TweetFollow Us on Twitter

June 91 - Data Access Language Unit for MacApp

Data Access Language Unit for MacApp

Mary Elaine Califf

A few months ago I began work on three Macintosh front-ends for administrative data. The data resides in Rdb databases on our VAXcluster and was to be accessed using Apple's Data Access Language (DAL).

Since all three projects were full-featured applications, I abandoned HyperCard-in which the previous incarnations of two of these front-ends were written-and started learning MacApp. When I couldn't find anyone else using DAL and MacApp together, UDAL was born.

This version of the UDAL unit uses the cl1_api interface provided with DAL instead of the Data Access Manager routines. Therefore, it works with both system 6.0.x and system 7.0. UDAL is currently written in MacApp 2.0, but conversion to MacApp 3.0 requires only minor changes. The UDAL unit may not handle everything you want to do with DAL, but should at least suggest a way to incorporate DAL into MacApp.

THE BASICS OF DATA ACCESS LANGUAGE

DAL is a client-server protocol for remote database access. The Macintosh client is a system extension-included with system 7.0, available separately for system 6.0.x. Servers are available with adapters for various databases on systems running VAX/VMS, MVS/TSO, and VM/CMS. Other servers are under development, including one for A/UX that has already been announced.

DAL uses an SQL-based language that includes some C-like programming constructs. DAL's greatest asset is in providing applications with a uniform access method across various systems, database management systems, and network connection types. Despite the network overhead and the translation of DAL into the databases' SQL dialects, DAL's performance is fairly good and is improving with newer versions of the servers.

OVERVIEW OF THE UDAL UNIT FOR MACAPP

UDAL defines two classes: TDALConnection and TDALRequest. TDALConnection maintains information about a DAL session and contains methods that call the DAL functions. TDALConnection also provides a Reconnect method that establishes a new session using a previous connection object whose session aborted. One TDALConnection object is needed for each concurrent DAL session.

TDALRequest manages DAL queries. This class is useful for long, complicated requests or for requests that need to be reissued. Besides the two object types, the UDAL unit provides an InitUDAL procedure, utility routines for converting between Pascal strings and the null-terminated strings used by DAL, and default error handling routines.

INSIDE TDALCONNECTION-INTERFACING TO THE DAL API

The TDALConnection class provides an object-oriented interface to the DAL API functions. Therefore, most of the object's methods are based on the API functions. Note: API functions all begin with the letters CL; DAL was originally called CL/1, and the original function names were retained for compatibility. Please refer to the TDALConnection interface listing for the complete source of the class interface.

I added functionality to TDALConnection by including an error handling procedure as one of the parameters to each of the methods based on an API function. This addition helps to hide the details of managing the DAL connections from applications interested only in the data retrieved. The procedure I usually pass to the functions, HandleDALError, is included in the UDAL unit. All functions except GetString return the result code from the API function called; this allows the calling method to handle error checking. GetString returns the result code in a var parameter instead of returning it as its function result.

IDALConnection

The IDALConnection method initializes the object and establishes a DAL session using the CLInit function. It takes the host name, user name, password, and connection options as parameters. The Reconnect method also establishes a DAL session using CLInit, but it uses the fields of the object to determine the parameters for CLInit since its purpose is to re-establish the user's connection with the host.

HandleDroppedConnection

HandleDroppedConnection calls CLEnd to ensure that the last session was properly closed and then displays an alert, enabling the user to reconnect immediately. My applications all provide menu choices allowing later reconnection. When the user discovers a dropped connection but wants to do something that doesn't require a current session (for example, printing an existing document), he shouldn't have to wait for the new session to be established. If the user does wish to reconnect immediately, HandleDroppedConnection calls Reconnect.

CloseConnection

CloseConnection calls CLEnd to close down a DAL session. It doesn't free the TDALConnection object, because the object could be used to handle another session after calling Reconnect.

SendSQL and ExecuteRequest

SendSQL and ExecuteRequest allow the application to talk to the host database. SendSQL sends a string to the host. The ExecuteRequest method tells the host to execute everything received since the last call to ExecuteRequest. Note: The host doesn't do anything with the commands received until the user tells it to execute them. Be careful to always complete a DAL programming construct between calls to ExecuteRequest-telling DAL to execute what it has when you're in the middle of a for loop always produces errors!

