Client architecture sync 2018 12 04

SecureDrop Client Architecture Discussion - December 4, 2018

Participants: Jen, Heartsucker, Jaysinh, Josh, Erik

Problems:

• Duplicate downloads -> how to prevent in an elegant manner? User clicks on file, then clicks it again -- currently this will initiate two downloads. Hacky solve would be to disable UI. Useful: keep track of what's being downloaded [needs issue]

• Use of tons of callbacks modifying state everywhere, leading to confusing/complicated application logic

• We are explicitly running e.g. update_conversation_view to refresh the UI when the underlying state changes

• What other problems do we have?

• non-persistent model (qt) (during application runtime)

  • data store used for e.g. ConversationView should be the underlying database object, not a new dict

• non-persistent widgets (during applications runtime)

  • We are deleting and recreating widgets to update the UI and losing data

Represent data in a http://pyqt.sourceforge.net/Docs/PyQt4/qabstractitemmodel.html and use delegates for display? See also: https://files.meetup.com/2179791/pyqt-model-view-framework-overview.pdf

Instead of elsewhere in the app using the DB objects directly, we'd just use them in the abstract model

Check out Calibre as a project that uses this approach: https://github.com/kovidgoyal/calibre/tree/master/src

To learn about Qt Model View programming: https://doc.qt.io/qt-5/model-view-programming.html

Refactoring application to use MVC may be precondition for larger changes such as use of QAbstractItemModel [needs issue] Potentially pause reply work for now, take on in current sprint -> Start with conversation view, go from there? -> Jen will coordinate

A modest proposal:

• We use actor model at least for API calls (e.g. the ReplyFetchActor can prevent the downloading of duplicate replies)
  • https://github.com/freedomofpress/securedrop-client/issues/100
  • see josh's https://github.com/joshuathayer/spot/blob/master/spot/system.py
• We use qt's MVC framework such that the UI reflects the state of the data store (leveraging signals/slots in pyqt via the Abstract*Model classes, refs https://github.com/redshiftzero/python-qt-mvc/blob/master/example-no-db.py#L34)

-> Wait until we've done MVC refactor?

Another point of discussion:

• How to manage operations that have been optimistically completed on the client (e.g., "mark as unread") but may still be pending on the server (and may eventually error out)
• The relevant actor should update the state locally upon failure, the UI should then update to represent this state

Existing actor libraries:

• https://github.com/actor-framework/actor-framework/issues/775
• https://quantmind.github.io/pulsar/design.html

-> May need investigatory spike