Any architecture is a result of what that architecture should yield, the success of an architecture should be evaluated based on the requirements the architecture should meet. Let us start our journey into Axis2 looking at the requirements that are expected from Axis2.
In the SOAP terminology, a participant who is taking part in a Web Service interaction is known as a SOAP Node. Delivery of a single SOAP Message is defined based on two participants, SOAP Sender and SOAP Receiver. Each SOAP Message is sent by SOAP Sender and received by SOAP Receiver, and single SOAP delivery is the most basic unit that builds the Web Service interactions.
Each SOAP Node may be written in specific programming language, may it be Java, C++, .NET or Perl, the Web Services allow them to inter operate. This is possible because on the wire each Web Service interaction is done via SOAP, which is common to every SOAP Node.
Web Service middleware handles the complexity in SOAP messaging and lets the users work with the programming language they are accustomed to. Axis2 allows java users to invoke Web Services using java representations, and handles the SOAP messaging behind the curtain.
Axis2 handles SOAP processing along with numerous other tasks, this makes the life of the Web Service developer a whole lot easier. Following are the identified requirements:
Provide a framework to process the SOAP messages. The framework should be extensible and the users should be able to extend the SOAP processing per service or per operation basis. Furthermore it should be able to model different Message Exchange Patterns (MEP) using the processing framework.
Apart from the above functionalities, performance in terms of memory and speed is a major consideration for Axis2. Axis2 Core Architecture is built on three specifications, WSDL, SOAP and WS-Addressing. Other specifications like JAX-RPC and SAAJ are layered on top of the Core Architecture. The WS-Policy might join the core specifications in the near future.
Now that we have looked at the requirements of the Axis2 we can direct our attention to the Architecture.
Axis2 architecture lays out some principals to preserve the uniformity of the architecture. They are as follows.
Axis2 architecture separates the logic and the states. The code that does the processing is stateless inside Axis2. This allows the code to be executed freely by parallel threads.
Axis2 architecture is modular and is broken down into seven modules.
Information Model
Let us look in to the rationale behind each Module, and what each does.
Axis2 defines a model to handle the information and all the states are kept in this model. The model has a hierarchy for the information and the system manages the life cycle of the objects in this hierarchy.
Handling the SOAP Message is the most important and the most complex task, the efficiency of this is the single most important factor that decides the performance. It makes sense to delegate this task to a separate module, and that module(AXIOM) provides a simple API for SOAP and XML info-set while hiding the complexities of the efficient XML processing within the implementation.
SOAP Processing Model controls the execution of the processing, the model defines different phases the execution would walk through, and the user can extend the Processing Model at some specific places.
Axis2 define a transport framework that enables the user to use different transports. The transports fit into specific places in the SOAP processing model. The implementation provides a few common transports and the user may write new ones if and when it is needed.
Axis2 deployment model allows the user to deploy services, configure the transports, extend the SOAP Processing model per system basis, per service basis, and per operation basis.
Finally Axis2 provides a code generation tool that will generate server side and client side code along with a test case. The generated code would simplify the service deployment and the service invocation. This would make the Axis2 easier to use.
Information Model has two main hierarchies, the Contexts and Descriptions.
This uses UML notations ( A ----<> B means B has 1 or more objects of A. A------>B means the given relationship holds between A and B.)
The two hierarchies are connected as shown in the above figure. The Description hierarchy represents the static data. This data may be loaded from a configuration file that exists throughout the lifetime of Axis2. Examples for such data would be deployed Web Services, operations, etc. On the other hand, the context hierarchy holds more dynamic information about the things that have more than one instances (e.g.Message Context).
These two hierarchies creates a model that provides the ability to search for key value pairs. When the values are searched at a given level, they are searched while moving up the hierarchy until a match is found. In the resulting model the lower levels overrides the values in the upper levels. For an example when a value is looked up at the Message Context and it is not found, it would be looked up at the Operation Context etc, up the hierarchy. The Search is first done up the hierarchy, and if starting point is a Context then it is search in the Description hierarchy as well.
This allows the user to declare and override values, result being a very flexible configuration model. The flexibility could be the Achilles heel for the system, as the search, specially for something that does not exist is expensive, yet in the final analysis developers believe that the flexibility would serve better in this instant.