CheckState and GetString

CheckState and GetString retrieve information from the host. CheckState determines if a value is waiting for retrieval; GetString retrieves a value from the host (if it doesn't time out first) and returns it as a Pascal string. The timeout period is specified as a parameter to the method. The routine returns values as strings because I couldn't get DAL to give me anything else from my Rdb database, even when the value was clearly an integer. Therefore, it was more convenient for the routine to convert the C strings into Pascal strings.

StopRequest

The final method in TDALConnection (other than Fields) is StopRequest. This method should abort the current request if the abortSession parameter is false and abort the entire DAL session if abortSession is true. However, test the method thoroughly in your environment before making it available to users. The CLBreak function it is based on is not always reliable. In some environments, it occasionally hangs both the Macintosh and the process on the mainframe. Everyone I've spoken to who uses DAL with Rdb databases has experienced these crashes when using CLBreak. The function does, however, work with some DBMSs.

A "Nothing" example for TDALConnection

Drop the source listed in A simple example using TDALConnection into a view's draw method to provide a very simple example that shows how to use TDALConnection.

INSIDE TDALREQUEST-HANDLING COMPLEX REQUESTS

Many uses of DAL can be made using only the TDALConnection class-I handle a fair amount of database communication in my applications using SendSQL, ExecuteRequest, and GetString.

However, at least two situations require better request management and a subclass of TDALRequest: inserting a few user-provided values into a query template, and saving a query and repeating it with no modifications. (Please refer to the TDALRequest interface listing for the complete source of the class interface.)

I had these needs in my applications. The template for one of my queries is a STR# resource with 15 strings. Also, all my documents contain the results of a database query that the user should be able to update without having to remember the query.

TDALRequest saves the text of a DAL request in a linked list of strings. It also contains a reference to the DALConnection it is supposed to use. It provides three methods of interest: SendRequest, ShowRequest, and BuildRequest.

SendRequest and ShowRequest

SendRequest sends each string in the request list using SendSQL, and then calls ExecuteRequest.

ShowRequest provides for debugging by displaying a dialog box with a TTEView containing the text of the request. If you cancel the dialog, ShowRequest returns false. This helps you avoid sending a bad request to the host during debugging.

BuildRequest

Most programmers who use UDAL will want to override BuildRequest. UDAL provides a default version that takes the STR# resource with id 2000 and stuffs the strings into a TTEView in a dialog box. It then takes what is in the TTEView when the dialog box is dismissed (if the OK button is pressed) and breaks the text into the request list. I find this useful in early stages of my applications, but all requests in my finished applications are implemented by subclasses of TDALRequest.

Your version of BuildRequest should usually display a dialog box to get user information and then create the linked list of strings from a STR# resource, inserting the information in the appropriate places. Make sure that each place where you need to insert information falls between strings in your STR#. BuildRequest should return false if the user cancels the request or some other error occurs while building the request. Otherwise, BuildRequest should return true.

The following code example demonstrates the use of TDALRequest assuming that aDALConn exists and that TMyDocument.GetData retrieves the data from the request:

New(aDALRequest);
FailNIL(aDALRequest);
aDALRequest.IDALRequest(kDefaultRequestID,fConn);
if aDALRequest.BuildRequest then
if aDALRequest.ShowRequest then
    begin
        aDALRequest.SendRequest;
        aDocument.GetData;
    end;

INSIDE TRETRIEVER-HANDLING ASYNCHRONOUS REQUESTS

DAL performance is improving, but some requests can take a long time. My applications allow users to switch to another application while a request is pending, and they also allow users to perform any task inside my application that doesn't require a database lookup. If you handle more than one DAL session, you can even allow multiple lookups, but only one per session.

To handle the retrieval of the information in the background, I create a TRetriever class that overrides DoIdle. The instance of this class need only exist while a lookup is occurring.

