How does Azure Cosmos DB provide high availability
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Azure Cosmos DB provides high availability in two primary ways. First, Azure Cosmos DB replicates data across regions configured within a Cosmos account. Second, Azure Cosmos DB maintains 4 replicas of data within a region.
Azure Cosmos DB is a globally distributed database service and is a foundational service available in all regions where Azure is available. You can associate any number of Azure regions with your Azure Cosmos account and your data is automatically and transparently replicated. You can add or remove a region to your Azure Cosmos account at any time. Cosmos DB is available in all five distinct Azure cloud environments available to customers:
Azure public cloud, which is available globally.
Azure China 21Vianet is available through a unique partnership between Microsoft and 21Vianet, one of the country’s largest internet providers in China.
Azure Germany provides services under a data trustee model, which ensures that customer data remains in Germany under the control of T-Systems International GmbH, a subsidiary of Deutsche Telekom, acting as the German data trustee.
Azure Government is available in four regions in the United States to US government agencies and their partners.
Azure Government for Department of Defense (DoD) is available in two regions in the United States to the US Department of Defense.
Within a region, Azure Cosmos DB maintains four copies of your data as replicas within physical partitions as shown in the following image:
The data within Azure Cosmos containers is horizontally partitioned.
A partition-set is a collection of multiple replica-sets. Within each region, every partition is protected by a replica-set with all writes replicated and durably committed by a majority of replicas. Replicas are distributed across as many as 10-20 fault domains.
Each partition across all the regions is replicated. Each region contains all the data partitions of an Azure Cosmos container and can serve reads as well as serve writes when multi-region writes is enabled.
If your Azure Cosmos account is distributed across N Azure regions, there will be at least N x 4 copies of all your data. Having an Azure Cosmos account in more than 2 regions improves the availability of your application and provides low latency across the associated regions.
SLAs for availability
Azure Cosmos DB provides comprehensive SLAs that encompass throughput, latency at the 99th percentile, consistency, and high availability. The table below shows the guarantees for high availability provided by Azure Cosmos DB for single and multi-region accounts. For higher write availability, configure your Azure Cosmos account to have multiple write regions.
|Operation type||Single-region||Multi-region (single-region writes)||Multi-region (multi-region writes)|
In practice, the actual write availability for bounded staleness, session, consistent prefix and eventual consistency models is significantly higher than the published SLAs. The actual read availability for all consistency levels is significantly higher than the published SLAs.
High availability with Azure Cosmos DB in the event of regional outages
For the rare cases of regional outage, Azure Cosmos DB makes sure your database is always highly available. The following details capture Azure Cosmos DB behavior during an outage, depending on your Azure Cosmos account configuration:
With Azure Cosmos DB, before a write operation is acknowledged to the client, the data is durably committed by a quorum of replicas within the region that accepts the write operations. For more details, see Consistency levels and throughput
Multi-region accounts configured with multiple-write regions will be highly available for both writes and reads. Regional failovers are detected and handled in the Azure Cosmos DB client. They are also instantaneous and don't require any changes from the application.
Single-region accounts may lose availability following a regional outage. It's always recommended to set up at least two regions (preferably, at least two write regions) with your Azure Cosmos account to ensure high availability at all times.
Multi-region accounts with a single-write region (write region outage)
During a write region outage, the Azure Cosmos account will automatically promote a secondary region to be the new primary write region when enable automatic failover is configured on the Azure Cosmos account. When enabled, the failover will occur to another region in the order of region priority you've specified.
When the previously impacted region is back online, any write data that was not replicated when the region failed, is made available through the conflicts feed. Applications can read the conflicts feed, resolve the conflicts based on the application-specific logic, and write the updated data back to the Azure Cosmos container as appropriate.
Once the previously impacted write region recovers, it becomes automatically available as a read region. You can switch back to the recovered region as the write region. You can switch the regions by using PowerShell, Azure CLI or Azure portal. There is no data or availability loss before, during or after you switch the write region and your application continues to be highly available.
It is strongly recommended that you configure the Azure Cosmos accounts used for production workloads to enable automatic failover. Manual failover requires connectivity between secondary and primary write region to complete a consistency check to ensure there is no data loss during the failover. If the primary region is unavailable, this consistency check cannot complete and the manual failover will not succeed, resulting in loss of write availability for the duration of the regional outage.
Multi-region accounts with a single-write region (read region outage)
During a read region outage, Azure Cosmos accounts using any consistency level or strong consistency with three or more read regions will remain highly available for reads and writes.
Azure Cosmos accounts using strong consistency with three regions (one write, two read) will maintain write availability during a read region outage. For accounts with two regions and automatic failover enabled, the account will stop accepting writes until the region is marked as failed and automatic failover occurs.
The impacted region is automatically disconnected and will be marked offline. The Azure Cosmos DB SDKs will redirect read calls to the next available region in the preferred region list.
