pingora_proxy/

proxy_trait.rs

// Copyright 2026 Cloudflare, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

use super::*;
use pingora_cache::{
    key::HashBinary,
    CacheKey, CacheMeta, ForcedFreshness, HitHandler, PurgeAction,
    RespCacheable::{self, *},
};
use proxy_cache::range_filter::{self};
use std::any::Any;
use std::time::Duration;

/// Context for proxy warning logs that can be suppressed by
/// [`ProxyHttp::suppress_proxy_warn_log`].
///
/// These contexts are distinct from final proxy errors, which are handled by
/// [`ProxyHttp::suppress_error_log`].
///
/// Experimental: this API may change or be removed until indicated otherwise.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
#[non_exhaustive]
pub enum ProxyWarnLogContext {
    /// A proxy upstream attempt failed with a retryable error.
    UpstreamRetry,
    /// A downstream error was ignored so cache fill could continue.
    DownstreamCache,
}

/// The interface to control the HTTP proxy
///
/// The methods in [ProxyHttp] are filters/callbacks which will be performed on all requests at their
/// particular stage (if applicable).
///
/// If any of the filters returns [Result::Err], the request will fail, and the error will be logged.
#[cfg_attr(not(doc_async_trait), async_trait)]
pub trait ProxyHttp {
    /// The per request object to share state across the different filters
    type CTX;

    /// Define how the `ctx` should be created.
    fn new_ctx(&self) -> Self::CTX;

    /// Define where the proxy should send the request to.
    ///
    /// The returned [HttpPeer] contains the information regarding where and how this request should
    /// be forwarded to.
    async fn upstream_peer(
        &self,
        session: &mut Session,
        ctx: &mut Self::CTX,
    ) -> Result<Box<HttpPeer>>;

    /// Set up downstream modules.
    ///
    /// In this phase, users can add or configure [HttpModules] before the server starts up.
    ///
    /// In the default implementation of this method, [ResponseCompressionBuilder] is added
    /// and disabled.
    fn init_downstream_modules(&self, modules: &mut HttpModules) {
        // Add disabled downstream compression module by default
        modules.add_module(ResponseCompressionBuilder::enable(0));
    }

    /// Set up upstream modules.
    ///
    /// In this phase, users can add [HttpModules] that will process upstream responses
    /// **before** `upstream_compression`. This is the correct place to register modules
    /// that need to observe the raw (pre-compression) upstream response body, such as
    /// a dictionary store for shared dictionary compression.
    ///
    /// Upstream modules are ordered by [`HttpModuleBuilder::order()`]: higher values run
    /// first. They are invoked on each upstream response task (header, body, trailers)
    /// before `upstream_compression` processes the task.
    ///
    /// By default this method does nothing.
    ///
    /// This method requires the `upstream_modules` feature to be enabled.
    #[cfg(feature = "upstream_modules")]
    fn init_upstream_modules(&self, _modules: &mut HttpModules) {}

    /// Handle the incoming request.
    ///
    /// In this phase, users can parse, validate, rate limit, perform access control and/or
    /// return a response for this request.
    ///
    /// If the user already sent a response to this request, an `Ok(true)` should be returned so that
    /// the proxy would exit. The proxy continues to the next phases when `Ok(false)` is returned.
    ///
    /// By default this filter does nothing and returns `Ok(false)`.
    async fn request_filter(&self, _session: &mut Session, _ctx: &mut Self::CTX) -> Result<bool>
    where
        Self::CTX: Send + Sync,
    {
        Ok(false)
    }

    /// Handle the incoming request before any downstream module is executed.
    ///
    /// This function is similar to [Self::request_filter()] but executes before any other logic,
    /// including downstream module logic. The main purpose of this function is to provide finer
    /// grained control of the behavior of the modules.
    ///
    /// Note that because this function is executed before any module that might provide access
    /// control or rate limiting, logic should stay in request_filter() if it can in order to be
    /// protected by said modules.
    async fn early_request_filter(&self, _session: &mut Session, _ctx: &mut Self::CTX) -> Result<()>
    where
        Self::CTX: Send + Sync,
    {
        Ok(())
    }

