Starlette includes several middleware classes for adding behavior that is applied across
your entire application. These are all implemented as standard ASGI
middleware classes, and can be applied either to Starlette or to any other ASGI application.

## Using middleware

The Starlette application class allows you to include the ASGI middleware
in a way that ensures that it remains wrapped by the exception handler.

```python
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.httpsredirect import HTTPSRedirectMiddleware
from starlette.middleware.trustedhost import TrustedHostMiddleware

routes = ...

# Ensure that all requests include an 'example.com' or
# '*.example.com' host header, and strictly enforce https-only access.
middleware = [
    Middleware(
        TrustedHostMiddleware,
        allowed_hosts=['example.com', '*.example.com'],
    ),
    Middleware(HTTPSRedirectMiddleware)
]

app = Starlette(routes=routes, middleware=middleware)
```

Every Starlette application automatically includes two pieces of middleware by default:

* `ServerErrorMiddleware` - Ensures that application exceptions may return a custom 500 page, or display an application traceback in DEBUG mode. This is *always* the outermost middleware layer.
* `ExceptionMiddleware` - Adds exception handlers, so that particular types of expected exception cases can be associated with handler functions. For example raising `HTTPException(status_code=404)` within an endpoint will end up rendering a custom 404 page.

Middleware is evaluated from top-to-bottom, so the flow of execution in our example
application would look like this:

* Middleware
    * `ServerErrorMiddleware`
    * `TrustedHostMiddleware`
    * `HTTPSRedirectMiddleware`
    * `ExceptionMiddleware`
* Routing
* Endpoint

The following middleware implementations are available in the Starlette package:

## CORSMiddleware

Adds appropriate [CORS headers](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS) to outgoing responses in order to allow cross-origin requests from browsers.

The default parameters used by the CORSMiddleware implementation are restrictive by default,
so you'll need to explicitly enable particular origins, methods, or headers, in order
for browsers to be permitted to use them in a Cross-Domain context.

```python
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.cors import CORSMiddleware

routes = ...

middleware = [
    Middleware(CORSMiddleware, allow_origins=['*'])
]

app = Starlette(routes=routes, middleware=middleware)
```

The following arguments are supported:

* `allow_origins` - A list of origins that should be permitted to make cross-origin requests. eg. `['https://example.org', 'https://www.example.org']`. You can use `['*']` to allow any origin.
* `allow_origin_regex` - A regex string to match against origins that should be permitted to make cross-origin requests. eg. `'https://.*\.example\.org'`.
* `allow_methods` - A list of HTTP methods that should be allowed for cross-origin requests. Defaults to `['GET']`. You can use `['*']` to allow all standard methods.
* `allow_headers` - A list of HTTP request headers that should be supported for cross-origin requests. Defaults to `[]`. You can use `['*']` to allow all headers. The `Accept`, `Accept-Language`, `Content-Language` and `Content-Type` headers are always allowed for CORS requests.
* `allow_credentials` - Indicate that cookies should be supported for cross-origin requests. Defaults to `False`. Also, `allow_origins`, `allow_methods` and `allow_headers` cannot be set to `['*']` for credentials to be allowed, all of them must be explicitly specified.
* `expose_headers` - Indicate any response headers that should be made accessible to the browser. Defaults to `[]`.
* `max_age` - Sets a maximum time in seconds for browsers to cache CORS responses. Defaults to `600`.

The middleware responds to two particular types of HTTP request...

#### CORS preflight requests

These are any `OPTIONS` request with `Origin` and `Access-Control-Request-Method` headers.
In this case the middleware will intercept the incoming request and respond with
appropriate CORS headers, and either a 200 or 400 response for informational purposes.

#### Simple requests

Any request with an `Origin` header. In this case the middleware will pass the
request through as normal, but will include appropriate CORS headers on the response.

### CORSMiddleware Global Enforcement

When using CORSMiddleware with your Starlette application, it's important to ensure that CORS headers are applied even to error responses generated by unhandled exceptions. The recommended solution is to wrap the entire Starlette application with CORSMiddleware. This approach guarantees that even if an exception is caught by ServerErrorMiddleware (or other outer error-handling middleware), the response will still include the proper `Access-Control-Allow-Origin` header.