If none of the regions in the preferred region list is available, calls automatically fall back to the current write region.
No changes are required in your application code to handle read region outage. When the impacted read region is back online it will automatically sync with the current write region and will be available again to serve read requests.
Subsequent reads are redirected to the recovered region without requiring any changes to your application code. During both failover and rejoining of a previously failed region, read consistency guarantees continue to be honored by Azure Cosmos DB.
Even in a rare and unfortunate event when the Azure region is permanently irrecoverable, there is no data loss if your multi-region Azure Cosmos account is configured with Strong consistency. In the event of a permanently irrecoverable write region, a multi-region Azure Cosmos account configured with bounded-staleness consistency, the potential data loss window is restricted to the staleness window (K or T) where K=100,000 updates or T=5 minutes, which ever happens first. For session, consistent-prefix and eventual consistency levels, the potential data loss window is restricted to a maximum of 15 minutes. For more information on RTO and RPO targets for Azure Cosmos DB, see Consistency levels and data durability
Availability Zone support
In addition to cross region resiliency, Azure Cosmos DB also supports zone redundancy in supported regions when selecting a region to associate with your Azure Cosmos account.
With Availability Zone (AZ) support, Azure Cosmos DB will ensure replicas are placed across multiple zones within a given region to provide high availability and resiliency to zonal failures. Availability Zones provide a 99.995% availability SLA with no changes to latency. In the event of a single zone failure, zone redundancy provides full data durability with RPO=0 and availability with RTO=0. Zone redundancy is a supplemental capability to regional replication. Zone redundancy alone cannot be relied upon to achieve regional resiliency.
Zone redundancy can only be configured when adding a new region to an Azure Cosmos account. For existing regions, zone redundancy can be enabled by removing the region then adding it back with the zone redundancy enabled. For a single region account, this requires adding one additional region to temporarily failover to, then removing and adding the desired region with zone redundancy enabled.
When configuring multi-region writes for your Azure Cosmos account, you can opt into zone redundancy at no extra cost. Otherwise, please see the table below regarding pricing for zone redundancy support. For a list of regions where availability zones is available, see the Availability zones.
The following table summarizes the high availability capability of various account configurations:
|KPI||Single-region without AZs||Single-region with AZs||Multi-region, single-region writes with AZs||Multi-region, multi-region writes with AZs|
|Write availability SLA||99.99%||99.995%||99.995%||99.999%|
|Read availability SLA||99.99%||99.995%||99.995%||99.999%|
|Zone failures – data loss||Data loss||No data loss||No data loss||No data loss|
|Zone failures – availability||Availability loss||No availability loss||No availability loss||No availability loss|
|Regional outage – data loss||Data loss||Data loss||Dependent on consistency level. See Consistency, availability, and performance tradeoffs for more information.||Dependent on consistency level. See Consistency, availability, and performance tradeoffs for more information.|
|Regional outage – availability||Availability loss||Availability loss||No availability loss for read region failure, temporary for write region failure||No availability loss|
|Price (1)||N/A||Provisioned RU/s x 1.25 rate||Provisioned RU/s x 1.25 rate (2)||Multi-region write rate|
1 For Serverless accounts request units (RU) are multiplied by a factor of 1.25.
2 1.25 rate only applied to those regions in which AZ is enabled.
Availability Zones can be enabled via:
Building highly available applications
Review the expected behavior of the Azure Cosmos SDKs during these events and which are the configurations that affect it.
To ensure high write and read availability, configure your Azure Cosmos account to span at least two regions with multiple-write regions. This configuration will provide the highest availability, lowest latency, and best scalability for both reads and writes backed by SLAs. To learn more, see how to configure your Azure Cosmos account with multiple write-regions.
For multi-region Azure Cosmos accounts that are configured with a single-write region, enable automatic-failover by using Azure CLI or Azure portal. After you enable automatic failover, whenever there is a regional disaster, Cosmos DB will automatically failover your account.
Even if your Azure Cosmos account is highly available, your application may not be correctly designed to remain highly available. To test the end-to-end high availability of your application, as a part of your application testing or disaster recovery (DR) drills, temporarily disable automatic-failover for the account, invoke the manual failover by using PowerShell, Azure CLI or Azure portal, then monitor your application's failover. Once complete, you can fail back over to the primary region and restore automatic-failover for the account.
Within a globally distributed database environment, there is a direct relationship between the consistency level and data durability in the presence of a region-wide outage. As you develop your business continuity plan, you need to understand the maximum acceptable time before the application fully recovers after a disruptive event. The time required for an application to fully recover is known as recovery time objective (RTO). You also need to understand the maximum period of recent data updates the application can tolerate losing when recovering after a disruptive event. The time period of updates that you might afford to lose is known as recovery point objective (RPO). To see the RPO and RTO for Azure Cosmos DB, see Consistency levels and data durability
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