diff --git a/services/document-updater/app/js/DocumentManager.js b/services/document-updater/app/js/DocumentManager.js
index 8ff285c5ff..4af82ae586 100644
--- a/services/document-updater/app/js/DocumentManager.js
+++ b/services/document-updater/app/js/DocumentManager.js
@@ -222,6 +222,13 @@ module.exports = DocumentManager = {
               user_id: userId,
             },
           }
+          // Keep track of external updates, whether they are for live documents
+          // (flush) or unloaded documents (evict), and whether the update is a no-op.
+          Metrics.inc('external-update', 1, {
+            status: op.length > 0 ? 'diff' : 'noop',
+            method: alreadyLoaded ? 'flush' : 'evict',
+            path: source,
+          })
           UpdateManager.applyUpdate(projectId, docId, update, error => {
             if (error) {
               return callback(error)
diff --git a/services/document-updater/test/unit/js/DocumentManager/DocumentManagerTests.js b/services/document-updater/test/unit/js/DocumentManager/DocumentManagerTests.js
index f4203f5997..e65e8bd96c 100644
--- a/services/document-updater/test/unit/js/DocumentManager/DocumentManagerTests.js
+++ b/services/document-updater/test/unit/js/DocumentManager/DocumentManagerTests.js
@@ -9,6 +9,7 @@ describe('DocumentManager', function () {
     tk.freeze(new Date())
     this.Metrics = {
       Timer: class Timer {},
+      inc: sinon.stub(),
     }
     this.Metrics.Timer.prototype.done = sinon.stub()
 
@@ -42,6 +43,7 @@ describe('DocumentManager', function () {
     this.unflushedTime = Date.now()
     this.lastUpdatedAt = Date.now()
     this.lastUpdatedBy = 'last-author-id'
+    this.source = 'external-source'
   })
 
   afterEach(function () {
@@ -517,6 +519,16 @@ describe('DocumentManager', function () {
             .should.equal(true)
         })
 
+        it('should increment the external update metric', function () {
+          this.Metrics.inc
+            .calledWith('external-update', 1, {
+              status: 'diff',
+              method: 'flush',
+              path: this.source,
+            })
+            .should.equal(true)
+        })
+
         it('should flush the doc to Mongo', function () {
           this.DocumentManager.flushDocIfLoaded
             .calledWith(this.project_id, this.doc_id)
@@ -568,6 +580,16 @@ describe('DocumentManager', function () {
             .should.equal(true)
         })
 
+        it('should increment the external update metric', function () {
+          this.Metrics.inc
+            .calledWith('external-update', 1, {
+              status: 'diff',
+              method: 'evict',
+              path: this.source,
+            })
+            .should.equal(true)
+        })
+
         it('should not flush the project history', function () {
           this.HistoryManager.flushProjectChangesAsync
             .calledWithExactly(this.project_id)