For example, instead of adding CORSMiddleware as an inner `middleware` via the Starlette middleware parameter, you can wrap your application as follows:

```python
from starlette.applications import Starlette
from starlette.middleware.cors import CORSMiddleware

import uvicorn

app = Starlette()
app = CORSMiddleware(app=app, allow_origins=["*"])

# ... your routes and middleware configuration ...

if __name__ == '__main__':
    uvicorn.run(
        app,
        host='0.0.0.0',
        port=8000
    )
```

## SessionMiddleware

Adds signed cookie-based HTTP sessions. Session information is readable but not modifiable.

Access or modify the session data using the `request.session` dictionary interface.

The following arguments are supported:

* `secret_key` - Should be a random string.
* `session_cookie` - Defaults to "session".
* `max_age` - Session expiry time in seconds. Defaults to 2 weeks. If set to `None` then the cookie will last as long as the browser session.
* `same_site` - SameSite flag prevents the browser from sending session cookie along with cross-site requests. Defaults to `'lax'`.
* `path` - The path set for the session cookie. Defaults to `'/'`.
* `https_only` - Indicate that Secure flag should be set (can be used with HTTPS only). Defaults to `False`.
* `domain` - Domain of the cookie used to share cookie between subdomains or cross-domains. The browser defaults the domain to the same host that set the cookie, excluding subdomains ([reference](https://developer.mozilla.org/en-US/docs/Web/HTTP/Cookies#domain_attribute)).


```python
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.sessions import SessionMiddleware

routes = ...

middleware = [
    Middleware(SessionMiddleware, secret_key=..., https_only=True)
]

app = Starlette(routes=routes, middleware=middleware)
```

## HTTPSRedirectMiddleware

Enforces that all incoming requests must either be `https` or `wss`. Any incoming
requests to `http` or `ws` will be redirected to the secure scheme instead.

```python
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.httpsredirect import HTTPSRedirectMiddleware

routes = ...

middleware = [
    Middleware(HTTPSRedirectMiddleware)
]

app = Starlette(routes=routes, middleware=middleware)
```

There are no configuration options for this middleware class.

## TrustedHostMiddleware

Enforces that all incoming requests have a correctly set `Host` header, in order
to guard against HTTP Host Header attacks.

```python
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.trustedhost import TrustedHostMiddleware

routes = ...

middleware = [
    Middleware(TrustedHostMiddleware, allowed_hosts=['example.com', '*.example.com'])
]

app = Starlette(routes=routes, middleware=middleware)
```

The following arguments are supported:

* `allowed_hosts` - A list of domain names that should be allowed as hostnames. Wildcard
domains such as `*.example.com` are supported for matching subdomains. To allow any
hostname either use `allowed_hosts=["*"]` or omit the middleware.
* `www_redirect` - If set to True, requests to non-www versions of the allowed hosts will be redirected to their www counterparts. Defaults to `True`.

If an incoming request does not validate correctly then a 400 response will be sent.

## GZipMiddleware

Handles GZip responses for any request that includes `"gzip"` in the `Accept-Encoding` header.

The middleware will handle both standard and streaming responses.

??? info "Buffer on streaming responses"
    On streaming responses, the middleware will buffer the response before compressing it.

    The idea is that we don't want to compress every small chunk of data, as it would be inefficient.
    Instead, we buffer the response until it reaches a certain size, and then compress it.

    This may cause a delay in the response, as the middleware waits for the buffer to fill up before compressing it.

```python
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.gzip import GZipMiddleware


routes = ...

middleware = [
    Middleware(GZipMiddleware, minimum_size=1000, compresslevel=9)
]

app = Starlette(routes=routes, middleware=middleware)
```

The following arguments are supported:

* `minimum_size` - Do not GZip responses that are smaller than this minimum size in bytes. Defaults to `500`.
* `compresslevel` - Used during GZip compression. It is an integer ranging from 1 to 9. Defaults to `9`. Lower value results in faster compression but larger file sizes, while higher value results in slower compression but smaller file sizes.

The middleware won't GZip responses that already have either a `Content-Encoding` set, to prevent them from
being encoded twice, or a `Content-Type` set to `text/event-stream`, to avoid compressing server-sent events.

## BaseHTTPMiddleware

An abstract class that allows you to write ASGI middleware against a request/response
interface.

