I just read a great article on web services by Patrick Cooney at AListApart. However, something is still vague to me on why web service recommendation is important and how it works? Is this approach an alternative for UDDI and similar solutions?
UDDI provides the infrastructure for web service discovery. As the linked article from the comment says, it is basically the "yellow pages" for web services.
If I understand your reference to "web service recommendation" correctly, you mean a system that, given a list of equivalent candidate services, chooses the best one for you, probably based on matching a set of requirements against some kind of QoS metrics of these services.
Therefore, such a system can be used as either an addition to UDDI (if the consumer performs the basic discovery and only delegates the dynamic selection to the recommendation system) or replace it transparently for the consumer (the consumer asks the recommendation system for a service type with some given QoS parameters). In the second case, the recommendation system would use UDDI for discovery and then filter the answer for the consumer based on the QoS parameters.
Related
Little domain presentation
I m actually having two microservices :
User - managing CRUD on users
Billings - managing CRUD on billings, with a "reference" on a user concerned by the billing
Explanation
I need, when a billing is called in a HTTP request, to send the fully billing object with the user loaded. In that case, and in this specifical case, I really need this.
In a first time, I looked around, and it seems that it was a good idea to use message queuing, for asynchronicity, and so the billing service can send on a queue :
"who's the user with the id 123456 ? I need to load it"
So my two services could exchange, without really knowing each other, or without knowing the "location" of each other.
Problems
My first question is, what is the aim of using a service registry in that case ? The message queuing is able to give us the information without knowing anything at all concerning the user service location no ?
When do we need to use a service registration :
In the case of Aggregator Pattern, with RESTFul API, we can navigate through hateoas links. In the case of Proxy pattern maybe ? When the microservices are interfaced by another service ?
Admitting now, that we use proxy pattern, with a "frontal service". In this case, it's okay for me to use a service registration. But it means that the front send service know the name of the userService and the billing service in the service registration ? Example :
Service User registers as "UserServiceOfHell:http://80.80.80.80/v1/"
on ZooKeeper
Service Billing registers as "BillingService:http://90.90.90.90/v4.3/"
The front end service needs to send some requests to the user and billing service, it implies that it needs to know that the user service is "UserServiceOfHell". Is this defined at the beginning of the project ?
Last question, can we use multiple microservices patterns in one microservices architecture or is this a bad practice ?
NB : Everything I ask is based on http://blog.arungupta.me/microservice-design-patterns/
A lot of good questions!
First of all, I want to answer your last question - multiple patterns are ok when you know what you're doing. It's fine to mix asynchronous queues, HTTP calls and even binary RPC - it depends on consistency, availability and performance requirements. Sometimes you can see a good fit for simple PubSub and sometimes you need to have distributed lock - microservices are different.
Your example is simple: two microservices need to exchange some information. You chose asynchronous queue - fine, in this case they don't really need to know about each other. Queues don't expect any discovery between consumers.
But we need service discovery in other cases! For example, backing services: databases, caches and actually queues as well. Without service discovery you probably hardcoded the URL to your queue, but if it goes down you have nothing. You need to have high availability - cluster of nodes replicating your queue, for example. When you add a new node or existing node crashed - you should not change anything, service discovery tool should understand that and update the registry.
Consul is a perfect modern service discovery tool, you can just use custom DNS name for accessing your backing services and Consul will perform constant health checks and keep your cluster healthy.
The same rule can be applied to microservices - when you have a cluster running service A and you need to access it from service B without any queues (for example, for HTTP call) you have to use service discovery to be sure that endpoint you use will bring you to the healthy node. So it's a perfect fit for Aggregator or Proxy patterns from the article you mentioned.
Probably the most confusion is caused by the fact that you see "hardcoded" URLs in Zookeeper. And you think that you need to manage that manually. Modern tools like Consul or etcd allows you to avoid that headache and just rely on them. It's actually also achievable with Zookeeper, but it'll require more time and resources to have similar setup.
PS: please remember about the most important rule in microservices - http://martinfowler.com/bliki/MonolithFirst.html
I am a newbie in stateful webservices area. Please ignore if any of the below questions do not make sense.
I have mostly dealt with stateless webservices. One question in my mind is that can we pass session id to a web service thru the XML API. Is there already a provision for this(so that it becomes stateful)? Does a Java-XML api to invoke a web service has a provision to make use of cookies?
Are RESTful web services stateful? Is this the reason they are called RESTful?
Let's say I am using a stateful web-service. And I have two different operations where 'Atomicity' of operation has to maintained(take an example of Credit/Debit of card and cruz booking). Both have to be successful or neither.
Can such a thing be achieved using a web service?(because I dont know how transaction control can be applied to web services. It would have been easy if I had to deal with only services(ejb) layer).
If yes-this gives rise to another question-how transaction manager identify client context and does the same transaction context gets applied to these operations which should be part of one meaningful and atomic transaction?
Does this depend on the framework/programming language that we gonna use? For e.g. I want to achive this using JSF/SpringMVC UI and JAX-WS/Axis web service. If no, please let me know what other similar frameworks(from Java) have this and provides the solution. Servers - weblogic/jboss.
