Add semaphore to limit simultaneous requests to tile data

api/config.dev.py

@@ -29,5 +29,6 @@ ADDITIONAL_CORS_ORIGINS = [
     "http://localhost:8880/",  # for maputnik on 8880
     "http://localhost:8888/",  # for maputnik on 8888
 ]
+TILE_SEMAPHORE_SIZE = 4
 
 # vim: set ft=python :

api/config.py.example

@@ -61,4 +61,9 @@ TILES_FILE = None
 # default. Python list, or whitespace separated string.
 ADDITIONAL_CORS_ORIGINS = None
 
+# How many asynchronous requests may be sent to the database to generate tile
+# information. Should be less than POSTGRES_POOL_SIZE to leave some connections
+# to the other features of the API ;)
+TILE_SEMAPHORE_SIZE = 4
+
 # vim: set ft=python :

api/obs/api/app.py

@@ -1,3 +1,4 @@
+import asyncio
 import logging
 import re
 
@@ -23,7 +24,6 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from obs.api.db import User, make_session, connect_db
 from obs.api.cors import setup_options, add_cors_headers
 from obs.api.utils import get_single_arg
-from sqlalchemy.util import asyncio
 
 log = logging.getLogger(__name__)
 
@@ -58,6 +58,24 @@ app = Sanic(
 )
 configure_sanic_logging()
 
+app.config.update(
+    dict(
+        DEBUG=False,
+        VERBOSE=False,
+        AUTO_RELOAD=False,
+        POSTGRES_POOL_SIZE=20,
+        POSTGRES_MAX_OVERFLOW=40,
+        DEDICATED_WORKER=True,
+        FRONTEND_URL=None,
+        FRONTEND_HTTPS=True,
+        TILES_FILE=None,
+        TILE_SEMAPHORE_SIZE=4,
+    )
+)
+
+# overwrite from defaults again
+app.config.load_environment_vars("OBS_")
+
 if isfile("./config.py"):
     app.update_config("./config.py")
 
@@ -168,6 +186,9 @@ async def app_connect_db(app, loop):
     )
     app.ctx._db_engine = await app.ctx._db_engine_ctx.__aenter__()
 
+    if app.config.TILE_SEMAPHORE_SIZE:
+        app.ctx._tile_semaphore = asyncio.Semaphore(app.config.TILE_SEMAPHORE_SIZE)
+
 
 @app.after_server_stop
 async def app_disconnect_db(app, loop):

api/obs/api/routes/tiles.py

@@ -1,10 +1,12 @@
+import asyncio
+from contextlib import asynccontextmanager
 from gzip import decompress
 from sqlite3 import connect
 from datetime import datetime, time, timedelta
 from typing import Optional, Tuple
 
 import dateutil.parser
-from sanic.exceptions import Forbidden, InvalidUsage
+from sanic.exceptions import Forbidden, InvalidUsage, ServiceUnavailable
 from sanic.response import raw
 
 from sqlalchemy import select, text
@@ -85,8 +87,41 @@ def get_filter_options(
     return user_id, start, end
 
 
+@asynccontextmanager
+async def use_tile_semaphore(req, timeout=10):
+    """
+    If configured, acquire a semaphore for the map tile request and release it
+    after the context has finished.
+
+    If the semaphore cannot be acquired within the timeout, issue a 503 Service
+    Unavailable error response that describes that the map tile database is
+    overloaded, so users know what the problem is.
+
+    Operates as a noop when the tile semaphore is not enabled.
+    """
+    sem = getattr(req.app.ctx, "_tile_semaphore", None)
+
+    if sem is None:
+        yield
+        return
+
+    try:
+        await asyncio.wait_for(sem.acquire(), timeout)
+
+        try:
+            yield
+        finally:
+            sem.release()
+
+    except asyncio.TimeoutError:
+        raise ServiceUnavailable(
+            "Too many map content requests, database overloaded. Please retry later."
+        )
+
+
 @app.route(r"/tiles/<zoom:int>/<x:int>/<y:(\d+)\.pbf>")
 async def tiles(req, zoom: int, x: int, y: str):
+    async with use_tile_semaphore(req):
         if app.config.get("TILES_FILE"):
             tile = get_tile(req.app.config.TILES_FILE, int(zoom), int(x), int(y))