Focus FeatureSource around FeatureReader and FeatureWriter - STEMLab/geotools GitHub Wiki

Description

The Problem

Currently our data access api presents two ways to access data:

  1. query-based reader/writer access
  2. operation-based collection access

While providing two modes of data access does have some advantages in terms of flexibility it has some major problems:

API Confusion

Having two apis for accessing data makes it harder to approach for users. Either method does not present any decisive advantage over the other. Its just "more than one way to do it" (to coin a phrase from perl), which has led to more headache than benefit.

Duplication

FeatureCollection duplicates much of what was originally created in FeatureReader/FeatureWriter. The biggest violation being iterators. Not only does FeatureCollection provide a duplication of FeatureReader in the form of Iterator from java collections, it goes beyond and defines the features() method which returns a FeatureIterator, which is literally a one-to-one duplication of FeatureReader.

One of the biggest problems of this duplication has been with resect to decorators. Each time a decorator is implemented, it is implemented up to 3 times. To add to the mess a set of adapters have been created to go back and forth between the 3 types of iterator. The following diagram illustrates:

Implementation Complexity

Having two data access apis to implement makes it very hard to create a clean datastore, which ultimately leads to a high cost of maintainence. This had led to a mass of unsupported format drivers in Geotools. With a better organization and internal api for implementing datastores, many of these drivers could be saved.

Data store implementors having to implement FeatureCollection is something that has been a disaster. The following diagram illustrates:

The Solution

The approach presented in this proposal is two fold:

  • focus the DataStore/FeatureSource api around FeatureReader and FeatureWriter
  • make FeatureCollection a convenience wrapper around DataStore/FeatureSource

The benefits of this are:

  • 100% backwards compatable
  • Implementors can ignore FeatureCollection and implement the simpler FeatureReader/FeatureWriter api
  • FeatureCollection gets implemented once and all duplication is removed

The modifications to the current datastore api are strictly additions. Consider the following:

interface DataStore2 extends DataStore {
  FeatureSource getFeatureSource( String typeName, Transaction tx );
  FeatureSource getFeatureSource( Name typeName, Transaction tx );
}
interface FeatureSource2 extends FeatureSource {

  /** get a reader over the entire set of features */
  FeatureReader getReader();

  /** get a reader over a subset of features */
  FeatureReader getReader( Filter filter );
  FeatureReader getReader( Query query );

}
interface FeatureStore2 extends FeatureStore {

  /** get an inserting + updating writer over the entire set of features */
  FeatureWriter getWriter();

  /** get an inserting + updating writer over a subset set of features */
  FeatureWriter getWriter( Filter filter );
  FeatureWriter getWriter( Query query );

  /** get an updating writer */
  FeatureWriter getWriterUpdate();
  FeatureWriter getWriterUpdate( Filter filter );
  FeatureWriter getWriterUpdate( Query query );

  /** get an inserting writer */
  FeatureWriter getWriterInsert();
 
}

As for FeatureCollection, with this additional api it is possible to create a single implementation which delegates to the DataStore and FeatureSource api. Such an implementation can be found here.

The ContentDataStore class is an experimental abstract datastore implementation which is based on the api changes presented in this proposal. It is the base of the new jdbc datastore which has been used in GeoServer and is fully cite compliant (so we know it works).

Status

This proposal is under construction.

Voting has not started yet:

------------|--------------------|------------|-------------------------|-------------------------------| no progress | done | impeded | lack mandate/funds/time | volunteer needed |

GeoTools 2.5

GeoTools 2.7.x

  • Review the FeatureCollection implementation in H2 datastore that completly delegates to FeatureSource
  • Copy the FeatureSource implementation from H2 into data store modules one at a time
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