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js-scripts/compact-example.js

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+// assumption is we are primary
+
+assert( rs.isMaster().setName, "not a repl set" );  // assuming this is a repl set
+assert( rs.isMaster().ismaster, "we are not primary and we were assuming that");
+
+// we want to compact, but in the background
+
+// so step down...
+try {
+    rs.stepDown();
+} catch(e) { 
+    print("exception:" + e);
+}
+
+// after stepdown connections are dropped. do an operation to cause reconnect:
+rs.isMaster();
+
+// now ready to go.  
+
+// wait for another node to become primary -- it may need data from us for the last 
+// small sliver of time, and if we are already compacting it cannot get it while the 
+// compaction is running.
+
+while( 1 ) { 
+    var m = rs.isMaster();
+    if( m.ismaster ) {
+        print("ERROR: no one took over during our stepDown duration. we are primary again!");
+	assert(false);
+    }
+    if( m.primary )
+        break; // someone else is, great
+    print("waiting");
+    sleep(1000);
+}
+
+// someone else is primary, so we are ready to proceed with a compaction
+print("Compacting...");
+printjson( db.runCommand({compact:"mycollection",dev:true}) );