plus a SQL database [see <ahref="trog2030.html">Trog2030 proposal</a>]. This means that programmers would need to understand more SQL but would not need to understand "django". Arguably this
<p>The above discussion is extremely ignorant in a couple of respects. Now (April 2021) we can properly appreciate that the part of Django that interacts with a database is actually a small part of the system. The http request/response engine is not easily replaced. And the 90 or so HTML templates do not just reformat the data given to them in python dictionaries: they directly query and traverse the database to produce tabular output. So if we 'took out' the database, most of our templates would fail utterly and need completely rewriting. It could be done, but the manpower requirement is not trivial.
<p>There is a templating engine <ahref="https://mozilla.github.io/nunjucks/">Nunjucks</a>
which is a port to JavaScript of the Django templating system we use
(via <ahref="https://palletsprojects.com/p/jinja/">Jinja</a> - these are the same people who do Flask). This would be an obvious thing to use if we needed to go in that direction.
<p>We need a templating engine because so much of the troggle coorindation output is in tables of data from diffrerent sources, e.g. see <ahref=/survexfile/264">all survey data for 264</a>.
<p>Currently every troggle code operation uses the django ORM <var>search</var> and <var>filter</var> operations on the central database to find any object it needs. If we don't have a central database then we have to use direct object references and we need to think about the design of <ahref="https://medium.com/@geoffreykoh/implementing-the-factory-pattern-via-dynamic-registry-and-python-decorators-479fc1537bbe">a central registry object</a> to hold these. There is a well-studied design pattern that describes this design "<ahref="http://www.laputan.org/mud/">Big Ball of Mud</a>" which and the contributing actions "Piecemeal growth" and "Sweeping it under the rug".
<p>We are always using one object, e.g. a wallet, just to get at another object, e.g. a scan of some original notes, in order to check the data we are checking, e.g. a survex file. Maintaining two-way dependencies amoung all the objects is what "foreign keys" do in a database, but the problem doesn't go away when we don't have a database: it gets slightly harder. <p>One thing that is easier with troggle is that we don't have many object lifecycle issues. Everything is created once and lasts forever. There are only a few ephemeral objects during the initial data import from files.
<h4>Wiring-up components</h4>
<p>Troggle today doesn't need anything complex, a single <ahref="https://hub.packtpub.com/python-design-patterns-depth-singleton-pattern/">registry
singleton</a> would probably be fine (though hard to test), but if it evolves towards being a set of interacting services then a more sophisticated architecture would be needed.
<p>The Java community found "dependency resolution" very helpful for wiring-up loosely objects/components in the late 1990s with the "<ahref="http://picocontainer.com/inversion-of-control.html">Inversion of Control</a>" technique which can be implemented in several ways, most commonly using "<ahref="https://martinfowler.com/articles/injection.html">Dependency Injection</a>". But for troggle we must be careful that doing this the "right" way may make the code even more inaccessible to novice caver-programmers than django is. Which is the whole point of moving away from django. Fortunately python programmers have produced some recent guidance: <ahref="https://blog.benpri.me/blog/2020/05/13/python-dependency-injection-made-simple/">Python Dependency Injection Made Simple</a> and <ahref="https://python-dependency-injector.ets-labs.org/introduction/di_in_python.html">Dependency injection and inversion of control in Python</a>. We should probably use the simpler "<ahref="http://picocontainer.com/constructor-injection.html">Constructor Injection</a>" variation as we need to make all our code <ahref="http://picocontainer.com/mock-objects.html">more easily testable</a>. Flask uses that.
