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Hit-for-miss (sw-dynamic-cache-bypass) + observability header #12

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Merged
roboshyim merged 1 commit into from
Jan 31, 2026
Merged

Hit-for-miss (sw-dynamic-cache-bypass) + observability header #12

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180 changes: 137 additions & 43 deletions src/cache.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -79,53 +79,77 @@ pub async fn handle_cached(
// BAN logic: if URL is banned, skip cache.
let banned = state.cache.is_banned(&norm_uri);

if !banned {
if let Some((resp, stale)) = lookup(&state.cache, &cache_key, &parts.headers, &norm_uri)? {
if !stale {
if banned {
// Observability: explicitly mark ban bypass.
} else if let Some((mut resp, decision)) =
lookup(&state.cache, &cache_key, &parts.headers, &norm_uri)?
{
match decision {
CacheDecision::Hit => {
resp.headers_mut().insert(
http::header::HeaderName::from_static("x-codycache"),
http::HeaderValue::from_static("HIT"),
);
return Ok(resp);
}

// Stale within grace: try to refresh from origin; serve stale only if origin fails.
match fetch_upstream_raw(&state, &parts, &norm_uri, body_bytes.clone()).await {
Ok((status, mut resp_headers, bytes)) => {
// Origin errors => serve stale
if status.is_server_error() {
let mut resp = resp;
CacheDecision::HitForMiss => {
// Varnish-like hit-for-miss: bypass cache for a short time.
let (status, resp_headers, bytes) =
fetch_upstream_raw(&state, &parts, &norm_uri, body_bytes.clone()).await?;

let mut out = build_response(status, resp_headers, bytes, &norm_uri);
out.headers_mut().insert(
http::header::HeaderName::from_static("x-codycache"),
http::HeaderValue::from_static("BYPASS"),
);
return Ok(out);
}
CacheDecision::Stale => {
// Stale within grace: try to refresh from origin; serve stale only if origin fails.
match fetch_upstream_raw(&state, &parts, &norm_uri, body_bytes.clone()).await {
Ok((status, mut resp_headers, bytes)) => {
// Origin errors => serve stale
if status.is_server_error() {
resp.headers_mut().insert(
http::header::HeaderName::from_static("x-codycache"),
http::HeaderValue::from_static("STALE"),
);
return Ok(resp);
}

let ttl = ttl_from_headers(&resp_headers).unwrap_or(Duration::from_secs(0));
let cacheable = ttl.as_secs() > 0
&& (parts.method == http::Method::GET
|| parts.method == http::Method::HEAD);

if cacheable {
resp_headers.remove(http::header::SET_COOKIE);
store(
&state.cache,
&cache_key,
&norm_uri,
status,
&resp_headers,
&bytes,
ttl,
)?;
}

let mut out = build_response(status, resp_headers, bytes, &norm_uri);
out.headers_mut().insert(
http::header::HeaderName::from_static("x-codycache"),
http::HeaderValue::from_static("REFRESH"),
);
return Ok(out);
}
Err(_e) => {
// Serve stale
resp.headers_mut().insert(
http::header::HeaderName::from_static("x-codycache"),
http::HeaderValue::from_static("STALE"),
);
return Ok(resp);
}

let ttl = ttl_from_headers(&resp_headers).unwrap_or(Duration::from_secs(0));
let cacheable = ttl.as_secs() > 0
&& (parts.method == http::Method::GET
|| parts.method == http::Method::HEAD);

if cacheable {
resp_headers.remove(http::header::SET_COOKIE);
store(
&state.cache,
&cache_key,
&norm_uri,
status,
&resp_headers,
&bytes,
ttl,
)?;
}

return Ok(build_response(status, resp_headers, bytes, &norm_uri));
}
Err(_e) => {
// Serve stale
let mut resp = resp;
resp.headers_mut().insert(
http::header::HeaderName::from_static("x-codycache"),
http::HeaderValue::from_static("STALE"),
);
return Ok(resp);
}
}
}
Expand All @@ -147,9 +171,22 @@ pub async fn handle_cached(
== Some("1")
{
resp_headers.remove("sw-dynamic-cache-bypass");
return Ok(build_response(status, resp_headers, bytes, &norm_uri));

