Merge pull request redis#972 from VanTanev/improve-scan-example-code

Update SCAN example code

examples/scan.js

@@ -15,20 +15,33 @@ function scan() {
 
             // Update the cursor position for the next scan
             cursor = res[0];
+            // get the SCAN result for this iteration
+            var keys = res[1];
 
-            // From <http://redis.io/commands/scan>:
-            // 'An iteration starts when the cursor is set to 0,
-            // and terminates when the cursor returned by the server is 0.'
-            if (cursor === '0') {
-                return console.log('Iteration complete');
-            }
             // Remember: more or less than COUNT or no keys may be returned
             // See http://redis.io/commands/scan#the-count-option
             // Also, SCAN may return the same key multiple times
             // See http://redis.io/commands/scan#scan-guarantees
+            // Additionally, you should always have the code that uses the keys
+            // before the code checking the cursor.
+            if (keys.length > 0) {
+                console.log('Array of matching keys', keys);
+            }
 
-            if (res[1].length > 0) {
-                console.log('Array of matching keys', res[1]);
+            // It's important to note that the cursor and returned keys
+            // vary independently. The scan is never complete until redis
+            // returns a non-zero cursor. However, with MATCH and large
+            // collections, most iterations will return an empty keys array.
+
+            // Still, a cursor of zero DOES NOT mean that there are no keys.
+            // A zero cursor just means that the SCAN is complete, but there
+            // might be one last batch of results to process.
+
+            // From <http://redis.io/commands/scan>:
+            // 'An iteration starts when the cursor is set to 0,
+            // and terminates when the cursor returned by the server is 0.'
+            if (cursor === '0') {
+                return console.log('Iteration complete');
             }
 
             return scan();