TweetFollow Us on Twitter

Finding Fault Volume Number: 17 (2001)
Issue Number: 1
Column Tag: WebObjects

Finding Fault

By Sam Krishna and Patrick Taylor

Why the EOFaulting mechanism neatly solves a thorny problem in the Object-Relational paradigm

The Object-Relational paradigm is the conceptual foundation to the Enterprise Objects Framework (EOF). Concepts such as tables map nicely to classes, rows to objects, row-column values to instance variables (ivars), etc. The mapping is so neat and clean that one is rather surprised to find a terrible flaw marring the paradigm.

One difficulty is that few situations are easily encompassed by single, isolated tables. A relational approach is frequently needed. However, having started down the Object-Relational path, what happens to objects that are related to other objects? What happens if a large set of objects is related to a single object? How can these relationships be represented without compromising performance or overwhelming system memory?

In real life, relationships are numerous, complex and widespread. Consider a (simplified) model of a credit card reporting application.

How would such an application be able to traverse from Consumer to Purchases without swamping the memory footprint? If we tried to apply this model to 200 million individuals charging approximately US$10,000 per year on 3 credit cards (on average), the situation could get nasty in a hurry. Imagine fetching 600 million rows into an application in order to traverse the relationship graph for a credit card query. What kind of memory footprint would be needed to do a report on the purchasing information? When the program exceeds memory requirements, the machine will grind to a stop.

Without an efficient way of accessing our program's data, the value of the application comes into question. The types of applications created by WebObjects depend on a reasonable amount performance, but an elegant object-relational mapping is a useless affectation if it becomes an albatross. Object-orientation is not an end in itself, but rather a means - preferably a superior means.

Fortunately for us, the EOF engineering team solved this problem using a mechanism called the EOFault class and its EOFaultHandler. Together these classes form a system where light placeholder objects are substituted for regular objects before they are fetched. The EOFault is a raincheck, promising to fetch the real objects when actually needed.

Ready-Aim-Fire!

If you wish to fetch a related row in EOF, a fault has to be fired. Before you get the idea that this is extremely complicated in either theory or practice, stop fretting. It isn't. You simply "walk" the relationship graph and, most of the time, your WebObjects application will do this for you.

Take a look at the Movies example model that ships with WebObjects. Let's assume that you need to get a list of movie roles for any given movie that is selected. After fetching a particular movie, let's take a look at the movieRoles relationship before the fault is fired:

JavaDebug>> po movieRoles
movieRoles = <ArrayFault(0x195cfc8) source: [GID: Movie, (155)] relationship: movieRoles >

As you can see, movieRoles is an ArrayFault which knows to fetch all the rows on the destination side of a to-many relationship named movieRoles. The GID refers to an EOGlobalID, an object that contains the primary key information within Movie that is needed by the faulting system to fetch the related rows in the movieRoles table. When the fault is cleared or fired, EOF generates the SQL necessary to fetch the related rows. What is cool is that you don't have to write any code to fire these faults, WebObjects/EOF simply knows how to fault relationships automagically with very little (if any) programmer intervention.

At this point, your program might need to ask which studio produced which movie. It can do this by faulting the movie's studio relationship. Here's what the fault will look like before it is fired:

movie.studio is <EOGenericRecord(0x2057c98) Fault [GID: Studio, (9)]>

Once the studio fault is fired, EOF will again automatically generate the SQL, fetch the related row, stuff the results in a brand new Enterprise Object (EO), and return the new EO to the WebObjects Movies application.

Batch Faulting

If the Movies example was a real application with real users, then it's quite likely that someone would wish to see a list of credits for the movie's actors. This isn't an unreasonable demand since WebObjects will traverse the to-one relationship from the MovieRole EO to the Talent EO. The problem however is similar to the one we discussed in the credit card example. When you deal with a situation where many roles are getting fetched individually on an application getting a million hits per day, you need to do something to minimize the number of round-trip fetches to the database.

Every time a fault is fired, a round trip to the database is made. If your Movies application had 1000 users, each one of which looked at about 10 movies per visit, and each movie had a cast of 20 actors, your application would face approximately 200,000 round trips to the database for the cast list alone. Batch faulting is a great way to rein in your application's demands, it can potentially reduce the number of round trips to 10,000. The more relationships hanging off, the bigger the bang for batch faulting. I'm sure that you'll agree that a twenty-fold reduction is nothing to sneeze at!

Prefetching

Prefetching is an alternative method for faulting. It is used specifically with an EOFetchSpecification, which we will discuss in more detail next month when we cover the EOEditingContext. Suffice to say, it is a pre-empting action to the faulting mechanism. Based upon the key paths you feed to a particular EOFetchSpecification, you will be be able to prefetch EOs based upon the relationships from a particular set of entities you are fetching.

