January 2022 – Shekhar Gulati

Working with Configuration Masters in Microservices Architecture

These days I am working on building a next generation mobile banking platform. One of the solutions that I was designing this week was around how to handle configuration masters in Microservices. I am not talking about Microservices configuration properties here. I have not seen much written about this in the context of Microservices . So, I thought let me document the solution that I am going forward with. But, before we do that let’s define what these are configuration masters.

In my terminology configuration masters are those entities of the system that are static yet configurable in nature. Examples of these include IFSC codes for banks, error messages, bank and their icons, account types, status types, etc. In a reasonably big application like mobile banking there will be anywhere between 50-100 configuration master entities. These configuration master entities have three characteristics:

They don’t change often. This means they can be cached
They don’t change often but you still want the flexibility to update existing items or add new items if required. Typically, they are modified either using database scripts or by exposing APIs that some form of admin portal(used by IT operations people) uses to add new entries or modify existing entry
The number of rows per configuration master entity is not more than 1000. This make them suitable for local in-memory caching

Correctly using Postgres as queue

I am building a central notification dispatch system that is responsible for sending different kinds of notifications to the end customer. It relies on multiple third party APIs for sending the actual email/SMS notifications. At a high level architecture of the system is shown below.

NotificationSender exposes both REST and messaging interface for accepting consumer requests. Consumers here refer to the services that need to send the notification. This is what notification system does:

It accepts requests from upstream services and stores that in the Postgres database after doing validation. The notification event is written to the Postgres database in ENQUEUED state. It is returns back HTTP 202 ACCEPTED to the upstream services if the request is valid else it returns HTTP 400 Bad Request.
At a predefined frequency a poller that is part of the NotificationDispacther polls the Postgres database for new notification events i.e. events in ENQUEUED state. For now, it respects insertion time order.
If enqueued events are found then it processes them and sends actual notifications using the downstream SMS and Email services.
After processing the events it change state of the events to processed

Issues with mmap

A couple of months back I watched a video by Andy Pavlo, Associate Professor of Databases Carnegie Mellon, where he made a point that databases should not use mmap. He went on to say that if there is only one thing you should get from his database course is to never use mmap when building and designing database management systems. I have not used mmap before so I was intrigued to understand it in more detail. I was aware that MongoDB used to use an mmap based storage engine. It allowed them to achieve faster time to market but later they had to replace it with a new storage engine wiredtiger because of the issues they faced with mmap. MongoDB is not the only database that uses mmap. There are many databases that use mmap. Some of the databases that use mmap are RavenDB, ElasticSearch, LevelDB, InfluxDB, LMDB, BoltDB, moss (key-value store from Couchbase), etc.

Given that so many databases use mmap I wanted to understand why Andy recommended us to not use mmap. I will list all of the reasons I could find in my research and from Andy’s video in this post. But, before we do that let’s first understand mmap.

Useful Stuff I Read This Week

Here are 10 posts I thought were worth sharing this week.

#1. What we learnt by migrating from CircleCI to Buildkite – Link

This post covers how and why Hasura switched their CI service from CircleCI to Buildkite. They started by defining the requirements from their CI, then they evaluated different solutions, and finally introduced it in their ecosystem. Their main reason to switch CI service was cost. They reduced the cost by 50%. This required them to own some of the aspects of the CI operations. A couple of interesting things I learnt from this post:

Use of labels to trigger build. They used them to save costs.
The use of dynamic configuration. They wrote their build code in a Go program. This saved them from YAML hell. Interestingly, they use shellcheck t static analysis of shell script

#2 . Reframing tech debt – Link

It is all about perspective. Tech Debt brings negative emotions in people and it becomes difficult to sell it to higher management. In this post, the author suggests we reframe tech debt as tech wealth while communicating with stakeholders. Building tech wealth means getting more value out of the software we’re creating, as well as our efforts to develop and maintain it. Author suggests two ways we can plan for tech wealth:

Allocate time within each planning cycle
Dedicate the last few cycles in a quarter

In one of the products I worked on we used to schedule 1 day per sprint (2 weeks) for paying tech debt. We had sprint demo every alternate Thursday and the next day i.e. Friday was scheduled for working on tech debt items. One problem with 1 day every sprint is that bigger items can’t be handled. We used to create stories for them and pick them as part of the sprint backlog.

When to use JSON data type in database schema design?

Today, I was doing solution design for a system when I started to think when we should use JSON data type for columns. Coming up with the right schema design takes multiple iterations. I consider it more as an art than science. All the mainstream RDBMS have JSON data type support.

Postgres has JSON data type since version 9.2. The 9.2 version was released in September 2012
MySQL has JSON data type since version 5.7.8. The 5.7.8 version was released in August 2015
SQL Server has JSON data type since version SQL Server 2016. The 2016 version was released in June 2016
Oracle has JSON data type since version 19c. The 19c version was released in February 2019

They all support efficient insertion and querying of JSON data type. I will not compare their JSON capabilities today. Today, I want to answer a design question – when should a column have a JSON data type?

I use the JSON data type in design situations mentioned below. There could be other places as well where JSON is a suitable data type.

Dump request data that will be processed later
Support extra fields
One To Many Relationship where many side will not have to its own identity
Key Value use case
Simpler EAV design

Let’s talk about each of these use cases in more detail.