Tag Archive for workflow

Web 2.0 AJAX Portal using jQuery, ASP.NET 3.5, Silverlight, Linq to SQL, WF and Unity

Dropthings
– my open
source
Web 2.0 Ajax Portal has gone through a technology
overhauling. Previously it was built using ASP.NET AJAX, a little
bit of Workflow Foundation and Linq to SQL. Now Dropthings boasts
full jQuery front-end combined with ASP.NET AJAX
UpdatePanel, Silverlight widget, full
Workflow Foundation implementation on the business
layer, 100% Linq to SQL Compiled Queries on the
data access layer, Dependency Injection and Inversion of Control
(IoC) using Microsoft Enterprise Library 4.1 and
Unity. It also has a ASP.NET AJAX Web Test
framework that makes it real easy to write Web Tests that simulates
real user actions on AJAX web pages. This article will walk you
through the challenges in getting these new technologies to work in
an ASP.NET website and how performance, scalability, extensibility
and maintainability has significantly improved by the new
technologies. Dropthings has been licensed for commercial use by
prominent companies including BT Business, Intel, Microsoft IS,
Denmark Government portal for Citizens; Startups like Limead and
many more. So, this is serious stuff! There’s a very cool
open source implementation of Dropthings framework available at
National
University of Singapore
portal.

Visit: http://dropthings.omaralzabir.com


Dropthings AJAX Portal

I have published a new article on this on CodeProject:

http://www.codeproject.com/KB/ajax/Web20Portal.aspx

Get the source code

Latest source code is hosted at Google code:

http://code.google.com/p/dropthings

There’s a CodePlex site for documentation and issue
tracking:

http://www.codeplex.com/dropthings

You will need Visual Studio 2008 Team Suite with Service Pack 1
and Silverlight 2 SDK in order to run all the projects. If you have
only Visual Studio 2008 Professional, then you will have to remove
the Dropthings.Test project.

New features introduced

Dropthings new release has the following features:

  • Template users – you can define a user who’s pages
    and widgets are used as a template for new users. Whatever you put
    in that template user’s pages, it will be copied for every
    new user. Thus this is an easier way to define the default pages
    and widgets for new users. Similarly you can do the same for a
    registered user. The template users can be defined in the
    web.config.
  • Widget-to-Widget communication – Widgets can send message
    to each other. Widgets can subscribe to an Event Broker and
    exchange messages using a Pub-Sub pattern.
  • WidgetZone – you can create any number of zones in any
    shape on the page. You can have widgets laid in horizontal layout,
    you can have zones on different places on the page and so on. With
    this zone model, you are no longer limited to the Page-Column model
    where you could only have N vertical columns.
  • Role based widgets – now widgets are mapped to roles so
    that you can allow different users to see different widget list
    using ManageWidgetPersmission.aspx.
  • Role based page setup – you can define page setup for
    different roles. For ex, Managers see different pages and widgets
    than Employees.
  • Widget maximize – you can maximize a widget to take full
    screen. Handy for widgets with lots of content.
  • Free form resize – you can freely resize widgets
    vertically.
  • Silverlight Widgets – You can now make widgets in
    Silverlight!

Why the technology overhauling

Performance, Scalability, Maintainability and Extensibility
– four key reasons for the overhauling. Each new technology
solved one of more of these problems.

First, jQuery was used to replace my personal hand-coded large
amount of Javascript code that offered the client side drag &
drop and other UI effects. jQuery already has a rich set of library
for Drag & Drop, Animations, Event handling, cross browser
javascript framework and so on. So, using jQuery means opening the
door to thousands of jQuery plugins to be offered on Dropthings.
This made Dropthings highly extensible on the client side.
Moreover, jQuery is very light. Unlike AJAX Control Toolkit jumbo
sized framework and heavy control extenders, jQuery is very lean.
So, total javascript size decreased significantly resulting in
improved page load time. In total, the jQuery framework, AJAX basic
framework, all my stuffs are total 395KB, sweet! Performance is
key; it makes or breaks a product.

