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Postgres trigger to update Java cache

I have a Java web app (WAR deployed to Tomcat) that keeps a cache (Map<Long,Widget>) in memory. I have a Postgres database that contains a widgets table:
widget_id | widget_name | widget_value
(INT) (VARCHAR 50) (INT)
To O/R map between Widget POJOs and widgets table records, I am using MyBatis. I would like to implement a solution whereby the Java cache (the Map) is updated in real-time whenever a value in the widgets table changes. I could have a polling component that checks the table every, say, 30 seconds, but polling just doesn't feel like the right solution here. So here's what I'm proposing:
Write a Postgres trigger that calls a stored procedure (run_cache_updater())
The procedure in turns runs a shell script (run_cache_updater.sh)
The script base-64 encodes the changed widgets record and then cURLs the encoded record to an HTTP URL
The Java WAR has a servlet listening on the cURLed URL and handles any HttpServletRequests sent to it. It base-64 decodes the record and somehow transforms it into a Widget POJO.
The cache (Map<Long,Widget>) is updated with the correct key/value.
This solution feels awkward, and so I am first wondering how any Java/Postgres gurus out there would handle such a situation. Is polling the better/simpler choice here (am I just being stubborn?) Is there another/better/more standard solution I am overlooking?
If not, and this solution is the standard way of pushing changed records from Postgres to the application layer, then I'm choking on how to write the trigger, stored procedure, and shell script so that the entire widgets record gets passed into the cURL statement. Thanks in advance for any help here.

I can't speak to MyBatis, but I can tell you that PostgreSQL has a publish/subscribe system baked in, which would let you do this with much less hackery.
First, set up a trigger on widgets that runs on every insert, update, and delete operation. Have it extract the primary key and NOTIFYwidgets_changed, id. (Well, from PL/pgSQL, you'd probably want PERFORM pg_notify(...).) PostgreSQL will broadcast your notification if and when that transaction commits, making both the notification and the corresponding data changes visible to other connections.
In the client, you'd want to run a thread dedicated to keeping this map up-to-date. It would connect to PostgreSQL, LISTENwidgets_changed to start queueing notifications, SELECT * FROM widgets to populate the map, and wait for notifications to arrive. (Checking for notifications apparently involves polling the JDBC driver, which sucks, but not as bad as you might think. See PgNotificationPoller for a concrete implementation.) Once you see a notification, look up the indicated record and update your map. Note that it's important to LISTEN before the initial SELECT *, since records could be changed between SELECT * and LISTEN.
This approach doesn't require PostgreSQL to know anything about your application. All it has to do is send notifications; your application does the rest. There's no shell scripts, no HTTP, and no callbacks, letting you reconfigure/redeploy your application without also having to reconfigure the database. It's just a database, and it can be backed up, restored, replicated, etc. with no extra complications. Similarly, your application has no extra complexities: all it needs is a connection to PostgreSQL, which you already have.

Simple ways to note/log the last date/time when a database was updated, accessed, modified etc

I made Java/JDBC code which performs simple/basic operations on a database.
I want to add code which helps me to keep a track of when a particular database was accessed, updated, modified etc by this program.
I am thinking of creating another database inside my DBMS where these details or logs will be stored for each database involved.
Is this the best way to do it ? Are there any other ways (preferably simple) to do this ?
EDIT-
For now, I am using MySQL. But, I also want my code to work with at least
Oracle SQL and MS-SQL as well.

It is pretty standard to add a "last_modified" column to a table and then add an update trigger on the table to set it to the db current time. Then your apps don't need to worry about it. Also, a "create_time" is often used as well, populated by an insert trigger.
Update after comment:
Seems you are looking for audit logs. Some write apps where data manipulation only happens through stored procedures and not through inserts and updates. A fixed api. So you want to add an item to a table, you call the stored proc:
addItem(itemName, itemDescription)
Then the proc inserts into the item table and does what ever logging is necessary.
Another technique, if you are using some kind of framework for your jdbc access (say Spring) might be to intercept at that layer.

