plugin injection points

So, we have had quite a bit of discussion, and tried various approaches to implementing plugins in levelup

https://github.com/rvagg/node-levelup/issues/search?q=plugins

quick summation of what has happened so far, I started experimenting with a crude monkey patching based approach, but ran into trouble with handling ranges - each plugin had manage which ranges it affected, which was tricky. I later refactored this to create a subsection of the database, with level-sublevel. This is a great improvement, because it allows you to extend a range within leveldb as if it's a whole db.

@rvagg has also experimented with exposing various integration points into levelup, https://github.com/rvagg/node-levelup/issues/92

personally, I am highly in favor of combining these two, and even merging sublevel into levelup, or at least, adding integration points to levelup so that level-sublevel does not have to monkey patch it.

the question is: what is the list of integration points that we need?

• prehooks (intercept a mutation [batch, put, del])
• posthooks (intercept a mutation callback)
• encoding/key-encoding
• setup (register special jobs that run first, after the database opens,
• asynchronously delay mutations. **

** maybe. The ability to get the current values for keys before performing a mutation.
this would be useful for validation, and merging concurrent updates.

I have a plugin for this, but it hasn't been updated to work with level-sublevel yet. This differs from level-hooks, which only provides
a sync api.

A setup integration point will be useful for saving metadata about the database in the database, and maybe stuff like summaries about the current overall state - whether a schema change migration is complete, etc.

Any other suggestions?

Review the linked plugin discussion, issue #92, and the level-sublevel, level-update, and level-hooks references first. Define and agree on the required integration points and their behavior; the issue is done when the plugin interface scope has a decided design.