Usage of a Data Access Layer (DAL) in a SOA design

As we are in the beginning phases of rejuvenating our application in to SOA design I have some questions that I can not get a clear answer/picture on.

I have been doing a lot of reading, mostly around books from Thomas Erl and following that design pattern of understanding what Task Services, Entity Services and Utility Services are.

What I am stumbling on is the whole DAL concept of how that would look. So this is more of a verification of understanding or a clarification so as to help make the best approach for our platform.

So background. We currently have several web based e-commerce applications that have been pretty much been built in silos and are again pretty much a copy of each other. We have supporting applications such as Daemons and misc web services out there. Many of these applications are older then 5 years and are build on only technology (Model 1). All of our applications are centered around conducting auction sales. So during a sale event we will be taking bids from users, determine who is winning and display that information back开发者_如何学Python. Each sale event has a set amount of time that they will be available to the users.

The company is moving towards a SOA solution as a lot of things we end up doing can be shared across not only our group but across other groups.

So what I understand on the DAL is that it in itself is a service which will sit on top of Data, in this case different Databases - MSSQL, ORACLE, MSSQL. Each of these databases have different schema's (Oracle) etc.

So the services (Task, Entity, Utility and Presentation Tier if needed) will make calls to the DAL to retrieve data. It is the responsibility of the DAL to know, from the contents of the message to determine what it needs to do in order to fulfill the request.

So for example, we have a Security Service candidate. This service needs to authenticate with LDAP and to authorize from the data that is stored for that given application.

The thought here is that a Utility service will be created to wrap up all the operations required to communicate with LDAP and that the Security Service will call upon the Utility Service and to the DAL to fetch the authorization data. The DAL then has the responsibility to go to the correct database/schema to retrieve the information. The information will be in XML format (standard SOA communication).

So, am I on the right track here? Have others done similar things or not? What other things do I need to consider (Currently getting the statistics on how many bids we take in an hour - on average).

Should each service have its own DAL - for example should the Security Service have the DAL as part of the service or should DAL be a shared service in which all services can use?

In your case, the approach to use for a full SOA based deployment would be to use an ESB, Identity provider and a data services solution.

To break it down, the DAL should be implemented using data services, in this way, this service will be a globally accessibly service in a language neutral way, and will support re-use and loose coupling. So all your data access logic can be implemented as web service operations in a data service.

So for the authentication and authorization management, in the SOA world, there's a standard called XACML, which is used for fine grained authorization management. So what you will need is an XACML server, who would authorize the user according to a specific criteria, where this should also have the ability to authenticate with LDAP.

Then your "Security Service" will be implemented in a service at the ESB, where that service will query the identity provider for authentication/authorization and according it's response, it will call the appropriate operations in the data service, with suitable parameters to fetch the data, and return it to the user.

The above scenarios can be implemented using WSO2 Data Services Server, WSO2 Identity Server and WSO2 ESB respectively, which are open source products, and can be freely used and found here.

i once worked with (developing) an soa project that used a "data service". it was some time ago, and i was only involved marginally, but my recollection was that it ended up being too complicated and slow.

in particular we had no real need for a data service - it would have made more sense to have placed the same abstractions in a library layer, which would have given better efficiency and no real loss of functionality (for our particular needs). this was exacerbated by the fact that the data tended to be requested in many small "chunks".

i guess it comes down to the trade-offs involved in the implementation. in our case, with a relatively closed system and a single underlying database technology, we could have easily exploited the support for distributed access that the database provided; instead we ended up duplicating this in a slower, more general, message bus, which added nothing except complexity. but i can easily imagine different cases where access to data is more "distant".

How you have to use SOA for your design is depends on the its requirements.

In generally you can write coarse grain services and expose them as web services. In your case you can write some services which calls the databases and produce the results. In this case authorization logic can also be written with the service logic.

The other approach is to use an ESB or BPEL engine to write the integration logic and expose the integrated service as a web service. In this case you can use some data services to expose data base data in xml format and integrate them. You can use services for different sachems and call the correct service with the request data. And the authorization logic can also be added to service integration logic.

Security aspects such as authentication, confidentiality, integrity is considered as non functional requirements and hence can be engaged to any service without writing an explicit security service.

Following articles describes such sample possible integration of services as mentioned in the second approach.

http://wso2.org/library/articles/2011/05/integrate-business-rules-bpel

http://wso2.org/library/articles/2011/06/securing-web-service-integration