message-db - Client library for Eventide's Message DB

Collection of libraries for interacting with Eventide's Message DB via PostgreSQL. The purpose of these libraries are to make it easier to implement event sourcing systems in Haskell.

For an example of how this library is used, please see BankAccount.hs.

Disclaimers

• This library is an unofficial message-db client library for Haskell.
• Only message-db-temp has been published to hackage so far.
• If you want to use the message-db or message-db-monad packages then you need to add the following:
  • For cabal add the following to cabal.project

    source-repository-package
      type: git
      location: https://github.com/Disco-Dave/message-db.git
      tag: 359cb51b69efbf8f561c2709973db713c8154a34
      subdir: 
        message-db
        message-db-monad

  • For stack add the follow to stack.yaml

    extra-deps:
      - git: https://github.com/Disco-Dave/message-db.git
        commit: 359cb51b69efbf8f561c2709973db713c8154a34
        subdirs: 
          - message-db
          - message-db-monad

  • Note: you may replace 359cb51b69efbf8f561c2709973db713c8154a34 with any git ref you want

Repository Structure

This repository contains the following packages:

• message-db - Low-level library built on top of postgresql-simple.
• message-db-temp - Creates temporary message-db instances using tmp-postgres and the official message-db scripts. This package is also used by message-db tests for running integration tests.
• message-db-monad - Utilizes MonadUnliftIO to allow running message-db actions in a custom monad.

How to Get Help

If you find a bug, have a feature request, or simply have a question then please open an issue.

How to Build

Currently this project supports that last three major versions of ghc with cabal, and the last two major lts snapshots plus nightly for stack.

If you need to setup ghc, cabal, or stack; then I highly recommend using ghcup.

Preparation

Regardless of which tool chain you use, the first thing you'll need to do is clone the repo and initialize all of the submodules. This may be done with one command:

git clone https://github.com/Disco-Dave/message-db.git --recurse-submodules

Or if you already cloned the repo, then you may run the following inside of the repo folder to initialize the submodules:

git submodule update --init --recursive

You may also need some additional system dependencies in order to build, if you're on a Debian based system then run the following:

apt update && apt install -y libpq-dev

If you also wish to run the test-suite, then you need to make sure that initdb is on your path. If you're on a Debian based system then run the following:

apt update && apt install -y postgresql
export PATH=/usr/lib/postgresql/13/bin:$PATH

Note, your distro may have a postgresql version different from 13. If so, replace the 13 in your PATH with the appropriate number.

Cabal Instructions

1. cabal update - Ensures you have the latest package list from Hackage.
2. cabal build all - Builds all packages.
3. cabal test all - Runs the test-suite for all packages.

Stack Instructions

1. stack init - Initializes the stack project. You can also specify a specific resolver instead:
   • stack init --resolver lts-18
   • stack init --resolver lts-19
   • stack init --resolver nightly
2. stack build - Builds all packages.
3. stack test - Runs the test-suite for all packages.

Learn More about Eventide's Message DB

• Docs - Official Message DB user guide
• Reference - Official postgres client in Ruby
• Blog - Build Your Own Message DB Client
• Book - Practical Microservices