dashboard: snapshot lazy

1 files changed, 47 insertions, 18 deletions

diff --git a/src/api/dashboard/mod.rs b/src/api/dashboard/mod.rs
index aae514a..4ba25c7 100644
--- a/src/api/dashboard/mod.rs
+++ b/src/api/dashboard/mod.rs
@@ -56,10 +56,40 @@ const LINGER: Duration = Duration::from_secs(60);
 /// Members asked for their links at most, each round.
 const MAX_ASKED: usize = 512;
 
-/// A snapshot or a patch as published: its number, and its JSON.
+/// A snapshot or a patch as published: its number, and its JSON. A snapshot's text is
+/// made when first asked for (a client connecting, or one that fell behind), not every
+/// time the state changes: the whole state may be large, and mostly nobody new comes.
 struct Published {
     seq: u64,
-    json: String,
+    value: Option<Arc<JsonValue>>,
+    json: std::sync::OnceLock<String>,
+}
+
+impl Published {
+    fn patch(seq: u64, patch: &JsonValue) -> Self {
+        Self {
+            seq,
+            value: None,
+            json: std::sync::OnceLock::from(patch.to_string()),
+        }
+    }
+
+    fn snapshot(seq: u64, snapshot: Arc<JsonValue>) -> Self {
+        Self {
+            seq,
+            value: Some(snapshot),
+            json: std::sync::OnceLock::new(),
+        }
+    }
+
+    fn json(&self) -> &str {
+        self.json.get_or_init(|| {
+            self.value
+                .as_ref()
+                .map(|value| value.to_string())
+                .unwrap_or_default()
+        })
+    }
 }
 
 /// The producer and what it published, shared by all clients.
@@ -146,7 +176,9 @@ impl Hub {
     }
 
     async fn run(self: Arc<Self>) {
-        let mut last: Option<JsonValue> = None;
+        // The latest snapshot, and it without its ages (`without_ages`): what the next
+        // is told apart from.
+        let mut last: Option<(Arc<JsonValue>, JsonValue)> = None;
         let mut seq = 0u64;
         let mut graph = Graph::default();
         let mut graph_due = Instant::now();
@@ -162,24 +194,21 @@ impl Hub {
                 graph_due = Instant::now() + LINKS_EVERY;
             }
             let snapshot = build(&self.state, &graph, live).await;
-            let changed = last
-                .as_ref()
-                .is_none_or(|old| without_ages(old) != without_ages(&snapshot));
+            let stripped = without_ages(&snapshot);
+            let changed = last.as_ref().is_none_or(|(_, old)| *old != stripped);
             if changed {
                 seq += 1;
-                if let Some(old) = &last
+                if let Some((old, _)) = &last
                     && let Some(patch) = merge_patch(old, &snapshot)
                 {
-                    let _ = self.patches.send(Arc::new(Published {
-                        seq,
-                        json: patch.to_string(),
-                    }));
+                    let _ = self.patches.send(Arc::new(Published::patch(seq, &patch)));
                 }
-                self.latest.send_replace(Some(Arc::new(Published {
+                let snapshot = Arc::new(snapshot);
+                self.latest.send_replace(Some(Arc::new(Published::snapshot(
                     seq,
-                    json: snapshot.to_string(),
-                })));
-                last = Some(snapshot);
+                    Arc::clone(&snapshot),
+                ))));
+                last = Some((snapshot, stripped));
             }
             let poked = tokio::select! {
                 () = tokio::time::sleep(TICK) => false,
@@ -632,7 +661,7 @@ async fn current_state(Extension(hub): Extension<Arc<Hub>>) -> Response {
     };
     (
         [(CONTENT_TYPE, HeaderValue::from_static("application/json"))],
-        current.json.clone(),
+        current.json().to_owned(),
     )
         .into_response()
 }
@@ -694,7 +723,7 @@ async fn events(Extension(hub): Extension<Arc<Hub>>) -> impl IntoResponse {
                     yield Ok(Event::default()
                         .event("patch")
                         .id(patch.seq.to_string())
-                        .data(patch.json.clone()));
+                        .data(patch.json()));
                 }
                 Err(broadcast::error::RecvError::Lagged(_)) => {
                     let Some(current) = hub.current().await else {
@@ -724,7 +753,7 @@ fn snapshot_event(published: &Published) -> Event {
     Event::default()
         .event("snapshot")
         .id(published.seq.to_string())
-        .data(published.json.clone())
+        .data(published.json())
 }
 
 #[cfg(test)]