howmuch/README.md

# howmuch

A tricount like expense-sharing system written in Go

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It is a personal project to learn go and relative technologies.

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## Project Diary

### 2024/09/30

The idea comes from a discussion with my mom. I was thinking about doing
some personal budget management thing but she brought up the expense-sharing
application that could be a good idea. I explained why it was a terrible idea
and had no value but in fact it was a really a good idea.

First I have to set up a web server. I'm thinking about using `gin`, since I
have played with `chi` in other projects.

Then I have to add some basic support functions like system `logging`,
versioning, and other stuffs.

Next I need to design the API.

- User management: signup, login, logout.
- A logged-in user must be able to:
  - create an event
  - add other users to that event
  - A user can only view their own events, but not the events of other users'
  - A user can add an expense to the event (reason, date, who payed how much,
  who benefited how much)
  - Users in the event can edit or delete one entry
  - changes are sent to friends in the event
  - User can get the money they spent themselves and the money they must pay
  to each other
  - User can also get the total amount or the histories.

That is what I thought of for now.

Thus, Besides a web server, I must have a database that can store all the
data. ex. PostgreSQL. I need a message queue system (RabbitMQ?) to handle
changes for an event. That will results in a messaging service sending emails.

I also want to use `Redis` for cache management.

What else?

`OpenAPI` + `swagger` for API management.

And last but not least, `Docker` + `Kubernetes` for the deployment.

That is what I am thinking of for now. I will note down other ideas during
the project.

### 2024/10/01

A Go application has 3 parts:

- Config
- Business logic
- Startup framework

#### Config

The application provides a command-line tool with options to load configs
directly and it should also be able to read configs from the yaml/json files.
And we should keep credentials in those files for the security reasons.

To do this, we can use `pflag` to read command line parameters, `viper` to
read from config files in different formats, `os.Getenv` to read from
environment variables and `cobra` for the command line
tool.

The execution of the program is then just a command like `howmuch run`.

Moreover, in a distributed system, configs can be stored on `etcd`.

> [Kubernetes stores configuration data into etcd for service discovery and
cluster management; etcd’s consistency is crucial for correctly scheduling
and operating services. The Kubernetes API server persists cluster state
into etcd. It uses etcd’s watch API to monitor the cluster and roll out
critical configuration changes.](https://etcd.io/docs/v3.5/learning/why/)

#### Business logic

- init cache
- init DBs (Redis, SQL, Kafka, etc.)
- init web service (http, https, gRPC, etc.)
- start async tasks like `watch kube-apiserver`; pull data from third-party
services; store, register `/metrics` and listen on some port; start kafka
consumer queue, etc.
- Run specific business logic
- Stop the program
- others...

#### Startup framework

When business logic becomes complicated, we cannot spread them into a simple
`main` function. We need something to handle all thoses task, sync or async.
That is why we use `cobra`.

So for this project, we will use the combination of `pflag`, `viper` and
`cobra`.