[#270] Adaptive Ajax send retry

Slightly smarter strategy re: waiting for possible Ajax repolling.

In particular, should provide more reliable async broadcasting to
Ajax clients on poor connections that take very long to repoll. In
these cases, send buffering should also be increased.

Note that this doesn't change the general advice given in #270.
Applications that need guaranteed async broadcasts should still use
an appropriate application-level ack design when possible.

src/taoensso/sente.cljc

@@ -698,11 +698,7 @@
   Allows some time for possible Ajax poller reconnects."
   [conns_ uid buffered-evs-pstr]
   (tracef "send-buffered-server-evs>ajax-clients!: %s" buffered-evs-pstr)
-  (let [nmax-attempts 7
-        ms-base 90
-        ms-rand 90
-        ;; (* 7 (+ 90 (/ 90 2.0))) ~= 945ms
-
+  (let [ms-backoffs [90 180 360 720 1440] ; Mean 2790s
         ;; All connected/possibly-reconnecting client uuids:
         client-ids-unsatisfied (keys (get-in @conns_ [:ajax uid]))]
 
@@ -746,11 +742,12 @@
                 now-satisfied (into client-ids-satisfied ?newly-satisfied)]
 
             ;; (tracef "now-satisfied: %s" now-satisfied)
-            (when (and (< n nmax-attempts)
-                       (some (complement now-satisfied) client-ids-unsatisfied))
-              ;; Allow some time for possible poller reconnects:
-              (<! (async/timeout (+ ms-base (rand-int ms-rand))))
-              (recur (inc n) now-satisfied))))))))
+            (when-let [ms-backoff (get ms-backoffs n)]
+              (when (enc/rsome (complement now-satisfied) client-ids-unsatisfied)
+                (let [ms-timeout (+ ms-backoff (rand-int ms-backoff))]
+                  ;; Allow some time for possible poller reconnects:
+                  (<! (async/timeout ms-timeout))
+                  (recur (inc n) now-satisfied))))))))))
 
 ;;;; Client API