Backend job for printing

[usu]

Outsource PDF printing into background job worker

Proposed concept

• Use a separate git repository for print job

• Digitalocean App Worker (5$) pointing to the new repo
  https://docs.digitalocean.com/products/app-platform/how-to/manage-workers/

• PDF Print Worker based on swoole-jobs (easy worker management within PHP)
  https://github.com/kcloze/swoole-jobs

• Messaging from ecampApp to worker queue via RabbitMQ
  No need to maintain own server: CloudAMQP has a free tier. Officially not recommended for production, but I believe it should well cover the few messages we need it for.
  https://www.cloudamqp.com/plans.html

• Worker stores PDF to Digitalocean Spaces (5$), compatible with AWS S3 API
  https://www.digitalocean.com/products/spaces/

• Client can poll S3 bucket until PDF is generated

Why swoole: Without swoole, we cannot easily run multiple workers on the same node, which probably would lead to inefficient usage of CPU resources. The only way to scale would be to run multiple worker nodes (expensive) or finding another method to run multiple php workers in parallel (not found any good solutions).

Alternative

Use some linux tooling (e.g. supervisord) to run multiple instances of the PHP workers, e.g.
https://github.com/slashfan/docker-supervisor-php-workers

In that case we wouldn't need swoole but can just write our workers in plain old PHP, e.g something link this demo:
https://devcenter.heroku.com/articles/php-workers

Open points

• if we use swoole-jobs: I haven't fully checked the message format that is exchanged via RabbitMQ. In case this is a special format, then we might need swoole-jobs as a dependency in the main app to generate the messages