### Usage

To implement a middleware class using `BaseHTTPMiddleware`, you must override the
`async def dispatch(request, call_next)` method.

```python
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.base import BaseHTTPMiddleware


class CustomHeaderMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        response = await call_next(request)
        response.headers['Custom'] = 'Example'
        return response

routes = ...

middleware = [
    Middleware(CustomHeaderMiddleware)
]

app = Starlette(routes=routes, middleware=middleware)
```

If you want to provide configuration options to the middleware class you should
override the `__init__` method, ensuring that the first argument is `app`, and
any remaining arguments are optional keyword arguments. Make sure to set the `app`
attribute on the instance if you do this.

```python
class CustomHeaderMiddleware(BaseHTTPMiddleware):
    def __init__(self, app, header_value='Example'):
        super().__init__(app)
        self.header_value = header_value

    async def dispatch(self, request, call_next):
        response = await call_next(request)
        response.headers['Custom'] = self.header_value
        return response


middleware = [
    Middleware(CustomHeaderMiddleware, header_value='Customized')
]

app = Starlette(routes=routes, middleware=middleware)
```

Middleware classes should not modify their state outside of the `__init__` method.
Instead you should keep any state local to the `dispatch` method, or pass it
around explicitly, rather than mutating the middleware instance.

### Limitations

Currently, the `BaseHTTPMiddleware` has some known limitations:

- Using `BaseHTTPMiddleware` will prevent changes to [`contextvars.ContextVar`](https://docs.python.org/3/library/contextvars.html#contextvars.ContextVar)s from propagating upwards. That is, if you set a value for a `ContextVar` in your endpoint and try to read it from a middleware you will find that the value is not the same value you set in your endpoint (see [this test](https://github.com/Kludex/starlette/blob/621abc747a6604825190b93467918a0ec6456a24/tests/middleware/test_base.py#L192-L223) for an example of this behavior). Importantly, this also means that if a `BaseHTTPMiddleware` is positioned earlier in the middleware stack, it will disrupt `contextvars` propagation for any subsequent Pure ASGI Middleware that relies on them.

To overcome these limitations, use [pure ASGI middleware](#pure-asgi-middleware), as shown below.

## Pure ASGI Middleware

The [ASGI spec](https://asgi.readthedocs.io/en/latest/) makes it possible to implement ASGI middleware using the ASGI interface directly, as a chain of ASGI applications that call into the next one. In fact, this is how middleware classes shipped with Starlette are implemented.

This lower-level approach provides greater control over behavior and enhanced interoperability across frameworks and servers. It also overcomes the [limitations of `BaseHTTPMiddleware`](#limitations).

### Writing pure ASGI middleware

The most common way to create an ASGI middleware is with a class.

```python
class ASGIMiddleware:
    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
        await self.app(scope, receive, send)
```

The middleware above is the most basic ASGI middleware. It receives a parent ASGI application as an argument for its constructor, and implements an `async __call__` method which calls into that parent application.

Some implementations such as [`asgi-cors`](https://github.com/simonw/asgi-cors/blob/10ef64bfcc6cd8d16f3014077f20a0fb8544ec39/asgi_cors.py) use an alternative style, using functions:

```python
import functools

def asgi_middleware():
    def asgi_decorator(app):

        @functools.wraps(app)
        async def wrapped_app(scope, receive, send):
            await app(scope, receive, send)

        return wrapped_app

    return asgi_decorator
```

In any case, ASGI middleware must be callables that accept three arguments: `scope`, `receive`, and `send`.

* `scope` is a dict holding information about the connection, where `scope["type"]` may be:
    * [`"http"`](https://asgi.readthedocs.io/en/latest/specs/www.html#http-connection-scope): for HTTP requests.
    * [`"websocket"`](https://asgi.readthedocs.io/en/latest/specs/www.html#websocket-connection-scope): for WebSocket connections.
    * [`"lifespan"`](https://asgi.readthedocs.io/en/latest/specs/lifespan.html#scope): for ASGI lifespan messages.
* `receive` and `send` can be used to exchange ASGI event messages with the ASGI server — more on this below. The type and contents of these messages depend on the scope type. Learn more in the [ASGI specification](https://asgi.readthedocs.io/en/latest/specs/index.html).

