Howdy,
On 12/30/2014 2:49 PM, Ancoron Luciferis wrote:
Hi OSGi devs and experts,
I've got a problem which I really want to solve in an elegant way but I
think I haven't found the appropriate pieces yet.
The problem is the following:
I want to create some abstraction for a little data management system
which should be connected to different data backends at the same time
(e.g. S3, Dropbox, local/network filesystem, ...).
Now let's consider a simple example of the logic involving the following
standard CRUD operations (because I want to publish that in a single
ReST endpoint):
* create (e.g. upload)
* read (get metadata or list objects/files/buckets/... or just download)
* update (e.g. re-upload or change metadata)
* delete
So, what I actually want is the following:
1.) For creating/uploading a new object, a specific data backend may be
specified via HTTP header or determined automatically (e.g. based on
expected size or some other metadata).
2.) For listing existing objects all service instances/data backends
shall be queried at the same time (in parallel) and results combined
into a single list.
3.) For retrieving object metadata, downloading a specific object,
modifying, deleting it or executing some other operation with a specific
object, the correct service instance/data backend is called.
So, for case 1 I would need some way to evaluate the contextual
information of the upcoming service call (in this case the HTTP header).
If that is not available, I'll have to look at some service instance
information that helps me figuring out where to put some object (DNS-SD
or ZeroConf keep popping up conceptually here in my head).
For case 2 I just need to actually execute the same call for each
available service instance (like a network broadcast).
For case 3 I need to know somehow (this could be done by a local object
identity/location mapping) which service instance is responsible (or
allowed to) manage a specific object.
Now, I could code all this inside the ReST layer. However, what I really
want is to make it abstract in a way that I can hook it into other
places as well.
So, the initial caller 'A' should just have code like this:
private B b;
//...
myObjectId = b.create(newObject);
//...
List objects = b.list(filter, sort, paging);
//...
myObject = b.get(myObjectId);
//...
b.updateMetadata(myObjectMetadata);
//...
b.delete(myObjectId);
So that the ReST layer does not even have to know that there is more
than just one backend.
The magic should be done outside in a generic way if possible, so that I
could re-use it as it is for other types of services.
Does that idea sound familiar to someone? Has that been solved already
and I just missed something?
I first started with the idea of using an aggregating proxy plus a
FindHook and stuff, but this imposes the runtime issue that consumers
need to be refreshed which I want to avoid if possible.
The main problem I think here is to present a single service instance to
the consumer for which there actually exists no real service
registration as it is just some kind of invocation handler with a
specialized purpose (routing/broadcasting service requests).
I would be thankful for any ideas!
Full disclosure: I'm on the ECF [1] project team.
One approach that might suit your use case: Remote Services/Remote
Service Admin - defined by chapters 100 and and 122 in the OSGi R6
enterprise specification.
Remote Services expose an OSGi service (e.g. instances of 'B' in your
example above) for remote access. The specification defines standard
service properties that trigger the export of a service, but the spec
intentionally does not define the underlying distribution provider or
transport. Rather, one of the standard service properties (config type)
allows the distribution provider to be selected at service registration
time.
ECF's [1] implementation of remote services [2] allows new providers to
be easily created to customize the service proxy behavior...so that, for
example, an existing or new REST API can/could be used to implement the
proxy calls. There is a recently created tutorial [3] with an example
how to customize the proxy for accessing a pre-existing REST service.
You could easily create a custom distribution provider and use whatever
distribution/transport you wished...and/or change the distribution
provider over time....without changing the actual service interface 'B',
or any code that uses B.
Also, the RSA spec defines a standard discovery metadata format (called
EDEF, for Endpoint Description Extender Format) for publishing and
dynamically discovering remote services. ECF's impl of this spec
allows plugging in a discovery module of choice, e.g. zookeeper,
zeroconf, dnssd, etcd are already available as part of ECF, or you can
create/plugin a custom discovery module if you wish.
Hope this helps.
Scott
[1] http://www.eclipse.org/ecf
[2] https://wiki.eclipse.org/ECF#OSGi_Remote_Services
[3]
https://wiki.eclipse.org/Tutorial:_ECF_Remote_Services_for_Accessing_Existing_REST_Services
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