diff --git a/modules/engine/lib/engine.js b/modules/engine/lib/engine.js
index 915ba37b..1625eae5 100644
--- a/modules/engine/lib/engine.js
+++ b/modules/engine/lib/engine.js
@@ -187,6 +187,7 @@ util.inherits(Engine, LogEmitter);
  *         });
  *     });
  */
+// this execute wrapper is for logging purpose only.
 Engine.prototype.execute = function(){
     var grandParentEvent = null;
     if(arguments.length > 1){
@@ -204,6 +205,9 @@ Engine.prototype.execute = function(){
     arguments[1].parentEvent = parentEvent;
     this.doExecute.apply(this,arguments);
 }
+/*
+The real logic of engine execution
+ */
 Engine.prototype.doExecute = function() {
     var script, opts, func;
 
@@ -321,6 +325,8 @@ Engine.prototype.doExecute = function() {
 
     // Initialize the exec state
     var execState = {};
+    // initialize variable assignment by walking through the dependency tree.
+    // Unused variable will be treated as orphans and executed eventually.
     function init(statement) {
         function scopeInit(statement) {
             switch(statement.type){
@@ -373,6 +379,7 @@ Engine.prototype.doExecute = function() {
     init(plan.rhs);
 
     /* Skip assign statement. Make it undefined, and trigger listener
+       for if/else and try/catch/finally clause only
      */
     function skipVarList(statements){
         function skipVar(statement){
@@ -432,7 +439,9 @@ Engine.prototype.doExecute = function() {
         });
     }
 
-    // scope is complex to execute, handle separately
+    // scope is complex to execute, handle separately.
+    // If any line in the clause is hitted, execute every line of the calus
+    // for if/else, try/catch/finally only
     function execScope(statement, arg) {
         switch(statement.type) {
             case 'if' :
@@ -463,6 +472,10 @@ Engine.prototype.doExecute = function() {
     }
 
     var skip = false;
+    /*
+    Sweep a statement, execute it only if all its dependencies are done.
+    If not, sweep the dependencies first.
+     */
     function sweep(statement) {
         if(skip) {
             return;
@@ -505,7 +518,8 @@ Engine.prototype.doExecute = function() {
             execState[todo.id].count === 0 &&
             scopeDone) {
             execState[todo.id].state = eventTypes.STATEMENT_IN_FLIGHT;
-
+            // step1 is true only if run in debug mode and not the first step.
+            // put all unexecuted statements into a list and execute them one by one by clicking "step" on console.
             if (emitterID && !step1) {
                 // only used for debugger
                 unexecuted.push(statement);
@@ -587,7 +601,7 @@ Engine.prototype.doExecute = function() {
                 }
             }, engineEvent.event);
     }
-
+    // done with all sweep, this is the final wrap up.
     var fnDone = function(err, results) {
         execState[plan.rhs.id].state = err ? eventTypes.STATEMENT_ERROR : eventTypes.STATEMENT_SUCCESS;
 
