Block DevOps / SRE Interview Questions

Behavioral Questions (15)

1. 1.Tell me about a time you had to balance infrastructure reliability for two different user types with conflicting needs. How did you approach that tradeoff?

   easy~3 min

   What interviewers look for

   • Clearly articulated the distinct needs of both user groups and why they conflicted
   • Demonstrated data-driven decision making with metrics that mattered to both constituencies
   • Showed follow-through by monitoring impact on both groups after implementation

   Likely follow-ups

   • How did you measure success for each user group differently?
   • What would you have done if one group's metrics got significantly worse?

   Company context

   Block's Seller and Consumer Reasoning principle requires engineers to balance Square's seller needs against Cash App's consumer needs. SREs must design infrastructure that serves both ecosystems without favoring one over the other, which creates unique operational challenges around performance, availability, and resource allocation.

2. 2.You're deploying a critical Cash App payment service to production and need zero-downtime deployment with rollback capability. The service handles millions of transactions daily and must maintain PCI-DSS compliance. Walk me through your deployment strategy.

   easy~3 min

   What interviewers look for

   • Mentions blue-green or rolling deployment strategy with health checks and circuit breakers
   • Addresses PCI-DSS compliance requirements during deployment process
   • Discusses rollback automation and monitoring alerts for transaction integrity
   • References canary deployments or gradual traffic shifting to minimize blast radius

   Likely follow-ups

   • How would you handle database schema changes that need to be backwards compatible during this deployment?
   • What specific metrics would you monitor to determine if a rollback is needed for a payment service?

   Company context

   Block handles billions in payment volume across Square and Cash App, requiring zero-downtime deployments while maintaining strict PCI-DSS compliance. This tests understanding of Block's 'Money Movement Correctness' principle where payment reliability is non-negotiable.

3. 3.Design a metrics collection and alerting system for Square's payment processing infrastructure that handles millions of transactions daily across readers, online payments, and APIs. How would you ensure critical payment failures are detected within seconds while avoiding alert fatigue?

   easy~3 min

   What interviewers look for

   • Designs a tiered alerting system with different severity levels for payment-critical vs operational metrics
   • Proposes metrics aggregation strategy that handles Square's transaction volume while maintaining real-time visibility
   • Considers PCI-DSS compliance requirements for metrics data handling and retention

   Likely follow-ups

   • How would you handle metrics collection when Square Readers go offline but continue processing transactions locally?
   • What would your alerting strategy be for a scenario where payment success rate drops from 99.9% to 99.7% over 5 minutes?

   Company context

   Block's Square ecosystem processes billions in payments annually across hardware readers, online checkout, and APIs. Given Block's 'Be Direct' principle, the company needs alerting systems that surface real issues quickly without noise, especially for money movement which directly impacts seller revenue and Block's economic empowerment mission.

4. 4.Tell me about a time you had to push back on an engineering manager or senior engineer who wanted to take a shortcut that would impact system reliability. How did you handle the conversation?

   easy~3 min

5. 5.Square merchants report slow transaction processing during Black Friday. You discover CPU utilization is only 30% but payment confirmations are taking 8+ seconds instead of the usual 2 seconds. Walk me through your diagnostic approach.

   easy~3 min

6. 6.Walk me through the most complex data consistency issue you've debugged in a distributed payments system. What was broken and how did you fix it?

   medium~4 min

7. 7.Cash App's notification service is experiencing intermittent delays during peak hours, causing users to not receive payment confirmations immediately. The service uses Kafka for message processing across multiple AWS regions. How would you debug and resolve this?

   medium~4 min

8. 8.Cash App needs to migrate its transaction ledger database from a single-region PostgreSQL setup to a multi-region architecture to reduce latency for users across the US. Walk me through how you'd design this migration while ensuring zero data loss and maintaining ACID properties for money transfers.

   medium~4 min

9. 9.You're leading the effort to open-source an internal DevOps tool that Block has built for managing Kubernetes deployments. How would you coordinate across teams to prepare this for external release?

   medium~4 min

10. 10.Estimate how many additional AWS instances Block would need if Cash App's user base doubled overnight. Walk me through your assumptions about current infrastructure and scaling patterns.

    medium~4 min

11. 11.Describe a time you had to operate infrastructure that integrated with an external protocol or API that you couldn't control. What reliability challenges did that create?

    hard~5 min

12. 12.Square's point-of-sale system needs to process transactions even when the merchant's internet connection is unstable. Design a system that ensures transaction data integrity while handling offline scenarios and eventual reconciliation with Square's backend services.

    hard~5 min

13. 13.Design the infrastructure to support Block's open-source strategy where internal services like Misk and Wire need to be developed as both internal production systems and external open-source libraries. How would you handle the complexity of maintaining code that serves both Block's internal scale and external community needs?

    hard~5 min

14. 14.Describe a situation where you had to influence infrastructure decisions across multiple product teams at once when they had conflicting priorities. Walk me through your approach.

    hard~5 min

15. 15.You need to design monitoring for Block's Bitcoin mining infrastructure that Spiral operates. The system needs to detect both hardware failures and Bitcoin network protocol changes that could affect mining efficiency. How would you approach this?

    hard~5 min