405 Not Allowed -

Eli, a young traveler, stood before the heavy oak doors of the . He had a simple task: he wanted to leave a letter for a friend. He walked up to the reception desk, where a clerk sat behind a glass partition.

"I don't understand," Eli said. "Is the library closed? Is the address wrong?" 405 Not Allowed

The clerk didn't even look at the letter. Instead, he stamped a red mark on a form and slid it back. It read: Eli, a young traveler, stood before the heavy

The clerk pointed to a small plaque on the wall—the . It listed the only "verbs" the building accepted: GET, HEAD, OPTIONS . "I don't understand," Eli said

"The library is right here," the clerk droned, pointing to the shelves of books behind him. "You’ve got the right place. But this is an . It is a read-only establishment. You can GET a book, you can HEAD a title, but you cannot POST a letter here. We simply don't allow that method of interaction." "But I have to leave this letter!" Eli insisted.

In the bustling city of Silicon Circuit, every citizen is a and every building is a Server .