Thursday, June 21, 2012

An Inversion of Control Container in Dojo Javascript Framework



A year since my last article was published! I just opened my account for this year's articles yesterday. My article An Inversion of Control Container in Dojo Javascript framework got published in DZone.


Inversion of Control (IoC) / Dependency Injection (DI) has become a very popular design and architectural standard in enterprise programming. Conceptually it enables a container to manage the lifecycle and dependencies between objects - something which need not be purely exclusive to server-side enterprise development. In fact, loosely typed languages like JavaScript could be far more amenable to this concept as they obviate the need for reflection mechanisms for injecting and resolving dependencies. This article demonstrates how a simple Inversion of Control (IoC)/ Dependency Injection container can be easily prepared with JavaScript. Owing to the author's experience, the article uses Dojo framework, but it should be possible to do so using other object-oriented JavaScript frameworks or plain Javascript. The existence of open source JavaScript DI frameworks like Wire.js and Squirrel IOC are testimonial to the fact that IoC / DI is being seriously considered in the JavaScript community. This article seeks to demonstrate the design under the hood to make IoC or DI possible, the benefits of wiring using dependency injection and encourage its adoption with JavaScript application developers. It does so by walking through a code sample.


Read the full post at any of these DZone links - 


http://java.dzone.com/articles/inversion-control-container
http://architects.dzone.com/articles/inversion-control-container
http://css.dzone.com/articles/inversion-control-container


Thursday, June 30, 2011

High Performance and Smarter Logging

A long gap since I lasted posted something. Today my article High Performance and Smarter Logging has got published in DZone.

Logging, often a back-burner activity in the development cycle can actually be the very lifeline of the development team once the application is in production. Assuming that readers are well acquainted with various logging frameworks like Log4J, SLF4J, etc, this article lays guidelines for 'real-world' production logging, examines its influence on the application quality attributes and also rediscovers a forgotten high performance, smarter logging technique with codes and centralized logging.

Read the full post at DZone.

Tuesday, March 1, 2011

The Dilemma of The Framework Creator

Developing or contributing to a framework used by a majority of the software community is one of the prime sources of career satisfaction for a whole lot of software developers, designers and architects. A great framework can be an object of beauty to them, just as a fine cathedral is for its architects - an expression of their creativity.
But a framework can be sustainable and useful only if it is designed with certain principles. This post is an attempt to find what it takes to make a sustainable and widely accepted software framework.
Read the complete post here.

Saturday, August 14, 2010

Just FYI

This is to inform possible readers of my blog that my article "Do we really need Servlet Containers always?" was published in http://www.theserverside.com/discussions/thread.tss?thread_id=60607. Also got linked my post "How JBoss Netty helped me build a highly scalable HTTP Communication Server over GPRS" on the Netty community site.

I will be writing more posts on this blog shortly.

Saturday, July 17, 2010

Do we really need Servlet Containers always - concluding part

Building an HTTP server engine using JBoss Netty and using Apache Web Server for load-balancing with sticky-sessions

JBoss Netty has been my first choice for building a scalable HTTP server engine sans Servlet API and Containers, ever since I first discovered it around 1.5 years back. Its USP is its simplicity combined with extreme performance. It is primarily an NIO framework which provides ready-to-use HTTP Protocol decoder and encoder along with a basic API for HTTP request and response, with which we can very simply and quickly build a scalable HTTP server. The performance test results and testimonials are proof of its excellence, for various protocols, including HTTP, vis-à-vis other NIO frameworks as well as HTTP Servlet containers like Jetty and Tomcat.

The documentation is very comprehensive and intuitive with a number of examples.

Here, I shall show a simple example of creating HTTP server with JBoss Netty which creates and provides a session identity to the user. When the user connects by providing the session identity as a URL parameter and without the name, it welcomes the user with its name. We will use this simple server as an example to do reverse proxy and then load balancing with sticky-sessions with Apache Web Server’s mod_proxy and mod_proxy_balancer modules.