Configuration Context | Holds the current state of execution. A deep copy of this would essentially make a copy of Axis2. | Axis Configuration | Holds all global configurations. Transports, global modules, parameters and Services. |
Service Group Context | Holds information about a particular usage of the respective service group. The life of a Service Group Context starts when a user starts interacting with a service that belong to this service group. This can be used to share information between services(in the same service group) in a single interaction. | ServiceGroup Description | Holds deployment time information about a particular service group. |
Service Context | This context is available throughout the usage of the respective service. This can be used to share information between several MEPs that belong to the same service, within a single interaction. | Service Description | Hold the Operations and the service level configurations |
Operation Context | Holds the information about the current MEP instance, maintain the Messages in the current MEP etc. | Operation Description | Holds the operation level configurations |
Holds all the information about the Message currently being executed. | Message Description | Do not hold any information as yet, but can be used as a future extension point. |
The architecture identified two basic actions a SOAP processor should perform, sending and receiving SOAP messages. The architecture provides two Pipes (also named 'Flows'), to perform these two basic actions. Axis Engine or the driver of Axis2 defines two methods send() and receive() to implement these two Pipes. The two pipes are named In Pipe and Out Pipe, and the complex Message Exchange Patterns are constructed by combining these two pipes.
Extensibility of the SOAP processing model is provided through Handlers. When a SOAP message is being processed the Handlers that are registered would be executed. The Handlers can be registered in global, service, or operation scopes and the final handler chain is calculated combining the Handlers from all the scopes.
The Handlers act as interceptors and they process parts of the SOAP message and provide add on services. Usually Handlers work on the SOAP headers yet they may access or change the SOAP Body as well.
When a SOAP message is being sent through the Client API, an Out Pipe would begin, the Out Pipe invokes the Handlers and end with a Transport Sender that sends the SOAP message to the target endpoint. The SOAP message is received by a Transport Receiver at the target endpoint, which reads the SOAP message and starts the In Pipe. The In Pipe consists of handlers and ends with the Message Receiver, which consumes the SOAP message.
Above explained processing happens for each and every SOAP message exchanged. After processing one message Axis2 may decide to create other SOAP messages, in which case more complex message patterns emerge. However Axis2 always view the SOAP message in terms of processing of a single message. The combination of the messages are layered on top of that basic framework.
The two pipes does not differentiate between the Server and the Client, the SOAP Processing Model handles the complexity and provides two abstract pipes to the user. The different areas or the stages of the pipes are given names, and according to the Axis2 slang those are named 'Phases'. A Handler always runs inside a Phase, and the Phase provides a mechanism to specify the ordering of Handlers. Both Pipes have built in Phases, and both define the areas for 'User Phases' which can be defined by the user.
Following figure shows the two pipes with their predefined Phases, the user defined Phases would fit in to the User Phases.
Axis2 has the, some inbuilt Handlers that run in inbuilt Phases and they create the default configuration for the Axis2, we will be looking more in to how to extend the default processing Model in the next section.
There are four special handlers defined in Axis2.
Dispatchers - Finds the service and the operation the SOAP message is directed to, always run on the In-Pipe and inside the Dispatch Phase. The in-built dispatchers dispatches to a particular operation depending on various conditions like WS-Addressing information, URI information, SOAP action information, etc.,
Message Receiver - Consume the SOAP Message and run on the Message Processing Phase in the inflow
Transport Sender - Send the SOAP message to the SOAP endpoint the message is destined to. Always runs on the
Incoming SOAP Message is always received by a Transport Receiver waiting for the SOAP Messages, once the SOAP Message is arrived the transport Headers are parsed and a Message Context is created for the incoming SOAP Message. The the In Pipe is executed with the Message Context. Let us see what would happen at the each Phase of the execution, this process my happen in either in the server or the Client, there is a special case of using the two way transport where the first four phases in the In-Phase most likely to do nothing.
Transport Phase - The Handlers in the transport Phase are taken from the transport configuration associated, they are executed according to the Phase rules.
Pre-Dispatch Phase- The Handlers that goes there must be engaged globally (for all services) as the Service does not known at this point. The best example for them would be, Addressing Handlers and may be security Handlers if the Addressing Headers are encrypted.
Dispatch Phase - The Dispatchers are run in this Phases and find the Service if the service is not found already.
