Hi,
as you might know all UI and REST operations nowadays grab a global r/w lock
to protect the previously thread unsafe catalog facade from corruption.
The REST ones grab a read or write lock according to their nature (GET vs POST/PUT/DELETE),
the UI ones are opaque and thus always get a write lock.
The new catalog facade is thread safe, however, it’s still possible to get into
an inconsistent state because the catalog does not have transactions around its modifications.
Now, the global lock causes issues per se, because the config subsystem can be get locked
for extended amounts of times, for example:
- When uploading a large file via REST on store creation resources
- When computing a bbox in the UI, of a large database table
This pull request tries to reduce the pain a bit by grabbing always a READ lock instead,
and upgrading it to a WRITE lock on the first actual write operation:
https://github.com/geoserver/geoserver/pull/2104
So the above cases are taken care of at least in part, allowing at least read only usage during
those operations instead of going full exclusive mode from the request start.
There is however a gotcha, the upgrade is now supported natively by the JVM, so one has
to release the read lock and get the write one, which might fail/timeout… if that happens
the operation will fail (I’ve extended the try period a bit, from 5 to 30 seconds, right
now the UI really shows the “busy page” too often, we also got complaints about that).
The change could maybe improved a bit further by also grabbing the read lock at the first
catalog read, but I’m not sure how much that would buy us, both the UI and REST
operations tend to grab something from the catalog anyways.
I believe I won’t have time to make it better than this before the release so…
take or leave, or pitch in something of yours 
Cheers
Andrea
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Ing. Andrea Aime
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