First, we need to create a handler, much like a Servlet that would receive HTTP requests. HTTP requests can be chunked and would have to be combined at the server side before processing. This chunking is not typical of high-bandwidth Internet requests, but is very much possible with GPRS medium. So first, we have an abstract template handler for receiving HTTP requests.


package apachetest;

import org.jboss.netty.buffer.ChannelBuffer;
import org.jboss.netty.buffer.ChannelBuffers;
import org.jboss.netty.channel.Channel;
import org.jboss.netty.channel.ChannelHandlerContext;
import org.jboss.netty.channel.ChannelPipelineCoverage;
import org.jboss.netty.channel.ChannelStateEvent;
import org.jboss.netty.channel.ExceptionEvent;
import org.jboss.netty.channel.MessageEvent;
import org.jboss.netty.channel.SimpleChannelUpstreamHandler;
import org.jboss.netty.channel.group.ChannelGroup;
import org.jboss.netty.channel.group.DefaultChannelGroup;
import org.jboss.netty.handler.codec.http.HttpChunk;
import org.jboss.netty.handler.codec.http.HttpRequest;

@ChannelPipelineCoverage("one")
public abstract class AbstractHttpHandler
extends SimpleChannelUpstreamHandler {

protected HttpRequest currentRequest;

public static ChannelGroup allChannels = new DefaultChannelGroup
("HttpServer");

@Override
public void messageReceived(ChannelHandlerContext ctx,
MessageEvent e)
throws Exception {
Object message = e.getMessage();
Channel userChannel = e.getChannel();
boolean processMessage = false;
if (message instanceof HttpChunk) {
HttpChunk httpChunk = (HttpChunk) message;
if (currentRequest==null)
throw new
IllegalStateException("No chunk start");
ChannelBuffer channelBuffer = currentRequest
.getContent();
if (channelBuffer==null)
throw new
IllegalStateException("No chunk start");
ChannelBuffer compositeBuffer =
ChannelBuffers.wrappedBuffer(channelBuffer,
httpChunk.getContent());
currentRequest.setContent(compositeBuffer);
processMessage = httpChunk.isLast();
} else if (message instanceof HttpRequest){
currentRequest = (HttpRequest) message;
processMessage = !currentRequest.isChunked();
}
if (processMessage) {
handleRequest(currentRequest, userChannel);
}
}

protected abstract void handleRequest(HttpRequest request,
Channel userChannel) throws Exception;

@Override
public void channelOpen(ChannelHandlerContext ctx,
ChannelStateEvent e) throws Exception {
allChannels.add(e.getChannel());
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx,
ExceptionEvent e) throws Exception {
e.getCause().printStackTrace();
e.getChannel().close();
}
}

The above AbstractHttpHandler extends JBoss Netty’s SimpleChannelUpstreamHandler and extends its messageReceived method to assemble HTTP chunks, if required to a complete HTTP Request by simply appending the content body. Once a complete HTTP Request is available, it calls the abstract handleRequest method, whose implementation is left to base classes, with the HttpRequest and the channel from which the request was obtained. The ChannelPipelineCoverage is kept as one so that for every channel connection, a new handler will be assigned. This helps us to ensure that we are dealing with a single channel in every handler and so we can safely assemble their chunked requests.

When a new channel (socket) is opened, it is added to an allChannels group. This group enables to close all client and server sockets gracefully during shutdown with a single method call. We will see this in more detail shortly in another code snippet.

Now, we will create an implementation of this AbstractHttpHandler which assigns unique sessions to users and later identifies the users from their sessions.


package apachetest;
import java.util.List;
import java.util.Map;

import org.jboss.netty.buffer.ChannelBuffers;
import org.jboss.netty.channel.Channel;
import org.jboss.netty.channel.ChannelFuture;
import org.jboss.netty.channel.ChannelFutureListener;
import org.jboss.netty.channel.ChannelPipelineCoverage;
import org.jboss.netty.handler.codec.http.DefaultHttpResponse;
import org.jboss.netty.handler.codec.http.HttpHeaders;
import org.jboss.netty.handler.codec.http.HttpRequest;
import org.jboss.netty.handler.codec.http.HttpResponse;
import org.jboss.netty.handler.codec.http.HttpResponseStatus;
import org.jboss.netty.handler.codec.http.HttpVersion;
import org.jboss.netty.handler.codec.http.QueryStringDecoder;

@ChannelPipelineCoverage("one")
public class SimpleHttpHandler extends AbstractHttpHandler {

private String serverInstanceId;

private String sessionInstanceSeparator;

private static ThreadLocal sessionIdBufLocal
= new ThreadLocal(){

@Override
public StringBuffer get() {
StringBuffer strBuf=super.get();
strBuf.setLength(0);
return strBuf;
}

@Override
protected StringBuffer initialValue() {
return new StringBuffer(50);
}

};

public SimpleHttpHandler(String serverInstanceId,
String sessionInstanceSeparator) {
this.serverInstanceId = serverInstanceId;
this.sessionInstanceSeparator = sessionInstanceSeparator;
}