Post-Dispatch Phase - This phase check weather the service is found, if the service has not found by this point the execution will halt and send a "service not found error". Policy Determination Phase - This Phase does nothing for the time being, this is placed for the implementing the Policy
User Defined Phases - User defined Phases are executed here.
Message Validation Phase - Once the user level execution is taken place, this Phase will validates has the SOAP Message Processing has taken place correctly. For an example the must understand processing would happen here.
Message Processing Phase - The Business logic of the SOAP message, executed here, the a Message Receiver is registered with a each Operation. The Message receiver associated with the each operation would be executed as the last Handler of this Phase.
There may be other handlers in the any of the these Phases, users may employ custom Handlers to override the mechanics in the each of these Phases. If there is a response message, that would be initiated by the Message Receiver, yet the architecture is not aware of the response message and merely invoke the Message Receiver.
Out pipe is simpler because the Service and the Operation to dispatch is known by the time the pipe is executed. The Out pipe may be initiated by the Message Receiver or the Client API implementation.
Message Initialize Phase - Fist Phase of the out pipe, this serves as the placeholder for the custom Handlers
Policy Determination Phase - Just like in the in-pipe this is not implemented and suppose to serve as a extension point
User Phases - This executes Handlers in user define Phases
Transports Phase - Execute any transport Handlers taken from the associated transport configuration and the last handler would be a transport Sender which would send the SOAP message to the target end point
We discussed the default processing model of the Axis2, ability to extend the model has been the whole point of spending the energy on the SOAP processing model. We shall discuss the extension mechanism for the SOAP processing model now.
Idea behind making each step of the SOAP processing in terms of Handlers (inbuilt ones we discuss earlier) and placing them in the Phases is to allow Handlers to be placed between those Handlers and to override or affect the default mechanics. There are two ways the to extend the SOAP Processing Model.
The Handlers can specify the Phase they need to be run, further more they can specify the there location inside a phase via the following information.
Handler should run as the first in the phases
Handler should run as the last in the Phases
Handler should run before a given Handlers
Handler should run after a Given Handler
SOAP processing Model defines a logical entity called a module that encapsulates two entities, Handlers and Web Service Operations. The Handlers will act in the same way as explained in the first method.
Apart from the extension mechanism based on the Handlers, the WS-* specifications suggest a requirement for add new Operations using modules. For an example once a user add a Reliable Messaging capability to a Service, the "Create Sequence" operation needs to be available to the service end point. This can be implemented by letting the Modules define the operations and once the module is engaged to a service the operations will be added to that service.
A service, operations or the system may engage a module, once the module is engaged the handlers and the operations defined in the module are added to the entity that engages them. Modules can not be added while the Axis2 is running but later they will be available once the system is restarted.
There deployment Model provides a concrete mechanism to configure Axis2. Deployment Model has four entities that provide the configuration.
This file holds the global configuration for the client and server, and provide following information.
The global parameters
Registered transports in and transport outs
User defined Phase names
Modules that are engaged globally
Globally defines Message Receivers
Service archive must have a META-INF/services.xml file and may contain the dependent classes. the services.xml file has following information.
Service level parameters
Modules that are engaged Service level
Operations inside the Service
Module archive must have a META-INF/module.xml file and dependent classes the module.xml file has Module parameters and the Operations defined in the module.
When the system started up the Axis2 ask the deployment model to create a Axis Configuration, the Deployment Model first find a axis2.xml file and build the global configuration. Then the Deployment check for the Module archives and then for the service archives, the corresponding services and Modules are added to the Axis Configuration. System will build Contexts on top of the Axis Configurations and the Axis2 is ready to send or receive the SOAP Message. The Hot deployment is allowed only for the Service and in that case a thread will check the repository repeatedly, and add the Service corresponds to the new found Service archives to the repository.
Although the basic objective of the code generation tool has not changed, the Code generation module of Axis2 has taken a different approach to generate code. Primarily the change is in the use of templates, namely XSL templates which gives the code generator the flexibility to generate code in multiple languages.
The basic approach is to set the code generator to generate an XML and parse it with a template to generate the code file. The following figure shows how this shows up in the architecture of the tool.
