(Java) How can I pass Schema-validated XML documents as parameters between distributed components (e.g. web services or sockets)?

Question

Here is a description of the scenario and I would appreciate also any comments on the approach used

The core of my application is a set of web services backed by a P2P database. One service accepts a simple XML-based record (I have designed a generic schema for it). The service processes this data (mainly creating keys based on certain criteria) and pass the original data along with the created keys to a listening SocketServer in one of the listening P2P nodes. This key,data pair is routed to the proper node, which stores the data (associated with the key as an ID) in an XML database.

A second service accepts a query document that is structured based on the same schema, but with optional values that would be used for searching and matching from the previously stored ones. So the second service would pass this query (with the proper keys) to the P2P part, get back the results and pass them back to the service client.

E.g. if the original record submitted to the first service was < attr1 >value1 < /attr1 > < attr2 > value2 < / attr2 > (attribute list along with some other metadata mandated by the schema) then the second service should retrieve that record if the query received was < attr2 >value2 < / attr2 >

(I could later think about using more complex XPath or XQuery queries as the underlying XML database allows instead of exact matches for values here but that is not important at this stage. there is also a third service I am working on but it depends on getting the first two in proper shape first)

So my questions are:

1) What data type should I use as the parameters of the web services? How to utilize my schema for this usage? I was considering various XML binding frameworks (especially JAXB and SDO) for this but didn't know how to proceed.

2) How can I enhance the two services (call them store and search) to use dynamically created templates based on the original generic schema? The service would still accept documents of the main schema type but has the inner attribute list based on a template say template1 only requires whose values are ints while template2 require (float) and (string). The current JSP-based prototype manually creates this template but as an XML document that is assembled by hand (<>tags dispersed in text) and there is no type checking what so ever so I thought I could do better!

3) Is it possible to generate a quick web app prototype for simple access to this system (again by using the schema (&templates) to edit the appropriate XML message structures? What I am looking for is for the (human) user to choose a template and then just "fill in the blanks" and submit, no need for any fancy look and feel.

4) Can I or how can I also use this XML message type for communicating across sockets?

5) Does it matter if I deploy the services as stateless EJBs or not? Do I need them to be EJBs or servlets would be more than enough?

I currently have a rudimentary implementation (from previous developers) that were meant for a subset of my current requirements (I am improving on the services and adding new derived ones) but there was no schema nor validation and the data is passed all along as basic strings, thus providing weak typing and difficult to update manual parsing. The reason I want to update this to a stronger bound typing is to introduce changes in the data schema that would be passed along the whole system easily. Basically I want the system to be as less coupled to the data format/schema used as possible; the current prototype is too coupled to the data that I am finding it extremely difficult to change the data without breaking the system.

My initial investigation led me to consider JAXB but it supports only static typing (cannot create a schema/types dynamically at runtime that I want to persist for later usage). So I came across SDO which has both dynamic and static typing. The problem is just that there is not enough community and/or examples of using this approach so it seems risky (the examples of Apache Tuscany and Eclipselink implementations are very scarce and I could not find complete examples that are not 5+ years old (like this http://www.ibm.com/developerworks/java/library/j-sdo/) and also tackles the XML use case of SDO (most seem to focus on the relational usage of SDO).

This is my first time asking for programming help (here and elsewhere) so please bear with me. I searched a lot on the net but I could not find anything useful but pieces here and there that did not add up.

Any comment or hint is really appreciated.

trfndr

EDIT

I forgot one thing: how would the search service get back the results? Since it is opening a client socket connection, there is no way to get back any results synchronously. The current implementation tackles this by having the service client opening a listening socket on a random port and putting this contact info in the query document. After the search web service sends the query to the p2p part it finishes. The p2p sends the results as a WS call to another service which sends them back to the service client socket. I don't like this approach much, is there any more elegant solution?

Answer 1

I lead the EclipseLink JAXB & SDO implementations and represent Oracle on those specifications so hopefully I can help you out. This question is very similar to talk I'm giving at JavaOne in September.

1) What data type should I use as the parameters of the web services? How to utilize my schema for this usage? I was considering various XML binding frameworks (especially JAXB and SDO) for this but didn't know how to proceed.

This depend's on what web service framework you are using. JAXB is much easier to use with JAX-WS, and while JAXB is still easier to use with JAX-RS SDO, is a possible alternative.

2) How can I enhance the two services (call them store and search) to use dynamically created templates based on the original generic schema? The service would still accept documents of the main schema type but has the inner attribute list based on a template say template1 only requires whose values are ints while template2 require (float) and (string). The current JSP-based prototype manually creates this template but as an XML document that is assembled by hand (<>tags dispersed in text) and there is no type checking what so ever so I thought I could do better!

I'm not 100% what you mean here, but the following may be helpful:

• Using @XmlAnyElement to Build a Generic Message

3) Is it possible to generate a quick web app prototype for simple access to this system (again by using the schema (&templates) to edit the appropriate XML message structures? What I am looking for is for the (human) user to choose a template and then just "fill in the blanks" and submit, no need for any fancy look and feel.

JAX-RS is a nice framework for creating quick prototypes. Below is an example I created:

• Part 1 - The Database
• Part 2 - Mapping the Database to Objects
• Part 3 - Mapping the Objects to XML
• Part 4 - The RESTful Service
• Part 5 - The Client

4) Can I or how can I also use this XML message type for communicating across sockets?

I prefer frameworks like JAX-RS that communicate over the HTTP protocol.

5) Does it matter if I deploy the services as stateless EJBs or not? Do I need them to be EJBs or servlets would be more than enough?

My preference is to use an EJB session bean for the service. If you are interacting with a database then you can leverage the Java Transaction API (JTA) to manage your database transactions.

• Part 4 - The RESTful Service

SDO

EclipseLink is the SDO 2.1.1 (JSR-235) reference implementation. We have some examples posted below. If you are looking how to do something specific, I will try to post a relevant example.

• http://wiki.eclipse.org/EclipseLink/Examples/SDO

JAXB

JAXB is static. It is also more popular than SDO. Recognizing this in EclipseLink we have implemented a dynamic JAXB feature. It gives you the dynamic aspect of SDO with a JAXB slant.

• http://wiki.eclipse.org/EclipseLink/Examples/MOXy/Dynamic

EDIT #1

Since you are dealing with JAX-WS and your model is almost entirely dynamic, I think you should skip the JAXB binding altogether. In the following link see the section "Switching Off Data Binding"

• http://java.sun.com/developer/technicalArticles/xml/jaxrpcpatterns3/

This will give us the body of the message as a javax.xml.transform.Source object. We will need to process the XML based on the dynamic templates. SDO would be a good choice here. You can constantly add new types to the HelperContext using XML schemas.

helperContext.getXSDHelper().define(schema1, null);
helperContext.getXSDHelper().define(schema2, null);

You wil be able to unmarshal the Source from the web service as follows:

XMLDocument doc = helperContext.getXMLHelper().load(source, null, null);
DataObject rootDataObject = doc.getRootObject();
String someValue = rootDataObject.getString("attr3/childAttr/anotherChildAttr");

You will also be able to use the XMLHelper to marshal your objects to XML when calling another service.