Document a few thoughts on celery

DESIGN-process-model.md

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+
+Just a few thoughts on the model we use for multi-processes, multi-threads, and how this relates to DB locks.
+
+
+Basic idea: if we run a single (gunicorn) process, locks can't ever become a problem.
+
+
+### Celery
+
+For now, we use Celery.
+It's the most out-of-the-box solution for async stuff (which we need for e.g. emails)
+
+I wonder if we'll still use this once we do a serious cutting up between ingest/digest, but we'll cross that bridge when
+we get there.
+
+For now I'm just running in always-eager mode to facilitate running debugserver. This will probably change at some point.
+
+If you'd need a worker, this is how you'd start it:
+
+```
+celery -A bugsink worker -l INFO
+```
+
+(I wonder: if single-worker is such a key concept, this may need to be applied on celery too... but that depends
+entirely on what we let celery do)