feat: add failed job trend chart to dashboard (#76)

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Failed job trend chart — "Failures — last 12 hours" sparkline added to the Solid Queue dashboard card below the throughput sparkline; bars render in the danger color to make failure spikes immediately visible
 - Error frequency report — `GET /failed_jobs/errors` groups all failed jobs by exception class and message prefix, showing count and an expandable sample backtrace per group; links through to a filtered failed jobs list via `?error_class=`; the failed jobs index gains an "Error Summary" button and shows an active-filter breadcrumb with a clear link
 
 ## [1.0.0] - 2026-05-27

app/assets/stylesheets/solid_stack_web/_07_dashboard.css

@@ -86,6 +86,10 @@ a.sqw-inline-stat:hover { opacity: 0.7; text-decoration: none; }
   color: var(--primary);
 }
 
+.sqw-sparkline-wrap--failures {
+  color: var(--danger);
+}
+
 .sqw-sparkline-label {
   display: block;
   font-size: 10px;

app/controllers/solid_stack_web/dashboard_controller.rb

@@ -5,6 +5,7 @@ def index
       @cache_stats = CacheStats.new.to_h
       @cable_stats = CableStats.new.to_h
       @throughput  = ThroughputSparkline.new
+      @failures    = FailedJobSparkline.new
     end
   end
 end

app/helpers/solid_stack_web/application_helper.rb

@@ -87,6 +87,17 @@ def queue_depth_sparkline_svg(sparkline)
       end
     end
 
+    def failed_job_sparkline_svg(sparkline)
+      build_sparkline_svg(sparkline, aria_label: "Failed jobs over the last 12 hours") do |count, i|
+        hours_ago = SolidStackWeb::FailedJobSparkline::HOURS - i
+        if hours_ago == 1
+          "#{count} #{count == 1 ? "failure" : "failures"} in the last hour"
+        else
+          "#{count} #{count == 1 ? "failure" : "failures"} (#{hours_ago}h–#{hours_ago - 1}h ago)"
+        end
+      end
+    end
+
     private
 
     def build_sparkline_svg(sparkline, css_class: "sqw-sparkline", aria_label: nil, &tooltip_text)

app/models/solid_stack_web/failed_job_sparkline.rb

@@ -0,0 +1,23 @@
+module SolidStackWeb
+  class FailedJobSparkline
+    HOURS = 12
+
+    def buckets
+      @buckets ||= begin
+        now    = Time.current
+        origin = now - HOURS.hours
+        times  = ::SolidQueue::FailedExecution.where(created_at: origin..now).pluck(:created_at)
+
+        HOURS.times.map do |i|
+          from = origin + i.hours
+          to   = origin + (i + 1).hours
+          times.count { |t| t >= from && t < to }
+        end
+      end
+    end
+
+    def max
+      buckets.max || 0
+    end
+  end
+end

app/views/solid_stack_web/dashboard/index.html.erb

@@ -68,6 +68,18 @@
         <span>now</span>
       </div>
     </div>
+    <div class="sqw-sparkline-wrap sqw-sparkline-wrap--failures" data-controller="sparkline-tooltip">
+      <span class="sqw-sparkline-label">Failures — last 12 hours</span>
+      <div class="sqw-sparkline-positioner">
+        <%= failed_job_sparkline_svg(@failures) %>
+        <div class="sqw-sparkline-tip" data-sparkline-tooltip-target="tip" hidden></div>
+      </div>
+      <div class="sqw-sparkline-axis">
+        <span>12h ago</span>
+        <span>6h ago</span>
+        <span>now</span>
+      </div>
+    </div>
   </div>
 
   <div class="sqw-gem-card sqw-gem-card--cache">

spec/models/solid_stack_web/failed_job_sparkline_spec.rb

@@ -0,0 +1,50 @@
+require "rails_helper"
+
+RSpec.describe SolidStackWeb::FailedJobSparkline do
+  def create_failed(created_at:)
+    SolidQueue::Job.skip_callback(:create, :after, :prepare_for_execution)
+    job = SolidQueue::Job.create!(class_name: "FailingJob", queue_name: "default",
+                                  priority: 0, arguments: {})
+    SolidQueue::FailedExecution.create!(
+      job: job,
+      error: { exception_class: "RuntimeError", message: "boom", backtrace: [] },
+      created_at: created_at
+    )
+  ensure
+    SolidQueue::Job.set_callback(:create, :after, :prepare_for_execution)
+  end
+
+  describe "#buckets" do
+    it "returns 12 buckets" do
+      expect(described_class.new.buckets.size).to eq(12)
+    end
+
+    it "returns all zeros when there are no failed executions" do
+      expect(described_class.new.buckets).to all(eq(0))
+    end
+
+    it "counts a failure within the window in the correct bucket" do
+      create_failed(created_at: 2.hours.ago)
+      buckets = described_class.new.buckets
+      expect(buckets[9]).to eq(1)
+      expect(buckets.sum).to eq(1)
+    end
+
+    it "excludes failures outside the 12-hour window" do
+      create_failed(created_at: 13.hours.ago)
+      expect(described_class.new.buckets).to all(eq(0))
+    end
+  end
+
+  describe "#max" do
+    it "returns 0 when there are no failures" do
+      expect(described_class.new.max).to eq(0)
+    end
+
+    it "returns the highest bucket count" do
+      3.times { create_failed(created_at: 1.hour.ago) }
+      create_failed(created_at: 5.hours.ago)
+      expect(described_class.new.max).to eq(3)
+    end
+  end
+end