// Store a short-lived hit-for-miss marker so subsequent requests bypass cache
// and we don't repeatedly attempt to cache this URL.
let hfm_ttl = Duration::from_secs(1);
store_hit_for_miss(&state.cache, &cache_key, &norm_uri, hfm_ttl)?;

let mut out = build_response(status, resp_headers, bytes, &norm_uri);
out.headers_mut().insert(
http::header::HeaderName::from_static("x-codycache"),
http::HeaderValue::from_static("BYPASS"),
);
return Ok(out);
}

let mut cache_status = if banned { "BAN" } else { "MISS" };

if cacheable {
// Strip Set-Cookie on cacheable responses
resp_headers.remove(http::header::SET_COOKIE);
Expand All @@ -163,9 +200,15 @@ pub async fn handle_cached(
&bytes,
ttl,
)?;
cache_status = if banned { "BAN" } else { "MISS" };
}

Ok(build_response(status, resp_headers, bytes, &norm_uri))
let mut out = build_response(status, resp_headers, bytes, &norm_uri);
out.headers_mut().insert(
http::header::HeaderName::from_static("x-codycache"),
http::HeaderValue::from_static(cache_status),
);
Ok(out)
}

fn build_cache_key(uri: &Uri, headers: &HeaderMap) -> String {
Expand Down Expand Up @@ -201,12 +244,19 @@ fn extract_cookie(headers: &HeaderMap, name: &str) -> Option<String> {
None
}

#[derive(Copy, Clone, Debug, Eq, PartialEq)]
enum CacheDecision {
Hit,
Stale,
HitForMiss,
}

fn lookup(
cache: &Cache,
key: &str,
req_headers: &HeaderMap,
uri: &Uri,
) -> Result<Option<(axum::response::Response, bool)>, String> {
) -> Result<Option<(axum::response::Response, CacheDecision)>, String> {
let inner = cache.inner.read();
let Some((meta, body)) = inner.disk.get(key)? else {
return Ok(None);
Expand All @@ -223,6 +273,16 @@ fn lookup(

let stale = !fresh;

// Hit-for-miss marker: do not serve, but signal caller to bypass cache for a short time.
if meta.hit_for_miss {
// Build a minimal response (unused body) so we can reuse some header logic if desired.
let resp = axum::response::Response::builder()
.status(http::StatusCode::OK)
.body(axum::body::Body::empty())
.unwrap();
return Ok(Some((resp, CacheDecision::HitForMiss)));
}

// VCL hit logic: pass if client states matches invalidation states
if let (Some(req_states), Some(obj_states)) = (
extract_cookie(req_headers, "sw-states"),
Expand All @@ -248,7 +308,12 @@ fn lookup(
let mut resp = resp;
*resp.headers_mut() = headers;

Ok(Some((resp, stale)))
let decision = if stale {
CacheDecision::Stale
} else {
CacheDecision::Hit
};
Ok(Some((resp, decision)))
}

async fn fetch_upstream_raw(
Expand Down Expand Up @@ -292,6 +357,34 @@ fn store(
headers: &HeaderMap,
body: &Bytes,
ttl: Duration,
) -> Result<(), String> {
store_inner(cache, key, url, status, headers, body, ttl, false)
}

fn store_hit_for_miss(cache: &Cache, key: &str, url: &Uri, ttl: Duration) -> Result<(), String> {
let empty_headers = HeaderMap::new();
store_inner(
cache,
key,
url,
http::StatusCode::OK,
&empty_headers,
&Bytes::new(),
ttl,
true,
)
}

#[allow(clippy::too_many_arguments)]
fn store_inner(
cache: &Cache,
key: &str,
url: &Uri,
status: http::StatusCode,
headers: &HeaderMap,
body: &Bytes,
ttl: Duration,
hit_for_miss: bool,
) -> Result<(), String> {
let grace = Duration::from_secs(60 * 60 * 24 * 3);

Expand All @@ -312,6 +405,7 @@ fn store(

let meta = disk::StoredMeta {
url: url.to_string(),
hit_for_miss,
stored_at_ms: disk::now_ms(),
ttl_ms: ttl.as_millis() as u64,
grace_ms: grace.as_millis() as u64,
Expand Down
6 changes: 6 additions & 0 deletions src/disk.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -24,6 +24,12 @@ pub struct StoredMeta {
#[serde(default)]
pub url: String,

/// Varnish-style hit-for-miss marker: entry exists but should never be served.
///
/// If true, lookups treat this as a cache bypass for a short TTL.
#[serde(default)]
pub hit_for_miss: bool,

pub stored_at_ms: u64,
pub ttl_ms: u64,
pub grace_ms: u64,
Expand Down