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Reliability in Azure Cognitive Search

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Across Azure, reliability means maintaining resiliency and availability if there's a service outage or degradation. In Cognitive Search, reliability is achieved when you:

  • Configure a service to use multiple replicas, paired with availability zone support.

  • Deploy multiple services across different geographic regions.

All search workloads are fully contained within a single service that runs in a single geographic region. On a service, you can configure multiple replicas that automatically run in different availability zones. This capability is how you achieve high availability.

For business continuity and recovery from disasters at a regional level, you should develop a strategy that includes a cross-regional topology, consisting of multiple search services having identical configuration and content. Your custom script or code provides the "fail over" mechanism to an alternate search service if one suddenly becomes unavailable.

High availability

In Cognitive Search, replicas are copies of your index. A search service is installed with at least one replica, and can have up to 12 replicas. Adding replicas allows Azure Cognitive Search to do machine reboots and maintenance against one replica, while query execution continues on other replicas.

For each individual search service, Microsoft guarantees at least 99.9% availability for configurations that meet these criteria:

  • Two replicas for high availability of read-only workloads (queries)

  • Three or more replicas for high availability of read-write workloads (queries and indexing)

No SLA is provided for the Free tier. For more information, see SLA for Azure Cognitive Search.

Availability zone support

Availability zones are an Azure platform capability that divides a region's data centers into distinct physical location groups to provide high-availability, within the same region. If you use availability zones for Cognitive Search, individual replicas are the units for zone assignment. A search service runs within one region; its replicas run in different zones.

You can utilize availability zones with Azure Cognitive Search by adding two or more replicas to your search service. Each replica is placed in a different availability zone within the region. If you have more replicas than availability zones, the replicas are distributed across availability zones as evenly as possible. There's no specific action on your part, except to create a search service in a region that provides availability zones, and then to configure the service to use multiple replicas.

Prerequisites

As noted, you must have multiple replicas for high availability: two for read-only query workloads, three for read-write workloads that include indexing.

Your service must be deployed in a region that supports availability zones. Azure Cognitive Search currently supports availability zones for Standard tier or higher, in one of the following regions:

Region Roll out
Australia East  January 30, 2021 or later
Brazil South May 2, 2021 or later
Canada Central  January 30, 2021 or later
Central India January 20, 2022 or later
Central US  December 4, 2020 or later
China North 3 September 7, 2022 or later
East Asia January 13, 2022 or later
East US January 27, 2021 or later
East US 2  January 30, 2021 or later
France Central October 23, 2020 or later
Germany West Central May 3, 2021, or later
Japan East  January 30, 2021 or later
Korea Central January 20, 2022 or later
North Europe  January 28, 2021 or later
Norway East January 20, 2022 or later
Qatar Central August 25, 2022 or later
South Africa North September 7, 2022 or later
South Central US April 30, 2021 or later
South East Asia  January 31, 2021 or later
Sweden Central January 21, 2022 or later
Switzerland North September 7, 2022 or later
UAE North September 9, 2022 or later
UK South  January 30, 2021 or later
US Gov Virginia April 30, 2021 or later
West Europe  January 29, 2021 or later
West US 2  January 30, 2021 or later
West US 3  June 02, 2021 or later

Availability Zones don't impact the Azure Cognitive Search Service Level Agreement. You still need three or more replicas for query high availability.

Multiple services in separate geographic regions

Service redundancy is necessary if operational requirements include:

  • Business continuity and disaster recovery (BCDR) (Cognitive Search doesn't provide instant failover in the event of an outage).

  • Global availability. If query and indexing requests come from all over the world, users who are closest to the host data center will have faster performance. Creating additional services in regions with close proximity to these users can equalize performance for all users.

If you need two or more search services, creating them in different regions can meet application requirements for continuity and recovery, as well as faster response times for a global user base.

Azure Cognitive Search doesn't provide an automated method of replicating search indexes across geographic regions, but there are some techniques that can make this process simple to implement and manage. These techniques are outlined in the next few sections.

