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

Macs Fan Control 1.4.7.0 - Monitor and c...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more
Boom 2 1.5.2 - $14.99
Boom 2 is a system-wide volume booster and equalizer app that is designed especially for OS X 10.10 Yosemite. It comes with a smart interface, self-calibrates itself according to your Mac, offers... Read more
MacFamilyTree 8.1.3 - Create and explore...
MacFamilyTree gives genealogy a facelift: modern, interactive, convenient and fast. Explore your family tree and your family history in a way generations of chroniclers before you would have loved.... Read more
WhiteCap 6.6 - Visual plug-in for iTunes...
WhiteCap is a sleek and sophisticated music visualizer and screensaver that features futuristic, wireframe mesh visuals with dynamic backgrounds and colors. WhiteCap contains thousands of visual... Read more
VOX 2.8.14 - Music player that supports...
VOX just sounds better! The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all audio formats you should ever need.... Read more
Paparazzi! 1.0b2 - Make user-defined siz...
Paparazzi! is a small utility for OS X that makes screenshots of webpages. This very simple tool takes screenshots of websites which do not fit on one screen. You specify the desired width, minimal... Read more
Carbon Copy Cloner 4.1.13 - Easy-to-use...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
TrailRunner 3.8.832 - Route planning for...
TrailRunner is the perfect companion for runners, bikers, hikers, and all people wandering under the sky. Plan routes on a geographical map. Import GPS or workout recordings and journalize your... Read more
VueScan 9.5.65 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Adobe Illustrator CC 2017 21.0.2 - Profe...
Illustrator CC 2017 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Illustrator customer). Adobe Illustrator CC 2017 is the industry... Read more

Z-Exemplar (Games)
Z-Exemplar 1.4 Device: iOS Universal Category: Games Price: $3.99, Version: 1.4 (iTunes) Description: | Read more »
5 dastardly difficult roguelikes like th...
Edmund McMillen's popular roguelike creation The Binding of Isaac: Rebirth has finally crawled onto mobile devices. It's a grotesque dual-stick shooter that tosses you into an endless, procedurally generated basement as you, the pitiable Isaac,... | Read more »
Last week on PocketGamer
Welcome to a weekly feature looking back on the past seven days of coverage on our sister website, PocketGamer. It’s taken a while for 2017 to really get going, at least when it comes to the world of portable gaming. Thank goodness, then, for... | Read more »
ROME: Total War - Barbarian Invasion set...
To the delight of mobile strategy fans, Feral Interactive released ROME: Total War just a few months ago. Now the game's expansion, Barbarian Invasion is marching onto iPads as a standalone release. [Read more] | Read more »
Yuri (Games)
Yuri 1.0 Device: iOS iPhone Category: Games Price: $3.99, Version: 1.0 (iTunes) Description: It's night. Yuri opens his eyes. He wakes up in a strange forest.The small, courageous explorer rides on his bed on casters in this... | Read more »
Space schmup Xenoraid launches on the Ap...
10Tons Xenoraid is out today on the App Store, bringing some high-speed space action to your mobile gadgets just in time for the weekend. The company's last premium title, another sci-fi game titled Neon Chrome, did quite well for itself, so... | Read more »
Star Wars: Force Arena Beginner's G...
Star Wars: Force Arena joined the populous ranks of Star Wars games on mobile today. It's a two-lane MOBA starring many familiar faces from George Lucas's famed sci-fi franchise. As with most games of this nature, Force Arena can be a little obtuse... | Read more »
Mysterium: The Board Game (Games)
Mysterium: The Board Game 1.0 Device: iOS Universal Category: Games Price: $6.99, Version: 1.0 (iTunes) Description: The official adaptation of the famous board game Mysterium! | Read more »
Sonny (Games)
Sonny 1.0.4 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.4 (iTunes) Description: Reimagined for iOS, cult-hit RPG Sonny brings challenging turn-based combat that requires strategy and mastery of each new skill to... | Read more »
Towaga (Games)
Towaga 1.0 Device: iOS iPhone Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: "It has been foretold that a masked being would stand atop the legendary Towaga Temple, dwelling among shadows to fulfil The Black Moon... | Read more »

Price Scanner via MacPrices.net

New MacBooks And MacBook Pros WIth Kaby Lake...
Digitimes’ Joseph Tsai cites a Chinese-language Economic Daily News (EDN) report that unnamed market watchers are predicting Apple MacBook shipments to grow 10 percent in 2017, and projecting 15... Read more
New 2016 13-inch MacBook Pros on sale for up...
B&H Photo has the new 2016 13″ MacBook Pros in stock today and on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.9GHz/512GB Touch Bar MacBook Pro... Read more
New 15-inch Touch Bar MacBook Pros in stock a...
B&H Photo has the new 2016 15″ Apple Touch Bar MacBook Pros in stock today and on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
Opera Announces Neon Concept Browser For Mac
Opera is inviting users to get a glimpse of what Opera for computers could become with its Opera Neon browser concept. Each Opera Neon feature is described as “an alternate reality” for the Opera... Read more
Tellini Releases TabView 3.0 Missing Tool fo...
Tellini has announced the release of TabView 3.0. TabView has been the first macOS viewer for PowerTab tablatures. PowerTab is a well-known and widely adopted tablature editor for Windows systems and... Read more
13-inch 1.6GHz/128GB MacBook Air on sale for...
Overstock.com has the 1.6GHz/128GB 13″ MacBook Air on sale for $130 off MSRP including free shipping: - 13″ 1.6GHz/128GB MacBook Air (MMGF2LL/A): $869.99 $130 off MSRP Their price is the lowest... Read more
12-inch 32GB Space Gray iPad Pro on sale for...
B&H Photo has 12″ Space Gray 32GB WiFi Apple iPad Pros on sale for $55 off MSRP including free shipping. B&H charges sales tax in NY only: - 12″ Space Gray 32GB WiFi iPad Pro: $744.44 $55 off... Read more
9-inch 32GB Space Gray iPad Pro on sale for $...
B&H Photo has the 9.7″ 32GB Space Gray Apple iPad Pro on sale for $549 for a limited time. Shipping is free, and B&H charges NY sales tax only. Read more
Apple iMacs on sale for up to $120 off MSRP,...
B&H Photo has 21″ and 27″ Apple iMacs on sale for up to $120 off MSRP, each including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2199 $100 off MSRP - 27″ 3.2GHz/1TB Fusion iMac... Read more
Apple refurbished Apple TVs available for up...
Apple has Certified Refurbished 32GB and 64GB Apple TVs available for up to $30 off the cost of new models. Apple’s standard one-year warranty is included with each model, and shipping is free: -... Read more

Jobs Board

*Apple* Retail - Multiple Positions (Multi-L...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* & PC Desktop Support Technician...
Apple & PC Desktop Support Technician job in Stamford, CT We have immediate job openings for several Desktop Support Technicians with one of our most well-known Read more
*Apple* macOS Systems Integration Administra...
…most exceptional support available in the industry. SCI is seeking an Junior Apple macOS systems integration administrator that will be responsible for providing Read more
*Apple* Premier Retailer - Service Technicia...
DescriptionSimply Mac is the largest premier retailer for Apple products and solutions. At Simply Mac we are all Apple , all the time. Same products. Same prices. Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.