Secondly, Linq to SQL queries are replaced with Compiled
Queries. Dropthings did not survive a load test when regular lambda
expressions were used to query database. I could only reach up to
12 Req/Sec using 20 concurrent users without burning up web server
CPU on a Quad Core DELL server.

Thirdly, Workflow Foundation is used to build operations that
require multiple Data Access Classes to perform together in a
single transaction. Instead of writing large functions with many
if…else conditions, for…loops, it’s better to
write them in a Workflow because you can visually see the flow of
execution and you can reuse Activities among different Workflows.
Best of all, architects can design workflows and developers can
fill-in code inside Activities. So, I could design a complex
operations in a workflow without writing the real code inside
Activities and then ask someone else to implement each Activity. It
is like handing over a design document to developers to implement
each unit module, only that here everything is strongly typed and
verified by compiler. If you strictly follow Single Responsibility
Principle for your Activities, which is a smart way of saying one
Activity does only one and very simple task, you end up with a
highly reusable and maintainable business layer and a very clean
code that’s easily extensible.

Fourthly, Unity
Dependency Injection (DI) framework is used to pave the path for
unit testing and dependency injection. It offers Inversion of
Control (IoC), which enables testing individual classes in
isolation. Moreover, it has a handy feature to control lifetime of
objects. Instead of creating instance of commonly used classes
several times within the same request, you can make instances
thread level, which means only one instance is created per thread
and subsequent calls reuse the same instance. Are these going over
your head? No worries, continue reading, I will explain later
on.

Fifthly, enabling API for Silverlight widgets allows more
interactive widgets to be built using Silverlight. HTML and
Javascripts still have limitations on smooth graphics and
continuous transmission of data from web server. Silverlight solves
all of these problems.

Read the article for details on how all these improvements were
done and how all these hot techs play together in a very useful
open source project for enterprises.

http://www.codeproject.com/KB/ajax/Web20Portal.aspx

Don’t forget to vote for me if you like it.

Memory Leak with delegates and workflow foundation

Recently after Load Testing my open source project Dropthings, I
encountered a lot of memory leak. I found lots of Workflow
Instances and Linq Entities were left in memory and never
collected. After profiling the web application using .NET Memory Profiler, it showed the real picture:


image

It shows you that instances of the several types are being
created but not being removed. You see the “New” column
has positive value, but the “Remove” column has 0. That
means new instances are being created, but not removed. Basically
the way you do Memory Profiling is, you take two snapshots. Say you
take one snapshot when you first visit your website. Then you do
some action on the website that results in allocation of objects.
Then you take another snapshot. When you compare both snapshots,
you can see how many instances of classes were created between
these two snapshots and how many were removed. If they are not
equal, then you have leak. Generally in web application many
objects are created on every page hit and the end of the request,
all those objects are supposed to be released. If they are not
released, then we have a problem. But that’s the scenario for
desktop applications because in a desktop application, objects can
remain in memory until app is closed. But you should know best from
the code which objects were supposed to go out of scope and get
released.

For beginners, leak means objects are being allocated but not
being freed because someone is holding reference to the objects.
When objects leak, they remain in memory forever, until the process
(or app domain) is closed. So, if you have a leaky website, your
website is continuously taking up memory until it runs out of
memory on the web server and thus crash. So, memory leak is a bad
– it prevents you from running your product for long duration
and requires frequent restart of app pool.

So, the above screenshot shows Workflow and Linq related classes
are not being removed, and thus leaking. This means somewhere
workflow instances are not being released and thus all workflow
related objects are remaining. You can see the number is same 48
for all workflow related objects. This is a good indication that,
almost every instance of workflow is leaked because there were
total 48 workflows created and ran. Moreover it indicates we have a
leak from a top Workflow instance level, not in some specific
Activity or somewhere deep in the code.