### Using pure ASGI middleware

Pure ASGI middleware can be used like any other middleware:

```python
from starlette.applications import Starlette
from starlette.middleware import Middleware

from .middleware import ASGIMiddleware

routes = ...

middleware = [
    Middleware(ASGIMiddleware),
]

app = Starlette(..., middleware=middleware)
```

See also [Using middleware](#using-middleware).

### Type annotations

There are two ways of annotating a middleware: using Starlette itself or [`asgiref`](https://github.com/django/asgiref).

* Using Starlette: for most common use cases.

```python
from starlette.types import ASGIApp, Message, Scope, Receive, Send


class ASGIMiddleware:
    def __init__(self, app: ASGIApp) -> None:
        self.app = app

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        if scope["type"] != "http":
            return await self.app(scope, receive, send)

        async def send_wrapper(message: Message) -> None:
            # ... Do something
            await send(message)

        await self.app(scope, receive, send_wrapper)
```

* Using [`asgiref`](https://github.com/django/asgiref): for more rigorous type hinting.

```python
from asgiref.typing import ASGI3Application, ASGIReceiveCallable, ASGISendCallable, Scope
from asgiref.typing import ASGIReceiveEvent, ASGISendEvent


class ASGIMiddleware:
    def __init__(self, app: ASGI3Application) -> None:
        self.app = app

    async def __call__(self, scope: Scope, receive: ASGIReceiveCallable, send: ASGISendCallable) -> None:
        if scope["type"] != "http":
            await self.app(scope, receive, send)
            return

        async def send_wrapper(message: ASGISendEvent) -> None:
            # ... Do something
            await send(message)

        return await self.app(scope, receive, send_wrapper)
```

### Common patterns

#### Processing certain requests only

ASGI middleware can apply specific behavior according to the contents of `scope`.

For example, to only process HTTP requests, write this...

```python
class ASGIMiddleware:
    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
        if scope["type"] != "http":
            await self.app(scope, receive, send)
            return

        ...  # Do something here!

        await self.app(scope, receive, send)
```

Likewise, WebSocket-only middleware would guard on `scope["type"] != "websocket"`.

The middleware may also act differently based on the request method, URL, headers, etc.

#### Reusing Starlette components

Starlette provides several data structures that accept the ASGI `scope`, `receive` and/or `send` arguments, allowing you to work at a higher level of abstraction. Such data structures include [`Request`](requests.md#request), [`Headers`](requests.md#headers), [`QueryParams`](requests.md#query-parameters), [`URL`](requests.md#url), etc.

For example, you can instantiate a `Request` to more easily inspect an HTTP request:

```python
from starlette.requests import Request

class ASGIMiddleware:
    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
        if scope["type"] == "http":
            request = Request(scope)
            ... # Use `request.method`, `request.url`, `request.headers`, etc.

        await self.app(scope, receive, send)
```

You can also reuse [responses](responses.md), which are ASGI applications as well.

#### Sending eager responses

Inspecting the connection `scope` allows you to conditionally call into a different ASGI app. One use case might be sending a response without calling into the app.

As an example, this middleware uses a dictionary to perform permanent redirects based on the requested path. This could be used to implement ongoing support of legacy URLs in case you need to refactor route URL patterns.

```python
from starlette.datastructures import URL
from starlette.responses import RedirectResponse

class RedirectsMiddleware:
    def __init__(self, app, path_mapping: dict):
        self.app = app
        self.path_mapping = path_mapping

    async def __call__(self, scope, receive, send):
        if scope["type"] != "http":
            await self.app(scope, receive, send)
            return

        url = URL(scope=scope)

        if url.path in self.path_mapping:
            url = url.replace(path=self.path_mapping[url.path])
            response = RedirectResponse(url, status_code=301)
            await response(scope, receive, send)
            return

        await self.app(scope, receive, send)
```

Example usage would look like this:

```python
from starlette.applications import Starlette
from starlette.middleware import Middleware

routes = ...

redirections = {
    "/v1/resource/": "/v2/resource/",
    # ...
}

middleware = [
    Middleware(RedirectsMiddleware, path_mapping=redirections),
]

app = Starlette(routes=routes, middleware=middleware)
```


#### Inspecting or modifying the request

Request information can be accessed or changed by manipulating the `scope`. For a full example of this pattern, see Uvicorn's [`ProxyHeadersMiddleware`](https://github.com/encode/uvicorn/blob/fd4386fefb8fe8a4568831a7d8b2930d5fb61455/uvicorn/middleware/proxy_headers.py) which inspects and tweaks the `scope` when serving behind a frontend proxy.