A developer must take care when using prefetching because it is too easy to fetch unnecessary EOs without thinking about it. It is better to profile the kinds of fetching patterns that occur during use of the web application before setting prefetching key paths upon the EOFetchSpecification, the class used with the EOEditingContext to fetch objects explicitly.

The method on the EOFetchSpecification class for prefetching, setPrefetchingRelationshipKeyPaths(), (setPrefetchingRelationshipKeyPaths: in Objective C), takes an NSArray of relationship keys. For example, if you wanted to prefetch both the movieRoles and studio relationships, you can pass the strings "movieRoles" and "studio" in an NSArray (@"movieRoles" and @"studio" in Objective C).

One thing to keep in mind, if you set an EOFetchSpecification to refresh through the method setRefreshesRefetchedObjects() (setRefreshesRefetchedObjects: in Objective C), any refetches are propagated through the object graph (meaning that both studio and movieRoles would be refetched as well to update the current state of the object graph as reflected in the database).

One way to batch fault is to set the number of items you want faulted in a to-many relationship using EOModeler's Advanced Relationship Inspector. While a terribly convenient approach since this allows you to fault lists without writing code, it unfortunately won't help us in this particular example.

We need to programmatically fault a whole array of Talent relationships based upon the MovieRole entity all at once. To achieve this, we message the EODatabaseContext with the batchFetchRelationship() method (batchFetchRelationship:forSourceObjects:editingContext: in Objective C). It isn't always quite clear how to get the EODatabaseContext from the EOF stack for a given WebObjects application, worse yet, even after you retrieve it, there are still a number of other objects you must retrieve in order to send the message to the EODatabaseContext. Here is an example of how we batch faulted the Talent EOs as they were related to an NSArray of MovieRoles for a given movie:

Java

Be sure to import com.apple.yellow.eoaccess.* as one of your import statements

   protected NSArray movieRoles() {
    // An example of how to batch fault programmatically in Java...
      NSArray movieRoles = 
                              (NSArray)movie.valueForKey("movieRoles");
   
      EOEditingContext ec = session().defaultEditingContext();
      EODatabaseContext dc = 
            EOUtilities.databaseContextForModelNamed(ec, "Movies");
      EOModel moviesModel = 
                  EOModelGroup.defaultGroup().modelNamed("Movies");
      EOEntity movieRoleEntity = 
                                 moviesModel.entityNamed("MovieRole");
      EORelationship talentRelationship = 
                           movieRoleEntity.relationshipNamed("talent");

   movieRoles.count();   // Fires the array fault to be used for the second argument
   dc.batchFetchRelationship(talentRelationship, movieRoles, ec);

         return movieRoles;
      }

Objective-C

Be sure to declare #import <EOAccess/EOAccess.h> as one of your import statements

   - (NSArray *)movieRoles
   {
      // Batch faulting example in Objective-C
      NSArray *movieRoles = 
                              [movie valueForKey:@"movieRoles"];
      EOEditingContext *ec = 
                        [[self session] defaultEditingContext];
      EODatabaseContext *dc = 
               [ec databaseContextForModelNamed:@"Movies"];
      EOModel *moviesModel = 
      [[EOModelGroup defaultGroup] modelNamed:@"Movies"];
      EOEntity *movieRoleEntity = 
                        [moviesModel entityNamed:@"MovieRole"];
      EORelationship *talentRelationship = 
               [movieRoleEntity relationshipNamed:@"talent"];
   
      [movieRoles count];   
            // Fires the array fault to be used for the forSourceObjects: argument
      [dc batchFetchRelationship:talentRelationship 
               forSourceObjects:movieRoles editingContext:ec];
      
      return movieRoles;
   }

There are quite a few objects to message as part of the EOF stack particularly the various elements of an EOModel. However this sample code yields the desired result which is a batch fault that fetches all the related TalentEOs simultaneously in a single round trip to the database. We decided to batch fault the talent relationship in this method for demo purposes only, in a professional development context, it would make more sense to refactor the batch faulting code into a separate method.

Deferred Faulting

New to WebObjects 4.5 is deferred faulting, a performance-focused approach to dealing with faults. While incredibly interesting, we aren't going to be able to do it justice within the confines of this more general article. Since WO4.5, all EOGenericRecords automatically do deferred faulting so you will definitely be hearing much more about it.

What to look for in code

A method will distinguish itself as a fault-firing method pretty quickly in a generated EO class file. In Java, all accessor methods will have the willRead() method in it. In Objective-C, you will see an accessor that reads [self storedValueForKey:@"someKey"]. Remember that all accessors in a generated EO class are fault-firing methods. If a fault cannot respond natively to a method without firing, it will fire and fetch the real object in order to fulfil the response.

