Add documentation about psycopg pool and application configuration

[drnextgis]

When I run titiler-pgstac using docker-compose and try to add tile service to QGIS I see these errors in console:

titiler-pgstac | [2021-11-17 19:08:48 +0000] [8] [ERROR] Exception in ASGI application
titiler-pgstac | Traceback (most recent call last):
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/uvicorn/protocols/http/httptools_impl.py", line 396, in run_asgi
titiler-pgstac |     result = await app(self.scope, self.receive, self.send)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/uvicorn/middleware/proxy_headers.py", line 45, in __call__
titiler-pgstac |     return await self.app(scope, receive, send)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/fastapi/applications.py", line 199, in __call__
titiler-pgstac |     await super().__call__(scope, receive, send)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/applications.py", line 112, in __call__
titiler-pgstac |     await self.middleware_stack(scope, receive, send)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/middleware/errors.py", line 181, in __call__
titiler-pgstac |     raise exc from None
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/middleware/errors.py", line 159, in __call__
titiler-pgstac |     await self.app(scope, receive, _send)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/middleware/base.py", line 25, in __call__
titiler-pgstac |     response = await self.dispatch_func(request, self.call_next)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/titiler/core/middleware.py", line 38, in dispatch
titiler-pgstac |     response = await call_next(request)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/middleware/base.py", line 45, in call_next
titiler-pgstac |     task.result()
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/middleware/base.py", line 38, in coro
titiler-pgstac |     await self.app(scope, receive, send)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/middleware/cors.py", line 78, in __call__
titiler-pgstac |     await self.app(scope, receive, send)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/exceptions.py", line 82, in __call__
titiler-pgstac |     raise exc from None
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/exceptions.py", line 71, in __call__
titiler-pgstac |     await self.app(scope, receive, sender)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/routing.py", line 580, in __call__
titiler-pgstac |     await route.handle(scope, receive, send)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/routing.py", line 241, in handle
titiler-pgstac |     await self.app(scope, receive, send)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/routing.py", line 52, in app
titiler-pgstac |     response = await func(request)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/fastapi/routing.py", line 219, in app
titiler-pgstac |     raw_response = await run_endpoint_function(
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/fastapi/routing.py", line 154, in run_endpoint_function
titiler-pgstac |     return await run_in_threadpool(dependant.call, **values)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/starlette/concurrency.py", line 40, in run_in_threadpool
titiler-pgstac |     return await loop.run_in_executor(None, func, *args)
titiler-pgstac |   File "/usr/local/lib/python3.8/concurrent/futures/thread.py", line 57, in run
titiler-pgstac |     result = self.fn(*self.args, **self.kwargs)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/titiler/pgstac/factory.py", line 157, in tile
titiler-pgstac |     data, _ = src_dst.tile(
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/titiler/pgstac/mosaic.py", line 216, in tile
titiler-pgstac |     mosaic_assets = self.assets_for_tile(
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/titiler/pgstac/mosaic.py", line 139, in assets_for_tile
titiler-pgstac |     return self.get_assets(Polygon.from_bounds(*bbox), **kwargs)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/cachetools/__init__.py", line 520, in wrapper
titiler-pgstac |     v = func(*args, **kwargs)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/titiler/pgstac/mosaic.py", line 175, in get_assets
titiler-pgstac |     conn = self.pool.getconn()
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/psycopg2/pool.py", line 169, in getconn
titiler-pgstac |     return self._getconn(key)
titiler-pgstac |   File "/usr/local/lib/python3.8/site-packages/psycopg2/pool.py", line 92, in _getconn
titiler-pgstac |     raise PoolError("connection pool exhausted")
titiler-pgstac | psycopg2.pool.PoolError: connection pool exhausted

[drnextgis]

Should we consider switching to psycopg3 pool as suggested here?

[vincentsarago]

maybe we should start by trying https://github.com/microsoft/planetary-computer-apis/blob/main/pctiler/pctiler/db.py#L62-L92

I'm not quite sure of the implication of using psycopg 3 yet.

[vincentsarago]

@drnextgis in #21, I've switched to psycopg3 but the pool exhausted message can still append. This just tells that there is a misconfiguration (too many connection to one pool) so this needs to be solved by changing the POOL configuration or the number of workers used.

I've used https://github.com/bdon/TileSiege to perform a benchmark. I'll push the code in #21. But the TLDR, is that you have to carefully choose how many worker (and concurrent request) an application can handle.

cc @lossyrob

[vincentsarago]

I've renamed the issue to make sure we add a note in the documentation

Links:

• https://github.com/brettwooldridge/HikariCP/wiki/About-Pool-Sizing
• https://www.psycopg.org/psycopg3/docs/advanced/pool.html#connection-pools
• http://www.uvicorn.org/settings/#resource-limits
• http://www.uvicorn.org/settings/#production
• https://github.com/stac-utils/titiler-pgstac/blob/master/benchmark/README.md