TRetriever = OBJECT(TEvtHandler)
fConn       : TDALConnection;
fDocument   : TFSDirDocument;
PROCEDURE TRetrever.IRetriever(conn: TDALConnection;
                         doc: TStuDirDocument);
FUNCTION TRetriever.DoIdle(phase:IdlePhase):BOOLEAN;
                        OVERRIDE;
END;

TRetriever's DoIdle method is fairly simple. It need only check to see if there is data waiting and call an appropriate method of the document to retrieve the data-in this case, GetPersonRecord. My DoIdle method also does some error handling-see the TRetriever.DoIdle implementation listing for the complete source.

Installing an instance of TRetriever is quite simple. The following five lines of code create the retriever and install it in the cohandler chain. You would include this code right after a call to TDALRequest.SendRequest or after TDALConnection.ExecuteRequest.

New(aRetriever);
FailNIL(aRetriever);
aRetriever.IRetriever(fConn,SELF);
aRetriever.SetIdleFreq(1);
gApplication.InstallCoHandler(aRetriever,true);

LOOKING FOR OTHER DAL USERS

The UDAL unit may not do everything you'd like to do with DAL, but I hope it will prove useful and suggest other ways that DAL can be used with MacApp. One future direction for the unit is the creation of a system 7.0-only version that takes advantage of the asynchronous capabilities of the new Data Access Manager routines. I welcome comments and questions. I would appreciate hearing from anyone using DAL.
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

ExpanDrive 4.3.2 - Access cloud storage...
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
RapidWeaver 6.0.8 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
Artlantis Studio 5.1.2.7 - 3D rendering...
Artlantis Studio is a unique and ideal tool for performing very high resolution rendering easily and in real time. The new FastRadiosity engine now lets you compute images in radiosity-even in... Read more
MacUpdate Desktop 6.0.5 - Search and ins...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
BitTorrent Sync 2.0.82 - Sync files secu...
BitTorrent Sync allows you to sync unlimited files between your own devices, or share a folder with friends and family to automatically sync anything. File transfers are encrypted. Your information... Read more
Google Drive 1.20 - File backup and shar...
Google Drive is a place where you can create, share, collaborate, and keep all of your stuff. Whether you're working with a friend on a joint research project, planning a wedding with your fiancé, or... Read more
Simon 4.0.3 - Monitor changes and crashe...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
Vitamin-R 2.23 - Personal productivity t...
Vitamin-R creates the optimal conditions for your brain to work at its best by structuring your work into short bursts of distraction-free, highly focused activity alternating with opportunities for... Read more
iDefrag 5.0.0 - Disk defragmentation and...
iDefrag helps defragment and optimize your disk for improved performance. Features include: Supports HFS and HFS+ (Mac OS Extended). Supports case sensitive and journaled filesystems. Supports... Read more
PCalc 4.2 - Full-featured scientific cal...
PCalc is a full-featured, scriptable scientific calculator with support for hexadecimal, octal, and binary calculations, as well as an RPN mode, programmable functions, and an extensive set of unit... Read more

New Publisher Allstar Games Heads West w...
Allstar Games has announced its first mobile title designed for western audiences, Allstar Heroes. The game will be a massive online battle arena (MOBA) that offers dozens of heroes for you to collect and pit against your opponents. As each hero has... | Read more »
RAD Boarding Review
RAD Boarding Review By Jennifer Allen on March 5th, 2015 Our Rating: :: NEARLY RADUniversal App - Designed for iPhone and iPad RAD Boarding isn’t quite one of the greats, but it has potential.   | Read more »
Presenting the International Mobile Gami...
11th Annual International Mobile Gaming Awards ceremony, hosted by actress Allison Haislip, gathered mobile game developers and publishers from around the world. They chose 13 winners out of the 93 nominations. British studio USTWO won the the Grand... | Read more »
AG Drive Review
AG Drive Review By Tre Lawrence on March 5th, 2015 Our Rating: :: FUTURISTIC STREET RACING.Universal App - Designed for iPhone and iPad Futuristic racing… interstellar style.   | Read more »
GDC 2015 – Nightmare Guardians is an Int...
GDC 2015 – Nightmare Guardians is an Interesting Hybrid of MOBA and Lane Defense Posted by Rob Rich on March 5th, 2015 [ permalink ] I have to say that lane defense (i.e. | Read more »
Overkill 3 Review
Overkill 3 Review By Tre Lawrence on March 5th, 2015 Our Rating: :: WHO'S NEXT?Universal App - Designed for iPhone and iPad Cover system gameplay in the third-person.   Developer: Craneballs Price: Free Version Reviewed: 1.1.6... | Read more »
Warner Bros. Interactive Entertainment A...
Warner Bros. has some exciting games coming down the pipe! | Read more »
GDC 2015 – Star Trek Timelines will Prob...
GDC 2015 – Star Trek Timelines will Probably Make Your Inner Trekkie Squeal With Glee Posted by Rob Rich on March 4th, 2015 [ permalink ] Any popular fictional universe has its fair share of fan fiction – where belo | Read more »
Protect Yourself from an Onslaught of Ca...
Surprise Attack Games has announced a Cat-astrophic new physics puzzler called Fort Meow! In the game, a young girl named Nia finds her grandfather’s journal which triggers an all mighty feline attack! Why do the cats want the journal? Who knows,... | Read more »
GDC 2015 – Jelly Reef will be Game Oven’...
GDC 2015 – Jelly Reef will be Game Oven’s Last Hurrah, and it Seems like a Good Note to Go Out on Posted by Rob Rich on March 4th, 2015 [ permalink ] It’s sad knowing that Game Oven ( | Read more »

