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.