@Override
protected void handleRequest(HttpRequest request,
Channel userChannel) throws Exception {
String requestUri = request.getUri();
QueryStringDecoder queryStringDecoder = new
QueryStringDecoder(requestUri);
Map> params =
queryStringDecoder.getParameters();
String sessionId = null;
int indexOf = requestUri.indexOf("sessionTest");
if (indexOf==-1) {
sendResponse(request, userChannel,
HttpResponseStatus.NOT_FOUND,
"Valid URI supported is /sessionTest");
return;
}

List sessionList = params.get("jsessionid");
if (sessionList!=null && !sessionList.isEmpty()) {
String sessionInList = sessionList.get(0);
if (sessionInList!=null &&
!"".equals(sessionInList.trim())) {
sessionId = sessionInList;
}
}
List userList = params.get("user");
String user = null;
if (userList!=null && !userList.isEmpty()) {
user = userList.get(0);
}
if (sessionId==null) {
if (user==null || "".equals(user)) {
sendResponse(request, userChannel,
HttpResponseStatus.UNUATHORIZED,
"No valid session, please login with user");
return;
}
UserSessionDetails userSessionDetails =
SessionIdHolder.userIdToUserMap.get(user);
if (userSessionDetails==null) {
userSessionDetails = new UserSessionDetails();
userSessionDetails.userChannel = userChannel;
userSessionDetails.userId = user;
SessionIdHolder.userIdToUserMap
.putIfAbsent(user, userSessionDetails);
userSessionDetails =
SessionIdHolder.userIdToUserMap.get(user);
}
synchronized(userSessionDetails) {
if (userSessionDetails.sessionId!=null) {
SessionIdHolder.sessionIdToUserMap.
remove(userSessionDetails.sessionId);
}
sessionId = userSessionDetails.sessionId =
IdGenerator.generateUniqueId();
userSessionDetails.sessionCreationTime =
System.currentTimeMillis();
userSessionDetails.lastAccessTime =
System.currentTimeMillis();
SessionIdHolder.sessionIdToUserMap.put(sessionId,
userSessionDetails);
SessionIdHolder.userIdToUserMap.put(user,
userSessionDetails);
}

StringBuffer sessionIdToReturn =
                            sessionIdBufLocal.get();
   sessionIdToReturn.append(sessionId);
if (serverInstanceId!=null &&
!"".equals(serverInstanceId)) {
sessionIdToReturn.append(
                              sessionInstanceSeparator)
.append(serverInstanceId);
}
sendResponse(request,userChannel,
HttpResponseStatus.OK,
"Session Id is "+sessionIdToReturn);

} else {
//strip off the identity of the server
int serverIdIndex =
sessionId.indexOf(sessionInstanceSeparator);
if (serverIdIndex>=0)
sessionId = sessionId.substring(0,
                              serverIdIndex);
UserSessionDetails userSessionDetails =
SessionIdHolder.sessionIdToUserMap
                              .get(sessionId);
if (userSessionDetails==null) {
sendResponse(request, userChannel,
HttpResponseStatus.UNUATHORIZED,
"No valid session, please login with user");
return;
}
synchronized(userSessionDetails) {
userSessionDetails.userChannel = userChannel;
userSessionDetails.lastAccessTime =
System.currentTimeMillis();
UserSessionDetails userDetails =
SessionIdHolder.userIdToUserMap
.get(userSessionDetails.userId);
if (userDetails==null ||
userDetails!=userSessionDetails) {
SessionIdHolder.userIdToUserMap
.put(userSessionDetails.userId,
userSessionDetails);
}
}
sendResponse(request,userChannel,
HttpResponseStatus.OK,
"Welcome user "+userSessionDetails.userId
+" to server "+serverInstanceId);
}
}

private void sendResponse(HttpRequest request,
                Channel userChannel,
HttpResponseStatus responseStatus, String responseBody)
throws Exception {
boolean close =
HttpHeaders.Values.CLOSE
.equalsIgnoreCase(request
.getHeader(HttpHeaders.Names.CONNECTION)) ||
request.getProtocolVersion()
.equals(HttpVersion.HTTP_1_0) &&
!HttpHeaders.Values.KEEP_ALIVE
.equalsIgnoreCase(request
.getHeader(HttpHeaders.Names.CONNECTION));
HttpResponse response = new DefaultHttpResponse
(HttpVersion.HTTP_1_1, responseStatus);
response.setHeader(HttpHeaders.Names.CACHE_CONTROL,
                  "no-store");
response.setHeader(HttpHeaders.Names.PRAGMA, "no-cache");
response.setHeader(HttpHeaders.Names.CONTENT_LENGTH,
""+((responseBody==null)?0:responseBody.length()));
byte[] array = responseBody.getBytes("UTF8");
if (responseBody!=null)
response.setContent(
                    ChannelBuffers.wrappedBuffer(array));
ChannelFuture future = userChannel.write(response);
if (close) {
future.addListener(ChannelFutureListener.CLOSE);
}
}
}