diff --git a/modules/engine/test/join-test.js b/modules/engine/test/join-test.js
index 841896b9..3551ed64 100644
--- a/modules/engine/test/join-test.js
+++ b/modules/engine/test/join-test.js
@@ -26,7 +26,7 @@ module.exports = {
         on select get from "http://api.bart.gov/api/stn.aspx?cmd=stninfo&orig=12th&key=MW9S-E7SL-26DU-VV8V"\
         resultset "root.stations.station" \
         create table myg  \
-        on select get from "http://maps.googleapis.com/maps/api/geocode/json?latlng={lo},{la}&sensor=true".  \
+        on select get from "https://maps.googleapis.com/maps/api/geocode/json?latlng={lo},{la}&sensor=true"  \
         select g.results from bart as b, myg as g where b.gtfs_latitude = g.lo and g.la = b.gtfs_longitude'
         engine.execute(script, function(emitter) {
             emitter.on('end', function(err, result) {
diff --git a/modules/engine/test/mock-routes/connectors/naive_connector.js b/modules/engine/test/mock-routes/connectors/naive_connector.js
new file mode 100644
index 00000000..839d57eb
--- /dev/null
+++ b/modules/engine/test/mock-routes/connectors/naive_connector.js
@@ -0,0 +1,47 @@
+/*
+example of using pipeline.
+On top of library when (https://github.com/cujojs/when.git)
+description of pipeline: https://github.com/cujojs/when/blob/master/docs/api.md
+
+Things must have:
+ customConnector.connectorName. this field tells the connector loader
+ .exec that verb.js would like to call when there is an execution for any connectors.
+ */
+var pipeline = require('/Users/hochang/workspace/ql.io-mp-apis/node_modules/when/pipeline')
+var customConnector = module.exports = function(table, statement, type, bag, path){
+    var foo = function(n){
+        return n+1
+    }
+    var bar = function(n){
+        return n+2
+    }
+    var wae = function(n){
+        return n*3
+    }
+    var beforePipeline = [foo]
+    var afterPipeline = [foo, bar,wae]
+    this.exec = function(args) {
+        var mypromise = pipeline(beforePipeline, 1);
+        mypromise.then(function(arg){
+            results = send(arg)
+            var myresult = pipeline(afterPipeline, results)
+            myresult.then(function(resultbody){
+                var result = {
+                    headers: {
+                        'content-type':  'application/json'
+                    },
+                    body: resultbody
+                };
+                return args.callback(null, result);
+            })
+        })
+
+    }
+
+
+function send(arg){
+    return arg+1
+}
+
+}
+customConnector.connectorName = 'aaa'
diff --git a/modules/engine/test/mock-routes/connectors/pipeline_connector.js b/modules/engine/test/mock-routes/connectors/pipeline_connector.js
new file mode 100644
index 00000000..1d058781
--- /dev/null
+++ b/modules/engine/test/mock-routes/connectors/pipeline_connector.js
@@ -0,0 +1,80 @@
+/*
+ example of using pipeline to make async calls..
+ On top of library when (https://github.com/cujojs/when.git)
+ description of pipeline: https://github.com/cujojs/when/blob/master/docs/api.md
+
+ put all chained functions into arrays. In my example, beforePipeline and afterPipeline
+ Calling pipeline to executed tasks in sequence, but without overlap. Result from prior task will be passed as argument to the latter tasks.
+ The final result can be accessed in the function passed in promise.then.
+
+
+
+ */
+var pipeline = require('/Users/hochang/workspace/ql.io-mp-apis/node_modules/when/pipeline'),
+    when = require('/Users/hochang/workspace/ql.io-mp-apis/node_modules/when'),
+    http = require('http');
+var customConnector = module.exports = function(){
+    var foo = function(n){
+        foo1(function(err, results){
+
+        })
+    }
+    /*
+    This is an aysnc call. In order to put it into pipeline,
+    a defer() is required to keep the call blocking during I/O
+     */
+    var foo = function(){
+        var deferred = when.defer()
+        var options = {
+            host: 'localhost',
+            port: 3000
+        }
+
+        callback = function(response) {
+            var str = '';
+
+            //another chunk of data has been recieved, so append it to `str`
+            response.on('data', function (chunk) {
+                str += chunk;
+            });
+
+            //the whole response has been recieved, so we just print it out here
+            response.on('end', function () {
+                deferred.resolve(parseInt(str))
+            });
+        }
+
+        http.request(options, callback).end();
+        return deferred.promise
+    }
+    var bar = function(n){
+        return n+2
+    }
+    var wae = function(n){
+        return n*3
+    }
+
+
+    this.exec = function(args) {
+        var cb = function(n){
+            var asd=args
+            var result = {
+                headers: {
+                    'content-type':  'application/json'
+                },
+                body: n
+            };
+            return args.callback(null, result);
+        }
+        var afterPipeline = [foo, bar,wae,cb]
+        pipeline(afterPipeline, 1);
+
+
+    function send(arg){
+        return arg+1
+    }
+
+
+}
+}
+customConnector.connectorName = 'bbb'