In almost all tables, I have the following columns:
CreatedBy
CreatedAt
These columns have default values of the current user and current time, respectively. They are populated when a row is added.
This solves only part of your problem. You can start adding triggers, but that gets complicated. Another method is to force modification access to the database through stored procedures, and then log the stored procedures. This has other advantages, in terms of controlling what users can do. But, you might want more flexibility.
A third possibility are auditing tools, that keep track of all queries being run on the database. I think most databases have a way of turning on internal auditing, although these are very specific to the database. There are also third party tools that allow you to see what has happened. Note, though, that these methods will affect performance if your database is doing high volume transactions.
For more information, you should revise your question to specify which database you are using or planning on using.

how to call a java method when a record inserted in mysql database

Please tell me is it possible to call a java method or a struts action class when a record is inserted in mysql database
Actuall my problem is i have 2 databases(both are mysql databases namely db1 and db2) running in two different locations.I need communication between them.When a record is inserted in X table in db1 at the same time some data must be insert in Y table in db2 depending up on the data inserted in db2 some data must be inserted in Z table in db1.
Actuall my project is patient care project
for a group of patients one care taker is there
for each and every patient one embeded button is there.
the mapping between the patient details and the embeded button and the corresponding care taker is there in db2 database
when the patient pressed the embeded button embeded information is inserted in db1
and inorder to send a mobile message to care taker i haveto get the data from db2 depending up on the embeded information inserted in db1 as patient and caretaker mapping is in db2
the businesslogic communicate with db1 iswritten in vc++
the businesslogic communicate with db2 iswritten in java
so i need communication between db1 and db2
technologies used
java,struts 1.2
database : mysql

I don't think it it is possible to trigger java code actively from db insert. What we do is the opposite:
when an insert occurs, we write the rowid to a dedicated table
we have a Java job that runs regularly and checks that table for new entries
You should not do any such thing in a web context, so forget struts in this context!

You can have a java process periodically query the table to check for new records. A better solution would be to have a database trigger perform the business logic you need (though you can't call the struts action directly). If you can provide more detail on what business logic you need performed after the row is inserted, we can maybe provide a much cleaner design.

Even though it's possible to use another Struts action (As it's a simple POJO class that can be instantiated like SomeAction action = new SomeAction()) I would not recommend it for few reasons:
You will have to pass various Struts configurations to the action in order to work.
Handle the ActionForward response.
My suggestion is to put your business logic in a service that you can either pass the same data to 2 databases or have strategy that understands that the data needs to be recorded to 2 databases.

Here's an extension for MySQL (UDF) that allows execution of other programs from MySQL.
It should be relatively straightforward how to use it. I don't know ins and outs of your application, but you can create a trigger that calls a program and passes certain parameters you require to establish your db communication.

Can MySQL triggers or stored procedures be written in Java?

I have two databases. Changes like edits, insertions to one, need to be made to the second one as well and vice versa.
Actually, one database is an old legacy database (with a very bad Entity-Relationship structure) and a legacy app front-end currently used by users.
The second database is a newly built, better restructure of the legacy with a separate app. front-end.
I want both apps (accessing the legacy and the new database respectively) to run simultaneously so users can have the option to use both applications and changes in one app are visible across the other.
I want to write triggers which call stored procedures, which restructure the data and put it in the opposite database.
My question is:
Is my line of execution as it is supposed to be? I mean, triggers >call> stored procedures >call> database.
Can triggers / stored procedures be written in Java?
Any good/ recommended tips, tutorials etc etc out there?
There are many links on google but none of them are useful. I wonder if MySQL and Java work together when it comes to MySQL triggers? is it possible or not?. Is there a better way of achieving what I need?

