Many thanks to Hanzík for the Czech translations!

tcc-case-title
extremely geeky  extremely geeky

Case 211

No Magic

One night the nun Hwídah was summoned to the Central Command Courtyard by master Bawan.

“There is curious behavior on our ‘Change Address’ page,” said Bawan. “The user fills out the form and elects to save their changes. No error is reported. No error is logged. The browser is sent to the ‘User Profile’ page as expected. Yet this page sometimes shows the old address instead of the new.”

“A swallowed exception, most likely,” said Hwídah, wishing to be somewhere else.

“There is more,” said the master, shaking his head. “If the ‘User Profile’ is then reloaded repeatedly, it will sometimes show the old address and sometimes the new. No pattern can be discerned.”

“A caching problem, then,” suggested the nun.

“There is still more,” said the master. “After several minutes have passed, only one of the two addresses will be shown forever after. But again, it is sometimes the old address and sometimes the new.”

“I am out of guesses,” sighed Hwídah. “A curse, perhaps, or an evil spirit.”

Bawan slapped her forehead. “I have no patience for country superstitions, nun!” he said, turning to depart. “There is no magic. And the only evil spirits in this room are the ones you will witness tomorrow, if I learn that Banzen's chosen pupil cannot be relied upon.”

- - -

The nun called up the troublesome page, and found that all was as the master described.

“Most curious,” Hwídah said to the screen. “Your face says sometimes this, and sometimes that. Yet what is in your heart, I wonder?”

The nun logged directly into the database, observing the record each time she changed it. But what she found made even less sense: for although the screen alternated randomly between the old address and the new, the database showed the old address steadily. Yet after a few minutes both would agree on either the old or new record forevermore.

“There is no magic,” Hwídah repeated. “Every behavior, no matter how odd, has some rational explanation.”

- - -

“...Yet I have no idea what that explanation is,” Hwídah complained to Yíwen. “My only suspicion is that the transaction does not commit properly, which is why the changes do not appear when viewed from another database session.”

“A thousand pardons,” yawned Yíwen, who was still in bed, squinting at Hwídah’s lantern. “But my only suspicion is that I was asleep moments ago, for it is after one in the morning. Is there no one else you may consult with?”

“The courtyard is empty, as are my brains,” said Hwídah. “Help me think! The Controller calls a Service method to save the address. The Service method is annotated as transactional. The method is invoked, yet chaos results. Why?”

“I know nothing about transaction management,” said Yíwen, falling back on her pillow with closed eyes. “I know only to put the annotations where they belong. The rest is magic.”

“There is no magic,” Hwídah said absently. Then like a bolt she was out the door, bounding for the courtyard.

- - -

“When does a Service method commit a transaction?” asked Hwídah with a broad smile.

“At four-thirty in the morning?” said master Bawan testily; for that was presently the time.

“It doesn’t,” said the nun. “The proxy object which decorates the Service object does. Now, when does that proxy object not do its job?”

“This is another trick question, isn’t it?” asked Bawan.

“When it isn’t used!” said Hwídah triumphantly. “A mistake in the application’s configuration caused the Controller to be injected with the Service object instead of its proxy.”

“Interesting,” said Bawan. “What of the page showing sometimes the old record and sometimes the new?”

“Since there was no transaction,” said Hwídah, “the database update was performed without a commit, on a connection borrowed from the connection pool. If a page happened to borrow that same connection it would show the new record. Otherwise it would show the old one.”

“Intriguing,” said the master. “And this behavior would cease after a while, because...?”

“If that connection is borrowed for some other transaction elsewhere in the application, then either a commit or a rollback will occur,” said Hwídah. “And if it goes unused for too long, we roll it back and close it.”

“Excellent,” said the master. “No evil spirits, then. No magic. Now go; get some sleep.”

- - -

Hwídah crept into her bed, taking care not to wake Yíwen. Yet sleep did not come quickly to her. Instead she lay on folded arms, staring up into the darkness.

What is magic, she wondered, but the achievement of things by hidden means? Is the master of sleight-of-hand any less a magician than the enslaver of spirits? Is it not perhaps more satisfying to peek behind the curtain, to understand the intricate clockwork behind the facade, and to realize that with that knowledge comes true power?

Hwídah was still pondering these things as she slipped, at last, into a deep slumber, undisturbed by the stirring of the Temple, the many ticks and tocks of the machine spinning up; water in the pipes, footsteps in the hall, gongs for breakfast, for meditation, for the start of a new day, a day like any other day, in this place of marvels.