add property cache for API

This caches results from querying nominatim_properties.

nominatim/api/connection.py

@@ -7,7 +7,7 @@
 """
 Extended SQLAlchemy connection class that also includes access to the schema.
 """
-from typing import Any, Mapping, Sequence, Union
+from typing import Any, Mapping, Sequence, Union, Dict, cast
 
 import sqlalchemy as sa
 from sqlalchemy.ext.asyncio import AsyncConnection
@@ -22,9 +22,11 @@ class SearchConnection:
     """
 
     def __init__(self, conn: AsyncConnection,
-                 tables: SearchTables) -> None:
+                 tables: SearchTables,
+                 properties: Dict[str, Any]) -> None:
         self.connection = conn
         self.t = tables # pylint: disable=invalid-name
+        self._property_cache = properties
 
 
     async def scalar(self, sql: sa.sql.base.Executable,
@@ -41,3 +43,44 @@ class SearchConnection:
         """ Execute a 'execute()' query on the connection.
         """
         return await self.connection.execute(sql, params)
+
+
+    async def get_property(self, name: str, cached: bool = True) -> str:
+        """ Get a property from Nominatim's property table.
+
+            Property values are normally cached so that they are only
+            retrieved from the database when they are queried for the
+            first time with this function. Set 'cached' to False to force
+            reading the property from the database.
+
+            Raises a ValueError if the property does not exist.
+        """
+        if name.startswith('DB:'):
+            raise ValueError(f"Illegal property value '{name}'.")
+
+        if cached and name in self._property_cache:
+            return cast(str, self._property_cache[name])
+
+        sql = sa.select(self.t.properties.c.value)\
+            .where(self.t.properties.c.property == name)
+        value = await self.connection.scalar(sql)
+
+        if value is None:
+            raise ValueError(f"Property '{name}' not found in database.")
+
+        self._property_cache[name] = cast(str, value)
+
+        return cast(str, value)
+
+
+    async def get_db_property(self, name: str) -> Any:
+        """ Get a setting from the database. At the moment, only
+            'server_version', the version of the database software, can
+            be retrieved with this function.
+
+            Raises a ValueError if the property does not exist.
+        """
+        if name != 'server_version':
+            raise ValueError(f"DB setting '{name}' not found in database.")
+
+        return self._property_cache['DB:server_version']

nominatim/api/core.py

@@ -7,7 +7,7 @@
 """
 Implementation of classes for API access via libraries.
 """
-from typing import Mapping, Optional, Any, AsyncIterator
+from typing import Mapping, Optional, Any, AsyncIterator, Dict
 import asyncio
 import contextlib
 from pathlib import Path
@@ -32,6 +32,7 @@ class NominatimAPIAsync:
         self._engine_lock = asyncio.Lock()
         self._engine: Optional[sa_asyncio.AsyncEngine] = None
         self._tables: Optional[SearchTables] = None
+        self._property_cache: Dict[str, Any] = {'DB:server_version': 0}
 
 
     async def setup_database(self) -> None:
@@ -64,11 +65,11 @@ class NominatimAPIAsync:
             try:
                 async with engine.begin() as conn:
                     result = await conn.scalar(sa.text('SHOW server_version_num'))
-                    self.server_version = int(result)
+                    server_version = int(result)
             except asyncpg.PostgresError:
-                self.server_version = 0
+                server_version = 0
 
-            if self.server_version >= 110000:
+            if server_version >= 110000:
                 @sa.event.listens_for(engine.sync_engine, "connect")
                 def _on_connect(dbapi_con: Any, _: Any) -> None:
                     cursor = dbapi_con.cursor()
@@ -76,6 +77,8 @@ class NominatimAPIAsync:
                 # Make sure that all connections get the new settings
                 await self.close()
 
+            self._property_cache['DB:server_version'] = server_version
+
             self._tables = SearchTables(sa.MetaData(), engine.name) # pylint: disable=no-member
             self._engine = engine
 
@@ -104,7 +107,7 @@ class NominatimAPIAsync:
         assert self._tables is not None
 
         async with self._engine.begin() as conn:
-            yield SearchConnection(conn, self._tables)
+            yield SearchConnection(conn, self._tables, self._property_cache)
 
 
     async def status(self) -> StatusResult:

nominatim/api/status.py

@@ -7,7 +7,7 @@
 """
 Classes and function releated to status call.
 """
-from typing import Optional, cast
+from typing import Optional
 import datetime as dt
 import dataclasses
 
@@ -37,10 +37,10 @@ async def get_status(conn: SearchConnection) -> StatusResult:
     status.data_updated = await conn.scalar(sql)
 
     # Database version
-    sql = sa.select(conn.t.properties.c.value)\
-            .where(conn.t.properties.c.property == 'database_version')
-    verstr = await conn.scalar(sql)
-    if verstr is not None:
-        status.database_version = version.parse_version(cast(str, verstr))
+    try:
+        verstr = await conn.get_property('database_version')
+        status.database_version = version.parse_version(verstr)
+    except ValueError:
+        pass
 
     return status