3 Understanding mod_plsql

3.6.1 Parameter Passing by Name (Overloaded Parameters)

Overloading allows multiple subprograms (procedures or functions) to have the same name, but differ in the number, order, or the datatype family of the parameters. When you call an overloaded subprogram, the PL/SQL compiler determines which subprogram to call based on the data types passed.

PL/SQL enables you to overload local or packaged subprograms. Standalone subprograms cannot be overloaded.

You must give parameters different names for overloaded subprograms that have the same number of parameters. Because HTML data is not associated with datatypes, mod_plsql does not know which version of the subprogram to call.

For example, although PL/SQL enables you to define two procedures using the same parameter names for the procedures, an error occurs if you use this with mod_plsql.

-- legal PL/SQL, but not for mod_plsql
CREATE PACKAGE my_pkg AS
  PROCEDURE my_proc (val IN VARCHAR2);
  PROCEDURE my_proc (val IN NUMBER);
END my_pkg;

To avoid the error, name the parameters differently. For example:

-- legal PL/SQL and also works for mod_plsql
CREATE PACKAGE my_pkg AS
  PROCEDURE my_proc (valvc2 IN VARCHAR2);
  PROCEDURE my_proc (valnum IN NUMBER);
END my_pkg;

The URL to invoke the first version of the procedure looks similar to:

http://www.acme.com:9000/pls/mydad/my_pkg.my_proc?valvc2=input

The URL to invoke the second version of the procedure looks similar to:

http://www.acme.com:9000/pls/mydad/my_pkg.my_proc?valnum=34

Overloading and PL/SQL Arrays

If you have overloaded PL/SQL procedures where the parameter names are identical, but the data type is owa_util.ident_arr (a table of varchar2) for one procedure and a scalar type for another procedure, mod_plsql can still distinguish between the two procedures. For example, if you have the following procedures:

CREATE PACKAGE my_pkg AS
  PROCEDURE my_proc (val IN VARCHAR2); -- scalar data type
  PROCEDURE my_proc (val IN owa_util.ident_arr); -- array data type
END my_pkg;

Each of these procedures has a single parameter of the same name, val.

When mod_plsql gets a request that has only one value for the val parameter, it invokes the procedure with the scalar data type.

http://www.acme.com:9000/pls/mydad/my_proc?val=john

http://www.acme.com:9000/pls/mydad/my_proc?val=john&val=sally

3.6.2 Flexible Parameter Passing

You can have HTML forms from which users can select any number of elements. If these elements have different names, you would have to create overloaded procedures to handle each possible combination. Alternatively, you could insert hidden form elements to ensure that the names in the query string are consistent each time, regardless of what elements the user chooses. mod_plsql makes this operation easier by supporting flexible parameter passing to handle HTML forms where users can select any number of elements.

To use flexible parameter passing for a URL-based procedure invocation, prefix the procedure with an exclamation mark (!) in the URL. You can use two or four parameters. The two parameter interface provides improved performance with mod_plsql. The four parameter interface is supported for compatibility.

procedure [proc_name] 
     (name_array IN [array_type],
     value_array IN  [array_type]);

http://www.acme.com:9000/pls/mydad/!scott.my_proc?x=john&y=10&z=doe

name_array ==> ('x', 'y', 'z')
value_array ==> ('john', '10', 'doe')

procedure [proc_name] 
     (num_entires IN NUMBER,
     name_array  IN  [array_type],
     value_array IN  [array_type],
     reserved in [array_type]);

http://www.acme.com:9000/pls/mydad/!scott.my_pkg.my_proc?x=a&y=b&x=c

num_entries ==> 3 
name_array ==> ('x', 'y', 'x');
value_array ==> ('a', 'b', 'c')
reserved ==> ()

3.6.3 Large Parameter Passing

The values passed as scalar arguments and the values passed as elements to the index-by table of varchar2 arguments can be up to 32K in size.

For example, when using flexible parameter passing (described in Section 3.6.2, "Flexible Parameter Passing"), each name or value in the query_string portion of the URL gets passed as an element of the name_array or value_array argument to the procedure being invoked. These names or values can be up to 32KB in size.

