doc: add brainstorming for mountpoints

doc/devlog.md

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+## 2024-10-16
+
+### Considerations for Mountpoints Feature
+
+- `_storage_upload` takes paramter `uuid` instead of `path`
+  - S3 bucket strategy needs the UUID
+    - If we do hashes, 10MB chunks should be fine
+      - we're already able to smooth out bursty traffic using the
+        EWA algorithm
+- Use of `systemFSEntryService`
+  - Is that normalized? Does everything go through this interface?
+- Storage interface has methods like `post_insert`
+  - as far as I can tell this doesn't pose any issue
+-  
+
+### Brainstorming Migration Strategies
+
+#### Interface boundary at HL<->LL filesystem methods
+
+-- **tags:** brainstorming
+
+From the perspectice of a trait-oriented implementation,
+which is not how LL/HL filesystem operations are currently implemented,
+the LL-class operations are implemented in separate traits.
+
+The composite trait containing all of these traits would be the trait
+that represents a filesystem implementation itself.
+
+Other filesystem interfaces that I've seen, such as FUSE and 9p,
+all usually have a monolithic interface - that is to say, an interface
+which includes all of the filesystem operations, rather than several
+interfaces each implementing a single filesystem operaiton.
+
+Something about the fact that the LL-class operations are in separate
+classes makes it difficult to reason about how to move.
+Is it simply that multiple files in a directory is just more
+annoying to think about? Maybe, but there must be something more.
+
+Perhaps it's that there are several references. Each implementation
+(that is, implemenation of a single filesystem operation) could have
+any number of different references across any number of different files.
+This would not be the case with a monolithic interface.
+
+I think the best of both worlds would be to have an interface representing
+the entire filesystem and, in one place, link of of the individual
+operation implementations to compose a filesystem implementation
+
+### Filesystem Brainstorming
+
+Puter's backend uses a service architecture. Each service is an instance
+of a class extending "Service". A service can listen to events of the
+backend's lifecycle, interact with other services, and interact with
+external interfaces such as APIs and databases.
+
+Puter's current filesystem, let's call it PuterFSv1, exists as the result
+of multiple services working together. We have LocalDiskStorageService
+which mimics an S3 bucket on a local system, and we have
+DatabaseFSEntryService which manages information about files, directories,
+and their relationships within the database, and therefore depends on
+DatabaseAccessService.
+
+It is now time to introduce a MountpointService. This will allow another
+service or a user's configuration to assign an instance of a filesystem
+implementation (such as PuterFSv1) to a specific path.
+
+The trouble here is that PuterFSv1 is composed of services, and the nature
+of a service is such that it exists for the lifecycle of the application.
+The class for a particular service can be re-used and registered with
+multiple names (creating multiple services with the same implementation
+but perhaps different configuration), but that's only a clean scenario when
+there is just one service. PuterFSv1, on the other hand, is like an
+imaginary service composed of other services.
+
+The following possibilities then should be discussed:
+- CompositeService base class for a service that is composed of
+  more than one service.
+- Refactor filesystem to not use service architecture.
+- Each filesystem service can manage state and configuration
+  for multiple mountpoints
+  (I don't like this idea; it feels messy. I wonder what software
+   principles this violates)
+
+We can take advantage of traits/interfaces here.
+PuterFSv1 depends on two interfaces:
+- An S3-like data storage implementation
+- An fsentry storage implementation
+
+Counterintuitively from what I first thought, "Refactor the filesystem"
+actually looks like the best solution, and it doens't even look like it
+will be that difficult. In fact, it'll likely make the filesystem easier
+to maintain and more robust as a result.
+
+Additionally, we can introduce PuterFSv2, which will introduce storing
+data in chunks identified by their hashes, and associated hashes with
+fsentries.
+
+PuterFSService will be a new service which registers 'PuterFSv1' with
+FilesystemService.
+
+An instance of a filesystem needs to be separate from a mountpoint.
+For example, PuterFSv1 will usually have only one instance but it may
+be mounted several different times. `/some-user` on Puter's VFS could
+be a mountpoint for `/some-user` in the instance of PuterFSv1.