Building Git by James Coglan

git is a widely successful version control system, it’s used in software companies large and small. It’s distributed nature changed software development in many ways. In Building Git, James Coglan re-implements a subset of git’s functionality from the ground up, using Ruby, which has a large standard-library and is higher-level than the original C.

git itself is a large project with a lot of functionality. The book covers a lot of ground, in a step-by-step fashion. Each line of code is explained both conceptually and syntactically.

In my previous posts about bi-temporal data, I dealt with a lot of queries that had where clauses that dealt with operations in dates. For example:

SELECT
    employee_id,
    committee_id
FROM
    committee_membership
WHERE
    valid_from <= '2020-05-02'
    AND '2020-05-02' < valid_up_to
    AND tx_applicable_from <= NOW()
    AND NOW() < tx_applicable_up_to

The underlying schema looks like this:

CREATE table committee_membership (
  employee_id int NOT NULL,
  committee_id int NOT NULL,
  valid_from date NOT NULL,
  valid_up_to date NOT NULL,
  tx_applicable_from date NOT NULL,
  tx_applicable_up_to date NOT NULL
)

The four dates in the table share the same structure. There are two prefixes valid and tx_applicable, and two suffixes from and up_to. This structure that hints that the dates represent two different concepts: An interval in time that delineates validity and an interval that delineates applicability.

I’ve been spending a lot of time thinking about bi-temporal data in the last few weeks. You can read what I’ve written about it in Bi-Temporal Data and Modeling Bi-Temporal Deletions.

The Case for Bitemporal Data

Craig Baumunk presents at the NJ SQL Server User Group on bi-temporal data. He goes over the differences between non-temporal, valid temporal, transaction temporal modeling and the different types of problems that they solve. He makes the case of why bi-temporal data is superior to all the previous issues and what the implications are. The presentation is from 2011, but it is still as relevant as ever. Note that the presentation is broken up into 7 different parts.

Bi-temporal data modeling with Envelope

Jeremy Beard covers the importance of bi-temporal data modeling, and the type of problems that it can solve. Using a credit score example, he builds up the modeling bit by bit in an intuitive way. The second portion of the article focuses specifically on the implementation in Cloudera EDH, which I don’t use.

Principles and priorities

Jeremy Keith writes on how to think about design principles. Sometimes, design principles can be truisms that can be less than useful (e.g. Make it usable.). Expressing principles as a set of priorities, makes them more useful and actionable (e.g. Usability, even over profitability). As an example, he uses the HTML design principles as:

Users, even over authors. Authors, even over implementors. Implementors, even over specifiers. Specifiers, even over theoretical purity.

Bi-temporal data refers to a modeling technique to store and retrieve data that changes on two different axes. The valid time axis refers to the range of time in which data is valid. Transaction time refers to when the system recorded the data. Keeping track of both with expose a very rich data model.

In the simplest data modeling, a system keeps track of the current state. Let’s assume that we are working with a system that keeps track of company personnel. In its people table, it will hold things like first and last names, date of birth, and social security numbers. At first sight, this seem like invariant facts, but upon closer inspection we can see that in reality they are not. People often change their name when their marital status changes, and birth date and social security number are technically facts that don’t change, but often need to be corrected. These two very different reasons for change are often conflated, or worse, not accounted for. Bi-temporal data provides a way to deal with both.