3.7.1 Document Table Definition

You can specify the document storage table for each DAD. The document storage table must have the following definition:

CREATE TABLE [table_name] (	
     NAME           VARCHAR2(256) UNIQUE NOT NULL,
     MIME_TYPE      VARCHAR2(128),
     DOC_SIZE       NUMBER,
     DAD_CHARSET    VARCHAR2(128),
     LAST_UPDATED   DATE,
     CONTENT_TYPE   VARCHAR2(128),
     [content_column_name] [content_column_type]
     [ , [content_column_name] [content_column_type]]
);

Users can choose the table_name. The content_column_type type must be either LONG RAW or BLOB.

The content_column_name depends on the corresponding content_column_type:

• If the content_column_type is LONG RAW, the content_column_name must be CONTENT.

• If the content_column_type is BLOB, the content_column_name must be BLOB_CONTENT.

An example of legal document table definition is:

CREATE TABLE MYDOCTABLE (
  NAME               VARCHAR(256)   UNIQUE NOT NULL, 
  MIME_TYPE          VARCHAR(128), 
  DOC_SIZE           NUMBER, 
  DAD_CHARSET        VARCHAR(128), 
  LAST_UPDATED       DATE, 
  CONTENT_TYPE       VARCHAR(128), 
  CONTENT            LONG RAW, 
  BLOB_CONTENT       BLOB ;
);

3.7.2 Old Style Document Table Definition

For backward capability with the document model used by older releases of WebDB 2.x, mod_plsql also supports the following old definition of the document storage table where the CONTENT_TYPE, DAD_CHARSET and LAST_UPDATED columns are not present.

/* older style document table definition (DEPRECATED) */
CREATE TABLE [table_name]
( 
    NAME         VARCHAR2(128),
    MIME_TYPE    VARCHAR2(128),
    DOC_SIZE     NUMBER,
    CONTENT      LONG RAW

);

3.7.3 Configuration Parameters for Document Upload/Downloading

The following configuration parameters in the DAD affect a document upload/download operation:

• "PlsqlDocumentTablename"

• "PlsqlDocumentPath (Document Access Path)"

• "PlsqlDocumentProcedure (Document Access Procedure)"

• "PlsqlUploadAsLongRaw"

PlsqlDocumentTablename   scott.my_document_table
PlsqlUploadAsLongRaw   html
PlsqlDocumentPath   docs
PlsqlDocumentProcedure   scott.my_doc_download_procedure

• mod_plsql will retrieve data from, or store to a database table called my_document_table in the scott schema.

• All file extensions except .html will be uploaded to the document table as BLOBs. All files with .html extension will be uploaded as Long Raw.

• All URLs which have the keyword docs immediately following the DAD location will result in invocation of the procedure scott.my_doc_download_procedure.

  Typically, this procedure will call wpg_docload.download_file to initiate a file download for a file whose name is based on the URL specification.

A simple example with the preceding configuration is:

http://www.acme.com:9000/pls/dad/docs/index.html

This results in downloading of the file index.html from the Long Raw column of the database table scott.my_document_table. Note that the application procedure has full control on the file download to initiate, and has the flexibility to define a more complex PlsqlDocumentProcedure that implements file-level access controls and versioning.

PlsqlDocumentTablename  [document_table_name]

PlsqlDocumentTablename  my_documents

PlsqlDocumentTablename  scott.my_document_table

http://www.acme.com:9000/pls/mydad/docs/myfile.htm 

PlsqlDocumentPath  [document_access_path_name]

PlsqlDocumentProcedure  [document_access_procedure_name]

PlsqlDocumentProcedure  my_access_procedure

PlsqlDocumentProcedure  scott.my_pkg.my_access_procedure

PlsqlUploadAsLongRaw  [file_extension]
PlsqlUploadAsLongRaw *

PlsqlUploadAsLongRaw  html 
PlsqlUploadAsLongRaw  txt
PlsqlUploadAsLongRaw  *

3.7.4 File Upload

To send files from a client machine to a database, create an HTML page that contains:

• A FORM tag whose enctype attribute is set to multipart/form-data and whose action attribute is associated with a mod_plsql procedure call, referred to as the "action procedure."