Price Scanner via MacPrices.net

Roundup of MacBook Air sale prices, models up...
B&H Photo has MacBook Airs on sale for up to $100 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 11″ 128GB MacBook Air: $799 100 off MSRP - 11″ 256GB MacBook Air: $999 $100... Read more
New Firstrade Mobile App Enables On-The-Go Tr...
Firstrade Securities Inc. has announced its new mobile app, which gives investors immediate access to the company’s trading platform on all mobile devices. The app was developed in-house and was... Read more
Sonnet Introduces USB 3.0 + eSATA Thunderbolt...
Sonnet has announced the launch of its new USB 3.0 + eSATA Thunderbolt Adapter for easy connectivity to USB 3.0 devices and eSATA storage, and USB 3.0 + Gigabit Ethernet Thunderbolt Adapter for easy... Read more
Apple restocks refurbished 27-inch 5K iMacs f...
The Apple Store has restocked Apple Certified Refurbished 27″ 3.5GHz 5K iMacs for $2119 including free shipping. Their price is $380 off the cost of new models, and it’s the lowest price available... Read more
Free Clean Reader Mobile App Hides Swear Word...
The new Clean Reader app, now available in the Apple App Store and Google Play, delivers the opportunity of reading any book without being exposed to profanity. By selecting how clean they want their... Read more
Kinsa Launches “Groups” App to Monitor Illnes...
Kinsa, makers of the first FDA approved app-enabled smartphone thermometer thst won the 2013 Cleveland Clinic Medical Innovation Grand Prize and recently appeared in Apple’s “Parenthood” TV... Read more
iPad: A More Positive Outlook – The ‘Book Mys...
It’s good to hear someone saying positive things about the iPad. I’ve been trying to bend my mind around how Apple’s tablet could have gone from zero to bestselling personal computing device on the... Read more
Mac Pros on sale for up to $279 off MSRP
Amazon has Mac Pros in stock and on sale for up to $279 off MSRP. Shipping is free: - 4-Core Mac Pro: $2725.87, $273 off MSRP (9%) - 6-Core Mac Pro: $3719.99, $279 off MSRP (7%) Read more
Sale! 13-inch Retina MacBook Pros for up to $...
B&H Photo has 13″ Retina MacBook Pros on sale for up to $205 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.6GHz/128GB Retina MacBook Pro: $1219.99 save $80 - 13″ 2.... Read more
Another Tranche Of IBM MobileFirst For iOS Ap...
IBM has announced the next expansion phase for  its IBM MobileFirst for iOS portfolio, with a troika of new apps to address key priorities for the Banking and Financial Services, Airline and Retail... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
Position Opening at *Apple* - Apple (United...
…Summary** As a Specialist, you help create the energy and excitement around Apple products, providing the right solutions and getting products into customers' hands. You Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** The Apple Store is a retail environment like no other - uniquely focused on delivering amazing customer experiences. As an Expert, you introduce people Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.