Few points to note (and possibly discuss):
- cfgmgr is build on top of LMDB, as it brings transaction (and so
thread safe) support out of the box
- LMDB keys are build in a way that we can support repeatable (see
https://pad.isc.org/p/cfgmgr-proposal-v2 even if most of it is
outdated, the section 2.0.2 about the way it's build is still
relevant, otherwise I hope the code changes here are clear enough)
- Each thread own its own cfgmgr context (which basically means LMDB
transaction, and where the API points to inside the configuration)
- In order to avoid allocations everytime we get/set a value (as well
as few other operations) a single buffer is pre-allocated per-thread
and per-transaction. Now, few internal helper functions directly use
it (instead or, let's say, work on a parameter) and this might be
confusing and error prone, I'm happy to change that if it is a
worry.
- The data type which can be read/wrote from cfgmgr is not exhaustive,
more data type will be added (i.e. duration type, uint64 if needed,
etc.)
- This current implementation does not support inheritance (i.e. a
non-specified view option won't use the option one). It's something
we need to discuss, I see some options that could be put on top of
that's here.
- The "default" values, however, should be fine out-of-the-box: I
think the default configuration in bin/named/config.c can be
parse/"added" in cfgmgr first then user one on top of that (because
writting an existing value override it). Obviously there would be
no-way to "go back" to the default config only, but I don't see such
use case in existing code. Even if we'd need such thing, that would
be quite invasive and a full config reload would be advisable. (so
we could add an API to entirely drop the cfgmgr data and start from
scratch. But I don't see such use case right now anyway, so it's not
there)
- I hope the things a user of cfgmgr must know should be clearly
explained in the cfgmgr.h file (if it's not the case, then I need
to fix it -- That said I'll likely re-work the doc anyway).
- I initially implemented a mechanism which would dynamically re-size
key buffers in case keys are very long, but I remove this as LMDB
doesn't supports key more than 511 bytes anyway. Instead I made
assertions every time we build a key to make sure we don't exceed
this value.
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