The SimpleHttpHandler is created with a server instance id and a session instance separator. Each server is provided a unique identity in a cluster of servers. Sessions created for users would be given a session id with the following format - . Thus, if the server instance name is ‘netty1’ and session instance separator is ‘.’ and the unique id created is ‘2C855D9FB0710111’, then the session id given to the user would be the concatenated value ‘2C855D9FB0710111.netty1’. This would help Apache Web Server to maintain sticky sessions, as it would be able to parse the session id and find that it has been created by netty1 instance and all the next requests with this session id must be passed to this instance.

In our handleRequest implementation, we simply generate a new session id for every new user using our own IdGenerator. Notice how we can use the request.getUri() to check that the requests have been made to the expected URI which is ‘/sessionTest’. The QueryStringDecoder from the Netty API helps in extracting request parameters as name-value(s) map. If the request provides the username, we simply create a new session id and store the user session details in our local memory map in SessionIdHolder. We send the response to the user – ‘Session ID is <…..>’. In the next request, the user can copy this session id from the response and send it as a request parameter. Then the server extracts the user session details mapped to the session id, after stripping off the server instance id, and finds out the corresponding user name. It sends the response ‘Welcome user <….> to server <…>’. If it does not find the session in its map, it returns the response ‘No valid session, please login with user’.

The sessions can be shared across servers using replicated caching or a shared database. However, to keep our example simple, and to illustrate load balancing with session stickiness, we have not replicated the sessions.

The sendResponse method sends a response to the user channel from which the request was received. It sets the desired response status – 200 for OK, 403 for UNAUTHORIZED. The HttpResponseStatus from Netty API provides all the HTTP response constants. We also set cache-control and pragma headers to ensure that responses are not cached by the client browser. We also set the content-length header. If the request had a CLOSE header or was using HTTP1.0 protocol with no KEEP_ALIVE header, we close the client socket channel after providing the response. If not, the same socket channel will be used for multiple HTTP request-response cycles between the client and server.

Now that we have the handlers ready, we write the main class to start the HTTP server.


package apachetest;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.util.concurrent.Executors;

import org.jboss.netty.bootstrap.ServerBootstrap;
import org.jboss.netty.channel.Channel;
import org.jboss.netty.channel.ChannelPipeline;
import org.jboss.netty.channel.ChannelPipelineFactory;
import org.jboss.netty.channel.Channels;
import org.jboss.netty.channel.group.ChannelGroupFuture;
import org.jboss.netty.channel.socket.nio.NioServerSocketChannelFactory;
import org.jboss.netty.handler.codec.http.HttpRequestDecoder;
import org.jboss.netty.handler.codec.http.HttpResponseEncoder;

public class SessionTestServer {

public static void main(String[] args) {
int port = 9080;
String[] arguments = new String[] {"","."};
if (args.length>0 && !"".equals(args[0])
                    && args[0]!=null) {
arguments[0] = args[0];
}
if (args.length>1 && !"".equals(args[1])
                    && args[1]!=null) {
arguments[1] = args[1];
}
if (args.length>2 && !"".equals(args[2])
                    && args[2]!=null) {
try {
int portGiven = Integer.parseInt(args[2]);
if (portGiven<=1024 || portGiven>9999);
else
port = portGiven;
} catch (Exception e) { }
}

String serverInstanceId = arguments[0];