As the workflows use Linq stuff, they held reference to the Linq
stuffs and thus the Linq stuffs leaked as well. Sometimes you might
be looking for why A is leaking. But you actually end up finding
that since B was holding reference to A and B was leaking and thus
A was leaking as well. This is sometimes tricky to figure out and
you spend a lot of time looking at the wrong direction.

Now let me show you the buggy code:

ManualWorkflowSchedulerService manualScheduler = 
workflowRuntime.GetService<ManualWorkflowSchedulerService>(); WorkflowInstance instance = workflowRuntime.CreateWorkflow(workflowType, properties); instance.Start(); EventHandler<WorkflowCompletedEventArgs> completedHandler = null; completedHandler = delegate(object o, WorkflowCompletedEventArgs e) { if (e.WorkflowInstance.InstanceId == instance.InstanceId) // 1. instance { workflowRuntime.WorkflowCompleted -= completedHandler; // 2. terminatedhandler // copy the output parameters in the specified properties dictionary Dictionary<string,object>.Enumerator enumerator =
e.OutputParameters.GetEnumerator(); while( enumerator.MoveNext() ) { KeyValuePair<string,object> pair = enumerator.Current; if( properties.ContainsKey(pair.Key) ) { properties[pair.Key] = pair.Value; } } } }; Exception x = null; EventHandler<WorkflowTerminatedEventArgs> terminatedHandler = null; terminatedHandler = delegate(object o, WorkflowTerminatedEventArgs e) { if (e.WorkflowInstance.InstanceId == instance.InstanceId) // 3. instance { workflowRuntime.WorkflowTerminated -= terminatedHandler; // 4. completeHandler Debug.WriteLine( e.Exception ); x = e.Exception; } }; workflowRuntime.WorkflowCompleted += completedHandler; workflowRuntime.WorkflowTerminated += terminatedHandler; manualScheduler.RunWorkflow(instance.InstanceId);

Can you spot the code where it leaked?

I have numbered the lines in comment where the leak is
happening. Here the delegate is acting like a closure
and those who are from Javascript background know closure is evil.
They leak memory unless very carefully written. Here the
delegate keeps a reference to the
instance object. So, if somehow delegate
is not released, the instance will remain in memory
forever and thus leak. Now can you find a situation when the
delegate will not be released?

Say the workflow completed. It will fire the completeHandler. But the
completeHandler will not release the
terminateHandler. Thus the
terminateHandler remains in memory and it also holds
reference to the instance. So, we have a leaky
delegate leaking whatever it is holding onto outside
it’s scope. Here the only thing outside the scope if the
instance, which it is tried to access from the parent
function.

Since the workflow instance is not released, all the properties
the workflow and all the activities inside it are holding onto
remains in memory. Most of the workflows and activities expose
public properties which are Linq Entities. Thus the Linq Entities
remain in memory. Now Linq Entities keep a reference to the
DataContext from where it is produced. Thus we have
DataContext remaining in memory. Moreover,
DataContext keeps reference to many internal objects
and metadata cacahe, so they remain in memory as well.

So, the correct code is:

ManualWorkflowSchedulerService manualScheduler = 
workflowRuntime.GetService<ManualWorkflowSchedulerService>(); WorkflowInstance instance = workflowRuntime.CreateWorkflow(workflowType, properties); instance.Start(); var instanceId = instance.InstanceId; EventHandler<WorkflowCompletedEventArgs> completedHandler = null; completedHandler = delegate(object o, WorkflowCompletedEventArgs e) { if (e.WorkflowInstance.InstanceId == instanceId) // 1. instanceId is a Guid { // copy the output parameters in the specified properties dictionary Dictionary<string,object>.Enumerator enumerator =
e.OutputParameters.GetEnumerator(); while( enumerator.MoveNext() ) { KeyValuePair<string,object> pair = enumerator.Current; if( properties.ContainsKey(pair.Key) ) { properties[pair.Key] = pair.Value; } } } }; Exception x = null; EventHandler<WorkflowTerminatedEventArgs> terminatedHandler = null; terminatedHandler = delegate(object o, WorkflowTerminatedEventArgs e) { if (e.WorkflowInstance.InstanceId == instanceId) // 2. instanceId is a Guid { x = e.Exception; Debug.WriteLine(e.Exception); } }; workflowRuntime.WorkflowCompleted += completedHandler; workflowRuntime.WorkflowTerminated += terminatedHandler; manualScheduler.RunWorkflow(instance.InstanceId); // 3. Both delegates are now released
workflowRuntime.WorkflowTerminated -= terminatedHandler; workflowRuntime.WorkflowCompleted -= completedHandler;

