Design of JDBC_PING
===================

Author: Sanne Ginovero


Goal
----

JDBC_PING is a discovery protocol making use of a shared database; during the discovery process,
new nodes read the addresses of existing members from a JDBC connection.

The design is derived from FILE_PING, and the implementation extends it as there are many similarities.
As with FILE_PING, also with JDBC_PING members write their addresses to the store and remove them on leaving the cluster.


Usage
------

A database is needed, and a single table will be used. Something like the following is expected:

CREATE TABLE JGROUPSPING (
   own_addr varchar(200) NOT NULL,
   cluster_name varchar(200) NOT NULL,
   ping_data varbinary(5000) DEFAULT NULL,
   PRIMARY KEY (own_addr, cluster_name)
   )

It's possible to change the table definitions, make sure to update the three SQL statements accordingly using the appropriate
configuration properties:

 initialize_sql - to customize the table creation SQL instructions
 insert_single_sql - to insert a new row
 delete_single_sql - to delete a single row
 select_all_pingdata_sql - to load all ping_data having a specific cluster_name
 
Connection properties to be set are either:

1) JDBC direct connection

 connection_url
 connection_username
 connection_password
 connection_driver
 
2) via a JNDI registered DataSource

 datasource_jndi_name


Design
------

Each node connects to the same database, and reads all Addresses related to his same cluster to find it's peers,
and adds himself to the list.

At stop(), a node attempts to cleanup the table by removing himself.

Members who crashed should be removed by the same strategy of FILE_PING, actually it should inherit each eventual
improvement.

Warning:
While the node stores the PingData instance of himself, to be retrieved by other nodes, it will nullify the View,
as there's no interest in restoring the View instance and this could potentially not fit in the fixed column dimension
allocated for a serialized PingData, or at least this variable is not fixed so storing View might make it hard to
select a proper size.
So consider that a restored PingData will have lost the information about the original View.