String sessionInstanceSeparator = arguments[1];
NioServerSocketChannelFactory factory = new
NioServerSocketChannelFactory(
Executors.newCachedThreadPool(),
Executors.newCachedThreadPool());
ServerBootstrap bootstrap = new ServerBootstrap(factory);
bootstrap.setPipelineFactory(new
HttpServerPipelineFactory(serverInstanceId,
sessionInstanceSeparator));
Channel serverChannel =
bootstrap.bind(new InetSocketAddress(port));
if (serverChannel==null) {
System.out.println("Unable to open
                            server at port "+port);
factory.releaseExternalResources();
return;
}
AbstractHttpHandler.allChannels.add(serverChannel);
System.out.println("Started server at port "+port
+" Press any key to stop server");
try {
System.in.read();
} catch (IOException e) {
} finally {
System.out.println("Shutting down server");
ChannelGroupFuture future =
AbstractHttpHandler.allChannels.close();
future.awaitUninterruptibly();
factory.releaseExternalResources();
}
}

private static class HttpServerPipelineFactory implements
ChannelPipelineFactory {

private String serverInstanceId;
private String sessionInstanceSeparator;

public HttpServerPipelineFactory(String serverInstanceId,
                String sessionInstanceSeparator) {
this.serverInstanceId = serverInstanceId;
this.sessionInstanceSeparator =
                          sessionInstanceSeparator;
}
@Override
public ChannelPipeline getPipeline() throws Exception {
ChannelPipeline pipeline = Channels.pipeline();
pipeline.addLast("decoder",
                             new HttpRequestDecoder());
pipeline.addLast("encoder",
                             new HttpResponseEncoder());
pipeline.addLast("handler", new
SimpleHttpHandler(serverInstanceId,
sessionInstanceSeparator));
return pipeline;
}
}
}

The program accepts the server instance id, the session separator and the server port number to bind for accepting client requests in the command-line arguments. These arguments are optional, the default value for session separator is ‘.’ and the server port number is 9080.

Like any other server-side implementation using Netty, we create a NioSocketChannelFactory and bootstrap the server with our ChannelPipelineFactory implementation and bind it to the acceptor port. We also add the server channel to the allChannels group and wait for user key stroke to stop the server. While stopping the server, we need to simply close the allChannels group and release external resources of our factory, which would stop underlying thread pools.

Now let us look at the ChannelPipelineFactory implementation. For every new client channel that connects to the server, a new pipeline (which is based on interceptor-chain design pattern) is created. The ready-made Netty HttpRequestDecoder and HttpResponseDecoder, and finally our very own SimpleHttpHandler are added to the pipeline in that order.

This is all that is needed to create a simple HTTP server using Netty API.

Now let us start the server using the command line

java apachetest.SessionTestServer netty1 . 9080

This starts a server listening for HTTP requests at port 9080 and with server instance id ‘netty1’ and session id separator as ‘.’ We get the command-line print-


Started server at port 9080 Press any key to stop server

We leave the server running and open a new browser window. We give the URL address as http://localhost:9080/sessionTest?user=Archanaa and press enter.

We get back a response on the browser window – ‘Session id is ….’

Now, we copy the session id and change the URL address at the browser to http://localhost:9080/sessionTest?jsessionid=0140C3892B4F5348.netty1

- where the ‘jsessionid’ request parameter value would be the session id which was assigned to us, in the last response.

We now get the response – ‘Welcome user Archanaa to server netty1’

At the console where netty1 process has been started, press any key (like Enter). The server process will stop with the message – ‘Shutting down server’

Now, let us use Apache Web Server for load-balancing two such HTTP servers with session stickiness.

After installing the Apache Web Server, which would listen at port 80 on your machine, go to conf/ directory and make the following changes in httpd.conf configuration file.

  • Uncomment the modules mod_proxy, mod_proxy_balancer, mod_proxy_http and mod_rewrite.
  • Add the following VirtualHost configuration to the end of the httpd.conf file.




ServerAdmin archanaa.panda@gmail.com
DocumentRoot "C:/apps/Apache Software Foundation/Apache2.2/htdocs"
ProxyRequests Off
ErrorLog logs/test.in-error_log
CustomLog logs/test.in-access_log common

Order deny,allow
Allow from all


ProxyPreserveHost On
ProxyPass /sessionTest/ balancer://mycluster/ stickysession=jsessionid
ProxyPass /sessionTest balancer://mycluster stickysession=jsessionid

Order deny,allow
Allow from all
BalancerMember http://localhost:9080/sessionTest route=netty1
BalancerMember http://localhost:9081/sessionTest route=netty2


SetHandler balancer-manager
Order deny,allow
Allow from all




We intend to start two servers with names netty1 and netty2 at ports 9080 and 9081, respectively. We have specified these URLs and their server instances in the BalancerMember configuration under Proxy tag. In the ProxyPass configuration, we have specified the stickysession as ‘jsessionid’, so that Apache would look for it in the HTTP request parameters or the cookies. We will pass the sessionid in the HTTP request parameter, as we had done earlier. We will choose the session id separator as ‘.’ Because that is the default separator recognized by Apache Web Server.