Besides, wrapping the `receive` ASGI callable allows you to access or modify the HTTP request body by manipulating [`http.request`](https://asgi.readthedocs.io/en/latest/specs/www.html#request-receive-event) ASGI event messages.

As an example, this middleware computes and logs the size of the incoming request body...

```python
class LoggedRequestBodySizeMiddleware:
    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
        if scope["type"] != "http":
            await self.app(scope, receive, send)
            return

        body_size = 0

        async def receive_logging_request_body_size():
            nonlocal body_size

            message = await receive()
            assert message["type"] == "http.request"

            body_size += len(message.get("body", b""))

            if not message.get("more_body", False):
                print(f"Size of request body was: {body_size} bytes")

            return message

        await self.app(scope, receive_logging_request_body_size, send)
```

Likewise, WebSocket middleware may manipulate [`websocket.receive`](https://asgi.readthedocs.io/en/latest/specs/www.html#receive-receive-event) ASGI event messages to inspect or alter incoming WebSocket data.

For an example that changes the HTTP request body, see [`msgpack-asgi`](https://github.com/florimondmanca/msgpack-asgi).

#### Inspecting or modifying the response

Wrapping the `send` ASGI callable allows you to inspect or modify the HTTP response sent by the underlying application. To do so, react to [`http.response.start`](https://asgi.readthedocs.io/en/latest/specs/www.html#response-start-send-event) or [`http.response.body`](https://asgi.readthedocs.io/en/latest/specs/www.html#response-body-send-event) ASGI event messages.

As an example, this middleware adds some fixed extra response headers:

```python
from starlette.datastructures import MutableHeaders

class ExtraResponseHeadersMiddleware:
    def __init__(self, app, headers):
        self.app = app
        self.headers = headers

    async def __call__(self, scope, receive, send):
        if scope["type"] != "http":
            return await self.app(scope, receive, send)

        async def send_with_extra_headers(message):
            if message["type"] == "http.response.start":
                headers = MutableHeaders(scope=message)
                for key, value in self.headers:
                    headers.append(key, value)

            await send(message)

        await self.app(scope, receive, send_with_extra_headers)
```

See also [`asgi-logger`](https://github.com/Kludex/asgi-logger/blob/main/asgi_logger/middleware.py) for an example that inspects the HTTP response and logs a configurable HTTP access log line.

Likewise, WebSocket middleware may manipulate [`websocket.send`](https://asgi.readthedocs.io/en/latest/specs/www.html#send-send-event) ASGI event messages to inspect or alter outgoing WebSocket data.

Note that if you change the response body, you will need to update the response `Content-Length` header to match the new response body length. See [`brotli-asgi`](https://github.com/fullonic/brotli-asgi) for a complete example.

#### Passing information to endpoints

If you need to share information with the underlying app or endpoints, you may store it into the `scope` dictionary. Note that this is a convention -- for example, Starlette uses this to share routing information with endpoints -- but it is not part of the ASGI specification. If you do so, be sure to avoid conflicts by using keys that have low chances of being used by other middleware or applications.

For example, when including the middleware below, endpoints would be able to access `request.scope["asgi_transaction_id"]`.

```python
import uuid

class TransactionIDMiddleware:
    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
        scope["asgi_transaction_id"] = uuid.uuid4()
        await self.app(scope, receive, send)
```

#### Cleanup and error handling

You can wrap the application in a `try/except/finally` block or a context manager to perform cleanup operations or do error handling.

For example, the following middleware might collect metrics and process application exceptions...

```python
import time

class MonitoringMiddleware:
    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
        start = time.time()
        try:
            await self.app(scope, receive, send)
        except Exception as exc:
            ...  # Process the exception
            raise
        finally:
            end = time.time()
            elapsed = end - start
            ...  # Submit `elapsed` as a metric to a monitoring backend
```

See also [`timing-asgi`](https://github.com/steinnes/timing-asgi) for a full example of this pattern.

