SOAP web service callback architecture?

Question

I am quite new to web services, JAX-WS etc. so maybe noob question...

So, I want to implement a web service to make two systems communicate. The "client" system is interested in events that are generated on the "server" system. But the "client system" is itself a server for a different app. The server is Java (WAR in tomcat). The client is .Net.

There should be just one client system, but several client processes inside the client system, each interested in distinct categories of events.

I will implement the server-side, and a test client. Somebody else will implement the .Net code.

The running sequence should be along this line :

1. Server is running...
2. Client initiates conversation, "registers" to the server, and requests some initial data.
3. Server keeps a list of registered clients' endpoints
4. In the server there is a listener that is notified when certain events happen. It will then go through the list of registered clients and forwards the event to each of them
5. At some point, the client can "unregister" no notify the server that it doesn't want to receive events any more.

First, does it sound like something reasonably doable ?

And is there a standard built-in mechanism, using SOAP (JAX-WS on the server, whatever is available with .Net n the client) - that the server can use to obtain a callback endpoint from the client ?

For example, I did something very similar using RMI, in this case the client can just send a remote reference to itself, that the server can just store ant refer to later.

Finally, is there a standard library to store endpoints references, make (collective) callbacks, and maybe keep the list up-to-date, removing the clients that don't respond so some "ping" call ?

Note for clarity : I need more than just asynchronous method with callback: one message from the client will generate many callback messages from server to client.

Sidestep

I would say its possible. Look into asynchronous Web Services. If you're going to use JAX-WS then WS-Addressing will be of help. – Kindrakindred

Answer 1

Seems you wish to implement a notification facility to inform arbitrary anonymous clients.

I suggest you first consider how you would pass the information using SOAP messages. Then you can consider how to achieve this using java - JAX-WS or additional non-standard libraries. The point is there may be significant limitations or assumptions required to transfer SOAP messages. E.g. firewalls might block your HTTP messages, clients might "just clients" with no ability to act in a server role to recieve SOAP notification requests

Note: An async callback mechanism is defined in JAX-WS 2.0, where the service obtains the endpoint reference of the client. This is the same sort of functionality provided by WebLogic/Fusion proprietary solution described by Deepak Bala. Websphere has a similar proprietary async solution. None of these meet your requirements, because they only allow a single response per request.

SOAP Options:

1. Proprietary SOAP messages - the "100% Do-It-Yourself Option"

   You design full SOAP payload schemas and message exchange pattern.

   You can push the notification from the server to the client if you know the client's SOAP endpoint address. The client can transfer it's SOAP endpoint address within original SOAP request payload. Sometime later the server can send a SOAP request to the client.

   Problems/Assumptions: (1) need a SOAP/HTTP communication path for requests from server-to-client - not guaranteed when firewalls exist; (2) the client needs to be aware of your notification schema - in fact the client needs to act as a Service endpoint to recieve the SOAP notification request. That's two big assumptions IF you are trying to support arbitrary anonymous clients - that's not something SOAP "just supports" both ends need to design all this in detail. In fact to do this in a service typesafe manner, the client should actually declare it's own service WSDL interface so that you can invoke it. (3) As hinted earlier, many clients are "just clients" - they might not have a HTTP server to accept SOAP requests.

   So for proprietary "push" notifications to work, both sides need to servers and both need to publish their SOA interfaces.

   Alternatively, you can pull the notification to the client. The client can use a notification request to the server that is either blocking or polling. The server can respond with the notification or nothing or error.

   Problems/Assumptions: (1) HTTP servers (i.e. the SOAP server) do not support blocking requests, as a rule, meaning you must poll; (2) the client needs to be aware of your notification schema - in fact the client needs to act as a Service endpoint to recieve the SOAP notification request. That's two very big assumptions for an arbitrary anonymous client - that's not something SOAP "just supports" both ends need to design all this in detail. In fact to do this in a service typesafe manner, the client should actually declare it's own service WSDL interface so that you can invoke it.

2. Same as above, but make include WS-addressing data in SOAP headers to inform either side of the other's endpoint address.

   Basically the same Problems/Assumptions as the first option. WS-addressing addresses will help you intelligently route SOAP messages to the right URL address, but no more.

3. Use WS-notification

   This spec was designed for your scenario.
   WS-BaseNotification sub-standard would meet your needs. It provides WSDL port definitions for notification producers and consumers. It provides a WS- standards compliant solution for subscription from a consumer to a producer, and a notification from producers to consumers.

   Problems/Limitations: (1) It does NOT define the format of notification payloads. The Notification payload is application-domain specific (proprietary design). The standard does not define any “standard” or “built-in” notification situations or messages. (2) It has the same problems with HTTP notifications passing through firewalls as mentioned above. (3) WS-Notification is not a supported standard of Java EE/JAX-WS (but there are plenty of app servers, open source and commercial, that support it as an extension).

