The art of the software post-mortem

Turning failures into learning opportunities

Photo by Gerrie van der Walt on Unsplash

Software development is a complex endeavor. Despite our best efforts, failures are inevitable. The key isn't to avoid failure altogether (which is impossible), but to learn from it and improve. This is where the software post-mortem comes in.

What is a post-mortem?

A post-mortem (also sometimes called a retrospective, though the terms aren't perfectly interchangeable) is a structured meeting or process conducted after an incident, project completion (successful or not), or significant event. Its purpose is to analyze what happened, why it happened, and what can be done to prevent similar issues in the future.

Why are post-mortems important?

Learning from mistakes: The most obvious benefit is the opportunity to learn from errors. By dissecting what went wrong, we can identify patterns, weaknesses in our processes, and areas where we need to improve our skills or tooling.

Improving processes: Post-mortems help us refine our development, testing, and deployment processes. They can reveal bottlenecks, inefficiencies, and areas where automation could be beneficial.

Building a culture of blamelessness: A crucial aspect of effective post-mortems is a blameless environment. The goal is not to assign blame to individuals, but to understand systemic issues that contributed to the problem. This encourages open communication and honest reflection.

Knowledge sharing: Post-mortems create a shared understanding of what happened and why. This knowledge can be disseminated throughout the team and organization to prevent similar issues from recurring.

Key elements of an effective post-mortem

An effective post-mortem should be conducted in a timely manner, as soon as possible after the event, while the details are still fresh in everyone's mind. It's important to define clear objectives, outlining the specific event or project being analyzed and the desired outcomes. Diverse participation is crucial, including representatives from all relevant teams such as development, testing, and operations, to ensure a comprehensive understanding from different perspectives. Data collection is essential, gathering relevant information like logs, error reports, performance metrics, and communication records. A structured discussion should be facilitated, using frameworks like "Start, Stop, Continue" to identify what to start, stop, and continue doing, the "5 Whys" to drill down to the root cause, or creating a timeline of key events. Finally, action items should be identified, assigning owners and deadlines to address the issues, and the entire process should be documented for easy access and future reference.

Creating a blameless environment

Creating a blameless environment is essential for honest and productive post-mortems. Here are some tips:

Emphasize systemic issues: Focus on identifying systemic issues rather than individual errors.

Use inclusive language: Avoid accusatory language. Use phrases like "What happened?" instead of "Who did this?".

Lead by example: Managers and team leaders should model blameless behavior by openly discussing their own mistakes.

Celebrate learning: Recognize and reward teams that demonstrate a commitment to learning from failures.

Incident recreation in controlled environments

Creating safe opportunities to reconstruct failures provides teams with profound learning advantages beyond traditional discussion-based post-mortems. By establishing isolated sandbox environments that mirror production settings, engineers can methodically reproduce the incident conditions without risking further system damage. This hands-on approach transforms abstract conversations into tangible demonstrations where team members can directly observe failure mechanisms and test theories in real-time. The recreation process itself often reveals subtle contributing factors that might be missed in purely theoretical analysis, as engineers watch the actual cascading effects unfold before them. These controlled recreations serve multiple purposes beyond immediate understanding—they become perfect testing grounds for proposed fixes, allowing verification that solutions actually address root causes rather than just symptoms. Additionally, documented recreations create invaluable onboarding and training resources, helping new team members understand complex system interactions through concrete examples rather than documentation alone. When combined with video recordings or step-by-step guides, these recreations build an institutional knowledge base that preserves hard-won insights for future teams.

Reverse chronology analysis

Approaching post-mortems through reverse chronological examination offers a fresh perspective that often uncovers insights missed by traditional forward-facing analysis. In this approach, teams begin with the final error state and methodically work backward through each preceding event, decision point, and system interaction that ultimately led to failure. This backward-facing technique naturally counteracts hindsight bias by positioning the team at each historical moment with only the information available at that time, revealing genuine intervention opportunities rather than idealized scenarios. As teams trace backward through the incident timeline, they naturally identify the earliest inflection points where alternative actions could have prevented cascading failures. This approach particularly excels at uncovering organizational and procedural weaknesses, as it highlights decision constraints that may have seemed reasonable in isolation but proved problematic in sequence. The method also focuses attention on practical prevention mechanisms rather than theoretical causation models, leading to more actionable recommendations. Teams commonly report that reverse chronology sessions generate unexpected insights about monitoring gaps, alert thresholds, and early warning indicators that might otherwise remain undiscovered through conventional analysis methods.

Following up

The post-mortem is not the end of the process. It's crucial to follow up on the action items and ensure that they are implemented. Regularly review the post-mortem documentation to track progress and identify any new issues that may arise.

Site Reliability Engineering (SRE) teams can play a pivotal role in this follow-up phase, serving as neutral third-party enforcers to ensure that critical improvements identified during post-mortems don't fall victim to competing priorities. By embedding post-mortem action items into engineering roadmaps and sprint planning sessions, SRE teams create accountability across the organization without appearing punitive. The SRE perspective brings valuable operational insight, helping development teams understand which fixes will have the most significant reliability impact. These teams can establish formal review cadences—weekly for critical issues, monthly for less severe findings—where action item owners must demonstrate progress, creating healthy pressure to complete remediation work.

Service Level Objectives (SLOs) provide concrete measurements to validate whether implemented fixes actually resolve the underlying issues. By establishing clear, measurable SLOs that directly connect to incident causes, teams gain quantifiable evidence of improvement rather than relying on subjective assessments. For example, if a post-mortem revealed database timeouts as the root cause of an outage, an SLO monitoring connection pool exhaustion rates provides clear evidence of whether the fix successfully addressed the problem. This data-driven approach transforms abstract promises into empirical results, enabling teams to confidently close post-mortem action items only when metrics demonstrate genuine improvement. Additionally, tracking SLO violations before and after implementing fixes creates a feedback loop that reinforces the value of thorough post-mortems and diligent follow-through, ultimately building organizational resilience against similar failures in the future.

Conclusion

Software post-mortems are a valuable tool for improving software quality, processes, and team performance. By embracing a culture of blamelessness and focusing on learning from mistakes, we can turn failures into opportunities for growth and innovation.

If you’re curious to see some template post-mortems have a look at this GitHub repository. Another great resource is public post-mortems published by companies. Have a look at this GitHub repository for a nice collection.