### Gotchas

#### ASGI middleware should be stateless

Because ASGI is designed to handle concurrent requests, any connection-specific state should be scoped to the `__call__` implementation. Not doing so would typically lead to conflicting variable reads/writes across requests, and most likely bugs.

As an example, this would conditionally replace the response body, if an `X-Mock` header is present in the response...

=== "✅ Do"

    ```python
    from starlette.datastructures import Headers

    class MockResponseBodyMiddleware:
        def __init__(self, app, content):
            self.app = app
            self.content = content

        async def __call__(self, scope, receive, send):
            if scope["type"] != "http":
                await self.app(scope, receive, send)
                return

            # A flag that we will turn `True` if the HTTP response
            # has the 'X-Mock' header.
            # ✅: Scoped to this function.
            should_mock = False

            async def maybe_send_with_mock_content(message):
                nonlocal should_mock

                if message["type"] == "http.response.start":
                    headers = Headers(raw=message["headers"])
                    should_mock = headers.get("X-Mock") == "1"
                    await send(message)

                elif message["type"] == "http.response.body":
                    if should_mock:
                        message = {"type": "http.response.body", "body": self.content}
                    await send(message)

            await self.app(scope, receive, maybe_send_with_mock_content)
    ```

=== "❌ Don't"

    ```python hl_lines="7-8"
    from starlette.datastructures import Headers

    class MockResponseBodyMiddleware:
        def __init__(self, app, content):
            self.app = app
            self.content = content
            # ❌: This variable would be read and written across requests!
            self.should_mock = False

        async def __call__(self, scope, receive, send):
            if scope["type"] != "http":
                await self.app(scope, receive, send)
                return

            async def maybe_send_with_mock_content(message):
                if message["type"] == "http.response.start":
                    headers = Headers(raw=message["headers"])
                    self.should_mock = headers.get("X-Mock") == "1"
                    await send(message)

                elif message["type"] == "http.response.body":
                    if self.should_mock:
                        message = {"type": "http.response.body", "body": self.content}
                    await send(message)

            await self.app(scope, receive, maybe_send_with_mock_content)
    ```

See also [`GZipMiddleware`](https://github.com/Kludex/starlette/blob/9ef1b91c9c043197da6c3f38aa153fd874b95527/starlette/middleware/gzip.py) for a full example implementation that navigates this potential gotcha.

### Further reading

This documentation should be enough to have a good basis on how to create an ASGI middleware.

Nonetheless, there are great articles about the subject:

- [Introduction to ASGI: Emergence of an Async Python Web Ecosystem](https://florimond.dev/en/posts/2019/08/introduction-to-asgi-async-python-web/)
- [How to write ASGI middleware](https://pgjones.dev/blog/how-to-write-asgi-middleware-2021/)

## Using middleware in other frameworks

To wrap ASGI middleware around other ASGI applications, you should use the
more general pattern of wrapping the application instance:

```python
app = TrustedHostMiddleware(app, allowed_hosts=['example.com'])
```

You can do this with a Starlette application instance too, but it is preferable
to use the `middleware=<List of Middleware instances>` style, as it will:

* Ensure that everything remains wrapped in a single outermost `ServerErrorMiddleware`.
* Preserves the top-level `app` instance.

## Applying middleware to groups of routes

Middleware can also be added to `Mount` instances, which allows you to apply middleware to a group of routes or a sub-application:

```python
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.gzip import GZipMiddleware
from starlette.routing import Mount, Route


routes = [
    Mount(
        "/",
        routes=[
            Route(
                "/example",
                endpoint=...,
            )
        ],
        middleware=[Middleware(GZipMiddleware)]
    )
]

app = Starlette(routes=routes)
```

Note that middleware used in this way is *not* wrapped in exception handling middleware like the middleware applied to the `Starlette` application is.
This is often not a problem because it only applies to middleware that inspect or modify the `Response`, and even then you probably don't want to apply this logic to error responses.
If you do want to apply the middleware logic to error responses only on some routes you have a couple of options:

* Add an `ExceptionMiddleware` onto the `Mount`
* Add a `try/except` block to your middleware and return an error response from there
* Split up marking and processing into two middlewares, one that gets put on `Mount` which marks the response as needing processing (for example by setting `scope["log-response"] = True`) and another applied to the `Starlette` application that does the heavy lifting.