The fact here is that it is the same information that is extracted from the WSDL no matter what code is generated. Code generator uses the WOM (WSDL Object Model) internally to manipulate the WSDL and passes that information to the emitter which emits an XML. the XML is then parsed with the relevant XSL to generate the code. No matter what the language, the process is the same except for the template that is being used
Axis2 M2 was released with code generation support but without data binding. The version 0.9 was shipped with data binding support with complete schema support. Such claim is made possible because of the fact that the data binding tool, xml-beans, has the full schema support. The original architecture of the code generation framework did not undergo significant changes because of the way that the code generation framework was originally designed. Data binding was incorporated as a pluggable extension to the code generation engine. Version 0.91 did not does not support SOAP encoding. It only supports RPC literal or document literal massages.
Xml-beans supports StAX API and AXIOM is based on a StAX API. Data binding in Axis2 is achieved through interfacing the AXIOM with the Xml-beans using the StAX API which is supported by both parties. At the time of the code generation there will be supporter classes for each WSDL operation that will have the utility methods that can de-serialize the from AXIOM to data bound object and serialize from data bound object to AXIOM. For example if the WSDL has an operation called "echoString", once the code is generated there will be an echoStringDatabindingSupporter.java class generated that will have methods that will look like the following.
public static org.apache.axis2.om.OMElement toOM(org.soapinterop.xsd.EchoStringParamDocument param) : This method will handle the serialization.
public static org.apache.xmlbeans.XmlObject fromOM(org.apache.axis2.om.OMElement param, java.lang.Class type) : This method will handle the de-serialization.
public static org.apache.xmlbeans.XmlObject getTestObject(java.lang.Class type) : This will be a utility method that can be used to create sample objects of the given data bound object.
There are three parameters that decide the nature of the Web Service interaction.
Message Exchange Pattern
The Behavior of the transport. Does it act one-way or two way
Synchronous/ Asynchronous behavior of the Client API
Variations of the three parameters can result in indefinite number of scenarios, even though Axis2 is built on a core that support any messaging interaction, the developers were compelled to support only two most widely used Message Exchange Patterns.
Two supported transports are One-Way and the Request-Response scenarios in
the Client API, the implementation is based on a class called
MEPClient and there are extensions for each Message Exchange
Pattern that Axis2 Client API supports.
The One-Way support is provided by the InOnlyMEPClient and
Axis2 provides a class called MessageSender that provides a much simpler
interface for the user. The Axis2 supports HTTP/SMTP and TCP transports, in
the case of the HTTP transport the return channel is not used and the HTTP
202 OK is returned in the return Channel.
The Request-Response support is provided by the
InOutMEPClient and Axis2 provides a class called
Call that provides a much simpler interface for the
user. The Client API has four ways to configure a given Message Exchange
Blocking or Non-Blocking nature - this
can be decided by using invokeBlocking() or
invokeNonBlocking() methods
Sender transport - transport use to send the SOAP Message
Listener transport - transport the Response is received
Use Separate Channel - does the response is send over a separate transport connection or not, this can be false only when sender an listener transport is same and is a two way transport.
Depend on the values for the above four parameter, Axis2 behave differently
Axis2 has two basic constructs for transports, named as Transport In Configuration and Transport Out Configuration. The Message Context has two fields to put the input and the out put transport to be used. Axis behaves according to the transport that is specified in each of the fields.
SOAP Message is arrived at the Server side, the incoming transport is decided by the Transport Listener that accepts the incoming SOAP Message. The transports for the subsequent SOAP Messages that are related to the first message, are decided based on the addressing parameters.
At the Client Side the user is free to specify the transport to be used, as in the Server side the transport for the subsequent SOAP Messages are decided by the addressing.
There Transport In Configuration and the Transport Out Configuration contains following information.
Transport Sender in Out Configuration, Transport Listener in the TransportIn Configuration
Parameters of the transport
Transport Handlers
Transport Sender send the SOAP Message over a given transport, each and every transport Out Configuration should define a transport Sender that send the transport.
Transport Receiver waits for the SOAP Messages and for each SOAP Message that arrives, uses the In Pipe to process the SOAP Message.
Axis2 Presently support the following transports
HTTP - The HTTP transport, the transport Listener is a Servlet or a Simple HTTP server provided by Axis2. The transport Sender uses sockets to connect and send the SOAP Message. Currently we have the commons-HTTP-client based HTTP Transport sender as the default transport
TCP - This is the most simplest transport, but needed the addressing support to be functional.
SMTP - This work off a single email account, Transport Receiver is a tread that checks for emails in fixed time intervals.