Now let us start the two server instances like so-

java apachetest.SessionTestServer netty1 . 9080
java apachetest.SessionTestServer netty2 . 9081

After that, let us start the Apache httpd

With this, the Apache Web Server will now act as a reverse-proxy as well as load balancer with sticky-sessions for the two server instances.

To test that, let us connect with the browser URL http://localhost/sessionTest?user=Archanaa
We are provided with a user session id –

Similarly, in a different browser window or tab, let us connect with a different user http://localhost/sessionTest?user=Ankur and we would get a new session id.

Now let us change the request to give the jsessionid parameter in the request instead of the user and see the results.

No matter how many times we connect with the same session id and in whatever order, we always get the response from the correct server showing that session stickiness is working properly.

Now let us stop one particular server instance, say netty1 and try to connect its user. Now if we fire a request from our browser with netty1’s session, it would be failed over to the next instance, that is netty2. Since we have not replicated our session, we get the response –

Meanwhile, the other user is able to connect as before successfully with netty2’s session.

Thus we see that our simple experiment has worked well.

There is no limit to how we can use JBoss Netty to build our own HTTP server. In the above example, we have kept the handler synchronous. But there is no restriction for saving the HttpRequest along with a unique asynchronous completion token in a map and doing asynchronous request processing in a different thread. Once the request processing is done, the request can be retrieved from the map and we can send the response as we have done in this simple example above. To add robustness, we can keep a time-to-live for requests and on timeout, remove the corresponding request and send a REQUEST_TIMEOUT response to the client.

With that I conclude this series and I hope I have been able to showcase a fine alternative to using Servlet API and Containers for building simple HTTP servers.


Sunday, July 11, 2010

Do we really need Servlet Containers always? Part 4

Other Features

Session Management, Replication and load-balancing with sticky sessions

The HttpSession provided by Servlet specification is, for most cases, an ideal context for storing user state and data in memory. However, this context can only be obtained from the reference of the request object.

Suppose you want to push frequent updates to the user from the server-side, this context would not be available without access to the request object. If such data is stored in the database, for every user request, the database would have to be accessed to check for and retrieve updates, leading to severe loss of performance and scalability. The ideal place to store this data is in memory. In the absence of access to the user’s HTTP request, you would look for some other means to store this data – possibly a distributable and replicable cache such as JBossCache or Terracotta. Frameworks such as Hibernate can be configured to use such caches for second level caching. This somewhat limits the usefulness of sessions to store user related data in memory. Of course, they still have their usefulness with respect to transparent access to cookies.

But the point of this discussion is anyway not restricted to the usual browser-based thin HTML GUI web applications, but rather with respect to other special cases that we have enumerated earlier in the premise. In fact, web-services exposed for client applications using either SOAP or REST APIs are designed to be as idempotent and stateless as possible. So, in such applications, one would not need HttpSessions anyway.

If you need session identifiers just for tracking that the connected device or client has been authenticated before, this can also be achieved simply by using custom logic for creating and storing sessions in memory and even replicating them across the cluster. It is also fairly trivial for the client application to use URL-rewriting or URL parameter or embed the session identifier in the application protocol itself to use this session identifier. Or, one may use JBossCache and Terracotta for this purpose as well. For a simple use like this, a custom solution may offer more freedom and developer control than using HttpSession.

This brings us to the discussion of load-balancing with a web-server reverse proxy using sticky sessions. Most application servers and containers provide plug-ins for major web-servers like Apache HTTPD. With these plug-ins the web-servers can do a reverse-proxy for application servers or containers and in case of fronting an entire cluster, they can identify the correct backend server to stick the user to. However, by using Apache HTTPD’s mod_proxy and mod_proxy_balancer, one may easily achieve these objectives for any custom HTTP server or engine. I have successfully done so myself and would soon be posting an article for the same.

JSP Support

For the scenarios under discussion, JSP support is clearly not required.

Declarative security (by adhering to JAAS Security Standards)

Servlet containers provide adherence to Java Authentication and Authorization Service (JAAS) Security standards. This feature is undeniably extremely useful for web-site based applications. However, as a user of JAAS myself, I have found it very difficult to understand and extend, especially when I did not have the option of using ready-made Servlet container JAAS plug-ins. For example, I once had to authenticate and authorize users from an Oracle Internet Directory credential repository, for which I had to make a custom JAAS module. Configuring custom JAAS modules is not standardized across containers and can be quite non-trivial.

