TweetFollow Us on Twitter

Changing Frameworks

Volume Number: 14 (1998)
Issue Number: 3
Column Tag: Rhapsody

Handling a Changing Framework

by Kris Jensen

What to do when the latest update to your framework no longer provides an object essential to your project

Data storage and retrieval is an important part of most computer applications. In some cases, writing a document to a file is sufficient; in others, the information to be stored and retrieved is more complex, and your program may rely on a package of objects that implement data storage and retrieval for you. What do you do if that set of objects is no longer available?

This article shows how the modularity of object-oriented programming allows one to quickly modify programs without modifying code by plugging in a different set of objects and how the high-level Foundation Kit objects can be used to quickly code interim solutions.

An OS update broke our application

Stone Design's database management application, DataPhile, suffered from an updated OS removing objects essential to the application. DataPhile was originally built using the Indexing Kit package of data storage and indexing objects, a package that was included with the original NeXTSTEP programming tools. The Indexing Kit was never problem-free at its higher levels, and, at its lower levels, was difficult to port to other platforms. As NeXTSTEP became OPENSTEP and migrated to other hardware, the Indexing Kit disappeared.

DataPhile needed a new back-end. While we were researching the possibilities for replacing or porting the Indexing Kit, we also wanted to show that we could get the front-end up and running under OpenStep and Rhapsody. The solution was a quick and dirty back-end built using OpenStep Foundation Kit objects. We did not, at this point, want to change DataPhile's data storage paradigm, nor did we want to build a functional and efficient b-tree package; we just wanted to "plug in" some quickly built objects that would provide the surface functionality that we had been using in the Indexing Kit. Efficiency, either in space or time, was not an issue; speed in implementing something that would allow us to demo our front-end under Rhapsody was our goal.

An advantage of working in an object-oriented environment is that you can use a speedily-written, temporary set of objects that provides the functionality of the objects that are no longer available or provides a general storage model that works for your application. Later, you can re-implement the objects more efficiently. Or you can use your objects as glue to implement your storage model using another set of objects. As long as you don't change your objects' interface, you don't need to touch the rest of your code.

Our solution may interest you if:

  • You're porting an application and your data storage package is not available for your new platform.
  • You're developing an application and you haven't yet decided on what you're going to use for your data storage needs.
  • You're developing a small-scale project and you need a simple, free persistent data store.

Implementing a fix with Foundation Kit

Our solution followed these steps:

  1. Identify which classes and methods in the obsolete package are being used. We used five classes: IXStore, IXStoreFile, IXStoreDirectory, IXBTree and IXBTreeCursor and we used most of the public API of those classes. Our data storage model was based on being able to store key-value pairs (with the keys and values being of any size) and to retrieve the values either directly (via the key) or sequentially.
  2. Implement those classes and methods. Don't worry about how they "really" work; our goal is to provide functionality without modifying our client code. For example, IXStore was based on the idea of "blocks" of storage; I kept the idea of blocks, but they just became id's used for access into a dictionary.
  3. To do the implementation, figure out what you need to do and look for Foundation Kit classes that will provide that functionality. We needed three things: to store key-value pairs to disk and to access those pairs directly (by key value) and sequentially. The Foundation Kit includes NSPPLs (persistent property lists). The NSPPL class description in the Rhapsody Foundation Kit documentation, states:

"Like serialization, a persistent property list stores data in a binary format, provides fast access, and is lazy. It also allows you to make incremental changes to an NSPPL (even one that contains tens of megabytes of data), while still ensuring that your data is never corrupted. In this sense, an NSPPL is analogous to a database. Because of their ability to incrementally store and retrieve data, NSPPLs are particularly well-suited for working with large amounts of data (that is, data that has several elements, that occupies a large number of bytes, or both)."

NSPPLs can store NSDictionaries and NSArrays. NSDictionaries provide fast key-value lookup, but don't allow sequential access in any kind of sorted order. NSArrays can provide sequential access. I decided to use an NSPPL to implement an IXStoreFile, an NSDictionary to implement an IXStoreDirectory, and both an NSDictionary and an NSArray to implement an IXBTree.

IXStore is "a transaction based, compacting storage allocator designed for data-intensive applications." and IXStoreFile puts the storage on disk. For a temporary back-end, I decided to forego transactions and compacting and concurrency control and just implement the calls my client code used: the transaction calls from IXStore and the initialization methods from IXStoreFile. Even though I didn't plan to implement transactions, I didn't want to remove the transaction-oriented code from DataPhile, so the methods had to be there.

IXStoreDirectory and IXBTree conform to the IXBlockAndStore protocols, which use the notion of a handle to a block of storage. Because of this, I kept the idea of block numbers allocated by IXStore, but eliminated the idea of blocks as constant-sized storage units (meaning that something stored in an IXStore might extend over multiple blocks). Instead, each IXStore client would be assigned a block number which would provide access to that client's storage. So in IXStore and IXStoreFile, I needed to map block numbers to data. Since an NSPPL incorporates an NSDictionary to provide access to its storage, I decided to use that dictionary to store block number / data pairs.

