Introduction
The Edition-Based Redefinition (EBR) feature of Oracle Database 11gR2 is ideal for web sites
that aim for no downtime when releasing application enhancements.
Editioning allows certain database objects to have multiple versions
all available at the same time. Applications can decide at runtime
which version of the objects should be used.
In this example I show how Oracle editioning can be used to upgrade a
"live" PHP application. The changes can be made and tested on the
production database and then enabled for all users with a one keyword
change to the application.
The objects that can be editioned by Oracle are:
- synonyms
- views
- PL/SQL object types:
- function
- library
- package and package body
- procedure
- trigger
- type and type body
Tables themselves can't be editioned but there is support for moving
and viewing data across editions.
The Initial PHP Application
To start the example, run SQL*Plus as the user SYSTEM and grant
edition access to the application user CJ:
sqlplus system/systempassword
grant create any edition to cj;
alter user cj enable editions;
As the user CJ create a table of employees for the application:
sqlplus cj/cj
drop table myemp;
create table myemp (name varchar2(10), hoursworked number);
insert into myemp (name, hoursworked) values ('alison', 200);
insert into myemp (name, hoursworked) values ('kris', 200);
insert into myemp (name, hoursworked) values ('wenji', 200);
commit;
Now create the stored function that calculates the number of days of
vacation an employee is eligible for. This function is stored in the
database so all Oracle applications can reuse the same logic:
create or replace function
vacationdaysleft(p_name in varchar2) return number as
vdl number;
begin
-- For every 40 hours worked, you get 1 day of vacation
select floor(hoursworked / 40) into vdl
from myemp
where myemp.name = p_name;
return vdl;
end;
/
Test the function in SQL*Plus by calling:
select name, vacationdaysleft(name) from myemp;
This returns:
NAME VACATIONDAYSLEFT(NAME)
------------------------------ ----------------------
alison 5
kris 5
wenji 5
In PHP OCI8 1.3 the function could be used in an example file,
vacation.php:
<?php
$c = oci_connect('cj', 'cj', 'localhost/orcl');
$s = oci_parse($c, 'alter session set edition = ora$base');
oci_execute($s);
$s = oci_parse($c, "begin :vdl := vacationdaysleft(:name); end;");
oci_bind_by_name($s, ":vdl", $vdl, 10);
oci_bind_by_name($s, ":name", $name, 10);
$name = 'alison';
oci_execute($s);
echo "$name has ".$vdl." days vacation left" . PHP_EOL;
?>
This makes a simple call to the stored function and returns in $vdl
the number of days vacation for person $name.
Note the ALTER SESSION call. I've added it because I know I'll be
using the editioning feature. The 'ora$base' token means to use the
root or first edition of objects, i.e. the PL/SQL function
vacationdaysleft() we just created. The ALTER statement should appear
pretty much immediately after the connection is made - you don't want
parts of the application to be using the wrong edition.
The PHP script produces output like:
$ php vacation.php
alison has 5 days vacation left
This is good. We put the application into production and employees
start using it. All is well for a while.
Enhancing the Application
Now assume the rate used to calculate vacation hours needs to be
changed so that now for every 30 hours worked, employees get one day
of vacation. Also we want the calculation to include how many days
vacation they have already taken.
First, we need a new column to store the vacation they have taken.
Adding this column won't affect the running PHP application since it
doesn't know about it (this is a good reminder never to do "select *"
in an application). The new column is:
alter table myemp add daysvacationtaken number;
We need to populate that column manually, here with values I've
invented:
update myemp set daysvacationtaken = 2 where name = 'alison';
update myemp set daysvacationtaken = 3 where name = 'kris';
update myemp set daysvacationtaken = 0 where name = 'wenji';
commit;
For more complex scenarios the editioning feature has CROSSEDITION
triggers and editioning views to help ensure the appropriate data is
available and used correctly in the old and new editions of the
application. Remember our current and future enhanced versions of the
PL/SQL function can be accessing the same table concurrently - only
the PL/SQL code has multiple versions.
Now we need to create the new version of the PL/SQL function. In
SQL*Plus create a new edition and set it to be in use for this
session:
sqlplus cj/cj
create edition e2;
alter session set edition = e2;
Now create the updated version of the procedure. Because the SQL*Plus
session is in edition 'e2' this won't affect the PHP application which
runs using edition 'ora$base'. The enhanced function can be created
with:
create or replace function
vacationdaysleft(p_name in varchar2) return number as
vdl number;
begin
-- For every 30 hours worked, you get 1 day of vacation
select floor(hoursworked / 30) - daysvacationtaken into vdl
from myemp
where myemp.name = p_name;
return vdl;
end;
/
Querying it shows the updated values:
select name, vacationdaysleft(name) from myemp;
NAME VACATIONDAYSLEFT(NAME)
------------------------------ ----------------------
alison 4
kris 3
wenji 6
This is all done while users continue to use the PHP application live
and get the original results. You can check that the script still
returns the "old" value:
$ php vacation.php
alison has 5 days vacation left
Now edit vacation.php and change the ALTER SESSION statement to the
new edition:
$s = oci_parse($c, 'alter session set edition = e2');
When run, the script output now shows the updated calcuation of
vacation time:
$ php vacation.php
alison has 4 days vacation left
You can see that the roll out of application updates can be made
faster and have more thorough testing, increasing the availability and
reliability of web sites.
Summary
Oracle Database 11g Release 2 Editioning helps PHP web sites meet the
goals of minimal downtime with the frequent upgrade cycle needed by
modern web applications. It allows PHP applications, their often
complex stored business logic, and large data sets to be updated ready
for going live in production without impacting the operation of
existing users.
After setting up the edition objects, the simple process of rolling
out a new PHP script with the updated edition name allows application
upgrades to occur rapidly.
The Oracle manuals have more information about using and managing
editions.
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