    /// Returns whether this session is allowed to spawn subrequests.
    ///
    /// This function is checked after [Self::early_request_filter] to allow that filter to configure
    /// this if required. This will also run for subrequests themselves, which may allowed to spawn
    /// their own subrequests.
    ///
    /// Note that this doesn't prevent subrequests from being spawned based on the session by proxy
    /// core functionality, e.g. background cache revalidation requires spawning subrequests.
    fn allow_spawning_subrequest(&self, _session: &Session, _ctx: &Self::CTX) -> bool
    where
        Self::CTX: Send + Sync,
    {
        false
    }

    /// Handle the incoming request body.
    ///
    /// This function will be called every time a piece of request body is received. The `body` is
    /// **not the entire request body**.
    ///
    /// The async nature of this function allows to throttle the upload speed and/or executing
    /// heavy computation logic such as WAF rules on offloaded threads without blocking the threads
    /// who process the requests themselves.
    async fn request_body_filter(
        &self,
        _session: &mut Session,
        _body: &mut Option<Bytes>,
        _end_of_stream: bool,
        _ctx: &mut Self::CTX,
    ) -> Result<()>
    where
        Self::CTX: Send + Sync,
    {
        Ok(())
    }

    /// This filter decides if the request is cacheable and what cache backend to use
    ///
    /// The caller can interact with `Session.cache` to enable caching.
    ///
    /// By default this filter does nothing which effectively disables caching.
    // Ideally only session.cache should be modified, TODO: reflect that in this interface
    fn request_cache_filter(&self, _session: &mut Session, _ctx: &mut Self::CTX) -> Result<()>
    where
        Self::CTX: Send + Sync,
    {
        Ok(())
    }

    /// This callback generates the cache key.
    ///
    /// This callback is called only when cache is enabled for this request.
    ///
    /// There is no sensible default cache key for all proxy applications. The
    /// correct key depends on which request properties affect upstream responses
    /// (e.g. `Vary` headers, custom request filters that modify the origin host).
    /// Getting this wrong leads to cache poisoning.
    ///
    /// See `pingora-proxy/tests/utils/server_utils.rs` for a minimal (not
    /// production-ready) reference implementation.
    ///
    /// # Panics
    ///
    /// The default implementation panics. You **must** override this method when
    /// caching is enabled.
    fn cache_key_callback(&self, _session: &Session, _ctx: &mut Self::CTX) -> Result<CacheKey> {
        unimplemented!("cache_key_callback must be implemented when caching is enabled")
    }

    /// This callback is invoked when a cacheable response is ready to be admitted to cache.
    fn cache_miss(&self, session: &mut Session, _ctx: &mut Self::CTX) {
        session.cache.cache_miss();
    }

    /// This filter is called after a successful cache lookup and before the
    /// cache asset is ready to be used.
    ///
    /// This filter allows the user to log or force invalidate the asset, or
    /// to adjust the body reader associated with the cache hit.
    /// This also runs on stale hit assets (for which `is_fresh` is false).
    ///
    /// The value returned indicates if the force invalidation should be used,
    /// and which kind. Returning `None` indicates no forced invalidation
    async fn cache_hit_filter(
        &self,
        _session: &mut Session,
        _meta: &CacheMeta,
        _hit_handler: &mut HitHandler,
        _is_fresh: bool,
        _ctx: &mut Self::CTX,
    ) -> Result<Option<ForcedFreshness>>
    where
        Self::CTX: Send + Sync,
    {
        Ok(None)
    }

    /// Decide if a request should continue to upstream after not being served from cache.
    ///
    /// returns: Ok(true) if the request should continue, Ok(false) if a response was written by the
    /// callback and the session should be finished, or an error
    ///
    /// This filter can be used for deferring checks like rate limiting or access control to when they
    /// actually needed after cache miss.
    ///
    /// By default the session will attempt to be reused after returning Ok(false). It is the
    /// caller's responsibility to disable keepalive or drain the request body if needed.
    async fn proxy_upstream_filter(
        &self,
        _session: &mut Session,
        _ctx: &mut Self::CTX,
    ) -> Result<bool>
    where
        Self::CTX: Send + Sync,
    {
        Ok(true)
    }

    /// Decide if the response is cacheable
    fn response_cache_filter(
        &self,
        _session: &Session,
        _resp: &ResponseHeader,
        _ctx: &mut Self::CTX,
    ) -> Result<RespCacheable> {
        Ok(Uncacheable(NoCacheReason::Custom("default")))
    }

