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January 2025

8

Watch our 'Azure Incident Retrospective' video about this incident:

What happened? 

Between 22:31 UTC on 8 January and 00:44 UTC on 11 January 2025, a networking configuration change in a single availability zone (physical zone AZ01) in East US 2 resulted in customers experiencing connectivity issues, prolonged timeouts, connection drops, and resource allocation failures across multiple Azure services. Customers leveraging Private Link with Network Security Groups (NSG) to communicate with Azure services may have also been impacted. Services that were configured to be zonally redundant and leveraging VNet integration may have experienced impact across multiple zones. 

The 'logical' availability zones used by each customer subscription may correspond to different physical availability zones. Customers can use the Locations API to understand this mapping, to confirm which resources run in this physical AZ -  . 

Impacted services included: 

  • Azure Databricks: between 22:55 UTC on 08 January and 15:00 UTC on 10 January, customers may have experienced issues launching clusters and serverless compute resources, leading to failures in dependent jobs or workflows. 
  • Azure OpenAI: between 04:29 UTC on 09 January and 15:00 UTC on 10 January, customers may have experienced stuck Azure OpenAI training jobs, and failures in Finetune model deployments and other jobs.  
  • Azure SQL Managed Instance, Azure Database for PostgreSQL flexible servers, Virtual Machines, Virtual Machine Scale Sets: between 22:31 UTC on 08 January and 22:19 UTC on 10 January, customers may have experienced service management operation failures.  
  • Azure App Service, Azure Logic Apps, Azure Function apps: between 22:31 UTC on 8 January and 20:05 UTC on 10 January, customers may have received intermittent HTTP 500-level response codes, timeouts or high latency when accessing App Service (Web, Mobile, and API Apps), App Service (Linux), or Function deployments hosted in East US 2. 
  • Azure Container Apps: between 22:31 UTC on 08 January and 21:01 UTC on 10 January, customers may have experienced intermittent failures when creating revisions, container app/jobs in a consumption workload profile, and scaling out a Dedicated workload profile in East US 2. 
  • Azure SQL Database: between 01:29 UTC on 09 January and 22:25 UTC on 10 January, customers may have experienced issues accessing services, and errors/timeouts for new database connections. Existing connections remained available to accept new requests, however re-established connections may have failed. 
  • Azure Data Factory and Azure Synapse Analytics: between 22:31 UTC on January 8 and 00:44 UTC on January 11, customers may have encountered errors when attempting to create clusters, job failures, timeouts during Managed VNet activities, and failures in activity runs for the ExecuteDataFlow activity type. 

Other impacted services included API Management, Azure DevOps, Azure NetApp Files, Azure RedHat OpenShift, and Azure Stream Analytics.  

What went wrong and why? 

The Azure PubSub service is a key component of the networking control plane, acting as an intermediary between resource providers and networking agents on Azure hosts. Resource providers, such as the Network Resource Provider, publish customer configurations during Virtual Machine or networking create, update, or delete operations. Networking agents (subscribers) on the hosts retrieve these configurations to program the hosts networking stack. Additionally, the service functions as a cache, ensuring efficient retrieval of configurations during VM reboots or restarts. This capability is essential for deployments, resource allocation, and traffic management in Azure Virtual Network (VNet) environments. 

Around 22:31 UTC on 08 January, during an operation to enable a feature, a manual recycling of internal PubSub replicas on certain metadata partitions was required. This recycling operation should have been completed in ‘series’ to ensure that data quorum for the partition is maintained. In this case, recycling was executed across the replicas in ‘parallel’, causing the partition to lose quorum. This resulted in the PubSub service to lose the indexing data stored on these partitions.  

Losing the indexing data on three partitions resulted in ~60% of network configurations not being delivered to agents from the control plane. This impacted all service management operations for resources with index data on the affected partitions, including the network resource provisioning for virtual machines in East US 2, Az01. Networking agents would have failed to receive updates on VM routing information for new and existing VMs that underwent any automated recovery. This manifested in a loss of connectivity affecting VMs that customers use, in addition to the VMs that underpin our PaaS service offerings. 