• An INPUT element whose type and name attributes are set to file. The INPUT type="file" element enables a user to browse and select files from the file system.

When a user clicks Submit, the following events occur:

1. The browser uploads the file specified by the user as well as other form data to the server.

2. mod_plsql stores the file contents in the database in the document storage table. The table name is derived from the PlsqlDocumentTablename DAD setting.

3. The action procedure specified in the action attribute of the FORM is run (similar to invoking a mod_plsql procedure without file upload).

   
   

   Note: The parsing of HTML documents is deprecated in mod_plsql. mod_plsql used to parse the content of an HTML file when it was uploaded, and identified other files that the HTML document was referring to. This information was then stored into a table. The table name was constructed by appending the name of the document table with "part". This functionality was found to be not of use to customers and has been deprecated, starting in version 9.0.4 of mod_plsql.

   
   

The following example shows an HTML form that lets a user select a file from the file system to upload. The form contains other fields to provide information about the file.

<html>
   <head>
      <title>test upload</title>
   </head>
   <body>
   <FORM 	enctype="multipart/form-data"
      action="pls/mydad/write_info"
      method="POST">
      <p>Author's Name:<INPUT type="text" name="who">
      <p>Description:<INPUT type="text" name="description"><br>
      <p>File to upload:<INPUT type="file" name="file"><br>
      <p><INPUT type="submit">
   </FORM>
   </body>
</html>

When a user clicks Submit on the form:

1. The browser uploads the file listed in the INPUT type="file" element.

2. The write_info procedure then runs.

3. The procedure writes information from the form fields to a table in the database and returns a page to the user.

   
   

   Note: The action procedure does not have to return anything to the user, but it is a good idea to let the user know whether the Submit succeeded or failed, as shown subsequently.

   
   

   procedure write_info (
        who         in varchar2,
        description in varchar2,
        file        in varchar2) as
   begin
        insert into myTable values (who, description, file);
        htp.htmlopen;
        htp.headopen;
        htp.title('File Uploaded');
        htp.headclose;
        htp.bodyopen;
        htp.header(1, 'Upload Status');
        htp.print('Uploaded ' || file || ' successfully');
        htp.bodyclose;
        htp.htmlclose;
   end;
   
   

The filename obtained from the browser is prefixed with a generated directory name to reduce the possibility of name conflicts. The "action procedure" specified in the form renames this name. So, for example, when /private/minutes.txt is uploaded, the name stored in the table by the mod_plsql is F9080/private/minutes.txt. The application can rename this in the called stored procedure. For example, the application can rename it to scott/minutes.txt.

3.7.5 Specifying Attributes (Mime Types) of Uploaded Files

In addition to renaming the uploaded file, the stored procedure can alter other file attributes. For example, the form in the example from Section 3.7.4, "File Upload" could display a field for allowing the user to input the uploaded document's Multipurpose Internet Mail Extension (MIME) type.

The MIME type can be received as a parameter in write_info. The document table would then store the mime type for the document instead of the default mime type that is parsed from the multipart form by mod_plsql when uploading the file.

3.7.6 Uploading Multiple Files

To send multiple files in a single submit, the upload form must include multiple <INPUT type="file" name="file"> elements. If more than one file INPUT element defines name to be of the same name, then the action procedure must declare that parameter name to be of type owa.vc_arr. The names defined in the file INPUT elements could also be unique, in which case, the action procedure must declare each of them to be of varchar2. For example, if a form contained the following elements:

<INPUT type="file" name="textfiles">
<INPUT type="file" name="textfiles">
<INPUT type="file" name="binaryfile">

As a result, the action procedure must contain the following parameters:

procedure handle_text_and_binary_files(textfiles IN owa.vc_arr, binaryfile IN varchar2).

3.7.7 File Download

After you have uploaded files to the database, you can download them from the database in three different ways, as described here:

• Define a PL/SQL procedure that calls wpg_docload.download_file(file_name) to download file file_name.