For array faults, the easiest way to cause the fault to fire is by sending the count() message to it (count in Objective C); this will fire any to-many relationship-based faults. If you need to fire a to-one fault, you can either send it a message that you know the original object will respond to, or you can send the fault a willRead() message. In Objective-C, you can send any variety of messages that the original class should respond to, or you can simply use the EOFault class object to fire the fault by sending the [EOFault clearFault:aFault] message. Almost any message that the original class will respond to should do the trick.

Reverting an object back into a fault

If you need to convert an object that was faulted back into its original fault (for example to redisplay the state of a relationship after recent editing), there are a few more hoops to jump through. You need to get the EOGlobalID for an EO, stuff the EOGlobalID into an NSArray, and send a message to the EOEditingContext telling it to invalidate the objects with that particular global ID.

Here is some sample code to refault the MovieRole objects that have been fetched:

Java

   public void refaultMovieRoles() {
   
      // Assume movieRoles is an ivar
      EOGlobalID gid = null;
      int i = -1;
      EOEnterpriseObject eo = null;
      NSMutableArray gids = new NSMutableArray();
      EOEditingContext ec = null;
      
      for (i = 0; i < movieRoles.count(); i++) {
         eo = movieRoles.objectAtIndex(i);
         ec = eo.editingContext();
         gid = ec.globalIDForObject(eo);
         gids.addObject(gid);
      }
      
      session().defaultEditingContext()
                              .invalidateObjectsWithGlobalIDs(gids);
      return;
   }

Objective C

  - (void)refaultMovieRoles
  {
      // Assume movieRoles is an ivar
      EOGlobalID *gid = nil;
      int i = -1;
      id eo = nil;
      NSMutableArray *gids = [NSMutableArray array];
      EOEditingContext *ec = nil;
      
      for (i = 0; i < [movieRoles count]; i++) {
         eo = [movieRoles objectAtIndex:i];
         ec = [eo editingContext];
         gid = [ec globalIDForObject:eo];
         [gids addObject:gid];
      }
      
      [[[self session] defaultEditingContext] 
                                 invalidateObjectsWithGlobalIDs:gids];
      return;
   }

The invalidateObjectsWithGlobalIDs() method (invalidateObjectsWithGlobalIDs: in Objective C) is very powerful. It invalidates the entire undo stack for those particular objects with the editing context. It also signals the underlying object stores that the objects will need to be refetched the next time those particular objects are accessed (just like what would happen for a pure fault).

There are other methods, such as invalidateAllObjects() (invalidateAllObjects in Objective C), which will completely discard all EOs in the editing context as well as dump the snapshots at the EODatabaseContext level. This is a massive snapshot dumping operation in a traditional session-based editing context application and should be used very sparingly.

Things to Look for

If you have an object model where important entities have many relationships hanging off of them, you will want to look for opportunities to create a batch fault and optimize the performance of your application. If you have an array of objects that also have important individual to-one relationship targets, think seriously about creating a batch fault for those as well.

Faulting is a very cool mechanism that significantly simplifies the problem of whether or not to fetch the entire object graph based upon an entity. If you don't need to fault an EO into memory, then it is best to leave it alone; however, if you do need an EO on the other side of a relationship, then just reach out and fault it.

 
AAPL
$102.13
Apple Inc.
+1.24
MSFT
$44.87
Microsoft Corpora
-0.14
GOOG
$571.00
Google Inc.
-6.86

MacTech Search:
Community Search:

Software Updates via MacUpdate

Viber 4.2.2 - 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,... Read more
Cocktail 7.6 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
LaunchBar 6.1 - 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
Maya 2015 - Professional 3D modeling and...
Maya is an award-winning software and powerful, integrated 3D modeling, animation, visual effects, and rendering solution. Because Maya is based on an open architecture, all your work can be scripted... Read more
BBEdit 10.5.12 - Powerful text and HTML...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
Microsoft Office 2011 14.4.4 - Popular p...
Microsoft Office 2011 helps you create professional documents and presentations. And since Office for Mac 2011 is compatible with Office for Windows, you can work on documents with virtually anyone... Read more
TextWrangler 4.5.10 - Free general purpo...
TextWrangler is the powerful general purpose text editor, and Unix and server administrator's tool. Oh, and also, like the best things in life, it's free. TextWrangler is the "little brother" to... Read more
BitTorrent Sync 1.4.72 - 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
Cyberduck 4.5.2 - FTP and SFTP browser....
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Tinderbox 6.0.3 - Store and organize you...
Tinderbox is a personal content management assistant. It stores your notes, ideas, and plans. It can help you organize and understand them. And Tinderbox helps you share ideas through Web journals... Read more

