Hey all,
This email is an overview of the Catalog system with lots of questions.
The Catalog+Tutorial needs some love. It's almost a great resource but
it needs some extra pointers and some reorganization. For example, it
has no overall introduction (what i have added is incomplete/wrong).
Also the "Implementing" section needs both an overview explaining the
pieces which need to be built and a modification of the skeletons which,
while they are great, need to be reorganized so we don't explain
resolve( * ) before the pieces which the method requires.
The lines starting with a '*)' below are questions for someone
to answer; the rest is my understanding which may be wrong, in
which case hopefully someone will comment.
*************************************************************************
General Issues and Questions
Creating each piece:
-------------------
Catalog:
- new DefaultCatalog()
ServiceFinder
- new DefaultServiceFinder( myCatalog )
*) new DefaultServiceFactory( myCatalog ) What's this?
How is it different from DefaultServiceFinder? Is the
finder to get the services originally and the factory to
create the services from the catalog?
Service:
- via ServiceFinder.acquire(serviceURI)
*) via the catalog? Does this require the use of
Servicefactory.acquire(serviceURI) as in the Catalog
+Tutorial (see below)? Is there another way?
GeoResource
- via ServiceFinder.acquire(resourceURI)
*) via the catalog? Does this require the use of
Servicefactory.acquire(resourceURI) as in the Catalog
+Tutorial (see below)? Is there another way?
*) Do we implicitly recommend that the user make a singleton
Catalog?
*) We don't use FactoryFinder and Factories to create Catalog
and ServiceFinder this module. Why not? I imagined the general
idea for the Geotools library was always to use the
FactoryFinder/Factory to easily transition to a GeoAPI setup
where the user grabbed one FactoryFinder and could eventually
get everything from that (be it Geotools or GeoAPI).
*) Are there other approaches to the creation of these objects?
Populate the catalog:
--------------------
- create the Catalog with new DefaultCatalog()
- create a ServiceFinder with new(myCatalog)
- create a List of services with
ServiceFinder.acquire(serviceURI)
- create a Service by getting the first element of the List
*) might the List ever have more than one element?
- add the service to the Catalog with Catalog.add(myService).
Use:
---
Catalog
- to search through the catalog services with .search()
(see next)
*) resolve( *) ? When would this be used?
Service
- to get a DataStore:
e.g. myService.resolve(DataStore.class)
GeoResource
- to get a FeatureSource/FeatureStore:
e.g. myGeoResource.resolve(FeatureSource.class)
- to get the Service parent and use that to get a
DataStore.
e.g. myService = myGeoResource.parent();
myService.resolve(DataStore.class)
*) Are those the general idea?
Searching:
---------
I presume searching is only on the resources obtainable from services
contained in the Catalog. That is, search is not against any resources
on the internet, nor against resources hard coded in the Geotools
library's code, but against entries in the services which the user has
added to their Catalog.
*) Is that correct?
Search is presumably against the strings contained in the
GetCapabilities documents, in the shapefiles or in the GML.
*) Is there anywhere an indication of what is being searched within the
different service types?
************************************************************************
Questions from the Catalog+Tutorial sections:
Using the Catalog
-----------------
Populating the Catalog
First code block:
*) Why don't we iterate through the list instead
of getting only the first element?
I would expect while list.hasNext() or
some such loop. Is there only ever a
single service returned?
Second code block:
*) How can the hash map have three identical
keys?
I don't understand why we can reuse the
same key, 'WFSDataStoreFactory.URL.key',
each time in the hash map. Should not
this be
'DataStoreFactorySpi.Param.PASSWORD' and
its ilk?
Looking up Services and Resources in the Catalog
First code block:
*) Should factory be finder?
We just got the List of services from a
ServiceFinder object above. Have we
really switched to a factory? What kind
of factory, a DefaultServiceFactory?
What's the difference?
Third code block:
*) The Resource should be GeoResource, correct?
Since there's no Resource in the Gt
2.2.0 API.
Implementing Catalog Support:
----------------------------
Implementing the Service
The canResolve(Class) code block:
*) Why does canResolve(null) return false when
the first statement in resolve( * ) is a
perfectly good return of null? The Resolve
*will* resolve null to null. Not that it's
interesting.
*) What does canResolve() tell you? It's only
telling you that if the internet weather is
right you *might* be able to get a class of the
kind you request, not that you *will* be able to
resolve() the object when you request for it,
correct? That is, your resolve() call could
still fail due to i/o, right? If so, what's the
defensive programming here?
Okay, enough for now (don't you agree). Thanks for any comments you may
have. Cheers,
--adrian
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