Question: I am working on a project that has a FastAPI back end with a React Frontend. When calling the back end via fetch I sometimes get the following: Access to fetch at 'http://localhost:8000/get-main-query-data' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.  This happens every so often, I can call one endpoint then the error gets thrown. Sometimes the error gets thrown for all endpoints I have set up Middleware in my main.py like so: (also at this line) # allows cross-origin requests from React origins = [ "http://localhost", "http://localhost:3000", ] app.add_middleware( CORSMiddleware, allow_origins=origins, allow_credentials=True, allow_methods=["*"], allow_headers=["*"], )  Could this be an issue with fetch it's self? I am worried at when I get to host this ill be getting CORS errors and my prototype won't be working :( The whole main.py is like so: Backend """ API to allow for data retrieval and manipulation. """ from typing import Optional from fastapi import FastAPI, HTTPException, status from fastapi.middleware.cors import CORSMiddleware from pydantic import BaseModel import models from db import Session app = FastAPI() """ Pydantic BaseModels for the API. """ class SignalJourneyAudiences(BaseModel): """SignalJourneyAudiences BaseModel.""" audienceId: Optional[int] # PK segment: str enabled: bool class SignalJourneyAudienceConstraints(BaseModel): """SignalJourneyAudienceConstraints BaseModel.""" uid: Optional[int] # PK constraintId: int audienceId: int # FK - SignalJourneyAudiences -> audienceId sourceId: int # FK - SignalJourneySources -> sourceId constraintTypeId: int # FK - SignalJourneyConstraintType -> constraintTypeId constraintValue: str targeting: bool frequency: int period: int class SignalJourneyAudienceConstraintRelations(BaseModel): """SignalJourneyAudienceConstraintRelations BaseModel.""" uid: Optional[int] # PK audienceId: int relation: str constraintIds: str class SignalJourneyConstraintType(BaseModel): """SignalJourneyConstraintType BaseModel.""" constraintTypeId: Optional[int] # PK constraintType: str class SingalJourneySources(BaseModel): """SignalJourneySources BaseModel.""" sourceId: Optional[int] # PK source: str # allows cross-origin requests from React origins = [ "http://localhost", "http://localhost:3000", ] app.add_middleware( CORSMiddleware, allow_origins=origins, allow_credentials=True, allow_methods=["*"], allow_headers=["*"], ) # database instance db = Session() @app.get("/") def index(): """Root endpoint.""" return { "messagee": "Welcome to Signal Journey API. Please use the API documentation to learn more." } @app.get("/audiences", status_code=status.HTTP_200_OK) def get_audiences(): """Get all audience data from the database.""" return db.query(models.SignalJourneyAudiences).all() @app.get("/audience-constraints", status_code=status.HTTP_200_OK) def get_audience_constraints(): """Get all audience constraint data from the database.""" return db.query(models.SignalJourneyAudienceConstraints).all() @app.get("/audience-constraints-relations", status_code=status.HTTP_200_OK) def get_audience_constraints_relations(): """Get all audience constraint data from the database.""" return db.query(models.SignalJourneyAudienceConstraintRelations).all() @app.get("/get-constraint-types", status_code=status.HTTP_200_OK) def get_constraints_type(): """Get all audience constraint data from the database.""" return db.query(models.SignalJourneyConstraintType).all() @app.post("/add-constraint-type", status_code=status.HTTP_200_OK) def add_constraint_type(sjct: SignalJourneyConstraintType): """Add a constraint type to the database.""" constraint_type_query = ( db.query(models.SignalJourneyConstraintType) .filter( models.SignalJourneyConstraintType.constraintType == sjct.constraintType.upper() and models.SignalJourneyConstraintType.constraintTypeId == sjct.constraintTypeId ) .first() ) if constraint_type_query is not None: raise HTTPException( status_code=status.HTTP_400_BAD_REQUEST, detail="Constaint type already exists.", ) constraint_type = models.SignalJourneyConstraintType( constraintType=sjct.constraintType.upper(), ) db.add(constraint_type) db.commit() return { "message": f"Constraint type {sjct.constraintType.upper()} added successfully." } @app.get("/get-sources", status_code=status.HTTP_200_OK) def get_sources(): """Get all sources data from the database.""" return db.query(models.SingalJourneySources).all() @app.post("/add-source", status_code=status.HTTP_200_OK) def add_source_type(sjs: SingalJourneySources): """Add a new source type to the database.""" source_type_query = ( db.query(models.SingalJourneySources) .filter(models.SingalJourneySources.source == sjs.source.upper()) .first() ) if source_type_query is not None: raise HTTPException( status_code=status.HTTP_400_BAD_REQUEST, detail="Source already exists.", ) source_type = models.SingalJourneySources(source=sjs.source.upper()) db.add(source_type) db.commit() return {"message": f"Source {sjs.source.upper()} added successfully."