DynamoDB: Replication and Partitioning – Part 4

In our previous post on DynamoDB, An Inside Look Into NoSQL, we mentioned the various distributed techniques used while architecting NoSQL data stores. A table nicely summarized these techniques and their advantages. In this article, we will go into the details of partitioning and replication.

Partitioning in DynamoDB

As mentioned earlier, the key design requirement for DynamoDB is to scale incrementally. In order to achieve this, there must be a mechanism in place that dynamically partitions the entire data over a set of storage nodes. DynamoDB employs consistent hashing for this purpose. As per the Wikipedia page, “Consistent hashing is a special kind of hashing such that when a hash table is resized and consistent hashing is used, only K/n keys need to be remapped on average, where K is the number of keys, and n is the number of slots. In contrast, in most traditional hash tables, a change in the number of array slots causes nearly all keys to be remapped.”

Consistent HashingConsistent Hashing (Credit)

Let’s understand the intuition behind it: Consistent hashing is based on mapping each object to a point on the edge of a circle. The system maps each available storage node to many pseudo-randomly distributed points on the edge of the same circle. To find where an object O should be placed, the system finds the location of that object’s key (which is MD5 hashed) on the edge of the circle; then walks around the circle until falling into the first bucket it encounters.

The result is that each bucket contains all the resources located between its point and the previous bucket point. If a bucket becomes unavailable (for example because the computer it resides on is not reachable), then the angles it maps to will be removed. Requests for resources that would have mapped to each of those points now map to the next highest point. Since each bucket is associated with many pseudo-randomly distributed points, the resources that were held by that bucket will now map to many different buckets. The items that mapped to the lost bucket must be redistributed among the remaining ones, but values mapping to other buckets will still do so and do not need to be moved. A similar process occurs when a bucket is added. You can clearly see that with this approach, the additions and deletions of a storage node only affect its immediate neighbors and all the other nodes remain unaffected.

The basic implementation of consistent hashing has its limitations:

  1. Random position assignment of each node leads to non-uniform data and load distribution.
  2. Heterogeneity of the nodes is not taken into account.

To overcome these challenges, DynamoDB uses the concept of virtual nodes. A virtual node looks like a single data storage node but each node is responsible for more than one virtual node. The number of virtual nodes that a single node is responsible for depends on its processing capacity.

Replication in DynamoDB

Every data item is replicated at N nodes (and NOT virtual nodes). Each key is assigned to a coordinator node, which is responsible for READ and WRITE operation of that particular key. The job of this coordinator node is to ensure that every data item that falls in its range is stored locally and replicated to N - 1 nodes. The list of nodes responsible for storing a certain key is called preference list. To account for node failures, the preference list usually contains more than N nodes. In the case of DynamoDB, the default replication factor is 3.

In the next article, we will look into Data Versioning.

Test your knowledge and learn how to use DynamoDB

We have some great DynamoDB content to test your knowledge on DynamoDB.

On the Working with DynamoDB course, you’ll learn to create DynamoDB tables, read and write data, and how to use queries and scans. Test the knowledge you’ve acquired in the course, in the Introduction to DynamoDB Hands-on Lab and learn how to create DynamoDB tables, with and without local or global secondary indexes, and how to manage your table data.

Avatar

Written by

47Line Technologies

47Line is building solutions solving critical business problems using “cloud as the backbone”. The team has been working in Cloud Computing domain for last 6 years and have proven thought leadership in Cloud, Big Data technologies.


Related Posts

Alisha Reyes
Alisha Reyes
— January 6, 2020

New on Cloud Academy: Red Hat, Agile, OWASP Labs, Amazon SageMaker Lab, Linux Command Line Lab, SQL, Git Labs, Scrum Master, Azure Architects Lab, and Much More

Happy New Year! We hope you're ready to kick your training in overdrive in 2020 because we have a ton of new content for you. Not only do we have a bunch of new courses, hands-on labs, and lab challenges on AWS, Azure, and Google Cloud, but we also have three new courses on Red Hat, th...

Read more
  • agile
  • AWS
  • Azure
  • Google Cloud Platform
  • Linux
  • OWASP
  • programming
  • red hat
  • scrum
Alisha Reyes
Alisha Reyes
— December 24, 2019

Cloud Academy’s Blog Digest: Azure Best Practices, 6 Reasons You Should Get AWS Certified, Google Cloud Certification Prep, and more

Happy Holidays from Cloud Academy We hope you have a wonderful holiday season filled with family, friends, and plenty of food. Here at Cloud Academy, we are thankful for our amazing customer like you.  Since this time of year can be stressful, we’re sharing a few of our latest article...

