# MTTD and MTTA

> Mean time to detect is the average time from the start of a failure until a signal shows it. Mean time to acknowledge is the average time from that signal until someone takes the incident. Google's SRE book counts both beside MTTR, because a team that fixes fast but notices late is still slow.

Reliability

## In Quorum

Every Incident records the sim second its first signal lands and the second a player first acts on it, and its Postmortem shows MTTD and MTTA beside MTTR. With Alerting the signal is the pager, which fires the Step a rule's threshold is crossed. Without Alerting the signal is the board, at the first second more than 1% of requests fail. On-call answers the page five calendar minutes after it lands, which is the MTTA a rotation holds. Without On-call, the player's first action is the acknowledgement.

## Design

[The incident loop](https://wiki.playquorum.dev/design/11-incidents.md#the-incident-loop)

## Read next

[MTTR](https://wiki.playquorum.dev/glossary/mttr.md), [SLO, SLA, SLI](https://wiki.playquorum.dev/glossary/slo-sla-sli.md), [Four golden signals](https://wiki.playquorum.dev/glossary/four-golden-signals.md)

## Learn more

- [Google SRE book: Being on-call](https://sre.google/sre-book/being-on-call/)
- [Wikipedia: Mean time to recovery](https://en.wikipedia.org/wiki/Mean_time_to_recovery)

## See also

- [Glossary](https://wiki.playquorum.dev/glossary.md)

Source: https://wiki.playquorum.dev/glossary/mttd-and-mtta