} """ Endpoints for populating the UI with data. These need to consist of some joins. Query to be used in SQL SELECT constraintid, sja.segment, sjs.source, sjct.constrainttype, constraintvalue, targeting, frequency, period FROM signaljourneyaudienceconstraints JOIN signaljourneyaudiences sja ON sja.audienceid = signaljourneyaudienceconstraints.audienceid; JOIN signaljourneysources sjs ON sjs.sourceid = signaljourneyaudienceconstraints.sourceid JOIN signaljourneyconstrainttype sjct ON sjct.constrainttypeid = signaljourneyaudienceconstraints.constrainttypeid """ @app.get("/get-main-query-data", status_code=status.HTTP_200_OK) def get_main_query_data(): """Returns data for the main query.""" return ( db.query( models.SignalJourneyAudienceConstraints.constraintId, models.SignalJourneyAudiences.segment, models.SingalJourneySources.source, models.SignalJourneyConstraintType.constraintType, models.SignalJourneyAudienceConstraints.constraintValue, models.SignalJourneyAudienceConstraints.targeting, models.SignalJourneyAudienceConstraints.frequency, models.SignalJourneyAudienceConstraints.period, ) .join( models.SignalJourneyAudiences, models.SignalJourneyAudiences.audienceId == models.SignalJourneyAudienceConstraints.audienceId, ) .join( models.SingalJourneySources, models.SingalJourneySources.sourceId == models.SignalJourneyAudienceConstraints.sourceId, ) .join( models.SignalJourneyConstraintType, models.SignalJourneyConstraintType.constraintTypeId == models.SignalJourneyAudienceConstraints.constraintTypeId, ) .all() )  Frontend I am calling my API endpoints like so: //form.jsx // pulls segments name from signaljourneyaudiences useEffect(() => { fetch('http://localhost:8000/audiences') .then((res) => res.json()) .then((data) => setSegmentNames(data)) .catch((err) => console.log(err)); }, []); // pulls field names from signaljourneyaudiences useEffect(() => { fetch('http://localhost:8000/get-constraint-types') .then((res) => res.json()) .then((data) => setConstraints(data)) .catch((err) => console.log(err)); }, []); // table.jsx useEffect(() => { fetch('http://localhost:8000/get-main-query-data') .then((res) => res.json()) .then((data) => { setTableData(data); }) .catch((err) => console.log(err)); }, []);  As you can see here the table has been populated by the endpoints but on the other hand, one of the dropdowns have not.  HTTP 500 error description INFO: 127.0.0.1:62301 - "GET /get-constraint-types HTTP/1.1" 500 Internal Server Error 2022-02-24 09:26:44,234 INFO sqlalchemy.engine.Engine [cached since 2972s ago] () ERROR: Exception in ASGI application Traceback (most recent call last): File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1702, in _execute_context context = constructor( File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 1013, in _init_compiled self.cursor = self.create_cursor() File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 1361, in create_cursor return self.create_default_cursor() File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 1364, in create_default_cursor return self._dbapi_connection.cursor() File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/pool/base.py", line 1083, in cursor return self.dbapi_connection.cursor(*args, **kwargs) sqlite3.ProgrammingError: SQLite objects created in a thread can only be used in that same thread. The object was created in thread id 6191820800 and this is thread id 6174994432. The above exception was the direct cause of the following exception: Traceback (most recent call last): File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/uvicorn/protocols/http/httptools_impl.py", line 372, in run_asgi result = await app(self.scope, self.receive, self.send) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/uvicorn/middleware/proxy_headers.py", line 75, in __call__ return await self.app(scope, receive, send) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/fastapi/applications.py", line 259, in __call__ await super().__call__(scope, receive, send) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/starlette/applications.py", line 112, in __call__ await self.middleware_stack(scope, receive, send) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/starlette/middleware/errors.py", line 181, in __call__ raise exc File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/starlette/middleware/errors.py", line 159, in __call__ await self.app(scope, receive, _send) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/starlette/middleware/cors.py", line 92, in __call__ await self.simple_response(scope, receive, send, request_headers=headers) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/starlette/middleware/cors.py", line 147, in simple_response await self.app(scope, receive, send) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/starlette/exceptions.py", line 