Latest Forum Discussions

See All

Mobile Convolution (Music)
Mobile Convolution 1.0.0 Device: iOS Universal Category: Music Price: $9.99, Version: 1.0.0 (iTunes) Description: | Read more »
Invaders! From Outer Space Review
Invaders! From Outer Space Review By Rob Thomas on August 27th, 2014 Our Rating: :: RETRO NOSTALGIAUniversal App - Designed for iPhone and iPad It’s a shame that Invaders! doesn’t offer deeper gameplay, as this retro-inspired... | Read more »
Spooklands Review
Spooklands Review By Jennifer Allen on August 27th, 2014 Our Rating: :: ONE-TOUCH SHOOTERUniversal App - Designed for iPhone and iPad One-touch simultaneously controls your direction and your weapon in this unique arena shooter.   | Read more »
Heroes of Order & Chaos Add Twitch I...
Heroes of Order & Chaos Add Twitch Integration, New Heroes, and More Posted by Ellis Spice on August 27th, 2014 [ permalink ] | Read more »
Foodie Yama Review
Foodie Yama Review By Jennifer Allen on August 27th, 2014 Our Rating: :: BRIEFLY HOOKSUniversal App - Designed for iPhone and iPad Foodie Yama will draw you in for a brief while, and you’ll never be entirely sure why.   | Read more »
Spotify Connect Turns One, Now Supports...
Spotify Connect Turns One, Now Supports New Devices Posted by Ellis Spice on August 27th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
The Rise of PicsaStock and How You Can M...
We all take plenty of photos, right? That’s the joy of having a reasonably powerful camera in your pocket, thanks to your trusty iPhone and a bevy of similarly useful apps. Wouldn’t it be great to make some money out of those snaps? While your... | Read more »
Appointment With F.E.A.R Review
Appointment With F.E.A.R Review By Jennifer Allen on August 27th, 2014 Our Rating: :: CAMP DELIGHTUniversal App - Designed for iPhone and iPad Ever wanted to be a superhero? Appointment With F.E.A.R is a fine way to live that dream... | Read more »
Indie MOBA ÆRENA’s 2.0 Summer Update Bri...
Indie MOBA ÆRENA’s 2.0 Summer Update Brings New Missions, Skins, and a Champion Posted by Jessica Fisher on August 27th, 2014 [ permalink | Read more »
Ultra Drift Review
Ultra Drift Review By Rob Thomas on August 27th, 2014 Our Rating: :: ULTRA DULLUniversal App - Designed for iPhone and iPad Ultra Drift is fine in concept, but the execution runs head-first into the wall. And so will you. Over and... | Read more »

Price Scanner via MacPrices.net

The iPad’s Real Competitive Challenger (Not S...
It’s been my contention for some time that the iPad is suffering from something of an identity crisis, and I suspect that may be a factor in slackening sales this year. Apple can’t seem to decide... Read more
13-inch 2.6GHz/256GB Retina MacBook Pro on sa...
B&H Photo has the 13″ 2.6GHz/256GB Retina MacBook Pro on sale for $1379 including free shipping plus NY sales tax only. Their price is $120 off MSRP. Read more
Life Inventory iOS Apps – Learn to Know Thyse...
James Hollender’s Life Inventory apps s are now on sale with 20% off thru Labor Day, 09/01/2014. This is a great opportunity to get started on that Moral Inventory you’ve been putting off doing for... Read more
Pocket Watch, LLC. Reveals Cloud Server For P...
Beaumont, Texas based Pocket Watch, LLC. has announced the availability of its new ActivePrint Cloud Server Powered by Raspberry Pi. With this small standalone box almost any USB printer or available... Read more
902it Simplifies Area Code Changes For Nova S...
The east coast Canadian provinces of Nova Scotia and Prince Edward Island are phasing in 10 digit telephone dialing, to be fully in place by November, in order to accommodate a second area code to... Read more
Boomerang iPad Stand Mounts Your iPad Anywher...
Boomerang, a Mountable Stand with Multiple Viewing Angles, is now available for iPad Air. Boomerang combines several functions that aim to expand your iPad’s potential in one, elegant product. The... Read more
Retina MacBook Pros available starting at $10...
The Apple Store has Apple Certified Refurbished 13″ and 15″ MacBook Pros available starting at $929. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook Pros (4GB RAM/... Read more
Apple 27-inch Thunderbolt Display (refurbishe...
The Apple Store has Apple Certified Refurbished 27″ Thunderbolt Displays available for $799 including free shipping. That’s $200 off the cost of new models. Read more
Apple offers free $25 iTunes gift card with p...
The Apple Store is offering a free $25 iTunes Gift Card with the purchase of a $99 Apple TV for a limited time. Shipping is free. Read more
Apple’s 2014 Back to School promotion: $100 g...
 Apple’s 2014 Back to School promotion includes a free $100 App Store Gift Card with the purchase of any new Mac (Mac mini excluded), or a $50 Gift Card with the purchase of an iPad or iPhone,... 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
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.