• Define a virtual path (PlsqlDocumentPath) for document downloads in the DAD configuration, and associate a user-defined procedure with that path (PlsqlDocumentProcedure). When mod_plsql detects, immediately after the DAD name, the virtual path specified by PlsqlDocumentPath, it automatically invokes the user-defined procedure (PlsqlDocumentProcedure), which in turn must call wpg_docload.download_file(file_name) to initiate download of file file_name. This user-defined procedure should have a prototype such that it be invoked without being passed any additional arguments.

  For example, if the DAD "mydad" specifies that PlsqlDocumentPath as "docs" and PlsqlDocumentProcedure is configured as "myschema.pkg.process_download", then the procedure "myschema.pkg.process_download" is invoked by mod_plsql whenever the URL is of the format http://www.acme.com:9000/pls/mydad/docs/myfile.htm.

  An example implementation of process_download is:

  procedure process_download is
  v_filename varchar2(255);
  begin
       -- getfilepath() uses the SCRIPT_NAME and PATH_INFO cgi
       -- environment variables to construct the full path name of
       -- the file URL, and then returns the part of the path name
       -- following '/docs/'
       v_filename := getfilepath;
       select name into v_filename from plsql_gateway_doc
                        where UPPER(name) = UPPER(v_filename);
       -- now we call docload.download_file to initiate
       -- the download.
       wpg_docload.download_file(v_filename);
       exception
          when others then
             v_filename := null;
  end process_download;
  
  

• Use the Direct Binary Large Object (BLOB) Download mechanism to download a BLOB from any database table. You do this by calling a PL/SQL procedure that streams the standard HTTP headers, such as mime-type and content-length, and then invokes wpg_docload.download_file(blob_name) to download BLOB b,ob_name, as shown here:

  1. Create a stored procedure that calls wpg_docload.download_file(blob) where blob is of data type BLOB. Since mod_plsql has no information about the contents in the BLOB, you must supply them.

  2. Setup the Content-Type and other headers.

     In the following example, the procedure uses the name from the argument to select a BLOB from a table and initiates the Direct BLOB download:

     create or replace procedure download_blob(name in varchar2) is
     myblob blob; 
     begin
     
     

     • Select the BLOB out of mytable using the name argument.

       select blob_data into myblob from mytable where blob_name = name;
       
       

     • Setup headers which describes the content.

       owa_util.mime_header('text/html', FALSE); 
       htp.p('Content-Length: ' || dbms_lob.getlength(myblob)); 
       owa_util.http_header_close;
       
       

     • Initiate Direct BLOB download.

       wpg_docload.download_file(myblob); 
       end;
       
       

       The structure of the mytable table is as follows:

       create table mytable 
       ( 
       blob_name varchar2(128), 
       blob_data blob 
       ); 
       
       

     When document downloads are initiated but the "Direct BLOB download" is not used, then the argument passed to wpg_docload.download_file should be able to uniquely identify the filename to download from the document table. While streaming back the content for such documents, mod_plsql will generate the HTTP response headers based on the information stored in other columns of the document table entry for the filename. The MIME_TYPE, DOC_SIZE, and LAST_UPDATED columns will be used to add response headers of "Content-Type", "Content-Length", and "If-Modified-Since" headers respectively.

Downloading Documents with Multibyte Characters

If you are using mod_plsql on a Windows platform, and are running against a multibyte database that contains the special character 0x5c as part of a multibyte character set (Japanese or Korean), you must edit the DAD used to access the application, by modifying the file dads.conf. To do this:

1. Open the file ORACLE_HOME/Apache/modplsql/conf/dads.conf.

2. Locate the DAD used to access the application.

3. Add the line "WindowsFileConversion Off" to this DAD entry.

4. Save the file.

5. If this Oracle HTTP Server is part of the Oracle Application Server, issue the following command:

   ORACLE_HOME/dcm/bin/dcmctl updateConfig -ct ohs
   
   

6. Restart Oracle HTTP Server.

If you do not update the DAD configuration, you will experience failures while downloading documents which contain 0x5c in the filename. For example:

• In OracleAS Portal, you will see the download error:

  Error: Document not found (WWC-46000)
  
  

• When using mod_plsql against your own PL/SQL application, file downloads will result in the error:

  HTTP-404 Not Found
  
  

3.9.1 Adding and Overriding CGI Environment Variables

The PlsqlCGIEnvironmentList DAD parameter is a one-entry-for-each-line list of name and value pairs that can override any environment variables or add new ones. If the name is one of the original environment variables (as listed in Section 3.9, "Common Gateway Interface (CGI) Environment Variables"), that environment variable is overridden with the given value. If the name is not in the original list, a new environment variable is added into the list with that same name and value given in the parameter.

If no value is specified for the parameter, then the value is obtained from the Oracle HTTP Server. With Oracle HTTP Server, you can pass the DOCUMENT_ROOT CGI Environment variable by specifying:

PlsqlCGIEnvironmentList DOCUMENT_ROOT

New environment variables passed in through this configuration parameter are available to the PL/SQL application through the owa_util.get_cgi_env interface.

PlsqlCGIEnvironmentList SERVER_NAME=myhost.mycompany.com
PlsqlCGIEnvironmentList REMOTE_USER=testuser

PlsqlCGIEnvironmentList MYENV_VAR=testing
PlsqlCGIEnvironmentList SERVER_NAME=
PlsqlCGIEnvironmentList REMOTE_USER=user2

3.9.2 PlsqlNLSLanguage

For mod_plsql, the Globalization Support setting is controlled by the DAD level setting of PlsqlNLSLanguage. If PlsqlNLSLanguage is not configured at the DAD level, the Globalization Support configuration is picked up from the environment setting of the Oracle NLS_LANG parameter. For details on this parameter, refer to the "mod_plsql" section in the Oracle HTTP Server Administrator's Guide.

owa_util.get_cgi_env('REQUEST_CHARSET');

owa_util.get_cgi_env('REQUEST_IANA_CHARSET');

3.10.1 Using the Validation Technique

In general, the validation technique basically asks the server if the page has been modified since it was last presented. If it has not been modified, the cached page will be presented to the user. If the page has been modified, a new copy will be retrieved, presented to the user and then cached.

There are two methods which use the Validation Technique: Last-Modified method, and the Entity Tag method. The next two sections show how these techniques are used in the HTTP protocol. Although the PL/SQL Gateway does not use the HTTP protocol, many of the same principles are used.

3.10.1.3 Using the Validation Technique for mod_plsql

Using HTTP validation caching as a framework, the following is the Validation Model for mod_plsql.

PL/SQL based Web applications that want to control the content being served should use this type of caching. This technique offers some moderate performance gains. One example of this would be a Web application that serves dynamic content that could change at any given time. In this case, the Web application needs full control over what is being served. Validation caching always asks the Web application whether the cached content is stale or not before serving it back to the browser.

Figure 3-2 shows the use of the validation technique for mod_plsql.

1. The Oracle HTTP Server receives a PL/SQL procedure request from a client server. The Oracle HTTP Server routes the request to mod_plsql.

2. mod_plsql prepares the request.

3. mod_plsql invokes the PL/SQL procedure in the Web application. mod_plsql passes the usual Common Gateway Interface (CGI) environment variables to the Web application.

4. The PL/SQL procedure generates content to pass back. If the PL/SQL procedure decides that the generated content is cacheable, it calls the owa_cache procedure from the PL/SQL Web Toolkit to set the tag and cache level:

   owa_cache.set_cache(p_etag, p_level);
   

Table 3-4 Validation Model Parameters

Parameter	Description
set_cache procedure	Sets up the headers to notify mod_plsql that the content being streamed back can be cached. Then, the mod_plsql caches the content on the local file system along with the tag and caching level information as it is streamed back to the browser.
p_etag	The string that the procedure generates to tag the content.
p_level	The caching level: SYSTEM for system level or USER for user level.

1. The HTML is returned to mod_plsql.

2. mod_plsql stores the cacheable content in its file system for the next request.