Thanks
Prakash
I have been fighting with this for a while now. I need to prototype SOA, and with it, the registry. I have been fiddling with jUDDIv3 on JBoss SOA Platform 5, but there don't appear to be any tools that allow me to publish to a v3 jUDDI registry. See my related questions here and here.
I realize after reading comments on those questions, and some articles on the internet (like this one) that UDDI is failing or dead, however my organization has some legacy tech we need to work with.
Also, my supervisor (I'm an intern) is adamant about sticking to standards. In principle, I agree with this, but perhaps a dead standard really isn't a much of a standard if nobody uses it.
In short, I need to provide the registry component of Service Oriented Architecture. It probably needs to be UDDI, so that it fits with the legacy tech, and satisfies the standard. Whatever the solution, it would be best if there were tools available that allow me to publish web services to that registry.
This problem has dragged on much longer than I would have liked. Any small piece of advice is really appreciated.
You may use WS-Discovery. WS-Discovery is a standard protocol for discovering services and service endpoints. This enables service clients to search for services based on a given criteria and bind with the discovered services. There are tow modes of WS-Discovery,
ad-hoc - servers advertise the services they have using a UDP multicast protocol
managed mode - servers and clients use an intermediary known as the discovery proxy for all service discovery purposes.
You can simply try this out with WSO2 Platform (free and open source under apache2 license). Please follow [1] to see a simple scenario of WS-Discovery in managed mode.
[1] http://charithaka.blogspot.com/2010/04/ws-discovery-with-wso2-carbon.html
I think I know the answer to this one, but I have just spotted someone doing exactly the opposite - who is a very skilled developer, were they just having a bad day?
Q: Is it OK to call a service from within a service in an SOA architected system?
I suspect not, but i'll ask anyway...
I can't think of any reason why not. I can think of multiple reasons why a service could call another service. I've architected and implemented multiple services that do this. I'm also aware of other architectures that set up this sort of system.
Definitely yes. This SOA pattern is commonly called choreography where one web service processes and then passes the message to another down a processing pipeline. Google it and you will find some good references.
Another case may be more technical reason like routing, where you have frontend webservices that routes your messaged to different backend services based on security policy, content, bridge different technology or transport protocol etc.
The answer to this is as always "it depends..." let me explain what I mean.
Calling another service in a SOA is of course a totally acceptable thing to do, it is at the heart of SOA to be able to compose new things out of existing services.
The more important part is how you call the services, consider a SOA system where multiple services collaborate in a call chain call each one enlisting the other in the transactional scope. Doing this sort of thing without careful planning will have a massive impact on your systems performance. The same call chain designed using well partitioned services that are scoped at the correct unit of work suffers less.
Consider system robustness, in a typical architecture one service tends to become more popular than others and lands up having many other services calling it. A failure of this one service brings down the entire system due to all the other services being dependent on the call to this one service.
Consider the difference between synchronous and asynchronous calls, when do you use what? That is the impact of each?
How do you design and partition services to limit the cost of crossing the service boundary on each call?
Lots of questions, but if you look around there are many places to find answers, I suggest starting with these.
I would suggest you read articles by Thomas Erl and Roger Sessions, this will give you a firm handle on what SOA is all about.
Building a SOA
SOA Design Pattern
Achieving integrity in a SOA
Why your SOA should be like a VW Beetle
SOA explained for your boss
WCF Service Performance
think about the architectural goal of "separation of concerns". instead of each service knowing how to do everything, it can rely on other specialized services for shared pieces of functionality
I have some different scenario. What if you want to implement service level clustering in your SOA? For example, say persistence service is resides on one machine and is responsible for handling all persistence tasks in the cluster. So, on other machine which needs persistence functionality, need to connect to the that machine only(ignore fail over factor).
Now while login, if User service has instance of Persistence service directly, then you will not be able to implement service level clustering.
We have our SOA middleware and I called service from each other directly. But when we implemented service level clustering using JMS/ActiveMQ, we faced the problem for those interconnected services.
What exactly is web service composition?
Composition refers to the way something is build, the new term at the moment is mash-up which basically means utilising a variety of different services in a composite application. So that functionality of disparate application can be used in one application.
I think your referring to service granularity - which means how much functionality a service exposes. a coarse grained service will expose a whole process as a consumable unit whereas a fine grained service will expose a specific unit of logic from a larger process. Obviously, it is up to the service architects to determine what granularity of service works best in the given environment.
This also, in a way has to do with the style of SOAP message you are using whether it is RPC style or document and that a service should be atomic and not hold external state. Meaning it does not need to know any more information other than that in the SOAP message to perform its function.
Hope this gives you a good starting point. The trouble with service-orientation is that it differs depending on who you read, but the main points stay the same!
Jon
Some web services which are provided for clients are abstract and composition of some smaller web services and it's called web service composition.
Sometimes there are more than one web service in order to use as the mentioned small web services, so we choose them based on QoS (Quality of Service) and many researches have been done on this subject.
Web service composition involves integration of two or more web service to achieve more added value of business functionality. A work flow composer is responsible of aggregating different web services to act as a single service according to functional requirements as well as QoS constrains. BPEL is one of the popular composers uses XML language to perform service composition. Fine-grained services perform single business task and provides higher flexibility and reusability. However, coarse-grained service involves performing complex business functionality leading to lower flexibility