The goal of a geo-distributed set of search services is to have two or more indexes available in two or more regions, where a user is routed to the Azure Cognitive Search service that provides the lowest latency:

Cross-tab of services by region

You can implement this architecture by creating multiple services and designing a strategy for data synchronization. Optionally, you can include a resource like Azure Traffic Manager for routing requests.

Tip

For help in deploying multiple search services across multiple regions, see this Bicep sample on GitHub that deploys a fully configured, multi-regional search solution. The sample gives you two options for index synchronization, and request redirection using Traffic Manager.

Synchronize data across multiple services

There are two options for keeping two or more distributed search services in sync:

Option 1: Use indexers for updating content on multiple services

If you're already using indexer on one service, you can configure a second indexer on a second service to use the same data source object, pulling data from the same location. Each service in each region has its own indexer and a target index (your search index isn't shared, which means data is duplicated), but each indexer references the same data source.

Here's a high-level visual of what that architecture would look like.

Single data source with distributed indexer and service combinations

Option 2: Use REST APIs for pushing content updates on multiple services

If you're using the Azure Cognitive Search REST API to push content to your search index, you can keep your various search services in sync by pushing changes to all search services whenever an update is required. In your code, make sure to handle cases where an update to one search service fails but succeeds for other search services.

Use Azure Traffic Manager to coordinate requests

Azure Traffic Manager allows you to route requests to multiple geo-located websites that are then backed by multiple search services. One advantage of the Traffic Manager is that it can probe Azure Cognitive Search to confirm availability and route users to alternate search services if a service is down. In addition, if you're routing search requests through Azure Web Sites, Azure Traffic Manager can help you load balance cases where the web site is up, but search isn't. Here's an example of an architecture that uses Traffic Manager.

Cross-tab of services by region, with central Traffic Manager

Data residency

When you deploy multiple search services in different geographic regions, your content is stored in your chosen region.

Azure Cognitive Search won't store data outside of your specified region without your authorization. Specifically, the following features write to an Azure Storage resource: enrichment cache, debug session, knowledge store. The storage account is one that you provide, and it could be in any region.

If both the storage account and the search service are in the same region, network traffic between search and storage uses a private IP address and occurs over the Microsoft backbone network. Because private IP addresses are used, you can't configure IP firewalls or a private endpoint for network security. Instead, use the trusted service exception as an alternative when both services are in the same region.

Disaster recovery and service outages

As stated in the Service Level Agreement (SLA), Microsoft guarantees a high level of availability for index query requests when an Azure Cognitive Search service instance is configured with two or more replicas, and index update requests when an Azure Cognitive Search service instance is configured with three or more replicas. However, there's no built-in mechanism for disaster recovery. If continuous service is required in the event of a catastrophic failure outside of Microsoft’s control, we recommend provisioning a second service in a different region and implementing a geo-replication strategy to ensure indexes are fully redundant across all services.

Customers who use indexers to populate and refresh indexes can handle disaster recovery through geo-specific indexers that retrieve data from the same data source. Two services in different regions, each running an indexer, could index the same data source to achieve geo-redundancy. If you're indexing from data sources that are also geo-redundant, be aware that Azure Cognitive Search indexers can only perform incremental indexing (merging updates from new, modified, or deleted documents) from primary replicas. In a failover event, be sure to redirect the indexer to the new primary replica.

If you don't use indexers, you would use your application code to push objects and data to different search services in parallel. For more information, see Keep data synchronized across multiple services.

Back up and restore alternatives

Because Azure Cognitive Search isn't a primary data storage solution, Microsoft doesn't provide a formal mechanism for self-service backup and restore. However, you can use the index-backup-restore sample code in this Azure Cognitive Search .NET sample repo to back up your index definition and snapshot to a series of JSON files, and then use these files to restore the index, if needed. This tool can also move indexes between service tiers.

Otherwise, your application code used for creating and populating an index is the de facto restore option if you delete an index by mistake. To rebuild an index, you would delete it (assuming it exists), recreate the index in the service, and reload by retrieving data from your primary data store.

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