3. The Oracle HTTP Server sends the response to the client browser.

Figure 3-2 Validation Technique

Description of "Figure 3-2 Validation Technique"

3.10.1.4 Second Request Using the Validation Technique

Using the Validation Technique for mod_plsql, a second request is made by the client browser for the same PL/SQL procedure.

Figure 3-3 shows the second request using the Validation Technique.

1. mod_plsql detects that it has a cached content for the request.

2. mod_plsql forwards the same tag and caching level information (from the first request) to the PL/SQL procedure as part of the CGI environment variables.

3. The PL/SQL procedure uses these caching CGI environment variables to check if the content has changed. It does so by calling the following owa_cache functions from the PL/SQL Web Toolkit:

   owa_cache.get_etag;
   owa_cache.get_level;
   
   

   These owa functions get the tag and caching level.

4. The Web application sends the caching information to mod_plsql.

5. Based on that information determines whether the content needs to be regenerated or can be served from the cache.

   1. If the content is still the same, the procedure calls the owa_cache.set_not_modified procedure and generates no content. This causes mod_plsql to use its cached content. The cached content is directly streamed back to the browser.

   2. If the content has changed, it generates the new content along with a new tag and caching level. mod_plsql replaces its stale cached copy with a new one and updates the tag and caching level information. The newly generated content is streamed back to the browser.

Figure 3-3 Validation Technique-Second Request

Description of "Figure 3-3 Validation Technique-Second Request"

3.10.2 Using the Expires Technique

In the validation model, mod_plsql always asks the PL/SQL procedure if it can serve the content from the cache. In the expires model, the procedure pre-establishes the content validity period. Therefore, mod_plsql can serve the content from its cache without asking the procedure. This further improves performance because no interaction with the database is required.

This caching technique offers the best performance. Use if your PL/SQL based Web application is not sensitive to serving stale content. One example of this is an application that generates news daily. The news can be set to be valid for 24 hours. Within the 24 hours, the cached content is served back without contacting the application. This is essentially the same as serving a file. After 24 hours, mod_plsql will again fetch new content from the application.

Assume the same scenario described for the Validation model, except the procedure uses the Expires model for caching.

Figure 3-4 shows the use of the expires technique for mod_plsql.

1. The Oracle HTTP Server receives a PL/SQL Server Page request from a client server. The Oracle HTTP Server routes the request to mod_plsql.

2. The request is forwarded by mod_plsql to the Oracle Database.

3. mod_plsql invokes the PL/SQL procedure in the application and passes the usual Common Gateway Interface (CGI) environment variables to the application.

4. The PL/SQL procedure generates content to pass back. If the PL/SQL procedure decides that the generated content is cacheable, it calls the owa_cache procedure from the PL/SQL Web Toolkit to set the validity period and cache level:

   owa_cache.set_expires(p_expires, p_level);
   

1. The HTML is returned to mod_plsql.

2. mod_plsql stores the cacheable content in its file system for the next request.

3. The Oracle HTTP Server sends the response to the client browser.

Figure 3-4 The Expires Technique

Description of "Figure 3-4 The Expires Technique"

Second Request Using the Expires Technique

Using the same expires model explained earlier, a second request is made by the client browser for the same PL/SQL procedure.

Figure 3-5 shows the second request using the Expires Technique.

1. mod_plsql detects that it has a cached copy of the content that is expires-based.

2. mod_plsql checks the content's validity by taking the difference between the current time and the time this cache file was created.

   1. If this difference is within the validity period, the cached copy is still fresh and will be used without any database interaction. The cached content is directly streamed back to the browser.

   2. If the difference is not within the validity period, the cached copy is stale. mod_plsql invokes the PL/SQL procedure and generates new content. The procedure then decides whether to use expires-based caching again. If so, it also determines the validating period for this new content. The newly generated content is streamed back to the browser.

Figure 3-5 The Expires Technique-Second Request

Description of "Figure 3-5 The Expires Technique-Second Request"

3.10.3 System- and User-level Caching with PL/SQL Based Web Applications

A PL/SQL procedure determines whether generated content is system-level content or user-level. This helps the PL/SQL Gateway cache to store less redundant files if more than one user is looking at the same content. It decides this by:

• For system-level content, the procedure passes the string SYSTEM as the caching level parameter to the owa_cache functions (set_cache for validation model or set_expires for expires model). This is for every user that shares the cache.

