Harvest, Yield, and Scalable Tolerant Systems

21 Sep 2020

Introduction

A classic paper that looks into strategies for scaling large systems that can tolerate graceful degradation.
Link to the paper

CAP refers to strong Consistency, high Availability, and Partitionability.
Strong consistency refers to single copy ACID consistency.
High availability means any consumer can access the data anytime. Generally, this is achieved by adding one or more data replicas.
Partitionability means that the system can survive a partition between the different replicas.
Strong CAP theorem states that any system can have only two out of three properties.
Weak CAP theorem says that stronger are the guarantees about any two properties, weaker are the third property’s guarantees.

Assume that the clients are making a request to a server.
There are two quantities of interest here:
- Yield - the probability of completing a request.
- Harvest - completeness of answer to a query.
In the presence of faults, a tradeoff can is made between yield and harvest. This tradeoff applies to both read and update queries.

In a hundred node cluster (without replication), a single-node failure reduces harvest by 1 %, and in the case of multi-node failure, the harvest degrades linearly.
The probability of losing high-priority data can be reduced by replicating it. However, replicating all the data would not n guarantee 100% harvest and yield despite significant costs.

Decompose a large application into subcomponents so that each component can be provisioned separately. Strong consistency can only be applied only on the components that need it, instead of the application as a whole.
Further, failure of one or more components need not cause the application to fail as a whole.
Decomposition also provides the opportunity to use orthogonal mechanisms, i.e., mechanisms independent of other mechanisms with no runtime interface.
Composition of orthogonal subsystems improves the robustness of runtime interactions by locally containing the errors. For example, the orthogonal components can be restarted /replaced independently without affecting other running components.