Multi-tenant PaaS services that used VNet integration had a dependency on the partitions for the networking provisioning state. After the index data was lost, the dependency on this data that no longer existed caused impact to these PaaS services to expand beyond the impacted zone. 

How did we respond? 

The issue was detected by monitoring on the PubSub service at 22:44 UTC, with impacted services raising alerts shortly thereafter. The PubSub team identified the trigger as the three partitions with indexing data loss due to the recycling. 

At 23:10 UTC, we started evaluating different options for data recovery. Several workstreams were evaluated including: 

  • Rehydrating the networking configurations to the partitions from the network resource provider.  
  • Restoring from backup to recover the networking configurations. 
  • Recovering data from the unaffected partitions. 

Due to limitations in feasibility for first two options, we started to recover data by rebuilding the mappings form the backend data store. Scanning the data store to extract source mappings is a time intensive process. It involves not only scanning the data, but also making sure that the mappings to the existing resources are still valid. To achieve this, we had to develop validation and mapping transformation scripts. We also throttled the mappings extraction to avoid impacting the backend data store. This scanning process took about 5-6 hours to complete. Once we obtained these mappings, we then had to restore the impacted index partitions.  

As we initiated the build phase, there were unforeseen challenges that we had to work around. Typically, each build takes 3-4 hours. We spent some time looking for ways to expedite this process, as our plan involved applying multiple builds. 

Independently on the builds taking longer than expected, we also observed a high number of failure requests due to the outage itself. As a result, our systems were generating a lot of errors which contributed to high CPU usage. This exacerbated the slowness in the restore process. One of the ways that we overcame this this was to deploy patches to block requests to the impacted partitions which mitigated the high CPU usage.  

Unfortunately, at 3:36 UTC on 10 January 2025, we discovered the first mapping source files that we generated were insufficient as we referenced an incorrect endpoint. Once this was understood, we had to repeat the mapping file generation and restore process. By 19:18 UTC on 10 January, all three partitions were fully recovered. Additional workstreams that were executed while this main workstream was underway included:

  • An immediate remediation step was to direct the Azure control plane service running on all host machines in this region to PubSub endpoints located in other non-impacted availability zones. This restored the host agent’s ability to look up the correct routing information resulting in mitigating VM to VM connectivity issues. 
  • We initiated a workstream to route all new allocations requests to other availability zones in East US 2. However, as mentioned, the services that were using VNet integration, which had a fixed dependency on the impacted zone, would have continued to see failures. 
  • To remediate impact to customers leveraging Private Link with NSGs to communicate with Azure services, we redirected the Private Link mapping to avoid the dependency on the impacted zone.  

By 19:18 UTC on 10 January, all three impacted partitions were fully recovered. Following this, we started working with impacted services to validate their health and remediate any data gaps that we discovered. At 00:30 UTC on the 11 January, we initiated a phased approach to gradually enable Az01 to accept new allocation requests. By 00:44 UTC on 11 January, all services confirmed mitigation. We reintroduced Az01 into rotation fully at 04:30 UTC, at which point we declared full mitigation. The impact window for Azure Services varied between service. 

Below is a timeline of events: 