I initially planned for the stored data to be the actual structures needed by the store clients: for example, to store the IXBTree's structure (an NSDictionary of key/data pairs) directly in the root dictionary. However, all NSDictionaries stored directly in an NSPPL must use NSStrings as keys: this didn't match the IXBTree's paradigm of arbitrary data for both keys and data. So I used the rootDictionary of the NSPPL to map block numbers to NSData objects; the NSData contained an archived version of the object to be stored.

IXStoreDirectory "provides access to store clients by name instead of by block handle." This obviously called for an NSDictionary mapping names to a structure that would keep track of an object, its class, and its block number.

An IXBTree "provides ordered associative storage and retrieval of untyped data. It identifies and orders data items, called values, by key using a comparator function". The information in an IXBTree can be accessed directly by key or sequentially, by using an IXBTreeCursor to traverse the b-tree. It's a fairly standard undergraduate computer science project to write a b-tree program, but this is for a quick back-end, so I decided to use prefab parts. Direct access could be provided by an NSDictionary mapping NSData keys to NSData values. Sequential access could be provided by a sorted NSArray of keys. The sorted array could be built easily by using the sortedArrayUsingFunction NSArray method. A cursor can be represented as an integer index into the sorted key array. It can be positioned (for a key that's in the b-tree) using the method indexOfObject. If the key is not already present, positioning becomes a little more complicated; I implemented a quick and dirty binary search on the array contents. If contents are added to or deleted from the key array, any cursors pointing into the array become invalid; I use an NSNotification to notify the IXBTreeCursors that are accessing the b-tree.

Conclusions

Putting all this together, I have a cover for the parts of the Indexing Kit that we needed (b-trees, cursors, and storage) that will allow us to continue to develop the DataPhile front end while we look for a replacement back end. It's not particularly robust or efficient, but it was built very quickly using Foundation objects. When we decide on another storage package, we can rewrite the guts of the cover objects without rewriting the code that uses the cover objects. You can take a look at my b-tree solution in the example code provided at the MacTech ftp site ftp://ftp.mactech.com/.

I also found that a simple way to store and index record objects of some type is to create a b-tree that maps an id number to an NSData that holds the record (serialized in whatever way you wish). Then create index b-trees that map keys (have your records create their own keys) to the record id number. This is demonstrated in the sample code that uses the b-tree code.


Kris Jensen, at the time an attorney for the State of California, bought an Apple II in 1980. It changed her life. She taught herself BASIC, started taking computer science courses, moved to New Mexico, entered the Ph.D. program in Computer Science at the University of New Mexico, bought a Macintosh, passed her comprehensive exams, became president of the local Apple Users Group in Albuquerque, met Andrew Stone, got NeXT representatives to demonstrate the NeXT cube at a users group meeting, joined Andrew in becoming NeXT developers, and didn't finish her Ph.D. because she was designing and writing NeXTSTEP software. Kris is happy to be back in the Macintosh world with Rhapsody. In her spare time, Kris calls square dances around the country.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Microsoft Remote Desktop 8.0.34 - Connec...
With Microsoft Remote Desktop, you can connect to a remote PC and your work resources from almost anywhere. Experience the power of Windows with RemoteFX in a Remote Desktop client designed to help... Read more
Data Rescue 4.2.3 - Powerful hard drive...
Use Data Rescue to recover: crashed, corrupted or non-mounting hard drive deleted, damaged, or lost files reformatted or erased hard drive One powerful new feature found in Data Rescue 4 is... Read more
Viber 6.2.0 - Send messages and make cal...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device, so... Read more
Lyn 1.7.2 - Lightweight image browser an...
Lyn is a fast, lightweight image browser and viewer designed for photographers, graphic artists, and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it delivers... Read more
Lyn 1.7.2 - Lightweight image browser an...
Lyn is a fast, lightweight image browser and viewer designed for photographers, graphic artists, and Web designers. Featuring an extremely versatile and aesthetically pleasing interface, it delivers... Read more
Tunnelblick 3.6.7beta02 - GUI for OpenVP...
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
jAlbum Pro 13.4 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. You can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly... Read more
calibre 2.65.1 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
jAlbum 13.4 - Create custom photo galler...
With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code! Beginner-friendly, with pro results - Simply drag and drop photos into groups, choose a design... Read more
Backblaze 4.2.0.966 - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac. With unlimited storage available for $5 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more