The `Route`/`WebSocket` class also accepts a `middleware` argument, which allows you to apply middleware to a single route:

```python
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.gzip import GZipMiddleware
from starlette.routing import Route


routes = [
    Route(
        "/example",
        endpoint=...,
        middleware=[Middleware(GZipMiddleware)]
    )
]

app = Starlette(routes=routes)
```

You can also apply middleware to the `Router` class, which allows you to apply middleware to a group of routes:

```python
from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.gzip import GZipMiddleware
from starlette.routing import Route, Router


routes = [
    Route("/example", endpoint=...),
    Route("/another", endpoint=...),
]

router = Router(routes=routes, middleware=[Middleware(GZipMiddleware)])
```

## Third party middleware

#### [asgi-auth-github](https://github.com/simonw/asgi-auth-github)

This middleware adds authentication to any ASGI application, requiring users to sign in
using their GitHub account (via [OAuth](https://developer.github.com/apps/building-oauth-apps/authorizing-oauth-apps/)).
Access can be restricted to specific users or to members of specific GitHub organizations or teams.

#### [asgi-csrf](https://github.com/simonw/asgi-csrf)

Middleware for protecting against CSRF attacks. This middleware implements the Double Submit Cookie pattern, where a cookie is set, then it is compared to a csrftoken hidden form field or an `x-csrftoken` HTTP header.

#### [AuthlibMiddleware](https://github.com/aogier/starlette-authlib)

A drop-in replacement for Starlette session middleware, using [authlib's jwt](https://docs.authlib.org/en/latest/jose/jwt.html)
module.

#### [BugsnagMiddleware](https://github.com/ashinabraham/starlette-bugsnag)

A middleware class for logging exceptions to [Bugsnag](https://www.bugsnag.com/).

#### [CSRFMiddleware](https://github.com/frankie567/starlette-csrf)

Middleware for protecting against CSRF attacks. This middleware implements the Double Submit Cookie pattern, where a cookie is set, then it is compared to an `x-csrftoken` HTTP header.

#### [EarlyDataMiddleware](https://github.com/HarrySky/starlette-early-data)

Middleware and decorator for detecting and denying [TLSv1.3 early data](https://tools.ietf.org/html/rfc8470) requests.

#### [PrometheusMiddleware](https://github.com/perdy/starlette-prometheus)

A middleware class for capturing Prometheus metrics related to requests and responses, including in progress requests, timing...

#### [ProxyHeadersMiddleware](https://github.com/encode/uvicorn/blob/main/uvicorn/middleware/proxy_headers.py)

Uvicorn includes a middleware class for determining the client IP address,
when proxy servers are being used, based on the `X-Forwarded-Proto` and `X-Forwarded-For` headers. For more complex proxy configurations, you might want to adapt this middleware.

#### [RateLimitMiddleware](https://github.com/abersheeran/asgi-ratelimit)

A rate limit middleware. Regular expression matches url; flexible rules; highly customizable. Very easy to use.

#### [RequestIdMiddleware](https://github.com/snok/asgi-correlation-id)

A middleware class for reading/generating request IDs and attaching them to application logs.

#### [RollbarMiddleware](https://docs.rollbar.com/docs/starlette)

A middleware class for logging exceptions, errors, and log messages to [Rollbar](https://www.rollbar.com).

#### [StarletteOpentracing](https://github.com/acidjunk/starlette-opentracing)

A middleware class that emits tracing info to [OpenTracing.io](https://opentracing.io/) compatible tracers and
can be used to profile and monitor distributed applications.

#### [SecureCookiesMiddleware](https://github.com/thearchitector/starlette-securecookies)

Customizable middleware for adding automatic cookie encryption and decryption to Starlette applications, with
extra support for existing cookie-based middleware.

#### [TimingMiddleware](https://github.com/steinnes/timing-asgi)

A middleware class to emit timing information (cpu and wall time) for each request which
passes through it.  Includes examples for how to emit these timings as statsd metrics.

#### [WSGIMiddleware](https://github.com/abersheeran/a2wsgi)

A middleware class in charge of converting a WSGI application into an ASGI one.