Communications of the ACM

Conclusion

Applications are no longer islands. Not only do they frequently run distributed and replicated over many cloud-based computers, but they also run over many handheld computers. This makes it challenging to talk about a single truth at a single place or time. In addition, most modern applications interact with other applications. These interactions settle out to impact understanding. Over time, a shared opinion emerges just as new interactions add increasing uncertainty. Many business, personal, and computational “facts” are, in fact, uncertain. As some changes settle, others meander from place to place.

With all the regular, irregular, and uncleared checks, my understanding of our personal joint checking account is a bit hazy. While I try to convince myself I will someday understand it, I have reconciled myself that it’s really consistently eventual.

Snapping Uncertainty into
“What We Know So Far”

Our bank sends us monthly statements. They represent the debits and credits to our account that cleared a strongly consistent location as of a deadline. That strongly consistent location is the bank’s centralized computer system. The debits and credits that have arrived at the clearinghouse by the monthly deadline get scooped up into the account’s statement.

Quarterly reports for public companies take a similar but different approach. At midnight of the last day of the quarter, new business and new expenses start being allotted to the next quarter. The company begins gathering and organizing all the income and expenses from the now-closed quarter. Then all records of what was spent and earned are swept into a big mélange that results in a public quarterly profit and loss report. This usually happens within 40 days or so after the quarter closes.

Some transactions during the quarter, however, may not be sent to the accountants in a timely fashion. One contributing factor may be the employment of software engineers, who are notoriously bad at the punctual submission of expense reports. So, the results published for the quarter are approximately correct but not perfect.

After the quarterly report, corrections will dribble in to the accountants. They will either categorize them as minor and issue a slight correction to the numbers for the previous quarter or issue a restatement. For a public company, a restatement showing a noticeable difference from the published report is embarrassing and rarely happens. Minor corrections are common.

You can’t really know what happened until you have heard everything.

The longer you wait, the more you
hear and the more accurate your opin-
ion. Eventually, you give up waiting for
new information and declare your opin-
ion of what happened a few months ago
is accurate enough.

In the bank account statement, the definition of certainty is provided at the bank’s centralized computer system at end-of-day when the month closes. Uncertainty from the perspective of the bank is eliminated. In the corporate quarterly reports, uncertainty is gauged by how much of the underlying truth of the business filters its way back to the accountants. The quarterly report is not definitive, just pretty darned close—at least usually.

Trust, Timeouts, and Escalation Working across trust boundaries is always eventual. Because you may not trust another entity, you are not going to do a distributed two-phase commit4 and lock up your database waiting for that other company. Instead, you have a workflow in which partial trust is used to get your cooperative business done. Throughout this process, there are long windows in which you are just waiting and waiting … still more examples of being consistently eventual.

This eventual nature of uncertainty continues through the steps of the workflow. You agree to reserve 200 widgets from your inventory on the receipt of a deposit from your purchaser. If you hear nothing back to consummate the purchase of the widgets, you are stuck. While your disappointment is somewhat tempered by the deposit you keep, it’s not enough to pay for the widgets. Darn! You lost out on selling them to another customer!

Working across trust boundaries, cooperative work functions using timeouts and escalation. If you do not call to cancel your room reservation at a hotel with 72 hours advance notice, you are stuck with the first night’s room charge. The hotel, however, is likely stuck with the remaining six days of an empty room from your one-week reservation.

Indeed, the hotel has an ongoing parade of eventually resolved room sales. By the time it knows what is happening on Tuesday, the confusion of Wednesday’s occupancy is about to be resolved. Again, payments to the hotel are consistently eventual.