  By using system-level caching, you can save both space in your file system and time for all users in the system. One example of this would be a Web application that generates content that is intended for everybody using the Web application. By caching the content with the system-level setting, only one copy of the content is cached in the file system. Furthermore, every user on that system benefits since the content is served directory from the cache.

• For user-level content, it passes the string USER as the parameter for the caching level. This is for a specific user that is logged in. The stored cache is unique for that user. Only that user can use the cache. The type of user is determined by the authentication mode. Refer to the following table for the different types of users.

For example, if no user customizes a PL/SQL based Web application, then the output can be stored in a system-level cache. There will be only one cache copy for every user on the system. User information is not used since the cache can be used by multiple users.

However, if a user customizes the application, a user-level cache is stored for that user only. All other users still use the system level cache. For a user-level cache hit, the user information is a criteria. A user-level cache always overrides a system-level cache.

PL/SQL Web Toolkit Functions (owa_cache package)

Your decision whether to use the Validation technique or the Expires technique determines which owa_cache functions to call.

The owa_cache package contains procedures to set and get special caching headers and environment variables. These allow developers to use the PL/SQL Gateway cache more easily. This package should already be installed in your database.

Table 3-7 lists the primary functions to call.

3.11.1 Connection Pooling with mod_plsql

On UNIX platforms, the Database Server connection pooling logic supplied with mod_plsql can be best explained with an example. Consider the following typical scenario:

1. The Oracle HTTP Server listener is started. There are no database connections in the connection pool maintained by mod_plsql.

2. A browser makes a mod_plsql request (R1) for Database Access Descriptor (DAD) D1.

3. One of the Oracle HTTP Server processes (httpd process P1) starts servicing the request R1.

4. mod_plsql in process P1 checks its connection pool and finds that there are no database connections in its pool for that user request.

5. Based on the information in DAD D1, mod_plsql in process P1 opens a new database connection, services the PL/SQL request, and adds the database connection to its pool.

6. From this point on, all subsequent requests to process P1 for DAD D1 can now make use of the database connection pooled by mod_plsql.

7. If a request for DAD D1 gets picked up by another process (process P2), then mod_plsql in process P2 opens its own database connection, services the request, and adds the database connection to its pool.

8. From this point on, all subsequent requests to process P2 for DAD D1 can now make use of the database connection pooled by mod_plsql.

9. Now, assume that a request R2 is made for DAD D2 and this request gets routed to process P1.

10. mod_plsql in process P1 does not have any database connections pooled for DAD D2, and a new database session is created for DAD D2 and pooled after servicing the request. Process P1 now has two database connections pooled, one for DAD D1 and another for DAD D2.

    The important details in the example shown in steps 1-10 are:

    1. Each Oracle HTTP Server process serves all types of requests, such as static files requests, servlet requests, and mod_plsql requests. There is no control on which Oracle HTTP Server process services the next request.

    2. One Oracle HTTP Server process cannot use or share the connection pool created by another process.

    3. Each Oracle HTTP Server process pools at most one database connection for each DAD.

    4. User sessions are switched within a pooled database connection for a DAD. For DADs based on Oracle Application Server Single Sign-On (SSO), proxy authentication is used to switch the user session. For non-SSO users, using HTTP basic authentication with the username and password not in the DAD, users are re-authenticated on the same connection.

    5. Multiple DADs may point to the same database instance, but database connections are not shared across DADs even within the same process.

    6. Unused DADs do not result in any database connections.

In the worst-case scenario, the total number of database connections pooled by mod_plsql is a factor of the total number of active DADs multiplied by the number of Oracle HTTP Server (httpd) processes running at any given time for a single Oracle HTTP Server instance. If you have configured the Oracle HTTP Server processes to a high number, you need to configure the back-end database to handle a corresponding number of database sessions, and remember that this configuration value needs to be multiplied times the number of Oracle HTTP Server instances that use the back-end database.