    /// Decide how to generate cache vary key from both request and response
    ///
    /// None means no variance is needed.
    fn cache_vary_filter(
        &self,
        _meta: &CacheMeta,
        _ctx: &mut Self::CTX,
        _req: &RequestHeader,
    ) -> Option<HashBinary> {
        // default to None for now to disable vary feature
        None
    }

    /// Decide if the incoming request's condition _fails_ against the cached response.
    ///
    /// Returning `Ok(true)` means that the response does _not_ match against the condition, and
    /// that the proxy can return `304 Not Modified` downstream.
    ///
    /// An example is a conditional GET request with `If-None-Match: "foobar"`. If the cached
    /// response contains the `ETag: "foobar"`, then the condition fails, and `304 Not Modified`
    /// should be returned. Else, the condition passes which means the full `200 OK` response must
    /// be sent.
    fn cache_not_modified_filter(
        &self,
        session: &Session,
        resp: &ResponseHeader,
        _ctx: &mut Self::CTX,
    ) -> Result<bool> {
        Ok(
            pingora_core::protocols::http::conditional_filter::not_modified_filter(
                session.req_header(),
                resp,
            ),
        )
    }

    /// This filter is called when cache is enabled to determine what byte range to return (in both
    /// cache hit and miss cases) from the response body. It is only used when caching is enabled,
    /// otherwise the upstream is responsible for any filtering. It allows users to define the range
    /// this request is for via its return type `range_filter::RangeType`.
    ///
    /// It also allow users to modify the response header accordingly.
    ///
    /// The default implementation can handle a single-range as per [RFC7232].
    ///
    /// [RFC7232]: https://www.rfc-editor.org/rfc/rfc7232
    fn range_header_filter(
        &self,
        session: &mut Session,
        resp: &mut ResponseHeader,
        _ctx: &mut Self::CTX,
    ) -> range_filter::RangeType {
        const DEFAULT_MAX_RANGES: Option<usize> = Some(200);
        proxy_cache::range_filter::range_header_filter(
            session.req_header(),
            resp,
            DEFAULT_MAX_RANGES,
        )
    }

    /// Modify the request before it is sent to the upstream.
    ///
    /// Unlike [Self::request_filter()], this filter allows changing the request headers sent to
    /// the upstream. Automatic upstream request-header policy configured on the selected peer is
    /// applied before this callback. Headers deliberately added by this callback are treated as
    /// application-controlled upstream behavior, including framing and protocol-upgrade fields.
    /// For an HTTP/1 upstream, if the downstream request has a non-empty body and this callback
    /// leaves neither `Content-Length` nor `Transfer-Encoding`, Pingora adds
    /// `Transfer-Encoding: chunked`.
    async fn upstream_request_filter(
        &self,
        _session: &mut Session,
        _upstream_request: &mut RequestHeader,
        _ctx: &mut Self::CTX,
    ) -> Result<()>
    where
        Self::CTX: Send + Sync,
    {
        Ok(())
    }

    /// Adjust upstream modules before they process the response header.
    ///
    /// This filter is called when the upstream response header arrives, before upstream modules
    /// (such as `upstream_compression`) run their response header filter. Use this to configure
    /// module behavior based on the response, e.g. setting a dictionary for dictionary-based
    /// content encoding.
    ///
    /// This filter may be called more than once per request if the upstream sends informational
    /// (1xx) response headers before the final response. Implementations can check
    /// [`upstream_response.status.is_informational()`](http::StatusCode::is_informational) to
    /// distinguish informational headers from the final response if needed.
    ///
    /// `end_of_stream` indicates whether the response header is also the end of the response
    /// (e.g. for HEAD responses or 304s with no body).
    ///
    /// The response header is provided as an immutable reference. To modify the response header
    /// itself, use [`Self::upstream_response_filter()`] instead.
    ///
    /// This filter requires the `upstream_modules` feature to be enabled.
    #[cfg(feature = "upstream_modules")]
    async fn adjust_upstream_modules(
        &self,
        _session: &mut Session,
        _upstream_response: &ResponseHeader,
        _end_of_stream: bool,
        _ctx: &mut Self::CTX,
    ) -> Result<()>
    where
        Self::CTX: Send + Sync,
    {
        Ok(())
    }

