From 5265db0673f0f6d02a32cb2b9b11cd4c11ddfa53 Mon Sep 17 00:00:00 2001
From: Christopher Baines <mail@cbaines.net>
Date: Thu, 14 Mar 2024 10:21:13 +0000
Subject: Instrument the recent changes count

---
 nar-herder/recent-changes.scm | 17 +++++++++++++++++
 nar-herder/server.scm         |  5 +++++
 2 files changed, 22 insertions(+)

diff --git a/nar-herder/recent-changes.scm b/nar-herder/recent-changes.scm
index 0ded58d..d95410e 100644
--- a/nar-herder/recent-changes.scm
+++ b/nar-herder/recent-changes.scm
@@ -23,6 +23,7 @@
   #:use-module (fibers)
   #:use-module (fibers channels)
   #:use-module (logging logger)
+  #:use-module (prometheus)
   #:use-module (web uri)
   #:use-module (guix narinfo)
   #:use-module (nar-herder database)
@@ -30,6 +31,7 @@
             start-recent-change-listener-fiber))
 
 (define (start-recent-change-removal-and-database-dump-fiber database
+                                                             metrics-registry
                                                              database-dump-filename
                                                              check-interval
                                                              recent-changes-limit)
@@ -49,6 +51,9 @@
       (simple-format (current-error-port)
                      "updated database dump\n")))
 
+  (define recent-changes-count-metric
+    (metrics-registry-fetch-metric metrics-registry "recent_changes_count"))
+
   (spawn-fiber
    (lambda ()
      (while #t
@@ -74,6 +79,10 @@
                          (database-delete-recent-changes-with-id-below
                           database
                           recent-changes-id-for-deletion)))))
+
+                 (metric-decrement recent-changes-count-metric
+                                   #:by deleted-recent-changes)
+
                  (simple-format (current-error-port)
                                 "deleted ~A recent changes\n"
                                 deleted-recent-changes)))
@@ -82,8 +91,12 @@
          #:unwind? #t)))))
 
 (define (start-recent-change-listener-fiber database
+                                            metrics-registry
                                             addition-channel
                                             removal-channel)
+  (define recent-changes-count-metric
+    (metrics-registry-fetch-metric metrics-registry "recent_changes_count"))
+
   (define (process-addition-change change-details)
     (let ((narinfo
            (call-with-input-string
@@ -121,6 +134,10 @@
                (lambda ()
                  (let ((recent-changes
                         (database-select-recent-changes database after)))
+
+                   (metric-increment recent-changes-count-metric
+                                     #:by (length recent-changes))
+
                    (for-each
                     (lambda (change-details)
                       (when (not (member change-details
diff --git a/nar-herder/server.scm b/nar-herder/server.scm
index a978ce2..9b9ecbe 100644
--- a/nar-herder/server.scm
+++ b/nar-herder/server.scm
@@ -611,6 +611,9 @@
              #f)
            (download-database)))))
 
+  ;; Used elsewhere
+  (make-gauge-metric metrics-registry "recent_changes_count")
+
   (let ((recent-changes-metric
          (make-gauge-metric metrics-registry "recent_changes_limit")))
     (metric-set recent-changes-metric (assq-ref opts 'recent-changes-limit)))
@@ -691,6 +694,7 @@
 
             (start-recent-change-removal-and-database-dump-fiber
              database
+             metrics-registry
              (let ((filename (assq-ref opts 'database-dump)))
                (if (string=? filename "disabled")
                    #f
@@ -760,6 +764,7 @@
 
               (start-recent-change-listener-fiber
                database
+               metrics-registry
                addition-channel
                removal-channel))
 
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