Databases Reference
In-Depth Information
Vector Clock: Joe (ver 1), Hillary (ver 1), Ajay (ver 0), Eric (ver 2)
Venue: Los Altos
Vector Clock: Joe (ver 1), Hillary (ver 0), Ajay (ver 1), Eric (ver 1)
Version 0 is created for Hillary and Ajay in the vector clocks for the venues they had not suggested
but are now aware of. Now, vector clocks descend from each other and Mountain View is the
venue for the meet up. From the example, you would have observed that vector clocks not only help
determine the order of the events but also help resolve any inconsistencies by identifying the root
causes for those problems.
Apart from object versioning, Dynamo uses gossip-based membership for the nodes and uses hinted
handoff for consistency.
Gossip-Based Membership and Hinted Handoff
A gossip protocol is a style of communication protocol inspired by the form of gossip or rumor in
social networks and offi ces. A gossip communication protocol involves periodic, pair wise, inter-
process interactions. Reliability is usually low and peer selection is often random.
In hinted handoff, instead of a full quorum during message write for durability, a relaxed quorum is
allowed. Write is performed on the healthy nodes and hints are recorded to let the failed node know
when it's up again.
SUMMARY
This chapter was a brief introduction to the basic principles of NoSQL databases. The essentials
of data models, storage schemes, and confi guration in some of the popular NoSQL stores were
explained. Typical parts of column-oriented databases were presented and examples from HBase
were used to illustrate some of the common underlying themes. Then the internals of both document
databases and key/value stores were covered. Finally, eventually consistent databases were introduced.
