Replay Backlog Calculation Audit
Replay Backlog Calculation Audit
Ada re-derives this chapter’s own numbers step by step, at full precision
ADA · CALCULATION AUDIT
Replay Backlog Calculation Audit
The chapter’s regional water-monitoring system has 50 stations sending a record every 5 seconds; a 20-minute WAN outage produces 12,000 records, a 3.6 MB backlog. That backlog looks small, but replaying it at 10 records/second takes another 20 minutes. This audit works the replay backlog to show why the architecture must record event time, receive time, and replay state, not just a buffer size.
Companion to the chapter Edge, Fog, and Cloud: Architecture — every number here comes from that chapter.
A fog gateway replay plan is a queue physics problem: records arrive during the outage, bytes occupy storage, and replay consumes time that can collide with live traffic.
See the relationship before changing it
The figure reads from left to right. The blue card is reporting stations. The middle card applies this page's rule. The green card is outage backlog. Walk the arrows once: set the input, apply the rule, then read the result with its unit.
The retained audit below checks several chapter fixtures. This model keeps those stated values fixed and changes only reporting stations, so the numeric fixture does not switch without explanation.
Derive the baseline in four named moves
- 1
Name the input. The chapter baseline is 50 stations.
- 2
Name the relationship. backlog = stations x 12 records/min x 20 min
- 3
Substitute with units. 50 x 12 x 20 = 12,000 records
- 4
Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.
Predict, then change reporting stations
Try Predict the direction of backlog = stations x 12 records/min x 20 min. Test another reporting stations, then compare outage backlog.
Observe More stations create a larger replay backlog during the same twenty-minute outage. Reset reporting stations to 50 and compare outage backlog.
Explain More stations create a larger replay backlog during the same twenty-minute outage.
Check yourself
What should you do before trusting a moved-control result?
What does this small model leave out?
1. Convert the outage into record opportunities. The example already states 50 stations, one record every 5 seconds, and a 20-minute outage. Since one minute has 60 seconds, each station creates 60 / 5 = 12 records per minute.
| Check | Arithmetic | Review result |
| Records per station per minute | 60 s/min / 5 s = 12 |
Each station contributes 12 normalized records per minute. |
| Total outage records | 50 x 12 x 20 = 12,000 |
The chapter's record count is correct. arithmetic verified |
| Backlog bytes | 12,000 x 300 B = 3,600,000 B |
Using decimal megabytes, 3,600,000 B = 3.6 MB, matching the example. |
| Replay duration | 12,000 records / 10 records/s = 1,200 s |
1,200 s / 60 = 20 min, so catch-up lasts about as long as the outage. |
2. The physics implication is not the storage size alone. A 3.6 MB backlog is small, but replaying it at 10 records per second occupies another 20 minutes of recovery time. That is why the architecture record needs event time, receive time, replay state, and quality flags, not only a buffer size.
3. The bounded claim is replay evidence. These calculations prove the example's backlog and replay timing only for the stated record rate, outage length, payload size, and replay rate. A different station count, sample interval, envelope size, or replay throttle reopens the arithmetic.
The replay model deliberately does not simulate live traffic during catch-up, retry storms, disk I/O limits, record reordering, deduplication, or gateway failure; it assumes a constant outage arrival rate and a constant ten-record-per-second replay service.
Work the audit first, then check the displayed derivation.
Every number above is taken from the chapter’s own material and re-derived step by step.