There are two changes – in both delegates, the
instanceId variable is passed, instead of the
instance. Since instanceId is a Guid,
which is a struct type data type, not a class, there’s no
issue of referencing. Structs are copied, not referenced. So, they
don’t leak memory. Secondly, both delegates are
released at the end of the workflow execution, thus releasing both
references.

In Dropthings, I am using the famous CallWorkflow Activity by John Flanders, which
is widely used to execute one Workflow from another synchronously.
There’s a CallWorkflowService class which is
responsible for synchronously executing another workflow and that
has similar memory leak problem. The original code of the service
is as following:

public class CallWorkflowService : WorkflowRuntimeService
{
    #region Methods

    public void StartWorkflow(Type workflowType,Dictionary<string,object> inparms, 
Guid caller,IComparable qn) { WorkflowRuntime wr = this.Runtime; WorkflowInstance wi = wr.CreateWorkflow(workflowType,inparms); wi.Start(); ManualWorkflowSchedulerService ss =
wr.GetService<ManualWorkflowSchedulerService>(); if (ss != null) ss.RunWorkflow(wi.InstanceId); EventHandler<WorkflowCompletedEventArgs> d = null; d = delegate(object o, WorkflowCompletedEventArgs e) { if (e.WorkflowInstance.InstanceId ==wi.InstanceId) { wr.WorkflowCompleted -= d; WorkflowInstance c = wr.GetWorkflow(caller); c.EnqueueItem(qn, e.OutputParameters, null, null); } }; EventHandler<WorkflowTerminatedEventArgs> te = null; te = delegate(object o, WorkflowTerminatedEventArgs e) { if (e.WorkflowInstance.InstanceId == wi.InstanceId) { wr.WorkflowTerminated -= te; WorkflowInstance c = wr.GetWorkflow(caller); c.EnqueueItem(qn, new Exception("Called Workflow Terminated",
e.Exception), null, null); } }; wr.WorkflowCompleted += d; wr.WorkflowTerminated += te; } #endregion Methods }

As you see, it has that same delegate holding reference to
instance object problem. Moreover, there’s some queue stuff
there, which requires the caller and qn
parameter passed to the StartWorkflow function. So,
not a straight forward fix.

I tried to rewrite the whole CallWorkflowService so
that it does not require two delegates to be created per Workflow.
Then I took the delegates out. Thus there’s no chance of
closure holding reference to unwanted objects. The result looks
like this:

