Wednesday, July 22, 2009

APEX application running on "another" database

I have attended some APEX presentations/seminars and one question seems to be repeating and got my attention. Can APEX run on another database?

Well, APEX itself or APEX application itself cannot run in any way on another database besides Oracle. APEX is PL/SQL application and no other database supports Oracle PL/SQL language.

But it's possible to select and modify data from another database from your APEX application.

First of all you need to set up a database link to the other database using Oracle Heterogeneous Services/Transparent Gateways. I'm not going to get into that right now, please use documentation/google for that.
I'm going to name my database link HSDB:

create database link hsdb 
  connect to "lowercaseusername" 
  identified by "randomCasepaSSword"
  using 'MYSQL_DB_SERVICE';

I'm going to use MySQL database and I'll create table employees on MySQL database:

CREATE TABLE `employees` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `first_name` varchar(100) NOT NULL,
  `last_name` varchar(100) NOT NULL,
  `personal_code` varchar(20) NOT NULL,
  `birthday` date DEFAULT NULL,
  `salary` decimal(10,2) NOT NULL,
  `is_active` tinyint(4) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Test that table employees is accessible from Oracle:

select * from "employees"@hsdb;

NB! I created the table in MySQL with a lower case table name, but Oracle uses upper case identifiers by default, unless you force the correct case with double-quotes.

One other thing to look out for: check that ODBC/Oracle do not translate any of the columns to the deprecated LONG datatype. LONG columns seems to be used for long/unlimited text fields (*text in MySQL, text in PostgreSQL). You can check it by running DESC command from SQL*PLUS.

desc "employees"@hsdb
 Name                       Null?    Type
 -------------------------- -------- -----------------
 id                                  NUMBER(10)
 first_name                 NOT NULL VARCHAR2(100)
 last_name                  NOT NULL VARCHAR2(100)
 personal_code              NOT NULL VARCHAR2(20)
 birthday                   DATE
 salary                     NOT NULL NUMBER(10,2)
 is_active                  NOT NULL NUMBER(3)

Make sure, that none of the data types is LONG. If you have LONG column, then you need to modify the data type from the source database or create a "Oracle friendly" view on top of it.
In PostgreSQL ODBC driver its also possible to modify the maximum VARCHAR length before its mapped to LONG.
Anyway, this is the most critical part in my opinion. Make sure that you are happy with the data type mappings before continuing!

Now create an Oracle view on top that MySQL table. This view will translate all column names to upper case and will simplify writing the SQL code.

  CREATE VIEW EMPLOYEES_MYSQL AS 
  SELECT   "id" id,
            "first_name" first_name,
            "last_name" last_name,
            "personal_code" personal_code,
            "birthday" birthday,
            "salary" salary,
            "is_active" is_active
     FROM   "employees"@hsdb;

desc employees_mysql
 Name                    Null?    Type
 ----------------------- -------- ----------------
 ID                               NUMBER(10)
 FIRST_NAME              NOT NULL VARCHAR2(300)
 LAST_NAME               NOT NULL VARCHAR2(300)
 PERSONAL_CODE           NOT NULL VARCHAR2(60)
 BIRTHDAY                DATE
 SALARY                  NOT NULL NUMBER(10,2)
 IS_ACTIVE               NOT NULL NUMBER(3)

Simple report

Creating a simple report is very easy, just use the created EMPLOYEES_MYSQL view in APEX like any other normal Oracle table/view.

Complex report

Writing vanilla-SQL against non-Oracle table over database link usually works, when the SQL is really simple. But there are pretty serious limitations and in some cases the performance "may not be very good" (indexes not used for some data type conversions).
To solve that problem, Oracle has a package DBMS_HS_PASSTHROUGH, that sends unmodified native SQL query to the linked database.

For this example I'll use the following native MySQL SQL statement for a report:

select sql_cache id, first_name, last_name from employees limit 10

To be able to use this result in a SQL statement, I'm going to use PIPELINED function and for that I first need to create TYPE, that will define the structure of the query output. And after that I'll create the actual function that will query the remote database.

create or replace type mysql_hstest_type as object (
  id number,
  first_name varchar2(250),
  last_name varchar2(250)
);
/

create or replace type tab_mysql_hstest_type is 
  table of mysql_hstest_type;
/

create or replace FUNCTION mysql_hstest_query 
  RETURN tab_mysql_hstest_type PIPELINED
IS
  p_row mysql_hstest_type:= mysql_hstest_type(null, null, null);
  p_c binary_integer;
BEGIN
  p_c:= DBMS_HS_PASSTHROUGH.OPEN_CURSOR@hsdb;
  DBMS_HS_PASSTHROUGH.PARSE@hsdb(p_c, 
    'select sql_cache id, first_name, last_name from employees limit 10');
  WHILE DBMS_HS_PASSTHROUGH.FETCH_ROW@hsdb(p_c) > 0 LOOP
    DBMS_HS_PASSTHROUGH.GET_VALUE@hsdb(p_c, 1, p_row.id);
    DBMS_HS_PASSTHROUGH.GET_VALUE@hsdb(p_c, 2, p_row.first_name);
    DBMS_HS_PASSTHROUGH.GET_VALUE@hsdb(p_c, 3, p_row.last_name);
    PIPE ROW(p_row);
  END LOOP;
  DBMS_HS_PASSTHROUGH.CLOSE_CURSOR@hsdb(p_c);
  RETURN;
END;
/

Now, to use it in an APEX form just use the following SQL in report:

select * from table(mysql_hstest_query)

Form

First create a form on a view EMPLOYEES_MYSQL with the wizard like for any normal table. This will just create all the necessary page elements quickly.