Read more
  • AWS
  • azure best practices
  • blog digest
  • Cloud Academy
  • Google Cloud
Avatar
Guy Hummel
— December 12, 2019

Google Cloud Platform Certification: Preparation and Prerequisites

Google Cloud Platform (GCP) has evolved from being a niche player to a serious competitor to Amazon Web Services and Microsoft Azure. In 2019, research firm Gartner placed Google in the Leaders quadrant in its Magic Quadrant for Cloud Infrastructure as a Service for the second consecuti...

Read more
  • AWS
  • Azure
  • Google Cloud Platform
Alisha Reyes
Alisha Reyes
— December 10, 2019

New Lab Challenges: Push Your Skills to the Next Level

Build hands-on experience using real accounts on AWS, Azure, Google Cloud Platform, and more Meaningful cloud skills require more than book knowledge. Hands-on experience is required to translate knowledge into real-world results. We see this time and time again in studies about how pe...

Read more
  • AWS
  • Azure
  • Google Cloud
  • hands-on
  • labs
Alisha Reyes
Alisha Reyes
— December 5, 2019

New on Cloud Academy: AWS Solution Architect Lab Challenge, Azure Hands-on Labs, Foundation Certificate in Cyber Security, and Much More

Now that Thanksgiving is over and the craziness of Black Friday has died down, it's now time for the busiest season of the year. Whether you're a last-minute shopper or you already have your shopping done, the holidays bring so much more excitement than any other time of year. Since our...

Read more
  • AWS
  • AWS solution architect
  • AZ-203
  • Azure
  • cyber security
  • FCCS
  • Foundation Certificate in Cyber Security
  • Google Cloud Platform
  • Kubernetes
Avatar
Cloud Academy Team
— December 4, 2019

Understanding Enterprise Cloud Migration

What is enterprise cloud migration? Cloud migration is about moving your data, applications, and even infrastructure from your on-premises computers or infrastructure to a virtual pool of on-demand, shared resources that offer compute, storage, and network services at scale. Why d...

Read more
  • AWS
  • Azure
  • Data Migration
Wendy Dessler
Wendy Dessler
— November 27, 2019

6 Reasons Why You Should Get an AWS Certification This Year

In the past decade, the rise of cloud computing has been undeniable. Businesses of all sizes are moving their infrastructure and applications to the cloud. This is partly because the cloud allows businesses and their employees to access important information from just about anywhere. ...

Read more
  • AWS
  • Certifications
  • certified
Avatar
Andrea Colangelo
— November 26, 2019

AWS Regions and Availability Zones: The Simplest Explanation You Will Ever Find Around

The basics of AWS Regions and Availability Zones We’re going to treat this article as a sort of AWS 101 — it’ll be a quick primer on AWS Regions and Availability Zones that will be useful for understanding the basics of how AWS infrastructure is organized. We’ll define each section,...

Read more
  • AWS
Avatar
Dzenan Dzevlan
— November 20, 2019

Application Load Balancer vs. Classic Load Balancer

What is an Elastic Load Balancer? This post covers basics of what an Elastic Load Balancer is, and two of its examples: Application Load Balancers and Classic Load Balancers. For additional information — including a comparison that explains Network Load Balancers — check out our post o...

Read more
  • ALB
  • Application Load Balancer
  • AWS
  • Elastic Load Balancer
  • ELB
Albert Qian
Albert Qian
— November 13, 2019

Advantages and Disadvantages of Microservices Architecture

What are microservices? Let's start our discussion by setting a foundation of what microservices are. Microservices are a way of breaking large software projects into loosely coupled modules, which communicate with each other through simple Application Programming Interfaces (APIs). ...

Read more
  • AWS
  • Docker
  • Kubernetes
  • Microservices
Nisar Ahmad
Nisar Ahmad
— November 12, 2019

Kubernetes Services: AWS vs. Azure vs. Google Cloud

Kubernetes is a popular open-source container orchestration platform that allows us to deploy and manage multi-container applications at scale. Businesses are rapidly adopting this revolutionary technology to modernize their applications. Cloud service providers — such as Amazon Web Ser...

Read more
  • AWS
  • Azure
  • Google Cloud
  • Kubernetes
Avatar
Stuart Scott
— October 31, 2019

AWS Internet of Things (IoT): The 3 Services You Need to Know

The Internet of Things (IoT) embeds technology into any physical thing to enable never-before-seen levels of connectivity. IoT is revolutionizing industries and creating many new market opportunities. Cloud services play an important role in enabling deployment of IoT solutions that min...

Read more
  • AWS
  • AWS IoT Events
  • AWS IoT SiteWise
  • AWS IoT Things Graph
  • IoT