JAAS is not the only option. For example, one may use Spring security (formerly known as Acegi). Especially for the case when you do not have to extend the authentication and authorization for container resources like EJB or use it in JSP scripting with Servlet request API like isUserInRole(..) or getUserPrincipal(..), it is better to implement a simple and custom security module rather than overly complicating the whole thing just to adhere to standards. Besides, as discussed in the case of Communications, Multi-threading and Request-Response lifecycle, you cannot be absolutely aware of how the Servlet Container of your choice actually operates beneath the layers and for all you know, it could be sub-optimal for your scenario.

Deployment contract

By definition, a container is one that would run the whole show- managing the life-cycle of your application and resources. It is the container that starts up and passes control to your application, not the other way round. While this helps in having a standard deployment and packaging contract for your application, it can be sometimes an unnecessary pain, especially for the scenarios enlisted in the premise. Containers may have elaborate steps to deploy and update applications and may take time to startup and shutdown due to the vast array of services that they have to gracefully start and stop. It is rather a welcome feature that some containers like Jetty or Tomcat can run as embedded engines, that is they can be started up by the application by using appropriate API.

There is one other aspect that becomes trivialized by the deployment contract of Application Servers and Containers. Typically Application Servers can deploy any number of web and other JavaEE applications within a single instance and all these applications can share the server resources. This is not necessarily a good thing because, after all, each instance is a single JVM or process and if it crashes, it can bring down all the applications hosted in it. Also, the applications need not always share the same application server resources and the sharing would in fact be detrimental, considering that, unless properly configured, all applications may be sharing the same thread-pool or JDBC connection pool and competing with each other for resources. These points are often overlooked and application developers forget that they have to plan how to deploy their components in different application server instances.

Standard HTTP request-response API

The Servlet Specification provides a standard API for HTTP server side applications, albeit with certain constraints and loss of freedom. If however, you choose to use a Server or Engine, they too would provide facilities to access HTTP headers, URL parameters, request body and so on. The lock-in to specific API has not been so much of a concern to users of Apache Commons HTTP Client, Spring, Hibernate, as long as the API suits the purpose to a T.


Saturday, July 10, 2010

Do we really need Servlet Containers always? Part 3


Request-Response Lifecycle and Multi-threading


Whenever a Servlet’s service()/doGet()/doPost() method is invoked, the container provides it an HTTP request and response object. What happens, when the Servlet method returns? As per the Servlet specification up to 2.5, the request and response objects can be recycled by the container and hence –

Each request/response object is valid only within the scope of a Servlet’s service method, or within the scope of a filter’s doFilter method. Containers commonly recycle request/response objects in order to avoid the performance overhead of request object creation. The developer must be aware that maintaining references to request/response objects outside the scope described above may lead to non-deterministic behavior.



The pooling of objects used to be a best practice to mitigate the performance overhead of early JVMs. But it is rendered less useful with modern JVMs and this approach now presents a serious constraint. The constraint is that you cannot save the reference to the request or response object and pass it on to a different thread context. As soon as the Servlet method returns, for all you know, the request and response objects would be recycled and reused for a completely different connection. The non-deterministic behavior, if you try to asynchronously handle the request and response, can be for example that the response you give may be delivered to a completely different client or else that the request you are trying to read may be suddenly over-written by some other client’s request. So you are then forced to do all the processing in the context of the container thread.



Is that a problem? Well, most of us have developed applications with a database backend. This can be a real bottleneck. Suppose, there is a database query which, even with maximum optimization, can only give results in 2 seconds. For those two seconds, the Servlet Container thread is blocked and cannot be used for handling another incoming request. Similarly other threads may also be similarly blocked waiting for their results and some other threads may be blocked waiting for JDBC Connections to be available to the pool. This, therefore, brings down the throughput (number of transactions per unit time that can be handled). So, what’s the solution? Increasing the thread pool size of the container to a 100? But can you also similarly increase the maximum number of connections you can have in the JDBC connection pool? So, there is no use increasing the thread pool of the Servlet Container when you have limited backend resources.



Moreover, you cannot use asynchronous techniques like passing pending requests to a JMS or simple Java queue, since the Servlet specification constraint implies that the moment the method call to the Servlet returns, it has lost the response object and therefore it cannot render the response to the user’s request in a different thread. You could possibly use a round-about technique of handing an Asynchronous completion token for the initial request and then use Ajax polling with the token to check if the corresponding response is ready. But this is rather like arm-twisting the application just to fit the Servlet Container constraints and would also result in lots of unnecessary pings from the client-side which would anyway bring down the actual throughput.