Its necessary to write optimistic locking feature, because "Automatic Row Processing (DML)" process cannot be used and optimistic locking is a "silent" built in feature of that process. If for some weird reason you do not want the optimistic locking feature, then you can skip the checksum and validation steps.
For checksumming I'll create hashing function (this one needs execute privileges on DBMS_CRYPTO):

CREATE OR REPLACE FUNCTION form_md5_checksum(
  p1 varchar2 default null, p2 varchar2 default null,
  p3 varchar2 default null, p4 varchar2 default null,
  p5 varchar2 default null, p6 varchar2 default null,
  p7 varchar2 default null, p8 varchar2 default null,
  p9 varchar2 default null, p10 varchar2 default null)
RETURN varchar2 DETERMINISTIC IS
BEGIN
  RETURN rawtohex(DBMS_crypto.hash(UTL_RAW.CAST_TO_RAW(
    p1||'|'||p2||'|'||p3||'|'||p4||'|'||
    p5||'|'||p6||'|'||p7||'|'||p8||'|'||
    p9||'|'||p10), DBMS_CRYPTO.HASH_MD5));
END;
/

Now create a new hidden and protected item in the APEX page, I'll call it P3_CHECKSUM.
Then create a PL/SQL anonymous block process:

  Name: Calculate checksum
  Sequence: 20 (just after Automated Row Fetch)
  Process Point: On Load - After Header
  Process:
BEGIN
  :p3_checksum:= form_md5_checksum(
    :p3_ID, :p3_FIRST_NAME, :p3_LAST_NAME, :p3_PERSONAL_CODE, 
    :p3_BIRTHDAY, :p3_SALARY, :p3_IS_ACTIVE);
END;

Then I removed "Database Action" from the form buttons and changed the Button Name (the page submit REQUEST value):

Delete - DELETE
Apply Changes - UPDATE
Create - INSERT

The default "Automatic Row Processing (DML)" process cannot be used for saving the chages back to the database, because the ODBC database/driver lacks the support for SELECT FOR UPDATE. Because of it, delete the existing "Automatic Row Processing (DML)" process.

To save the changes, a new procedure is needed:

CREATE OR REPLACE PROCEDURE modify_employees_mysql(
  p_action IN VARCHAR2, p_row IN employees_mysql%rowtype,
  p_md5 varchar2)
IS
  PRAGMA AUTONOMOUS_TRANSACTION;
  p_new_md5 varchar2(50);
  p_new_row employees_mysql%rowtype;
BEGIN
  -- Calculate checksum
  IF p_action IN ('UPDATE','DELETE') AND p_row.id IS NOT NULL THEN
    -- Lock the row
    UPDATE employees_mysql SET first_name = first_name
      WHERE id = p_row.id;
    -- Calculate new checksum
    SELECT * INTO p_new_row FROM employees_mysql WHERE id = p_row.id;
    p_new_md5:= form_md5_checksum(p_new_row.ID, p_new_row.FIRST_NAME,
      p_new_row.LAST_NAME, p_new_row.PERSONAL_CODE, 
      p_new_row.BIRTHDAY, p_new_row.SALARY, p_new_row.IS_ACTIVE);
    -- Check if the checksum has changed
    IF NVL(p_new_md5, '-') <> NVL(p_md5, '-') THEN
      ROLLBACK;
      raise_application_error(-20000, 'Data has changed');
    END IF;
    --
  END IF;
  -- Do the data modifications
  IF p_action = 'INSERT' THEN
    INSERT INTO employees_mysql VALUES p_row;
  ELSIF p_action = 'UPDATE' AND p_row.id IS NOT NULL THEN
    UPDATE employees_mysql SET ROW = p_row WHERE id = p_row.id;
  ELSIF p_action = 'DELETE' AND p_row.id IS NOT NULL THEN
    DELETE FROM employees_mysql WHERE id = p_row.id;
  ELSE
    raise_application_error(-20099, 'Invalid action.');
  END IF;
  commit;
END;
/

Note the "PRAGMA AUTONOMOUS_TRANSACTION" in the above code. I used the default open source MySQL ODBC driver that lacks the support for 2PC (Two Phase Commit). The symptom for this "ORA-02047: cannot join the distributed transaction in progress" when running the procedure inside APEX transaction.
If you are using some commercial ODBC driver with 2PC support or drivers supplied by Oracle HS or Oracle Transparent Gateways, then you don't need autonomous transaction for this procedure and you also need to remove commit/rollback statements from the procedure.

And finally put this procedure to the APEX page flow.
Create a new PL/SQL anonymous block process:

  Name: Save changes
  Sequence: 30
  Process Point: On Submit - After Computations and Validations
  Process:
DECLARE
  p_row employees_mysql%rowtype;
BEGIN
  p_row.id:= :P3_ID;
  p_row.first_name:= :P3_FIRST_NAME;
  p_row.last_name:= :P3_LAST_NAME;
  p_row.personal_code:= :P3_PERSONAL_CODE;
  p_row.birthday:= :P3_BIRTHDAY;
  p_row.salary:= :P3_SALARY;
  p_row.is_active:= :P3_IS_ACTIVE;
  modify_employees_mysql(:REQUEST, p_row, :p3_checksum);
END;

And now you have it - APEX running on another database :)

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