  • 22:31 UTC on 08 January 2025 – Customer impact begins following the recycling of the PubSub partitions.  
  • 22:44 UTC on 08 January 2025 – The issue is identified as an incorrect networking configuration. 
  • 23:10 UTC on 08 January 2025: We start to evaluate options for the index data recovery to the PubSub service. 
  • 23:43 UTC on 08 January 2025: Targeted customer communications from the earliest reported impacted services started going out via Azure Service Health. 
  • 00:35 UTC on 09 January 2025: We begin scanning the backend data store.  
  • 02:37 UTC on 09 January 2025: Azure Status page updated with the details of a wider networking impact to the region. 
  • 03:05 UTC on 09 January 2025: A workstream to route all new allocation requests to other availability zones in East US 2 is initiated. 
  • 03:45 UTC on 09 January 2025: The workstream for the host agent’s ability to look up the correct routing information was completed. 
  • 04:30 UTC on 09 January 2025: Developing scripts to restore the impacted partitions started. 
  • 05:00 UTC on 09 January 2025: The workstream to route all new allocation requests to other availability zones in East US 2 was completed. 
  • 07:45 UTC on 09 January 2025: Validation and testing of scripts for restoring mapping data to the impacted partitions is completed. 
  • 08:51 UTC on 09 January 2025: Completed scanning backend generated source mapping files. 
  • 09:14 UTC on 09 January 2025: Build process completed for the scripts to restore mappings. 
  • 11:30 UTC on 09 January 2025: We observed high CPU and slow restoration speed. 
  • 11:58 UTC on 09 January 2025: The workstream to mitigate impacted services with Private link and NSG dependencies is initiated. 
  • 13:12 UTC on 09 January 2025: We updated one of the internal PubSub services to stop sending requests which was contributing to the high CPU usage.  
  • 16:10 UTC on 09 January 2025: We completed another update to reduce request volumes to impacted partitions. This helped to improve the restore speed further.  
  • 18:17 UTC on 09 January 2025: The workstream to redirect Private Link mapping to avoid the dependency on the impacted zone is completed. 
  • 19:15 UTC on 09 January 2025: We created another patch to speed up the restore process. The new patch restores the mappings inside the partition rather than calling APIs to add mappings.  
  • 23:40 UTC on 09 January 2025: This patching completed with the new optimization step, and the time for rebuilding one partition was reduced even further. Our overall restore time was reduced by >75%. 
  • 02:10 UTC on 10 January 2025: Restoration of the first two partitions completed.  
  • 03:00 UTC on 10 January 2025: Restoration of the third partition completed. 
  • 03:36 UTC on 10 January 2025: Following the restoration, during our validation steps, we determined that the source mapping files generated were incorrect. As a result, we started generating all new source mapping files. 
  • 05:00 UTC on 10 January 2025: We validated the data of source mappings for first partition. Continued extraction of the rest of the mappings. Meanwhile, we had to clean the previously restored partitions. 
  • 08:30 UTC on 10 January 2025: The correct source mapping file is ready, and we started the partition restore process. 
  • 11:04 UTC on 10 January 2025: The first partition is back online while troubleshooting and mitigating data anomalies.  
  • 19:18 UTC on 10 January 2025 – All three of the impacted partitions are fully recovered and validated – At this stage most customers and services would have seen full mitigation.  
  • 00:30 UTC on 11 January 2025 – We initiated a phased approach to gradually enable the impacted zone for new allocations. 
  • 00:44 UTC on 11 January 2025 - All services confirmed mitigation. 

How are we making incidents like this less likely or less impactful? 

  • We are working on reducing the recovery time for any data loss scenarios by improving the PubSub data recovery mechanism. This will bring our RTO to under 2 hours (Estimated completion: February 2025). 
  • While PubSub production changes already go through our Safe Deployment Practices (SDP), we have added a step with additional layers of scrutiny. These will include, but are not limited to, verifying rollback plans, ensuring error budgets compliance, and performance validations (Completed).  
  • All manual configuration changes will be moved to an automation pipeline which will require elevated privileges. (Estimated completion: February 2025). 
  • Significant impact to the Pubsub service can occur if certain operations are performed incorrectly. We will block these operations from being executed, except by designated experts (Estimated completion: January 2025).  
  • We are re-examining zonal services to ensure that all downstream dependencies are zone resilient. (Estimated completion: October 2025) 
  • During the incident, we recommended that customers invoke a failover. This was an inappropriate communication. We are making corrections to our processes to ensure that our communications inform customers about the state of the impacted services, zones and regions. This, coupled with any other information that we have about the incident, including ETAs, should be taken into consideration as customers evaluate their own business continuity decisions (Estimated completion: January 2025)  

How can customers make incidents like this less impactful? 

  • Consider evaluating the reliability of your applications using guidance from the Azure Well-Architected Framework and its interactive Well-Architected Review:
  • Consider ensuring that the right people in your organization will be notified about any future service issues - by configuring Azure Service Health alerts. These can trigger emails, SMS, push notifications, webhooks, and more:
  • The impact times above represent the full incident duration, so are not specific to any individual customer. Actual impact to service availability varied between customers and resources – for guidance on implementing monitoring to understand granular impact: 

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