Mummy madness in new action game Tomb He...
Hot on the tail of Bump Hero, ZPlay is giving gamers another reason to get screen bashing with a brand new release. Tomb Heroes is a challenging action game in which you battle enemies in various tombs around the world. You can select from nine... | Read more »
Siralim 2 (RPG / Roguelike) (Games)
Siralim 2 (RPG / Roguelike) 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Siralim 2 is an old-school monster catching RPG. Summon and customize hundreds of creatures to fight for you as... | Read more »
Clean Text (Productivity)
Clean Text 1.0 Device: iOS Universal Category: Productivity Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
Gemini - A Journey of Two Stars (Games)
Gemini - A Journey of Two Stars 1.0.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.1 (iTunes) Description: *** SPECIAL LAUNCH SALE: $2.99 (25% off) *** "A mesmerizing and unexpectedly emotional journey." -- Los... | Read more »
How to get four NFL superstars for your...
Even though you're probably well on your way to building a top notch squad for the new season in Madden NFL Mobile, let's say you could beef it up by adding Rob Gronkowski, Antonio Brown, Von Miller, and Todd Gurley to your roster. That's... | Read more »
Cartoon Network Superstar Soccer: Goal!!...
Cartoon Network Superstar Soccer: Goal!!! – Multiplayer Sports Game Starring Your Favorite Characters 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Become a soccer superstar with your... | Read more »
NFL Huddle: What's new in Topps NFL...
Can you smell that? It's the scent of pigskin in the air, which either means that cliches be damned, pigs are flying in your neck of the woods, or the new NFL season is right around the corner. [Read more] | Read more »
FarmVille: Tropic Escape tips, tricks, a...
Maybe farming is passé in mobile games now. Ah, but farming -- and doing a lot of a other things too -- in an island paradise might be a little different. At least you can work on your tan and sip some pina coladas while tending to your crops. [... | Read more »
Become the King of Avalon in FunPlus’ la...
King Arthur is dead. Considering the legend dates back to the 5th century, it would be surprising if he wasn’t. But in the context of real-time MMO game King of Avalon: Dragon Warfare, Arthur’s death plunges the kingdom into chaos. Evil sorceress... | Read more »
Nightgate (Games)
Nightgate 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: *** Launch Sale: 25% OFF for a limited time! *** In the year 2398, after a great war, a network of intelligent computers known as... | Read more »

Price Scanner via MacPrices.net

9-inch 32GB iPad Pros on sale for $70 off MSR...
B&H Photo has 9″ 32GB WiFi Apple iPad Pros on sale for $70 off MSRP, each including free shipping. B&H charges sales tax in NY only: - 9″ Space Gray 32GB WiFi iPad Pro: $529 $70 off MSRP - 9... Read more
Mac minis on sale for up to $140 off MSRP
Adorama has Mac minis on sale for up to $100 off MSRP including free shipping plus NY & NJ sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP - 2.6GHz Mac mini: $649 $50 off MSRP Amazon has the... Read more
Back To School with OtterBox Essentials
Back to school means back to an environment that is tough on tech. OtterBox has the back to school essentials you need to keep tech safe from drops, bumps, scratches and hallway havoc. Check out the... Read more
VRS Design Releases New iPhone 7 Plus Case Li...
With a device as large and costly as the iPhone 7 Plus, it is primal instinct to protect it from potential damage. According to a study by SquareTrade in 2012, iPhone damages cost Americans roughly $... Read more
MacBook Airs on sale for up to $101 off MSRP
Amazon has 11″ and 13″ MacBook Airs on sale for up to $101 off MSRP for a limited time. Shipping is free: - 11″ 1.6GHz/128GB MacBook Air (model MJVM2LL/A): $798 $101 off MSRP - 11″ 1.6GHz/256GB... Read more
Apple certified refurbished iPad mini 4s avai...
Apple has certified refurbished iPad mini 4s now available for up to $120 off the cost of new models. An Apple one-year warranty is included with each iPad, and shipping is free. The following models... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Global Tablet Shipments Projected to Increase...
Digitimes’ Jim Hsiao reports that global tablet shipments will increase by 16.3 percent sequentially to reach nearly 47 million units in 2016′s third quarter, but that volume will still be down over... Read more
Apple’s 2016 Back to School promotion: Free B...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more
Apple refurbished iPad Air 2s available start...
Apple has Certified Refurbished iPad Air 2 available starting at $339. Apple’s one-year warranty is included with each model, and shipping is free: - 128GB Wi-Fi iPad Air 2: $499 - 64GB Wi-Fi iPad... Read more

Jobs Board

*Apple* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 51443201 Mishawaka, Indiana, United States Posted: Aug. 25, 2016 Weekly Hours: 40.00 **Job Summary** As an Apple Read more
*Apple* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 51218354 Fredericksburg, Virginia, United States Posted: Aug. 18, 2016 Weekly Hours: 40.00 **Job Summary** As an Apple Read more
*Apple* Retail - Multiple Positions Birmingh...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
SW Engineer *Apple* TV - Apple Inc. (United...
The Apple TV team is looking for excellent software engineers with experience in hardware, media management, media playback, content delivery and a passion for Read more
*Apple* Retail - Multiple Positions Victor,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.