    /// Modify the response header from the upstream
    ///
    /// The modification is before caching, so any change here will be stored in the cache if enabled.
    ///
    /// Responses served from cache won't trigger this filter. If the cache needed revalidation,
    /// only the 304 from upstream will trigger the filter (though it will be merged into the
    /// cached header, not served directly to downstream).
    async fn upstream_response_filter(
        &self,
        _session: &mut Session,
        _upstream_response: &mut ResponseHeader,
        _ctx: &mut Self::CTX,
    ) -> Result<()>
    where
        Self::CTX: Send + Sync,
    {
        Ok(())
    }

    /// Modify the response header before it is send to the downstream
    ///
    /// The modification is after caching. This filter is called for all responses including
    /// responses served from cache.
    async fn response_filter(
        &self,
        _session: &mut Session,
        _upstream_response: &mut ResponseHeader,
        _ctx: &mut Self::CTX,
    ) -> Result<()>
    where
        Self::CTX: Send + Sync,
    {
        Ok(())
    }

    // custom_forwarding is called when downstream and upstream connections are successfully established.
    #[doc(hidden)]
    async fn custom_forwarding(
        &self,
        _session: &mut Session,
        _ctx: &mut Self::CTX,
        _custom_message_to_upstream: Option<mpsc::Sender<Bytes>>,
        _custom_message_to_downstream: mpsc::Sender<Bytes>,
    ) -> Result<()>
    where
        Self::CTX: Send + Sync,
    {
        Ok(())
    }

    // received a custom message from the downstream before sending it to the upstream.
    #[doc(hidden)]
    async fn downstream_custom_message_proxy_filter(
        &self,
        _session: &mut Session,
        custom_message: Bytes,
        _ctx: &mut Self::CTX,
        _final_hop: bool,
    ) -> Result<Option<Bytes>>
    where
        Self::CTX: Send + Sync,
    {
        Ok(Some(custom_message))
    }

    // received a custom message from the upstream before sending it to the downstream.
    #[doc(hidden)]
    async fn upstream_custom_message_proxy_filter(
        &self,
        _session: &mut Session,
        custom_message: Bytes,
        _ctx: &mut Self::CTX,
        _final_hop: bool,
    ) -> Result<Option<Bytes>>
    where
        Self::CTX: Send + Sync,
    {
        Ok(Some(custom_message))
    }

    /// Similar to [Self::upstream_response_filter()] but for response body
    ///
    /// This function will be called every time a piece of response body is received. The `body` is
    /// **not the entire response body**.
    fn upstream_response_body_filter(
        &self,
        _session: &mut Session,
        _body: &mut Option<Bytes>,
        _end_of_stream: bool,
        _ctx: &mut Self::CTX,
    ) -> Result<Option<Duration>> {
        Ok(None)
    }

    /// Similar to [Self::upstream_response_filter()] but for response trailers
    fn upstream_response_trailer_filter(
        &self,
        _session: &mut Session,
        _upstream_trailers: &mut header::HeaderMap,
        _ctx: &mut Self::CTX,
    ) -> Result<()> {
        Ok(())
    }

    /// Similar to [Self::response_filter()] but for response body chunks
    fn response_body_filter(
        &self,
        _session: &mut Session,
        _body: &mut Option<Bytes>,
        _end_of_stream: bool,
        _ctx: &mut Self::CTX,
    ) -> Result<Option<Duration>>
    where
        Self::CTX: Send + Sync,
    {
        Ok(None)
    }

    /// Similar to [Self::response_filter()] but for response trailers.
    /// Note, returning an Ok(Some(Bytes)) will result in the downstream response
    /// trailers being written to the response body.
    ///
    /// TODO: make this interface more intuitive
    async fn response_trailer_filter(
        &self,
        _session: &mut Session,
        _upstream_trailers: &mut header::HeaderMap,
        _ctx: &mut Self::CTX,
    ) -> Result<Option<Bytes>>
    where
        Self::CTX: Send + Sync,
    {
        Ok(None)
    }

    /// This filter is called when the entire response is sent to the downstream successfully or
    /// there is a fatal error that terminate the request.
    ///
    /// An error log is already emitted if there is any error. This phase is used for collecting
    /// metrics and sending access logs.
    async fn logging(&self, _session: &mut Session, _e: Option<&Error>, _ctx: &mut Self::CTX)
    where
        Self::CTX: Send + Sync,
    {
    }

