Details of the February 22nd 2013 Windows Azure Storage Disruption

At 12:29 PM PST on February 22nd, 2013 there was a service interruption in all regions that affected customers who were accessing Windows Azure Storage Blobs, Tables and Queues using HTTPS. Availability was restored worldwide by 00:09 AM PST on February 23, 2013.

We apologize for the disruption of service to affected customers and are proactively issuing a service credit to those customers as outlined below.

We are providing more information on the components associated with the interruption, the root cause of the interruption, the recovery process, what we’ve learned from this case, and what we’re doing to improve the service reliability for our customers.

Windows Azure Overview

Before diving into the details of the service interruption, and to provide better context on what happened, we’d first like to share some information on the internal components of Windows Azure associated with this event.

Windows Azure runs many cloud services across various data centers and geographic regions around the globe. Windows Azure Storage runs as a cloud service on Windows Azure. There are multiple physical storage service deployments per geographic region, which we call stamps. Each storage stamp has multiple racks of storage nodes.

The Windows Azure Fabric Controller is the resource provisioning and management layer that manages the hardware, provides resource allocation, deployment and upgrade functions, and management for cloud services on the Windows Azure platform.

Windows Azure uses an internal service called the Secret Store to securely manage certificates needed to run the service. This internal management service automates the storage, distribution and updating of platform and customer certificates in the system. This internal management service automates the handling of certificates in the system so that personnel do not have direct access to the secrets for compliance and security purposes.

Root Cause Analysis

Windows Azure Storage uses a unique Secure Socket Layer (SSL) certificate to secure customer data traffic for each of the main storage types: blobs, tables and queues. The certificates allow for the encryption of traffic for all subdomains which represent a customer account (e.g. myaccount.blob.core.windows.net) via HTTPS. Internal and external services leverage these certificates to encrypt traffic to and from the storage systems. The certificates originate from the Secret Store, are stored locally on each of the Windows Azure Storage Nodes, and are deployed by the Fabric Controller. The certificates for blobs, tables and queues were the same for all regions and stamps.

The expiration times of the certificates in operation last week were as follows:

  • *.blob.core.windows.net      Friday, February 22, 2013 12:29:53 PM PST
  • *.queue.core.windows.net    Friday, February 22, 2013 12:31:22 PM PST
  • *.table.core.windows.net   Friday, February 22, 2013 12:32:52 PM PST

When the certificate expiration time was reached, the certificates became invalid prompting a rejection for those connections using HTTPS with the storage servers. Throughout, HTTP transactions were still operational.

While the expiration of the certificates caused the direct impact to customers, a breakdown in our procedures for maintaining and monitoring these certificates was the root cause. Additionally, since the certificates were the same across regions and were temporally close to each other, they were a single point of failure for the storage system.

Details of how the Storage Certificate was not updated

For context, as a part of the normal operation of the Secret Store, scanning occurs on a weekly basis for the certificates being managed. Alerts of pending expirations are sent to the teams managing the service starting 180 days in advance. From that point on, the Secret Store sends notifications to the team that owns the certificate. The team then refreshes a certificate when notified, includes the updated certificate in a new build of the service that is scheduled for deployment, and updates the certificate in the Secret Store’s database. This process regularly happens hundreds of times per month across the many services on Windows Azure.

In this case, the Secret Store service notified the Windows Azure Storage service team that the SSL certificates mentioned above would expire on the given dates. On January 7th, 2013 the storage team updated the three certificates in the Secret Store and included them in a future release of the service. However, the team failed to flag the storage service release as a release that included certificate updates. Subsequently, the release of the storage service containing the time critical certificate updates was delayed behind updates flagged as higher priority, and was not deployed in time to meet the certificate expiration deadline. Additionally, because the certificate had already been updated in the Secret Store, no additional alerts were presented to the team, which was a gap in our alerting system.

Recovering the Storage Service

The incident was detected at 12:44 PM PST through normal monitoring, and the expired certificates were diagnosed as the cause. By 13:15 PM PST, the engineering team had triaged the issue and established several work streams to determine the fastest path to restore the service.

During its normal operation, the Fabric Controller drives nodes to a desired state, also known as a “goal state”. The service definition of a service provides the desired state of the deployment, which enables the Fabric Controller to determine the goal state of nodes (servers) that are a part of the deployment. The service definition is comprised of role instances with their endpoints, configuration, and failure/update domains, as well as references to other artifacts such as code, Virtual Hard Disk (VHD) names, thumbprints of certificates, etc.