4. Use a message queuing solution (e.g. JMS) with traffic encapsulated in HTTP This requires proprietary design of payloads passing between client and server and back - forming contracts between the two sides. An advantage is that the client can be a pure client, with a message listener invoked in a thread when a message is recieved.

   Problems/Limitations: (1) It has the same problems with HTTP notifications passing through firewalls as mentioned above. (2) Is a do-it-yourself implementation. (3) Uses more technology then you currently use.

End Result:
You need at least part of your solution to be a proprietary design - the SOAP/WS standards do not meet your full requirements. In theory this proprietary design could leverage a product to provide much of the legwork, BUT the notification schema design would need to be created and integrated by you.

IF you wish to push notifications, you need some sort of contract for notifications passing to the client, the client needs to act as a SOA server, and you need the firewalls openned for your traffic. Most corporations disallow HTTP requests leaving a server and passing to a client - you normally need an extremely good reason to open firewall ports and even then, many corporations will disallow it...

IF you wish to have clients polling for notifications, you just need a basic WSDL interface on the server side that can be called frequently by clients.

Future Option: HTML5 Web Sockets
IF your client is a HTML5 app, then web sockets enabled servers can push traffic to the browser - and there is some chance corporations will open firewalls. SOAP messages could travel over HTTP web sockets, enabling you to push notifications.

Bargain

Many thanks for the detailed answer. You get the bounty. Given that 1. The clients are not just really "arbitrary", it's more of a system-to-another-system type of communication and so both ends are under our responsibility 2. Didn't want to do polling 3. Server runs on Tomcat, using JAX-WS, so not a real app server and no proprietary extension available and 4. Client is written in .Net so no JMS... (1/2 continued...) – Sidestep

(2/2) ... I ended up implementing the first solution you described, with the client passing its own enpoint in the "Registration message" payload. It seemed the easiest to implement rapidly if not the cleanest. For now it works fine. I might use WS addressing in the future to determine client's endpoint though. – Sidestep

@Glen can you please provide direction/help on my question that I think is similar to this one, sorry for taking your attention this way – Companionable

Answer 2

Async clients are supported for WSDL based services through the use of polling and callbacks. In your case I think the requirement is relatively more complicated.

The Oracle fusion middleware doc page has a scenario outlined that will help you. It details a method that allows clients to send requests which generate a HTTP 202 (accepted) and the clients then wait for a response on message queues. In your case the scenario can be tweaked from the one shown below.

Initiate several response queues for each category of callback. The clients can filter them by supplying a client and category ID for the queue. This will serve as a callback mechanism for each client or the processes that are governed under each client. The MDB can be backed by a file store or DB store to ensure reliability and one-time delivery.

Of course you do not need Oracle fusionware to implement this. You can use RabbitMQ or Redis (with transactions) to acknowledge receipt of a message on the client. If your client wishes to un-register it make a call and stop listening to the queue.

I'm unaware of any industry standard that will fit your scenario, but I believe this solution should work well for you.

Prostyle

Thanks fo the interesting answer. However for now we just went with a homemade solution with the client itself publishing a normal WSDL and the server calling it. We might consider something more elaborate such as RabbitMQ in the future. – Sidestep

Answer 3

Have you considered the simpler approach of "pub-sub" using a messaging product ? (Such as MQ, EMS, or ActiveMQ)

The requirements you describe does not seem to fit "classic" request/reply sync/async SOAP Web Service scenarios.

In a Pub/Sub solution, the client(s) subscribe to a topic once, and the publisher(s) (in your case, the Server) can post any number of relevant messages to all subscribers.

As a bonus, most messaging products include support for "durable subscribers", so the client can be off-line at times and receive all messages after re-connection.

In your case, the server could house a (free) ActiveMQ Server... Providing most of the feature you seem to seek.

If you go that way, I suggest you pick a JMS compliant product with support for .Net.

Armour

Interesting suggestion, we might consider it in the future but for now we just implemented it as described in glen best's answer (see my comment there). – Sidestep

Answer 4

For those getting here from search engines:

You can use a WebHook for Web APIs nowadays, which works essentially as OP described.

In REST for example, the client will have an HTTP endpoint itself that is dedicated to receiving POST events/notifications from the actual server. The client registers his endpoint with the actual server by giving it an URI on his notification endpoint. From that point on, the actual server can POST to the client’s notification endpoint. It is essentially a convoluted callback, if you are familiar with async terminology.

Maybe Java or .Net have libraries for WebHooks by now.

That said, SSE and Websockets standards provide actual push and real-time messages while being compatible with HTTP and most browsers.

Long polling variations were also used in the past, and are now sometimes called Comet as an aggregate.

Enduring