    /// Called after [`Self::logging`] when the downstream connection will be reused for another
    /// HTTP/1.x keepalive request. The returned value, if any, will be carried to the next
    /// request on this connection and delivered via [`Self::on_connection_reuse`].
    ///
    /// Use this to persist debugging or timing information across keepalive requests.
    /// This is only called for HTTP/1.x keepalive connections, not for HTTP/2.
    /// It is also called on error paths when the downstream connection is eligible for reuse.
    ///
    /// The default implementation returns `None` (no context persisted).
    fn persist_connection_context(
        &self,
        _session: &Session,
        _ctx: &Self::CTX,
    ) -> Option<Box<dyn Any + Send + Sync>> {
        None
    }

    /// Called at the start of a new request on a reused HTTP/1.x keepalive connection,
    /// before [`Self::early_request_filter`]. The `prev_ctx` argument is the value returned
    /// by [`Self::persist_connection_context`] from the previous request on this connection.
    ///
    /// This is only called for HTTP/1.x keepalive connections, not for HTTP/2.
    /// It is not called when `persist_connection_context` returned `None` on the previous request.
    ///
    /// Use this to transfer state from the previous request into the new request's context.
    fn on_connection_reuse(
        &self,
        _session: &mut Session,
        _ctx: &mut Self::CTX,
        _prev_ctx: Box<dyn Any + Send + Sync>,
    ) {
    }

    /// A value of true means that the log message will be suppressed. The default value is false.
    ///
    /// See also: [`Self::suppress_proxy_warn_log`].
    fn suppress_error_log(&self, _session: &Session, _ctx: &Self::CTX, _error: &Error) -> bool {
        false
    }

    /// A value of true means that the proxy warning log message will be suppressed.
    /// The default value is false.
    ///
    /// This hook currently applies to retryable proxy upstream failures and downstream errors
    /// ignored while cache fill continues. Final proxy errors are still handled by
    /// [`Self::suppress_error_log`].
    ///
    /// Suppressing retry warning logs can remove the only per-retry audit record. Callers that
    /// suppress these logs should provide alternative observability, such as metrics or logs in
    /// their implementation of this hook.
    ///
    /// This hook runs inline on retry and cache-error paths, so implementations should be cheap.
    ///
    /// Experimental: this API may change or be removed until indicated otherwise.
    fn suppress_proxy_warn_log(
        &self,
        _session: &Session,
        _ctx: &Self::CTX,
        _error: &Error,
        _context: ProxyWarnLogContext,
    ) -> bool {
        false
    }

    /// This filter is called when there is an error **after** a connection is established (or reused)
    /// to the upstream.
    ///
    /// By default, this hook forces retry to false, regardless of the incoming retry state, when
    /// the request method is non-idempotent or the body retry buffer was truncated. For eligible
    /// requests, [`pingora_error::RetryType::ReusedOnly`] errors are retried only on a reused
    /// connection.
    ///
    /// Implementations that override this hook replace the default policy and are responsible for
    /// deciding when a retry is safe.
    fn error_while_proxy(
        &self,
        peer: &HttpPeer,
        session: &mut Session,
        e: Box<Error>,
        _ctx: &mut Self::CTX,
        client_reused: bool,
    ) -> Box<Error> {
        let mut e = e.more_context(format!("Peer: {}", peer));
        if !session.req_header().method.is_idempotent() || session.as_ref().retry_buffer_truncated()
        {
            e.set_retry(false);
        } else {
            e.retry.decide_reuse(client_reused);
        }
        e
    }

    /// This filter is called when there is an error in the process of establishing a connection
    /// to the upstream.
    ///
    /// In this filter the user can decide whether the error is retry-able by marking the error `e`.
    ///
    /// If the error can be retried, [Self::upstream_peer()] will be called again so that the user
    /// can decide whether to send the request to the same upstream or another upstream that is possibly
    /// available.
    fn fail_to_connect(
        &self,
        _session: &mut Session,
        _peer: &HttpPeer,
        _ctx: &mut Self::CTX,
        e: Box<Error>,
    ) -> Box<Error> {
        e
    }

