Increase FastAPI thread pool and make configurable

The FastAPI thread pool size can now be configured by setting the environment variable DAF_BUTLER_SERVER_THREAD_POOL_SIZE.

It turns out the default value was 10, which is way too low for an I/O-bound service like Butler server.

pyproject.toml

@@ -51,7 +51,8 @@ remote = [
 ]
 server = [
     "fastapi",
-    "safir >= 3.4.0"
+    "safir >= 3.4.0",
+    "pydantic-settings"
 ]
 test = [
     "pytest >= 3.2",

python/lsst/daf/butler/remote_butler/server/_config.py

@@ -27,12 +27,10 @@
 
 from __future__ import annotations
 
-import dataclasses
-import os
+from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
-@dataclasses.dataclass(frozen=True)
-class ButlerServerConfig:
+class ButlerServerConfig(BaseSettings):
     """Butler server configuration loaded from environment variables.
 
     Notes
@@ -43,12 +41,20 @@ class ButlerServerConfig:
      `ButlerRepoIndex`.
     """
 
-    static_files_path: str | None
+    model_config = SettingsConfigDict(env_prefix="daf_butler_server_")
+
+    static_files_path: str | None = None
     """Absolute path to a directory of files that will be served to end-users
     as static files from the `configs/` HTTP route.
     """
 
+    thread_pool_size: int = 40
+    """
+    Maximum number of concurrent threads that may be spawned by FastAPI for
+    synchronous handlers.
+    """
+
 
 def load_config() -> ButlerServerConfig:
     """Read the Butler server configuration from the environment."""
-    return ButlerServerConfig(static_files_path=os.environ.get("DAF_BUTLER_SERVER_STATIC_FILES_PATH"))
+    return ButlerServerConfig()

python/lsst/daf/butler/remote_butler/server/_server.py

@@ -29,8 +29,10 @@
 
 __all__ = ("create_app",)
 
-from collections.abc import Awaitable, Callable
+from collections.abc import AsyncIterator, Awaitable, Callable
+from contextlib import asynccontextmanager
 
+import anyio
 import safir.dependencies.logger
 from fastapi import FastAPI, Request, Response
 from fastapi.staticfiles import StaticFiles
@@ -54,7 +56,17 @@ def create_app() -> FastAPI:
     """Create a Butler server FastAPI application."""
     config = load_config()
 
-    app = FastAPI()
+    @asynccontextmanager
+    async def lifespan(app: FastAPI) -> AsyncIterator[None]:
+        # Set the size of the threadpool used internally by FastAPI for
+        # synchronous handlers.  Because most Butler server endpoints are sync,
+        # this effectively sets the maximum number of concurrent requests that
+        # can be handled.
+        # See https://github.com/encode/starlette/issues/1724#issuecomment-1179063924
+        anyio.to_thread.current_default_thread_limiter().total_tokens = config.thread_pool_size
+        yield
+
+    app = FastAPI(lifespan=lifespan)
 
     # A single instance of the server can serve data from multiple Butler
     # repositories.  This 'repository' path placeholder is consumed by

requirements/main.in

@@ -19,3 +19,4 @@ httpx
 # Butler server dependencies sufficient for unit tests
 fastapi
 safir >= 3.4.0
+pydantic-settings