diff --git a/libs/sk_ps_discovery/src/lib.rs b/libs/sk_ps_discovery/src/lib.rs
index 069d411af9..0fbf21ab24 100644
--- a/libs/sk_ps_discovery/src/lib.rs
+++ b/libs/sk_ps_discovery/src/lib.rs
@@ -18,14 +18,23 @@ use utils::{
 #[derive(Debug, Default)]
 pub struct World {
     attachments: BTreeMap<TenantShardAttachmentId, NodeId>,
+    attachment_count: HashMap<TenantId, u16>,
+    // continously maintained aggregate for efficient decisionmaking on quiescing;
+    // quiesced timelines are always caught up
+    // can quiesce one == attachment_count (TODO: this requires enforcing foreign key relationship between attachments and remote_consistent_lsn)
+    caught_up_count: HashMap<TenantTimelineId, u16>,
 
+    // BEGIN quiescing/active split
     quiesced_timelines: BTreeMap<TenantTimelineId, Lsn>,
     // ^
     // either a timeline is in quiesced_timelines
-    // or it is in commit_lsns + remote_consistent_lsns
+    // or it is below
     // v
     commit_lsns: BTreeMap<TenantTimelineId, Lsn>,
     remote_consistent_lsns: BTreeMap<TimelineAttachmentId, Lsn>,
+    // END quiescing/active split
+
+    // other fields
 }
 
 #[derive(Debug, PartialEq, Eq, Hash, Clone, Copy, PartialOrd, Ord)]
@@ -97,6 +106,12 @@ impl World {
             }
             (Attach { ps_id }, Vacant(e)) => {
                 e.insert(ps_id);
+                // Keep attachmount_count up to date
+                let attachment_count = self
+                    .attachment_count
+                    .entry(tenant_shard_attachment_id.tenant_id)
+                    .or_default();
+                *attachment_count += attachment_count.checked_add(1).unwrap();
                 // New shards may start at an older LSN than where we quiesced => activate all quiesced timelines.
                 let activate_range =
                     TenantTimelineId::tenant_range(tenant_shard_attachment_id.tenant_id);
@@ -111,6 +126,12 @@ impl World {
             }
             (Detach, Occupied(e)) => {
                 e.remove();
+                // Keep attachment count up to date
+                let attachment_count = self
+                    .attachment_count
+                    .get_mut(&tenant_shard_attachment_id.tenant_id)
+                    .expect("attachment action initializes the hasmap entry");
+                *attachment_count = attachment_count.checked_sub(1).unwrap();
             }
             (Detach, Vacant(_)) => {
                 info!("detachment is already known");
@@ -132,6 +153,16 @@ impl World {
                 match (*current).cmp(&remote_consistent_lsn) {
                     Less => {
                         *current = remote_consistent_lsn;
+                        let caught_up_count = self
+                            .caught_up_count
+                            .get_mut(&attachment.tenant_timeline_id)
+                            .unwrap();
+                        *caught_up_count = caught_up_count.checked_add(1).unwrap();
+                        if *caught_up_count
+                            == self.attachment_count[&attachment.tenant_timeline_id.tenant_id]
+                        {
+                            self.quiesce_timeline(attachment.tenant_timeline_id);
+                        }
                     }
                     Equal => {
                         info!("ignoring no-op update, likely duplicate delivery");
@@ -172,6 +203,10 @@ impl World {
                 match (*current).cmp(&update) {
                     Less => {
                         *current = update;
+                        // We never allow remote_consistent_lsn to be ahead of commit_lsn.
+                        // Therefore, it is safe to say nothing is caught up anymore.
+                        let caught_up_count = self.caught_up_count.get_mut(&ttid).unwrap();
+                        *caught_up_count = 0;
                     }
                     Equal => {
                         // This code runs in safekeeper impl, no reason why there would be duplicate delivery.
@@ -281,6 +316,13 @@ impl World {
             assert_eq!(None, replaced);
         }
     }
+
+    fn quiesce_timeline(&mut self, tenant_timeline_id: TenantTimelineId) {
+        if self.quiesced_timelines.contains_key(&tenant_timeline_id) {
+            panic!("only call this function on active timelines");
+        }
+        todo!();
+    }
 }
 
 impl TimelineAttachmentId {