    /// This filter is called when the request encounters a fatal error.
    ///
    /// Users may write an error response to the downstream if the downstream is still writable.
    ///
    /// The response status code of the error response may be returned for logging purposes.
    /// Additionally, the user can return whether this session may be reused in spite of the error.
    /// Today this reuse status is only respected for errors that occur prior to upstream peer
    /// selection, and the keepalive configured on the `Session` itself still takes precedent.
    async fn fail_to_proxy(
        &self,
        session: &mut Session,
        e: &Error,
        _ctx: &mut Self::CTX,
    ) -> FailToProxy
    where
        Self::CTX: Send + Sync,
    {
        let code = match e.etype() {
            HTTPStatus(code) => *code,
            _ => {
                match e.esource() {
                    ErrorSource::Upstream => 502,
                    ErrorSource::Downstream => {
                        match e.etype() {
                            WriteError | ReadError | ConnectionClosed => {
                                /* conn already dead */
                                0
                            }
                            _ => 400,
                        }
                    }
                    ErrorSource::Internal | ErrorSource::Unset => 500,
                }
            }
        };
        if code > 0 {
            session.respond_error(code).await.unwrap_or_else(|e| {
                error!("failed to send error response to downstream: {e}");
            });
        }

        FailToProxy {
            error_code: code,
            // default to no reuse, which is safest
            can_reuse_downstream: false,
        }
    }

    /// Decide whether should serve stale when encountering an error or during revalidation
    ///
    /// An implementation should follow
    /// <https://datatracker.ietf.org/doc/html/rfc9111#section-4.2.4>
    /// <https://www.rfc-editor.org/rfc/rfc5861#section-4>
    ///
    /// This filter is only called if cache is enabled.
    // 5xx HTTP status will be encoded as ErrorType::HTTPStatus(code)
    fn should_serve_stale(
        &self,
        _session: &mut Session,
        _ctx: &mut Self::CTX,
        error: Option<&Error>, // None when it is called during stale while revalidate
    ) -> bool {
        // A cache MUST NOT generate a stale response unless
        // it is disconnected
        // or doing so is explicitly permitted by the client or origin server
        // (e.g. headers or an out-of-band contract)
        error.is_some_and(|e| e.esource() == &ErrorSource::Upstream)
    }

    /// This filter is called when the request just established or reused a connection to the upstream
    ///
    /// This filter allows user to log timing and connection related info.
    async fn connected_to_upstream(
        &self,
        _session: &mut Session,
        _reused: bool,
        _peer: &HttpPeer,
        #[cfg(unix)] _fd: std::os::unix::io::RawFd,
        #[cfg(windows)] _sock: std::os::windows::io::RawSocket,
        _digest: Option<&Digest>,
        _ctx: &mut Self::CTX,
    ) -> Result<()>
    where
        Self::CTX: Send + Sync,
    {
        Ok(())
    }

    /// This callback is invoked every time request related error log needs to be generated
    ///
    /// Users can define what is important to be written about this request via the returned string.
    fn request_summary(&self, session: &Session, _ctx: &Self::CTX) -> String {
        session.as_ref().request_summary()
    }

    /// Whether the request should be used to invalidate(delete) the HTTP cache
    ///
    /// - `true`: this request will be used to invalidate the cache.
    /// - `false`: this request is a treated as a normal request
    fn is_purge(&self, _session: &Session, _ctx: &Self::CTX) -> bool {
        false
    }

    /// What a purge request should do to the cached asset.
    ///
    /// Only consulted when [`ProxyHttp::is_purge`] returns `true`. The default deletes the asset.
    /// Returning [`PurgeAction::Expire`] asks to keep it and mark it stale instead, so it
    /// revalidates against the origin rather than being refetched in full. Storage that cannot
    /// mark an entry stale falls back to deleting it.
    fn purge_action(&self, _session: &Session, _ctx: &Self::CTX) -> PurgeAction {
        PurgeAction::Delete
    }

    /// This filter is called after the proxy cache generates the downstream response to the purge
    /// request (to invalidate or delete from the HTTP cache), based on the purge status, which
    /// indicates whether the request succeeded or failed.
    ///
    /// The filter allows the user to modify or replace the generated downstream response.
    /// If the filter returns `Err`, the proxy will instead send a 500 response.
    fn purge_response_filter(
        &self,
        _session: &Session,
        _ctx: &mut Self::CTX,
        _purge_status: PurgeStatus,
        _purge_response: &mut std::borrow::Cow<'static, ResponseHeader>,
    ) -> Result<()> {
        Ok(())
    }
}

/// Context struct returned by `fail_to_proxy`.
pub struct FailToProxy {
    pub error_code: u16,
    pub can_reuse_downstream: bool,
}