Handling an incident

Incidents are going to happen.

When to raise an incident

When in doubt, raise an incident. We'd much rather have declared an incident which turned out not to be an incident. Many incidents take too long to get called, or are missed completely because someone didn't ring the alarm when they had a suspicion something was wrong.

To declare an incident, type /incident anywhere in Slack. This will create a new channel and send updates.

Anyone can declare an incident, including non-engineers. If in doubt, check with your nearest engineer.

Some things that should definitely be an incident

• app.posthog.com (PostHog Cloud US) or eu.posthog.com (PostHog Cloud EU) being completely unavailable (not just for you)
• No insights can be created
• Feature flags are not being returned at all, or /decide is down
• Various alerts defined as critical, such as disk space full, OOM or >5 minute ingestion lag

Things that shouldn’t be an incident

• Insights returning incorrect data
• Events being < 5 minutes behind
• Unable to save insights, create feature flags
• Expected disruption which happens as part of scheduled maintenance

Planning some maintenance? Check the announcements section instead.

Incident severity

Please refer to the following guidance when choosing the severity for your incident. If you are unsure, it's usually better to over-estimate than under-estimate!

Minor

A minor-severity incident does not usually require paging people, and can be addressed within normal working hours. It is higher priority than any bugs however, and should come before sprint work.

Examples

• Broken non-critical functionality, with no workaround. Not on the critical path for customers.
• Performance degradation. Not an outage, but our app is not performing as it should. For instance, growing (but not yet critical) ingestion lag.
• A memory leak in a database or feature. With time, this could cause a major/critical incident, but does not usually require immediate attention.

If not dealt with, minor incidents can often become major incidents. Minor incidents are usually OK to have open for a few days, whereas anything more severe we would be trying to resolve ASAP.

Major

A major incident usually requires paging people, and should be dealt with immediately. They are usually opened when key or critical functionality is not working as expected.

Major incidents often become critical incidents if not resolved in a timely manner.

Examples

• Customer signup is broken
• Significantly elevated error rate

Critical

An incident with very high impact on customers, and with the potential to existentially affect the company or reduce revenue.

Examples

• Posthog Cloud is completely down
• A data breach, or loss of data
• Event ingestion totally failing - we are losing events

What happens during an incident?

The person who raised the incident is the incident lead. It’s their responsibility to:

• Make sure the right people join the call. This includes the current on call person. Optionally, add people from Infra and the feature owner and Marketing if relevant. Marketing can assist on running communication if required.
• Take notes in the incident channel. This should include time stamps, and is a brain dump of everything that we know, and everything that we are or have tried. This will give us much more of an opportunity to learn from the incident afterwards.
• Update the status banner on app. There are some templates below to make this easier.
• Update the status page - this is best done via the incident slack channel via the incident app actions.

If the person who raised the incident is the best person to debug the issue, they should hand over the incident lead role to someone else on the call.

You can find all of our production runbooks + specific strategies for debugging outages here (internal)

Customer communications

Major incidents such as the app being partially or fully unreachable, as well as ingestion delays of 30 minutes or longer should be clearly communicated to our customers so that they know what is going on and what we are doing to resolve it.

The main way to communicate an incident to customers is the status page which is updated via the incident slack channel using /incident sp. There you can set granular information on the status of the problem and which components are affected. If the incident is critical and clearly impacting users then it makes sense to also update the banner feature flag which will show an announcement at the top of the app. It's the responsibility of the incident lead to enable the banner, and to disable it when the incident is resolved.

All in-app banners should link to a resource offering more information, usually the status page. The banner should simply state the user impact and direct users to more detailed information. Keep it simple, and direct.

Example flag payloads: Events from the last 5 days may be duplicated due to an error. [More info](https://status.posthog.com/). Event ingestion is currently delayed by three hours. [More info](https://status.posthog.com/).

If in doubt, a generic message can suffice:

We're experiencing technical difficulties. Check [status.posthog.com](https://status.posthog.com) for updates.

Occasionally it may be desirable to do addditional customer communications, such as sending an email to impacted customers or making updates to the service page. Marketing will organize and write these communications for you, so please let them know if this is needed. Joe is usually the best initial point of contact.

When does an incident end?

When we’ve identified the root cause of the issue and put a fix in place. End the incident by typing /inc close in the incident channel.

Don't forget to disable the in-app banner too.

What happens after an incident? (Incident analysis)

24-48 hours after an incident, we should have a quick sync meeting with a small group of people involved in the incident, plus Tim. If you raised the incident, you can schedule this. If you don’t want to or can’t, Tim is happy to take over at this point, just let him know.

Before this meeting you should create a PR against the incidents analysis repository. You can use Incident.io's automatic analysis as a starting point. In general it is advised to post short point in time summaries of actions taken in the incident Slack channel so that you can more easily fill this out after the fact.

• If a post-mortem had significant impact on customers (like data loss or flags not being available), we should sanitize and copy the post mortem into the public post-mortems repository

During the incident analysis session, we’ll walk through the entire timeline as it happened. While doing that, we want to answer the following types of questions:

• In what ways did our systems surprise us?
• How did it make sense for someone to do what they did?

We'll collect learnings through this process.

During a process like this, trying to come up with action items means losing focus on the learnings. That's why it’s up to the individuals in the meeting to figure out whether they need to action any of the learnings from the session.

If an incident was pretty uneventful we can skip this step.

Thanks to Incident Review and Postmortem Best Practices from Pragmatic Engineer