Business continuity in Azure Data Explorer refers to the mechanisms and procedures that enable your business to continue operating in the face of a true disruption. There are some disruptive events that cannot be handled by ADX automatically such as:
This overview describes the capabilities that Azure Data Explorer provides for business continuity and disaster recovery. Learn about options, recommendations, and tutorials for recovering from disruptive events that could cause data loss or cause your database and application to become unavailable.
Users with table admin permissions or higher are allowed to drop tables. If one of those users accidentally drops the table then it is still recoverable using the .undo drop table command. This is successful given the recoverability property has been enabled in the retention policy (data will be recoverable 14 days after its deletion).
In regions that support Availability Zones (AZ) the ADX cluster can be setup to “span” all AZs in the region and use separated physical resources (different builds, power supplies, etc.). Doing so guarantees that the cluster remains available during incidents that involve the loss of a single availability zone in the region (but with degraded capacity of course).
This process is automated and nothing, but the initial setup process needs to be configured.
For more details on enabling availability zones on Azure Data Explorer please read - https://docs.microsoft.com/en-us/azure/data-explorer/create-cluster-database-portal
Azure Data Explorer currently does not support an automatic protection against the outage of an entire Azure region. Theoretically this can happen in case of a natural disaster like an earthquake. If you require a solution for this situation you must create two or more independent clusters, and make sure that the clusters are created in two Azure paired regions.
Once you created multiple ADX clusters you must make sure that:
ADX will make sure that maintenance operations (such as upgrades) are never conducted in parallel to two clusters if they are in Azure paired regions.
Azure Data Explorer currently does not support an automatic protection against the outage of an entire Azure region. Theoretically this can happen in case of a natural disaster like an earthquake. If you require a solution for this situation you must create two or more independent clusters,
This HowTo article presents an example how to create an architecture that takes business continuity into account under those heavy conditions.
In the first step you must create more than one cluster in more than one region in order to protect against regional outages.
Please take a look on how to create clusters. Make sure that at least two of them are created in two Azure paired regions. The drawing is showing three clusters in three different regions. In the rest of the article we are referencing the ADX clusters as replicas.
In order to have the same cluster configuration in every replica you must replicate the management activities.
To ingest data from Azure Event Hubs into each region's Azure Data Explorer cluster, first replicate your Azure Event Hubs setup in each region. Then configure each region's Azure Data Explorer replica to ingest data from its corresponding Event Hubs.
The ingestion via EventHub/IoTHub/storage is very robust. In case a cluster is not available for some time it will catch up on the to be inserted messages or blobs. The underlying technology makes use of checkpointing.
With the current state of the business continuity hardening you distributed your data and management to multiple regions and in case of a temporal outage the underlying technology of Azure Data Explorer will be able to catch up in the individual replicas.
As shown in the diagram your data sources are producing events to the failover configured EventHub and all Azure Data Explorer replicas are consuming from it.
On the other side the data visualization components like PowerBI, Grafana or SDK powered WebApps are querying one of the replicas.
The remaining part of this article sheds a light on the following optimizations
Having the exact Azure Data Explorer setup on all replicas including a 24/7 uptime on all of them is linearly increasing the cost by number of replicas. In order to optimize the cost this section explains a variant of the architecture shown which makes a compromise between time to failover and cost.
The cost optimization has been implemented by introducing passive Azure Data Explorer instances which are only turned on in case of a disaster in the primary region (i.e. region A). Some examples on how to start / stop Azure Data Explorer cluster:
As you can see in the drawing only one cluster is consuming from the EventHub. The primary cluster in Region A is performing a continuous export of all data to a storage account. The secondary replicas are getting access to the data using external tables.
Now the secondary clusters in Region B and C do not need to be turned on 24/7 which reduces the cost significantly. The drawback of this solution is that the performance on the secondary clusters will not be as good as in the primary cluster for most of the cases.
This section should demonstrate how to create an Azure App Service which supports a connection to a single primary and multiple secondary Azure Data Explorer cluster. The following picture is illustrating the setup (intentionally removed the management activities and the data ingest).
Having multiple connections to replicas in the same app service increases the availability of the overall solution (not only regional outages can cause an interruption of the service). Recently we pushed some boilerplate code for an app service to github : https://github.com/Azure/azure-kusto-bcdr-boilerplate. In order to implement a multi-ADX client the AdxBcdrClient class has been created. Each query that is executed using this client will be send first to the primary ADX and in case it fails to the secondaries.
In order to measure the performance and request distribution to primary/secondary cluster custom application insights metrics have been used. Some of the results which have been captured during the test:
The following picture shows that during the test multiple Azure Data Explorer cluster have been used. The reason for this is a simulated outage of primary and secondary clusters to verify that the app service BCDR client is behaving as intended.
The Azure Data Explorer cluster have been distributed across West Europe (2xD14v2 primary), South East Asia and East US (2xD11v2). The slower response time can be explained by the different SKUs and by doing cross planet queries.
One last extension to this architecure could be the dynamic or static routing of the requests using Azure Traffic Manager routing methods. Azure Traffic Manager is a DNS-based traffic load balancer that enables you to distribute the app service traffic optimally to services across global Azure regions, while providing high availability and responsiveness. Alternatively one could use Azure Front Door based routing as well. An excellent comparison between both can be found here.
Adding replicas to an active / active architecture increases the cost linearly. Besides the cost for compute-nodes, storage and markup one needs to take increased networking cost for bandwidth into consideration.
Using the optimized autoscale feature one can configure that the horizontal scaling for the secondary clusters. They should be dimensioned to be able to handle the load of the ingest. Once the primary cluster is not reachable, they will get more traffic and scale out according to the configuration. In my previous example this saved roughly 50% of the cost compared to having the same horizontal and vertical scale on all replicas.
Even if Azure Data Explorer is not offering an out-of-the-box business continuity and disaster recovery solution this article outlined different strategies to mitigate the risk. Depending on the investment, the user can minimize the impact of a regional outage.
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