Mack Earnhardt, Developer in Carmel, IN, United States
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Mack Earnhardt

Verified Expert  in Engineering

System Architecture Developer

Carmel, IN, United States
Toptal Member Since
November 17, 2020

As the founding or lead engineer of startups with combined funding of nearly $30 million, Mack is an executive-level technology strategist with deep technical skills. With 20+ years of experience in various industries, his ability to rapidly assess, plan, and execute enables key contributions to system stability and scalability. He enjoys mentoring less experienced team members and makes a point of keeping his skills sharp by learning as he teaches and by always pushing the knowledge envelope.


Agile Reasoning
SQL, Ruby, DevOps, System Architecture, Team Leadership, CTO
Ruby, MySQL, PostgreSQL, JavaScript, AWS Lambda
Upper Hand
Amazon Web Services (AWS), Redis, PostgreSQL, DevOps, React, Digital Payments...




Preferred Environment

Amazon Web Services (AWS), SQL, Linux, MacOS, Ruby

The most amazing...

...impact was to rewrite a Ruby scheduling system using SQL functions in key places, resulting in dramatic performance gains, and allowing a SaaS company to scale.

Work Experience

SaaS Advisor and Consultant

2007 - PRESENT
Agile Reasoning
  • Mentored first-time engineering leaders on both leadership and architecture.
  • Provided DevOps consulting for eCommerce operation of personalized business greeting cards and calendars company.
  • Developed custom software in various languages, including Ruby and SQL.
Technologies: SQL, Ruby, DevOps, System Architecture, Team Leadership, CTO

Senior Architect

2020 - 2022
  • Identified performance and scalability issues and created roadmaps to mitigate or migrate production loads as needed, typically achieving an order of magnitude or better improvements.
  • Identified opportunities for process improvements and worked with relevant leaders to plan and execute the change.
  • Identified mismatches between existing systems and desired future capabilities and created solutions to fill the gaps.
  • Created a plain text query language for subscriber segmentation to permit nested and/or syntax without the complexity of a QueryBuilder UI.
Technologies: Ruby, MySQL, PostgreSQL, JavaScript, AWS Lambda

VP of Technology

2018 - 2020
Upper Hand
  • Rearchitected scheduling engine at the heart of the application, using SQL functions for a dramatic improvement in performance, enabling retrieval of multiple months of availability in a single API request.
  • Created data ingestion and transformation (ETL) process resulting in the ability to rapidly onboard new customers by consuming data extracts from our largest competitors.
  • Led implementation of dual payment gateways, resulting in a reliable path for customers to transition to a new payment partner.
Technologies: Amazon Web Services (AWS), Redis, PostgreSQL, DevOps, React, Digital Payments, SQL, Heroku, Ruby on Rails API

Director of Engineering

2014 - 2016
  • Built an Agile engineering team from scratch and grew the platform from proof of concept to nearly 500 employers representing 500,000 individuals.
  • Led the transformation from proof of concept to production SaaS product.
  • Architected a DevOps solution using Amazon CloudFormation, EC2, and Chef.
  • Created a complex ETL solution using Ruby, Sidekiq, and SQL.
  • Architected a security model to protect personal health information (PHI), ensuring the product and cloud environment met HIPAA security requirements.
Technologies: Amazon Web Services (AWS), HIPAA Compliance, Chef, Sidekiq, Redis, SQL, Ruby on Rails (RoR), AWS CloudFormation

HumanReadable Gem
Human readable random tokens without ambiguous characters and optional emoji support.

Focus is readability in poor conditions or from potentially damaged printed documents rather than cryptographic uses. Despite this focus, SecureRandom is used to help avoid collisions.

Inspired by Douglas Crockford's Base 32, but attempts to correct mistakes by substituting the most likely misread. To make substitution safer, the token includes a check character generated using the Luhn mod N algorithm. The default character set is all caps based on this published study on text legibility, which matches Crockford.

Generates a Rails 6 application with opinionated choices, going beyond what an app template can create. It makes a quick and easy jumping-off point for new Rails apps with server-side rendering.

Here are some of the features:
* PostgreSQL database
* Bootstrap V5 front-end toolkit
* Devise authentication
* Pundit authorization
* Slim templating engine
* RSpec and Cucumber testing
* Bullet N+1 detection
* FactoryBot test data
* Kaminari pagination
* Ransack search
* MetaTags SEO
* Pry console
* Gravatar with fallback to LetterAvatar
* RuboCop and ESLint linters, with Prettier formatting
* Model annotation
* SimpleCov code coverage
* Ready for cloud deployment at Heroku



Stripe, REST APIs, Ruby on Rails API, Sidekiq, React


AWS CloudFormation, Chef


Ruby on Rails (RoR)


Ruby, SQL, JavaScript


DevOps, RESTful Development, HIPAA Compliance


Redis, PostgreSQL, MySQL


Linux, Amazon Web Services (AWS), Heroku, AWS Lambda


System Architecture, CTO, Team Leadership, PSM-1, Digital Payments

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