns are naturally pushed into application

logic. In part, our goal has been to recognize, formalize, and automate

application-level consistency patterns.

We describe three language variants that each tackle a different challenge in

distributed programming. Each variant is a modification of Bloom, a

declarative language for distributed programming we have developed at UC

Berkeley. The first variant of Bloom, BloomL, enables

deterministic distributed programming without the need for distributed

coordination. Second, Edelweiss allows distributed storage reclamation

protocols to be generated in a safe and automatic fashion. Finally,

BloomPO adds sophisticated ordering constraints that

we use to develop a declarative, high-level implementation of concurrent

editing, a particularly difficult class of loosely consistent programs.

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