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From conflue...@apache.org
Subject [CONF] Lucene Connector Framework > How to Write an Output Connector
Date Tue, 09 Mar 2010 15:11:00 GMT
Space: Lucene Connector Framework (http://cwiki.apache.org/confluence/display/CONNECTORS)
Page: How to Write an Output Connector (http://cwiki.apache.org/confluence/display/CONNECTORS/How+to+Write+an+Output+Connector)


Edited by Karl Wright:
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h1. Writing an Output Connector

An output connector furnishes the mechanism by which content that has been fetched from a
repository gets handed to a back-end repository for processing.  It also furnishes a mechanism
for removing previously-processed content from that back end repository.

As is the case with all connectors under the LCF umbrella, an output connector consists of
two parts:

* A class implementing an interface (in this case, _org.apache.lcf.agents.interfaces.IOutputConnector_)
* A set of JSP's that implement the crawler UI for the connector

h3. Key concepts

The output connector abstraction makes use of, or introduces, the following concepts:

|| Concept || What it is ||
| Configuration parameters | A hierarchical structure, internally represented as an XML document,
which describes a specific configuration of a specific output connector, i.e. *how* the connector
should do its job; see _org.apache.lcf.core.interfaces.ConfigParams_ |
| Output connection | An output connector instance that has been furnished with configuration
data |
| Document URI | The unique URI (or, in some cases, file IRI) of a document, which is meant
to be displayed in search engine results as the link to the document |
| Repository document | An object that describes a document's contents, including raw document
data (as a stream), metadata (as either strings or streams), and access tokens; see _org.apache.lcf.agents.interfaces.RepositoryDocument_
|
| Connection management/threading/pooling model | How an individual output connector class
instance is managed and used |
| Activity infrastructure | The framework API provided to specific methods allowing those
methods to perform specific actions within the framework, e.g. recording activities; see _org.apache.lcf.agents.interfaces.IOutputAddActivity_
and _org.apache.lcf.agents.interfaces.IOutputRemoveActivity_ |
| Output specification | A hierarchical structure, internally represented as an XML document,
which describes *what* a specific output connector should do in the context of a specific
job; see _org.apache.lcf.agents.interfaces.OutputSpecification_ |
| Output version string | A simple string, used for comparison purposes, that allows LCF to
figure out if an ingestion operation needs to be repeated as a result of changes to the output
specification in effect for a document |
| Service interruption | A specific kind of exception that signals LCF that the output repository
is unavailable, and gives a best estimate of when it might become available again; see _org.apache.lcf.agents.interfaces.ServiceInterruption_
|


h3. Implementing the Output Connector class

A very good place to start is to read the javadoc for the output connector interface.  You
will note that the javadoc describes the usage and pooling model for a connector class pretty
thoroughly.  It is very important to understand the model thoroughly in order to write reliable
connectors!  Use of static variables, for one thing, must be done in a very careful way, to
avoid issues that would be hard to detect with a cursory test.

The second thing to do is to examine some of the provided output connector implementations.
 The GTS connector, the SOLR connector, and the Null Output connector all are output connectors
which demonstrate (to some degree) the sorts of techniques you will need for an effective
implementation.  You will also note that all of these connectors extend a framework-provided
output connector base class, found at _org.apache.lcf.agents.output.BaseOutputConnector_.
 This base class furnishes some basic bookkeeping logic for managing the connector pool, as
well as default implementations of some of the less typical functionality a connector may
have.  For example, connectors are allowed to have database tables of their own, which are
instantiated when the connector is registered, and are torn down when the connector is removed.
 This is, however, not very typical, and the base implementation reflects that.

TODO: More implementation details

h3. Implementing a set of Output Connector JSPs

The output connector class you write provides, through one of its methods, a symbolic name
where the crawler UI will look for output connector UI components.  Your components will therefore
have the following path, relative to the crawler UI web application:

_output/<connector_symbolic_name>_

For an output connector, you need to furnish the following JSPs:

|| JSP name || Where it fits ||
| headerconfig.jsp | Called during the header section of output connector configuration editing
page |
| editconfig.jsp | Called during the body section of the output connector configuration editing
page |
| postconfig.jsp | Called when configuration editing page is posted, either on a repost or
on a save |
| viewconfig.jsp | Called when the connection configuration is being viewed |
| headerspec.jsp | Called during the header section of a job definition editing page, for
which this output connector has been selected |
| editspec.jsp | Called during the body section of a job definition editing page, for which
this output connector has been selected |
| postspec.jsp | Called whenever a job definition that uses this output connector is posted,
either for a repost or a save |
| viewspec.jsp | Called when a job definition that uses this output connector is viewed |

TODO: More implementation details

h3. Implementation support provided by the framework

LCF's framework provides a number of helpful services designed to make the creation of a connector
easier.  These services are summarized below.  (This is not an exhaustive list, by any means.)

* Lock management and synchronization (see _org.apache.lcf.core.interfaces.LockManagerFactory_)
* Cache management (see _org.apache.lcf.core.interfaces.CacheManagerFactory_)
* Local keystore management (see _org.apache.lcf.core.KeystoreManagerFactory_)
* Database management (see _org.apache.lcf.core.DBInterfaceFactory_)

For JSP UI component support, these too are very useful:

* Multipart form processing (see _org.apache.lcf.ui.multipart.MultipartWrapper_)
* HTML encoding (see _org.apache.lcf.ui.util.Encoder_)
* HTML formatting (see _org.apache.lcf.ui.util.Formatter_)

h3. DO's and DON'T DO's

It's always a good idea to make use of an existing infrastructure component, if it's meant
for that purpose, rather than inventing your own.  There are, however, some limitations we
recommend you adhere to.

* DO make use of infrastructure components described in the section above
* DON'T make use of infrastructure components that aren't mentioned, without checking first
* NEVER write connector code that directly uses framework database tables, other than the
ones installed and managed by your connector

If you are tempted to violate these rules, it may well mean you don't understand something
important.  At the very least, we'd like to know why.  Send email to connectors-dev@incubator.apache.org
with a description of your problem and how you are tempted to solve it.


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