message_fetch: Vary how much history we fetch based on date range.

To better make use of server load when fetching message history for
users, we limit how much history we fetch in the initial loading
experience based on how much time in the organization's message
history it corresponds to.

Co-authored-by: Tim Abbott <tabbott@zulip.com>

web/src/message_fetch.js

@@ -23,7 +23,7 @@ export let initial_narrow_offset;
 
 const consts = {
     // Because most views are centered on the first unread message,
-    // the user has a higher probability of wantingto scroll down
+    // the user has a higher probability of wanting to scroll down
     // than, so extra fetched history after the cursor is more likely
     // to be used and thus worth more to fetch.
     //
@@ -31,15 +31,27 @@ const consts = {
     // for a larger number of messages after the cursor is cheap.
     narrow_before: 60,
     narrow_after: 150,
-    initial_backfill_fetch_size: 1000,
-    maximum_initial_backfill_size: 15000,
+
+    // Batch sizes when at the top/bottom of a narrowed view.
     narrowed_view_backward_batch_size: 100,
     narrowed_view_forward_batch_size: 100,
-    recent_view_fetch_more_batch_size: 2000,
+
+    // Initial backfill parameters to populate message history.
+    initial_backfill_fetch_size: 1000,
     catch_up_batch_size: 2000,
+    // We fetch at least minimum_initial_backfill_size messages of
+    // history, but after that will stop fetching at either
+    // maximum_initial_backfill_size messages or
+    // target_days_of_history days, whichever comes first.
+    minimum_initial_backfill_size: 9000,
+    maximum_initial_backfill_size: 25000,
+    target_days_of_history: 180,
     // Delay in milliseconds after processing a catch-up request
     // before sending the next one.
     catch_up_backfill_delay: 150,
+
+    // Parameters for asking for more history in the recent view.
+    recent_view_fetch_more_batch_size: 2000,
 };
 
 function process_result(data, opts) {
@@ -487,6 +499,8 @@ export function set_initial_pointer_and_offset({narrow_pointer, narrow_offset})
 }
 
 export function initialize(finished_initial_fetch) {
+    const fetch_target_day_timestamp =
+        Date.now() / 1000 - consts.target_days_of_history * 24 * 60 * 60;
     // get the initial message list
     function load_more(data) {
         if (first_messages_fetch) {
@@ -501,6 +515,17 @@ export function initialize(finished_initial_fetch) {
             return;
         }
 
+        // Stop once we've hit the minimum backfill quantiy of
+        // messages if we've received a message older than
+        // `target_days_of_history`.
+        const latest_message = all_messages_data.first();
+        if (
+            all_messages_data.num_items() >= consts.minimum_initial_backfill_size &&
+            latest_message.timestamp < fetch_target_day_timestamp
+        ) {
+            return;
+        }
+
         if (all_messages_data.num_items() >= consts.maximum_initial_backfill_size) {
             return;
         }