Postoffice

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What's postoffice?

We can think about postoffice as a real post office. You send messages to a topic and publishers send it to anyone interested in this topic. In case the receiver is not available, we'll try to deliver the message later. It uses a pub/sub approach, so instead of handling receiver's addresses we use topics, and receivers must subscribe to them through Publishers. A publisher is isolated from others and it handles itself its own pending messages

Motivation

This project started as a solution to buffer messages in case some apps are deployed on-premise and could work with connectivity issues. Then it evolved to also offer a pub/sub mechanism.

What's not Postoffice?

This is not designed to be realtime. We use GenStage to process pending messages. We create a process tree for each Publisher. It looks like an ETL, and it's refreshed each 10 seconds.

Features

• Buffer messages in case receiver system is down or there is no connectivity for any reason.
• Deliver messages through:
  • Http.
  • Pub/Sub (GCloud).
• API to create topics/publishers.
• Web interface:
  • Manage topics/publishers.
  • Search messages to see when it was received and proccessed.
• Cluster nodes. Best effort to avoid sending messages more than once. (More info on clustering later)
• Endpoint to be used as health-check from k8s /api/health

How to install it locally

To start your Phoenix server:

• brew update
• brew install elixir
• Create the following environmet variables in order to start the application:
  • GCLOUD_PUBSUB_CREDENTIALS_PATH with the absolute path to the pubsub credentials file. We provide config/dummy-credentials.json to be able to start the app.
  • GCLOUD_PUBSUB_PROJECT_ID with the project_id used.
• mix local.hex
• mix archive.install hex phx_new 1.4.11
• Install dependencies with mix deps.get
• Inside docker directory, run docker-compose up -d to start a new postgres database
• Create and migrate your database with mix ecto.setup
• Execute npm install inside assets/
• Start Phoenix endpoint with mix phx.server Now you can visit localhost:4000 from your browser.

Clustering

Postoffice has been developed to be used forming a cluster. We use libcluster under the hood to create the cluster. You can take a look at its documentation in case you want to tune settings.

What's next?

Some desired features have been delayed until the first release:

• Set publishers as recoverable. This means any service sending messages to Postoffice will save messages that couldn't be delivered to us on their side, and expose an API so Postoffice will automatically consume those messages. API definition is still pending.
• Some code is duplicated in Handlers layer. Refactor to have just one handler module and N adapters.
• Be able to configure max_demand through environment variables.
• Provide a mechanism to run schema migrations.
• Do not try to create a k8s cluster by default, change how it's configured.
• Create docker images to make it easier to run Postoffice.