public class CallWorkflowService : WorkflowRuntimeService
{
    #region Fields

    private EventHandler<WorkflowCompletedEventArgs> _CompletedHandler = null;
    private EventHandler<WorkflowTerminatedEventArgs> _TerminatedHandler = null;
    private Dictionary<Guid, WorkflowInfo> _WorkflowQueue = 
new Dictionary<Guid, WorkflowInfo>(); #endregion Fields #region Methods public void StartWorkflow(Type workflowType,Dictionary<string,object> inparms,
Guid caller,IComparable qn) { WorkflowRuntime wr = this.Runtime; WorkflowInstance wi = wr.CreateWorkflow(workflowType,inparms); wi.Start(); var instanceId = wi.InstanceId; _WorkflowQueue[instanceId] = new WorkflowInfo { Caller = caller, qn = qn }; ManualWorkflowSchedulerService ss =
wr.GetService<ManualWorkflowSchedulerService>(); if (ss != null) ss.RunWorkflow(wi.InstanceId); } protected override void OnStarted() { base.OnStarted(); if (null == _CompletedHandler) { _CompletedHandler = delegate(object o, WorkflowCompletedEventArgs e) { var instanceId = e.WorkflowInstance.InstanceId; if (_WorkflowQueue.ContainsKey(instanceId)) { WorkflowInfo wf = _WorkflowQueue[instanceId]; WorkflowInstance c = this.Runtime.GetWorkflow(wf.Caller); c.EnqueueItem(wf.qn, e.OutputParameters, null, null); _WorkflowQueue.Remove(instanceId); } }; this.Runtime.WorkflowCompleted += _CompletedHandler; } if (null == _TerminatedHandler) { _TerminatedHandler = delegate(object o, WorkflowTerminatedEventArgs e) { var instanceId = e.WorkflowInstance.InstanceId; if (_WorkflowQueue.ContainsKey(instanceId)) { WorkflowInfo wf = _WorkflowQueue[instanceId]; WorkflowInstance c = this.Runtime.GetWorkflow(wf.Caller); c.EnqueueItem(wf.qn,
new Exception("Called Workflow Terminated", e.Exception),
null, null); _WorkflowQueue.Remove(instanceId); } }; this.Runtime.WorkflowTerminated += _TerminatedHandler; } } protected override void OnStopped() { _WorkflowQueue.Clear(); base.OnStopped(); } #endregion Methods #region Nested Types private struct WorkflowInfo { #region Fields public Guid Caller; public IComparable qn; #endregion Fields } #endregion Nested Types }

After fixing the problem, another Memory Profile result showed
the leak is gone:


image

As you see, the numbers vary, which means there’s no
consistent leak. Moreover, looking at the types that remains in
memory, they look more like metadata than instances of
classes. So, they are basically cached instances of metadata,
not instances allocated during workflow execution which are
supposed to be freed. So, we solved the memory leak!

Now you know how to write anonymous delegates without leaking
memory and how to run workflow without leaking them. Basically, the
principle theory is – if you are referencing some outside
object from an anonymous delegate, make sure that
object is not holding reference to the delegate in
some way, may be directly or may be via some child objects of its
own. Because then you have a circular reference. If possible, do
not try to access objects e.g. instance inside an
anonymous delegate that is declared outside the delegate. Try
accessing instrinsic data types like int, string, DateTime, Guid
etc which are not reference type variables. So, instead of
referencing to an object, you should declare local variables e.g.
instanceId that gets the value of properties (e.g.
instance.InstanceId) from the object and then use
those local variables inside the anonymous delegate.

Strongly typed workflow input and output arguments

When you run a Workflow using Workflow Foundation, you
pass arguments to the workflow in a Dictionary form where
the type of Dictionary is Dictionary.
This means you miss the strong typing features of .NET languages.
You have to know what arguments the workflow expects by looking at
the Workflow public properties. Moreover, there’s no
way to make arguments required. You pass parameter, expect it to
run, if it throws exception, you pass more arguments, hope it works
now. Similarly, if you are running workflow synchronously using
ManualWorkflowSchedulerService, you expect return arguments
from the Workflow immediately, but there again, you have to rely on
the Dictionary key and value pair. No strong typing there as
well.

In order to solve this, so that you could pass Workflow
arguments as strongly typed classes, you can establish a format
that every Workflow has only two arguments named
“Request” and “Response” and none other. Whatever
needs to be passed to the Workflow and expected out of it,
must be passed via Request and must be expected via Response
properties. Now the type of these arguments can be workflow
specific, it can be any class with one or more parameters. This
way, you could write code like this:


Running workflow with strongly typed argument

The advantages of these strongly typed approach are:

  • Compile time validation of input parameters passed to workflow.
    No risk of passing unexpected object in Dictionary’s
    object type value.
  • Enforce required values by creating Request objects with
    non-default constructor.
  • Establish a fixed contract for Workflow input and output via
    the strongly typed Request and Response classes or interfaces.
  • Validate input arguments for the Workflow directly from the
    Request class, without going through the overhead of running a
    workflow.

