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

Parallels Desktop 10.2.0 - Run Windows a...
Parallels Desktop is simply the world's bestselling, top-rated, and most trusted solution for running Windows applications on your Mac. With Parallels Desktop for Mac, you can seamlessly run both... Read more
LaunchBar 6.2 - Powerful file/URL/email...
LaunchBar is an award-winning productivity utility that offers an amazingly intuitive and efficient way to search and access any kind of information stored on your computer or on the Web. It provides... Read more
Firefox 37.0 - Fast, safe Web browser. (...
Firefox offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and casual... Read more
Arq 4.11 - Online backup to Google Drive...
Arq is super-easy online backup for the Mac. Back up to your own Google Drive storage (15GB free storage), your own Amazon Glacier ($.01/GB per month storage) or S3, or any SFTP server. Arq backs up... Read more
MacFamilyTree 7.3.4 - Create and explore...
MacFamilyTree gives genealogy a facelift: it's modern, interactive, incredibly fast, and easy to use. We're convinced that generations of chroniclers would have loved to trade in their genealogy... Read more
Yummy FTP 1.10.2 - FTP/SFTP/FTPS client...
Yummy FTP is an FTP + SFTP + FTPS file transfer client which focuses on speed, reliability and productivity. Whether you need to transfer a few files or a few thousand, schedule automatic backups, or... Read more
VueScan 9.5.08 - 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
Iridient Developer 3.0.1 - Powerful imag...
Iridient Developer (was RAW Developer) is a powerful image conversion application designed specifically for OS X. Iridient Developer gives advanced photographers total control over every aspect of... Read more
Monodraw 0.8.4.1 - Powerful ASCII art ed...
Monodraw allows you to easily create text-based art (like diagrams, layouts, flow charts) and visually represent algorithms, data structures, binary formats and more. Because it's all just text, it... Read more
Air Video Server HD 2.1.0 - Stream video...
Air Video Server HD streams videos instantly from your computer on your iPhone, iPad, iPod touch or Apple TV. No need to worry about converting or transferring files. We took everything that was... Read more

2K Announces WWE 2K, Mobile's First...
It seems like this month has been pretty big for wrestling. First Wrestlemania, then 2K has announces that they're releasing  WWE 2K for iOS. It's a simulation-based WWE game where you'll get to play with several WWE superstars such as John Cena, ... | Read more »
How the Apple Watch Could Change the Fac...
The Apple Watch is still a ways out, but my previous musings on the wearable’s various features got me thinking: what might it be like a year after launch? Two years? Five years? What if it becomes a symbiotic part of the iOS framework to the point... | Read more »
Pie In The Sky: A Pizza Odyssey (Games)
Pie In The Sky: A Pizza Odyssey 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: A game about delivering pizza. In space. | Read more »
Chosen Gives Hopeful Singers, Songwriter...
If YouTube videos and reality TV shows like The Voice have taught us one thing, it’s that there are a lot of people out there who are anxious to show the world their talents. And if they’ve taught us a second thing, it’s that there’s an almost... | Read more »
Android's Popular OfficeSuite Now A...
Once only available for Android devices, OfficeSuite has finally landed on the app store. The Mobile Systems app lets you view, edit, create, and share Word, Excel, and PowerPoint documents as well as convert them to/from PDFs. It's touted as being... | Read more »
Warhammer: Arcane Magic is Coming Soon,...
Turbo Tape Games has announced that they're joining forces with Games Workshop to bring the turn-based strategy board game, Warhammer: Arcane Magic, to life on the iOS. | Read more »
Fast & Furious: Legacy's Creati...
| Read more »
N-Fusion and 505's Ember is Totally...
| Read more »
These are All the Apple Watch Apps and G...
The Apple Watch is less than a month from hitting store shelves, and once you get your hands on it you're probably going to want some apps and games to install. Fear not! We've compiled a list of all the Apple Watch apps and games we've been able to... | Read more »
Appy to Have Known You - Lee Hamlet Look...
Being at 148Apps these past 2 years has been an awesome experience that has taught me a great deal, and working with such a great team has been a privilege. Thank you to Rob Rich, and to both Rob LeFebvre and Jeff Scott before him, for helping me... | Read more »

Price Scanner via MacPrices.net

13-inch 2.4GHz Retina MacBook Pro available f...
MacMall has the 2013 13″ 2.4GHz/128GB Retina MacBook Pro available for $949.99 for a limited time. Shipping is free. Their price is $350 off original MSRP, and it’s the only sub-$1000 new Retina... Read more
Adobe Brings Powerful Layout-Design Capabilit...
Adobe today announced the availability of Adobe Comp CC, a free iPad app that enables rapid creation of layout concepts for mobile, Web and print projects. With Comp CC, designers can rough out and... Read more
13-inch 2.6GHz/256GB Retina MacBook Pro avail...
Best Buy has clearance 2014 13″ 2.6GHz/256GB Retina MacBook Pros available for $1199.99 including free shipping. Their price is $300 off original MSRP, and it’s the lowest price for this model.... Read more
Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
21-inch 1.4GHz iMac on sale for $999, save $1...
Best Buy has the 21″ 1.4GHz iMac on sale for $999.99 on their online store. Choose free shipping or free local store pick up. Price is for online orders only, in-store prices may vary. Their price is... Read more
2.6GHz Mac mini on sale for $649, save $50
Amazon has the 2.6GHz Mac mini on sale for $649.99 including free shipping. Their price is $50 off MSRP, and it’s the lowest price available for this model. Read more
Textkraft Professional 3.2 Powerful iPad Text...
Finally it’s springtime, at least theoretically in my neck of the woods, where we’re still navigating canyons between towering snowbanks with temperatures well below freezing in winter weather that... Read more
Apple offering refurbished 27-inch 5K iMacs f...
The Apple Store is offering Apple Certified Refurbished 27″ 3.5GHz 5K iMacs for $2119 including free shipping. Their price is $380 off the price of new models, and it’s the lowest price available for... Read more
16GB iPad mini on sale for $199, save $50
Walmart has 16GB iPad minis (1st generation) available for $199.99 on their online store, including free shipping. Their price is $50 off MSRP. Online orders only. Read more
New 128GB MacBook Airs on sale for $50 off MS...
 B&H Photo has 128GB 11″ and 13″ 2015 MacBook Airs on sale today for $50 off MSRP including free shipping plus NY sales tax only: - 11″ 1.6GHz/128GB MacBook Air (Model #MJVM2LL/A): $849 $50 off... Read more

Jobs Board

DevOps Software Engineer - *Apple* Pay, iOS...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*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
Sr. Technical Services Consultant, *Apple*...
**Job Summary** Apple Professional Services (APS) has an opening for a senior technical position that contributes to Apple 's efforts for strategic and transactional Read more
Lead *Apple* Solutions Consultant - Retail...
**Job Summary** Job Summary The Lead ASC is an Apple employee who serves as the Apple business manager and influencer in a hyper-business critical Reseller's store Read more
*Apple* Pay - Site Reliability Engineer - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.