Inside the AWS DynamoDB Outage: What Really Went Wrong in us-east-1

Summary

This transcript explains the AWS US-East-1 outage where a race condition in DynamoDB’s DNS management caused hours-long service disruption. DynamoDB uses many NLBs and DNS records to route traffic to replicated nodes; a DNS planner creates versioned DNS plans and DNS enactors apply them to Route 53. Due to high processing delays, a stale “newer-plan” check failed: a delayed enactor applied an older plan (V1) after V2 was already active, and concurrent cleanup removed records—leaving Route 53 inconsistent and DynamoDB unreachable, impacting EC2, Lambda, and more. The incident highlights the need for robust coordination, fresh validation, and careful handling of race conditions in distributed systems.

Takeaways

• 😀 A recent AWS outage in the US East-1 region occurred due to a DNS record error in DynamoDB caused by a race condition.
• 😀 The issue stemmed from a faulty DNS record entry that caused failures in DynamoDB services, which then impacted other AWS services like EC2 and Lambda.
• 😀 Despite DynamoDB's high availability claim, this issue led to hours of service disruption, demonstrating the complexities in managing distributed systems.
• 😀 DynamoDB relies on DNS records for load balancing across a fleet of nodes in different availability zones to ensure scalability and fault tolerance.
• 😀 The need for DNS records in DynamoDB arises from its use of load balancers (NLBs) to distribute traffic across distributed nodes, ensuring good customer experience and handling hardware failures.
• 😀 DNS management architecture in DynamoDB involves components like the DNS planner, which monitors load balancer capacity, and the DNS enactor, which implements DNS plans.
• 😀 DNS planners periodically generate new DNS plans based on the load balancer's health and capacity, while the DNS enactor updates DNS records in Route 53.
• 😀 On the day of the outage, delays in the DNS enactor processes led to a race condition where an old DNS plan (V_sub_1) overwrote a newer one (V_sub_2), causing the system to become inconsistent.
• 😀 The problem occurred because the system failed to prevent an older plan from overwriting a newer plan, due to delays in processing that caused the check to become stale.
• 😀 AWS acknowledged that this race condition led to DynamoDB in US East-1 becoming unreachable, impacting services like EC2 and Lambda that depend on DynamoDB for backend operations.
• 😀 Manual intervention was required to fix the DNS issues after the race condition, highlighting the importance of robust monitoring and fail-safes in DNS management systems.

Q & A

What caused the AWS US East-1 outage?

The outage was caused by a faulty DNS record entry in DynamoDB, specifically related to a DNS record for 'dynamob.us-east-1.amazonaws.com'. This issue arose due to a race condition in the DNS management process, which led to a series of failures in the system.

How did the race condition affect DynamoDB's DNS records?

The race condition caused delays in the processing of DNS plans by the DNS enactors. As a result, an older DNS plan was able to overwrite a newer plan, leading to inconsistent DNS records. This inconsistency prevented DynamoDB from being accessible in the affected region.

Why does DynamoDB need DNS records in the first place?

DynamoDB is a distributed service that uses multiple load balancers across different availability zones and regions. DNS records are necessary to route traffic to the correct load balancers and ensure that data can be accessed reliably across different nodes and regions.

What is the role of the DNS planner in DynamoDB's DNS management architecture?

The DNS planner monitors the health and capacity of network load balancers in DynamoDB's architecture. It creates new DNS plans based on changes in load or the addition of new replica nodes, ensuring that DNS records are updated to reflect the current system configuration.

What is the role of the DNS enactor?

The DNS enactor implements the DNS plans created by the DNS planner. It updates DNS records in Amazon Route 53 to ensure that traffic is correctly routed to the appropriate load balancers and nodes.

How did the delays in the DNS enactors lead to a race condition?

Due to delays in the DNS enactor processing, one enactor applied an older DNS plan (V1) after a newer plan (V2) had been implemented. This caused the older plan to overwrite the newer one, resulting in incorrect DNS records being applied and disrupting access to DynamoDB.

What happened during the cleanup process that caused further issues?

After the enactor applied its DNS plan, it triggered a cleanup process that deleted older DNS plans. However, because the older plan (V1) had already overwritten the newer plan (V2), the cleanup process mistakenly deleted the valid records, leaving the system in an inconsistent state and preventing future updates.

What was the consequence of the inconsistent DNS records in Route 53?

The inconsistent DNS records prevented DynamoDB from being reachable, which impacted AWS services like EC2, Lambda, and others that depend on DynamoDB. This led to a service outage that lasted for hours.

How was the issue ultimately resolved?

The issue was only fixed after manual intervention. AWS engineers had to restore the DNS records and resolve the inconsistencies in the system that were caused by the race condition and faulty cleanup process.

What lessons can be learned from this AWS outage?

Key lessons include the importance of ensuring consistency in DNS record management, implementing more robust locking mechanisms to prevent race conditions, and improving monitoring systems for DNS health to detect issues before they escalate into service-wide outages.