Persist data

[odungern]

[odungern]

Poor man's persistence via export-JSONLD and import-JSONLD, see #27.

Other options to consider:

• Write a small file per item and use Git for versioning and synchronizing. Need to implement basic Git operations to synchronize. Real-time synchronization is not possible to my understanding, just periodic pull and push (which is similar to polling).
• Use Pouch-DB on client-side. Persists the data locally per browser. If a server-side Couch-DB is configured, real-time synchronization with multiple users is automatically done in the background.
• Implement a CASCaRA Server (backend) with API based on common code with the browser. The server can use any (document) database deemed appropriate, such as Mongo or ArangoDB. Has the possibility to use server-side transformation and import/export. Need to implement real-time synchronization with a publish-and-subscribe mechanism - otherwise poll the backend for changes. Also need to implement optimistic locking to avoid update collisions.