82, in __call__ raise exc File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/starlette/exceptions.py", line 71, in __call__ await self.app(scope, receive, sender) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/fastapi/middleware/asyncexitstack.py", line 21, in __call__ raise e File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/fastapi/middleware/asyncexitstack.py", line 18, in __call__ await self.app(scope, receive, send) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/starlette/routing.py", line 656, in __call__ await route.handle(scope, receive, send) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/starlette/routing.py", line 259, in handle await self.app(scope, receive, send) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/starlette/routing.py", line 61, in app response = await func(request) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/fastapi/routing.py", line 227, in app raw_response = await run_endpoint_function( File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/fastapi/routing.py", line 162, in run_endpoint_function return await run_in_threadpool(dependant.call, **values) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/starlette/concurrency.py", line 39, in run_in_threadpool return await anyio.to_thread.run_sync(func, *args) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/anyio/to_thread.py", line 28, in run_sync return await get_asynclib().run_sync_in_worker_thread(func, *args, cancellable=cancellable, File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 818, in run_sync_in_worker_thread return await future File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/anyio/_backends/_asyncio.py", line 754, in run result = context.run(func, *args) File "/Users/paul/Developer/signal_journey/backend/./main.py", line 109, in get_constraints_type return db.query(models.SignalJourneyConstraintType).all() File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/orm/query.py", line 2759, in all return self._iter().all() File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/orm/query.py", line 2894, in _iter result = self.session.execute( File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 1692, in execute result = conn._execute_20(statement, params or {}, execution_options) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1614, in _execute_20 return meth(self, args_10style, kwargs_10style, execution_options) File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/sql/elements.py", line 325, in _execute_on_connection return connection._execute_clauseelement( File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1481, in _execute_clauseelement ret = self._execute_context( File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1708, in _execute_context self._handle_dbapi_exception( File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 2026, in _handle_dbapi_exception util.raise_( File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/util/compat.py", line 207, in raise_ raise exception File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1702, in _execute_context context = constructor( File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 1013, in _init_compiled self.cursor = self.create_cursor() File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 1361, in create_cursor return self.create_default_cursor() File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 1364, in create_default_cursor return self._dbapi_connection.cursor() File "/Users/paul/.local/share/virtualenvs/backend-CF5omcRU/lib/python3.9/site-packages/sqlalchemy/pool/base.py", line 1083, in cursor return self.dbapi_connection.cursor(*args, **kwargs) sqlalchemy.exc.ProgrammingError: (sqlite3.ProgrammingError) SQLite objects created in a thread can only be used in that same thread. The object was created in thread id 6191820800 and this is thread id 6174994432. [SQL: SELECT "SignalJourneyConstraintType"."constraintTypeId" AS "SignalJourneyConstraintType_constraintTypeId", "SignalJourneyConstraintType"."constraintType" AS "SignalJourneyConstraintType_constraintType" FROM "SignalJourneyConstraintType"] [parameters: [{}]] (Background on this error at: https://sqlalche.me/e/14/f405) 

Answer: When a server side error occurs (a response code of 5xx), the CORS middleware doesn't get to add their headers since the request is effectively terminated, making it impossible for the browser to read the response. For your second problem, you should use a separate session for each invocation of your API. The reference guide has an example of how to do this: SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine) ... # Dependency def get_db(): db = SessionLocal() try: yield db finally: db.close() ... @app.post("/users/{user_id}/items/", response_model=schemas.Item) def create_item_for_user(..., db: Session = Depends(get_db)): return crud.create_user_item(db=db, item=item, user_id=user_id)