Triggers are named database objects. They define some action that the database should take when certain database related events occur. They are written in SQL. Their execution is transparent to the user. You write your Java JDBC code as usual and the DBMS will automatically execute the appropriate trigger whenever necessary.
mysql> delimiter //
mysql> CREATE TRIGGER insert_trigger BEFORE INSERT ON Customer
-> FOR EACH ROW
-> BEGIN
-> UPDATE Customer SET Price=Price-10 WHERE CustomerGroup=32 and CityCode=11;
-> END;
-> //
This example shows you how to make your trigger write to another database. Be careful with auto-increment attributes.
I think you should forget about Java stored procedures in MySQL, but you could always move the business logic to your own Java program.

keeping the history of table in java

I need the sample program in Java for keeping the history of table if user inserted, updated and deleted on that table. Can anybody help in this?
Thanks in advance.

If you are working with Hibernate you can use Envers to solve this problem.

You have two options for this:
Let the database handle this automatically using triggers. I don't know what database you're using but all of them support triggers that you can use for this.
Write code in your program that does something similar when inserting, updating and deleting a user.
Personally, I prefer the first option. It probably requires less maintenance. There may be multiple places where you update a user, all those places need the code to update the other table. Besides, in the database you have more options for specifying required values and integrity constraints.

Well, we normally have our own history tables which (mostly) look like the original table. Since most of our tables already have the creation date, modification date and the respective users, all we need to do is copy the dataset from the live table to the history table with a creation date of now().
We're using Hibernate so this could be done in an interceptor, but there may be other options as well, e.g. some database trigger executing a script, etc.

How is this a Java question?
This should be moved in Database section.
You need to create a history table. Then create database triggers on the original table for "create or replace trigger before insert or update or delete on table for each row ...."

I think this can be achieved by creating a trigger in the sql-server.
you can create the TRIGGER as follows:
Syntax:
CREATE TRIGGER trigger_name
{BEFORE | AFTER } {INSERT | UPDATE |
DELETE } ON table_name FOR EACH ROW
triggered_statement
you'll have to create 2 triggers one for before the operation is performed and another after the operation is performed.
otherwise it can be achieved through code also but it would be a bit tedious for the code to handle in case of batch processes.

You should try using triggers. You can have a separate table (exact replica of your table of which you need to maintain history) .
This table will then be updated by trigger after every insert/update/delete on your main table.
Then you can write your java code to get these changes from the second history table.

I think you can use the redo log of your underlying database to keep track of the operation performed. Is there any particular reason to go for the program?

You could try creating say a List of the objects from the table (Assuming you have objects for the data). Which will allow you to loop through the list and compare to the current data in the table? You will then be able to see if any changes occurred.
You can even create another list with a object that contains an enumerator that gives you the action (DELETE, UPDATE, CREATE) along with the new data.
Haven't done this before, just a idea.

Like #Ashish mentioned, triggers can be used to insert into a seperate table - this is commonly referred as Audit-Trail table or audit log table.
Below are columns generally defined in such audit trail table : 'Action' (insert,update,delete) , tablename (table into which it was inserted/deleted/updated), key (primary key of that table on need basis) , timestamp (the time at which this action was done)
It is better to audit-log after the entire transaction is through. If not, in case of exception being passed back to code-side, seperate call to update audit tables will be needed. Hope this helps.

If you are talking about db tables you may use either triggers in db or add some extra code within your application - probably using aspects. If you are using JPA you may use entity listeners or perform some extra logic adding some aspect to your DAO object and apply specific aspect to all DAOs which perform CRUD on entities that needs to sustain historical data. If your DAO object is stateless bean you may use Interceptor to achive that in other case use java proxy functionality, cglib or other lib that may provide aspect functionality for you. If you are using Spring instead of EJB you may advise your DAOs within application context config file.

Triggers are not suggestable, when I stored my audit data in file else I didn't use the database...my suggestion is create table "AUDIT" and write java code with help of servlets and store the data in file or DB or another DB also ...