During normal operation, a given service would update their build to include new certificates and then have the Fabric Controller deploy the service by systematically walking the update domains and deploying the service across all nodes. This process is designed to update the software in such a way that external customers experience seamless updates and meet the published Service Level Agreement (SLA). While some of this work is executed in parallel, the overall time to deploy updates to a global service is many hours.

During this HTTPS service interruption, the Windows Azure Storage service was still up and functioning for customers who were using HTTP to access their data and some customers had quickly mitigated their HTTPS issues by moving to HTTP temporarily. Care was taken not to impact customers using HTTP while restoring service for others.

After examining several options to restore HTTPS service, two approaches were selected: 1) an update of the certificate on each storage node, and, 2) a complete update of the storage service. The first approach optimized for restoring customer service as rapidly as possible.

1)     Update of the Certificate

The development team worked through the manual steps required to update the certificate to validate the remediation approach and restore service. This process was complicated by the fact that the Fabric Controller tries to return a node to its goal state. A process that successfully updated the certificates was developed and tested by 18:22 PM PST. A key learning from a previous outage was to take the time upfront to test and validate the fix sufficiently, to prevent complications or secondary outages that would impact other services. During testing of the fix, several issues were found and corrected before it was validated for production deployment.

Once the automated update process was validated, we applied it to the storage nodes in the US West Data Center at 19:20 PST, successfully restoring service there at 20:50 PM PST. We then subsequently rolled it out to all storage nodes globally. This process completed at 22:45 PM PST and restored HTTPS service to the majority of customers. Additional monitoring and validation was done and the Azure dashboard marked green at 00:09 AM PST on February 23rd, 2013.

2) Complete Update

In parallel to the update of the certificate, a complete update of the storage service with the updated certificate was scheduled and rolled out across the globe. The purpose of this update was to provide the final and correct goal state for all of the storage nodes and ensure the system was in a constant and normal state. This process was started on February 22nd at 23:00 PM PST and completed on February 23rd at 19:59 PM PST and as designed, it did not impact the availability SLA for customers.

Improving the Service

After an incident occurs, we always take the time to analyze the incident and look at ways we can improve our engineering, operations and communications. To learn as much as we can, we do a root cause analysis and analyze all aspects of the incident to improve the reliability of our platform for our customers.

This analysis is organized into four major areas, looking at each part of the incident lifecycle as well as the engineering process that preceded it:

  • Detection – how to rapidly surface failures and prioritize recovery
  • Recovery – how to reduce the recovery time and impact on our customers
  • Prevention – how the system can avoid, isolate, and/or recover from failures
  • Response – how to support our customers during an incident

Detection

We will be expanding our monitoring of certificates expiration to include not only the Secret Store, but the production endpoints as well, in order to ensure that certificates do not expire in production.

Recovery

Our processes for recovery worked correctly, but we continue to work to improve the performance and reliability of deployment mechanisms.

We will put in place specific mechanisms to do critical certificate updates and exercise these mechanisms regularly to provide a quicker response should an incident like this happen again.

Prevention

We will improve the detection of future expiring certificates deployed in production. Any production certificate that has less than 3 months until the expiration date will create an operational incident and will be treated and tracked as if it were a Service Impacting Event.

We will also automate any associated manual processes so that builds of services that contain certificate updates are tracked and prioritized correctly.  In the interim, all manual processes involving certificates have been reviewed with the teams.

We will examine our certificates and look for opportunities to partition the certificates across a service, across regions and across time so an uncaught expiration does not create a widespread, simultaneous event. And, we will continue to review the system and address any single points of failure.

Response

The multi-level failover procedures for the Windows Azure service dashboard functioned as expected and provided critical updates for customers through the incident. There were 59 progress updates over the period of the incident but we will continue to refine our ability to provide accurate ETAs for issues and updates.

We do our best to post what we know, real-time, on the Windows Azure dashboard and will continuously look for ways to improve our customer communications.

Service Credits

We recognize that this service interruption had a significant impact on affected customers. Due to the nature and duration of this event we will proactively provide SLA credits to affected customers. Credits will cover all impacted services. Customers that were running the following impacted services at the time of the outage will get a 25% service credit for any charges associated with these services for the impacted billing period:

  • Storage
  • Mobile Services
  • Service Bus
  • Media Services
  • Web Sites

Impacted customers will also receive a 25% credit on any data transfer usage. The credit will be calculated in accordance with our SLA and will be reflected on a subsequent invoice. Customers who have additional questions can contact Windows Azure Support for more information.

Conclusion

The Windows Azure team will continue to review the findings outlined above over the coming weeks and take all steps to continually improve our service.

We sincerely apologize and regret the impact this outage had on our customers. We will continue to work diligently to deliver a highly available service.

 -Mike