So, even if you have divided the application into presentation, business logic and data access layer, they become tightly coupled with respect to their dynamic execution and the slowness of one layer adversely impacts the throughput and scalability of another layer.


If you look at the CPU utilization of any particular server, it is bound to be under-utilized. If you increase the number of threads, the throughput does not increase but due to heavy context-switching, the CPU utilization would shoot up. What is the reason? Whenever there is any thread waiting for I/O to occur or receiving data from the database over a network call, it would be swapped for another thread which can do CPU based work. In the above case, nearly all threads that are working for the application would be blocked for more time than actually doing CPU related processing. This is not the best way of using the CPU resources.

To manage a high load with such architecture, you would have to replicate the application across several machines and do horizontal scaling. That would be good news for commercial application server and hardware vendors – you would have to multiply your hardware and licensing costs. It is bad news for application providers, unless they are already used to raising a large bill of material.

But for this constraint of the Servlet specification, these applications could have been designed in an asynchronous way. In contrast, look at the alternative of keeping the layers asynchronously decoupled as follows-



With the ability to store references to request and response references in memory, now one is free to loosely decouple the layers and queue up requests for processing and response data to be rendered.

The benefits can be profound. I have recently used the above approach of keeping the layers loosely coupled and found that I could attain a huge throughput from one machine itself whereas in the earlier approach I would have required multiple horizontally clustered machines and application servers. The reason, I believe, is that here we are decoupling the layers such that each layer has its own thread or thread pool concentrating on the specific responsibilities of that layer; rather than the earlier approach of making threads span through all three layers. The front layer concentrates on I/O and very little CPU processing; the business processing layer would typically have more of CPU-bound activities; whereas the Data Access/EAI layer would again have more of I/O related activities. This division enables the CPU to more optimally switch its attention to different layers. Moreover, one layer does not affect the lifecycle and processing of another layer. Thus the front layer is free to accept more and more HTTP requests and work on them.

To design such an application, though, one must be careful to appropriately time-out and clean up request and response objects, not to mention a more sophisticated programming and design with multiple threads to keep the layers loosely coupled.

In fact, possibly realizing this, Servlet 3.0 specification has sought to remedy this by introducing Asynchronous request processing. With Servlet 3.0, The HttpServletRequest has a new method startAsync() which returns an AsyncContext object that caches the request/response object pair. This AsyncContext object can be stored in a memory-repository and the servlet method may return immediately so that more requests can be handled. Additionally, the HttpServlet should be annotated with the asyncSupported attribute set to true so that the Servlet container does not commit the response when the Servlet method returns.


@WebServlet(name=”frontServlet”, urlPatterns={“/*.do”}, asyncSupported=true)
public class FrontServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response) {
AsyncContext aCtx = request.startAsync(request, response);
Queue bizLayerQueue = BusinessLayer.getRequestQueue();
bizLayerQueue.add(aCtx);
}
}
public class FrontResponseThread extends Thread {
Queue bizLayerResponseQueue = BusinessLayer.getResponseQueue();
public void run() {
while (true) {
AsyncContext asyncContext = bizLayerResponseQueue.take();
ServletRequest request = aCtx.getRequest();
//get parameters and render the response
asyncContext.forward(“/result.jsp”);
}
}
}

Servlet 3.0 would help standardize the asynchronous request processing API for the JavaEE developer community. But it would take some time for Application Servers to be JavaEE 6 compliant with stable versions of their product. The good news is that most of the application servers already support Asynchronous Request Processing with non-standard APIs and at the risk of sacrificing portability, one may use these APIs till then.

  • Tomcat 6 provides a CometProcessor interface which can be implemented by Servlets.
  • Jetty 6 provides Continuations
  • WebLogic 9.2 provides AbstractAsynchServlet and FutureResponseServlet
  • Websphere version 7 provides Asynchronous Request Dispatcher API

But the fact remains that most of the frameworks built around this Servlet model and based on the Core J2EE Patterns, be they presentation frameworks like Struts, JSF, Spring MVC or bridge frameworks like Seam, is synchronous and they have not yet advocated any samples using Asynchronous Request Processing paradigm. And most of the JavaEE developer community is heavily influenced by reference architectures and blueprints like the Java Pet Store application and does not follow out-of-the-box approaches.

In my opinion, this has been the result of the mentality that Servlet Containers are the be-all and end-all solution for HTTP server side applications and restricting oneself within the confines of the Servlet framework.