Customers API Lite microservice prototype 🔹

A daemon written in Crystal, designed and intended to be run as a microservice,
implementing a special Customers API prototype with a smart yet simplified data scheme

Rationale: This project is a direct Crystal port of the earlier developed Customers API Lite microservice prototype, written in V using veb web server library/framework, and tailored to be run as a microservice in a Docker container. The following description of the underlying architecture and logics has been taken from here almost as is, without any principal modifications or adjustment.

This repo is dedicated to develop a microservice that implements a prototype of REST API service for ordinary Customers operations like adding/retrieving a Customer to/from the database, also doing the same ops with Contacts (phone or email) which belong to a Customer account.

The data scheme chosen is very simplified and consisted of only three SQL database tables, but that's quite sufficient because the service operates on only two entities: a Customer and a Contact (phone or email). And a set of these operations is limited to the following ones:

• Create a new customer (put customer data to the database).
• Create a new contact for a given customer (put a contact regarding a given customer to the database).
• Retrieve from the database and list all customer profiles.
• Retrieve profile details for a given customer from the database.
• Retrieve from the database and list all contacts associated with a given customer.
• Retrieve from the database and list all contacts of a given type associated with a given customer.

As it is clearly seen, there are no mutating, usually expected operations like update or delete an entity and that's made intentionally.

The microservice incorporates the SQLite database as its persistent store. It is located in the data/db/ directory as an XZ-compressed database file with minimal initial data — actually having two Customers and by six Contacts for each Customer. The database file is automatically decompressed during build process of the microservice and ready to use as is even when containerized with Docker.

Generally speaking, this project might be explored as a PoC (proof of concept) on how to amalgamate Crystal REST API service backed by SQLite database, running standalone as a conventional daemon in host or VM environment, or in a containerized form as usually widely adopted nowadays.

Surely, one may consider this project to be suitable for a wide variety of applied areas and may use this prototype as: (1) a template for building similar microservices, (2) for evolving it to make something more universal, or (3) to simply explore it and take out some snippets and techniques from it for educational purposes, etc.

Table of Contents

• Building
• Running
• Consuming
  • Logging

Building

The microservice might be built and run successfully under Arch Linux (proven). — First install the necessary dependencies (base-devel, crystal, shards, sqlite, docker):

$ sudo pacman -Syu base-devel crystal shards sqlite docker
...

Then pull and install all the necessary third-party libraries (so-called shards):

$ shards
Resolving dependencies
Fetching https://github.com/crystal-community/toml.cr.git
Fetching https://github.com/chris-huxtable/syslog.cr.git
Fetching https://github.com/kemalcr/kemal.git
Fetching https://github.com/crystal-lang/crystal-sqlite3.git
Fetching https://github.com/crystal-lang/crystal-db.git
Fetching https://github.com/luislavena/radix.git
Fetching https://github.com/crystal-loot/exception_page.git
Fetching https://github.com/sija/backtracer.cr.git
Using toml (0.8.1)
Using syslog (0.1.2)
Using db (0.13.1)
Using sqlite3 (0.21.0)
Using radix (0.4.1)
Using backtracer (1.2.4)
Using exception_page (0.5.0)
Using kemal (1.7.1)

Build the microservice using the Shards utility:

$ shards build && \
  if [ -f data/db/customers-api-lite.db.xz ]; then \
      unxz data/db/customers-api-lite.db.xz; \
  fi
Dependencies are satisfied
Building: api-lited

Or build the microservice using GNU Make (optional, but for convenience — it covers the same Shards utility build workflow under the hood):

$ make clean
...
$ make all  # <== Building the daemon.
...

Running

Run the microservice using its executable directly, built previously by the Shards utility or GNU Make's all target:

$ ./bin/api-lited; echo $?
...

To run the microservice as a true daemon, i.e. in the background, redirecting all the console output to /dev/null, the following form of invocation of its executable can be used:

$ ./bin/api-lited > /dev/null 2>&1 &
...

Note: This will suppress all the console output only; logging to a logfile and to the Unix syslog will remain unchanged.

Consuming

The microservice should expose six REST API endpoints to web clients... They are all intended to deal with customer entities and/or contact entities that belong to customer profiles. The following table displays their syntax:

• The {customer_name} placeholder is a string — it usually means the full name given to a newly created customer.
• The {customer_id} placeholder is a decimal positive integer number, greater than 0.
• The {customer_contact} placeholder is a string — it denotes a newly created customer contact (phone or email).
• The {contact_type} placeholder is a string and can take one of two possible values, case-insensitive: phone or email.

TBD 💿

Logging

The microservice has the ability to log messages to a logfile and to the Unix syslog facility. To enable debug logging, the debug.enabled setting in the microservice main config file etc/settings.conf should be set to true before starting up the microservice. When running under Arch Linux (not in a Docker container), logs can be seen and analyzed in an ordinary fashion, by tailing the log/customers-api-lite.log logfile:

$ tail -f log/customers-api-lite.log
[2025-07-27][22:25:10] [DEBUG] [Customers API Lite]
[2025-07-27][22:25:10] [DEBUG] [#<DB::Database:0x7dc92c47ded0>]
[2025-07-27][22:25:10] [INFO ] Server started on port 8765
[2025-07-27][22:25:10] [INFO ] [development] Kemal is ready to lead at http://0.0.0.0:8765
[2025-07-27][22:25:20] [INFO ] 404 GET /v1/customers 209.23µs
[2025-07-27][22:25:30] [INFO ] Kemal is going to take a rest!

Messages registered by the Unix system logger can be seen and analyzed using the journalctl utility:

$ journalctl -f
...
Jul 27 22:25:10 <hostname> api-lited[<pid>]: [Customers API Lite]
Jul 27 22:25:10 <hostname> api-lited[<pid>]: [#<DB::Database:0x7dc92c47ded0>]
Jul 27 22:25:10 <hostname> api-lited[<pid>]: Server started on port 8765
Jul 27 22:25:30 <hostname> api-lited[<pid>]: Server stopped

TBD 💿

WIP 📀