In a sufficiently large organization, embrace Conway’s Law. Create software that mimic organizational structure, or modify organizational structure to mimic software architecture. Any other arrangement leads to chronic friction.
As a corollary, refactoring of organizational structure can be a forcing function for refactoring software architecture. It may take time, but your software will eventually reflect your new organization.
I made a topical cartoon.
Credit: Jeffmock CC-BY-SA-3.0
On this 15th anniversary of 9/11, I recall my most vivid memory of the events. I drove in to work at Honeywell Aerospace in Kansas City after watching the second aircraft crash into the Twin Towers on live TV. Partway through my drive, I saw something I’d never seen before or since: arcing contrails, all over the clear blue Kansas sky. It wasn’t after I got in to work that I found out that the FAA had grounded all aircraft in flight. Those contrails showed where aircraft turned around and landed where they came from.
We didn’t get much work done that day. Someone hooked up large screen TVs and we watched as the towers fell.
This is the sixth and final post in a series about Grand Central Dispatch.
Long time users of GCD will tell you: it’s easy to forget where you are. Which queue am I on? Should I dispatch_sync to a queue that protects my variables, or has my caller taken care of that?
In this post I will describe a simple naming convention that has kept me sane over the years. Follow it, and you shouldn’t ever deadlock or forget to synchronize access to your member variables again.
When designing thread-safe code, it helps to have a library mindset. You should distinguish between the external or public interface (API), and the internal or private interface. The public API is presented in public headers, and the private interface is presented in private headers, used only by the library’s developers.
The ideal thread-safe public API does not expose threading or queueing at all (unless, of course, thread or queue management is the point of your library’s utility). It should simply not be possible to induce a race condition or deadlock when using your library. Let’s take a look at this classic example:
// Public header
#import <Foundation/Foundation.h>
// Thread-safe
@interface Account: NSObject
@property (nonatomic, readonly, getter=isClosed) BOOL closed;
@property (nonatomic, readonly) double balance;
- (void)addMoney:(double)amount;
- (void)subtractMoney:(double)amount;
- (double)closeAccount; // Returns balance
@end
@interface Bank: NSObject
@property (nonatomic, copy) NSArray<Account *> *accounts;
@property (nonatomic, readonly) double totalBalanceInAllAccounts;
- (void)transferMoneyAmount:(double)amount
fromAccount:(Account *)fromAccount
toAccount:(Account *)toAccount;
@end
If it weren’t for the helpful comment, you wouldn’t be able to tell from this header that the library is thread-safe. In other words, you’ve defined thread-safety as an implementation detail.
See this photo in my flickr album
The lights went out at Seven Seas Park (darn copper thieves!). This gave me an opportunity to capture the full moon over the townhomes lining the park’s perimeter. If you look closely you can see a full halo around the moon, courtesy of some light cirrus clouds.
📷 Nikon D800, Nikon 14-24mm f/28 at 14mm, 20 seconds at f/5.6.
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