If we follow this approach, we create workflows with only two
DependencyProperty, one for Request and one for
Response. Showing you an example from my open source project
Dropthings, which uses Workflow for the entire
Business Layer. Below you see the Workflow that executes when a new
user visits Dropthings.com, creates a new user and setups all the
pages and widgets for the user. It has only two Dependency
property – Request and Response.


image

The Request parameters is of type
IUserVisitWorkflowRequest. So, you can pass any class as
Request argument that implements the interface.


image

Here I have used fancy inheritance to create Request object
hierarchy. You don’t need to do that. Just remember, you can
pass any class. You don’t even need to use interface for
Request parameter. It can be a class directly. I use all these
interfaces in order to facilitate Dependency Inversion.

Similarly, the Response object is also a class.


image

The Response returns quite some properties. So, it’s kinda
handy to wrap them all in one property.

So, there you have it, strongly typed Workflow arguments. You
can attach properties of the Request object to any activity
directly form the designer:


image

There’s really no compromise to make in this approach.
Everything works as before.

In order to make workflow execution simpler, I use a helper
method like the following, that takes the Request and Response
object and creates the Dictionary for me. This Dictionary always
contains one “Request” and one “Response”
entry.


image

This way, I can run Workflow in strongly typed fashion:


image

Here I can specify the Request, Response and Workflow type using
strong typing. This way I get strongly typed return object as well
as pass strongly type Request object. There’s no dictionary
building, no risky string key and object type value passing.
You can ignore the ObjectContainer.Resolve() stuff, because
that’s just returning me an existing reference of
WorkflowRuntime.

Hope you like this approach.

My first book – Building a Web 2.0 Portal with ASP.NET 3.5

My first book “Building a Web 2.0 Portal with ASP.NET 3.5” from
O’Reilly is published and available in the stores. This book
explains in detail the architecture design, development, test,
deployment, performance and scalability challenges of my open
source web portal Dropthings.com. Dropthings is a prototype of a web
portal similar to iGoogle or Pageflakes. But this portal is developed using
recently released brand new technologies like ASP.NET 3.5, C# 3.0,
Linq to Sql, Linq to XML, and Windows Workflow foundation. It makes
heavy use of ASP.NET AJAX 1.0. Throughout my career I have built
several state-of-the-art personal, educational, enterprise and mass consumer web
portals
. This book collects my experience in building all of
those portals.

O’Reilly Website:
http://www.oreilly.com/catalog/9780596510503/

Amazon:

http://www.amazon.com/Building-Web-2-0-Portal-ASP-NET/dp/0596510500

Disclaimer: This book does not show you how to build Pageflakes.
Dropthings is entirely different in terms of architecture,
implementation and the technologies involved.

You learn how to:

  • Implement a highly decoupled architecture following the popular
    n-tier, widget-based application model
  • Provide drag-and-drop functionality, and use ASP.NET 3.5 to
    build the server-side part of the web layer
  • Use LINQ to build the data access layer, and Windows Workflow
    Foundation to build the business layer as a collection of
    workflows
  • Build client-side widgets using JavaScript for faster
    performance and better caching
  • Get maximum performance out of the ASP.NET AJAX Framework for
    faster, more dynamic, and scalable sites
  • Build a custom web service call handler to overcome
    shortcomings in ASP.NET AJAX 1.0 for asynchronous, transactional,
    cache-friendly web services
  • Overcome JavaScript performance problems, and help the user
    interface load faster and be more responsive
  • Solve various scalability and security problems as your site
    grows from hundreds to millions of users
  • Deploy and run a high-volume production site while solving
    software, hardware, hosting, and Internet infrastructure
    problems

If you’re ready to build state-of-the art, high-volume web
applications that can withstand millions of hits per day, this book
has exactly what you need.