For example, if there are three Oracle HTTP Server instances configured to spawn a maximum of 50 httpd processes each, plus two active DADs, you need to set up the database to allow 300 (3*50*2) sessions. This number does not include any sessions that are needed to allow other Web applications to connect.

On Windows platforms, the Oracle HTTP Server runs as a single process. On such systems, the mod_plsql connection pool is shared across threads, and the total number of database connections is a factor of the number of concurrent requests for each DAD. Due to the sharing of database connections across threads, Section 4.3.4, "Two-Listener Strategy" does not apply to Windows systems.

3.11.2 Closing Pooled Database Sessions

Pooled database sessions are closed under the following circumstances:

1. When a pooled connection has been used to serve a configured number of requests.

   By default, each connection pooled by mod_plsql is used to service a maximum of 1000 requests, and then the database connection is shut down and reestablished on the next mod_plsql request. This is done to make sure that any resource leaks in the PL/SQL based Web application, or in the Oracle client server side, do not adversely affect the system. Change the default value of 1000 by tuning the DAD configuration parameter PlsqlMaxRequestsPerSession.

2. When a pooled connection has been idle for an extended period of time.

   By default, the cleanup thread in mod_plsql cleans up each pooled connection after 15 minutes of idle time. The value for idle session timeout is controlled by the configuration setting PlsqlIdleSessionCleanupInterval. If your site is accessing mod_plsql content less frequently, the idle session cleanup happens more frequently, and users will establish database connections that are not being used too often. In such situations, consider increasing the default setting of PlsqlIdleSessionTimeoutInterval for better performance. Note that keeping pooled database connections open for a longer time means additional load on the database to allow for more sessions.

3. On UNIX systems, when the Oracle HTTP Server process goes down.

   On UNIX systems, the Oracle HTTP Server configuration parameter MaxRequestsPerChild governs when an Oracle HTTP Server process will be shut down. For example, if this parameter is set to 5000, each Oracle HTTP Server process would serve exactly 5000 requests before it is shut down. Oracle HTTP Server processes could also start up and shut down as part of Oracle HTTP Server maintenance based on the configuration parameters MinSpareServers, MaxSpareServers, and MaxClients.

   An incorrect configuration of Oracle HTTP Server could result in a setup where Oracle HTTP Server processes are started up and shut down heavily, resulting in ineffective mod_plsql connection pooling. For best performance, configure Oracle HTTP Server such that each Oracle HTTP Server process remains active for a certain period of time, thus preventing the processes from shutting down.

   
   

   See Also: The section "mod_plsql" in the Oracle HTTP Server Administrator's Guide.

   
   

4. When mod_plsql detects dead connections in its connection pool.

   See Section 3.11.3, "Detecting Dead Database Connections in a Connection Pool" for more information.

3.11.3 Detecting Dead Database Connections in a Connection Pool

mod_plsql maintains a pool of connections to the database, and reuses established database connections for subsequent requests. If there is no response from a database connection in a connection pool, mod_plsql detects this, discards the dead connection, and creates a fresh database connection for subsequent requests.

The dead database connection detection feature of mod_plsql eliminates the occurrence of random errors when a database node or instance goes down. This feature is also extremely useful in high availability configurations like Real Application Cluster (RAC). If a node in an RAC cluster has gone down, mod_plsql detects this and immediately starts servicing requests using the other RAC nodes.

By default, when a RAC node or database instance goes down and mod_plsql had previously pooled connections to the node, the first mod_plsql request which uses a dead connection in its pool will result in a failure response of HTTP-503 being sent back to the end-user. This failure is then used by mod_plsql to trigger the detection and removal of all dead connections in its pool. mod_plsql pings all connection pools that were created before the node failure, and this ping operation is performed at the time of processing the next request that uses a pooled connection. If the ping operation fails, the database connection is discarded, and a new connection is created and processed.

mod_plsql provides two configuration options for tuning the dead database connection detection feature:

• Specifying the Option to Detect Dead Database Connections

• Specifying the Timeout Period