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Reliability in Azure Cognitive Search

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Across Azure, reliability means resiliency and availability if there's a service outage or degradation. In Cognitive Search, reliability can be achieved within a single service or through multiple search services in separate regions.

  • Deploy a single search service and scale up for high availability. You can add multiple replicas to handle higher indexing and query workloads. If your search service supports availability zones, replicas are automatically provisioned in different physical data centers for extra resiliency.

  • Deploy multiple search services across different geographic regions. All search workloads are fully contained within a single service that runs in a single geographic region, but in a multi-service scenario, you have options for synchronizing content so that it's the same across all services. You can also set up a load balancing solution to redistribute requests or fail over if there's a service outage.

For business continuity and recovery from disasters at a regional level, plan on 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. In Cognitive Search, individual replicas are the units for zone assignment. A search service runs within one region; its replicas run in different physical data centers (or zones) within that region.

Availability zones are used when you add 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 available zones in the search service region, the replicas are distributed across 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

  • Service tier must be Standard or higher.
  • Service region must be in a region that has available zones (listed in the following table).
  • Configuration must include multiple replicas: two for read-only query workloads, three for read-write workloads that include indexing.

Availability zones for Cognitive Search are supported in 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

Note

Availability zones don't change the terms of 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 your operational requirements include:

  • Business continuity and disaster recovery (BCDR) requirements (Cognitive Search doesn't provide instant failover if there's an outage).

  • Fast performance for a globally distributed application. If query and indexing requests come from all over the world, users who are closest to the host data center experience faster performance. Creating more 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, and 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 distinct search services in sync:

To configure either option, we recommend using the sample Bicep script in the azure-search-multiple-region repository, modified to your regions and indexing strategies.

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 each index has its own copy of the data), 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.

Fail over or redirect query requests

If you need redundancy at the request level, Azure provides several load balancing options:

  • Azure Traffic Manager, used to route requests to multiple geo-located websites that are then backed by multiple search services.
  • Application Gateway, used to load balance between servers in a region at the application layer.
  • Azure Front Door, used to optimize global routing of web traffic and provide global failover.

Some points to keep in mind when evaluating load balancing options:

  • Search is a backend service that accepts query and indexing requests from a client.

  • Requests from the client to a search service must be authenticated. For access to search operations, the caller must have role-based permissions or provide an API key on the request.

  • Service endpoints are reached through a public internet connection by default. If you set up a private endpoint for client connections that originate from within a virtual network, use Application Gateway.

  • Cognitive Search accepts requests addressed to the <your-search-service-name>.search.windows.net endpoint. If you reach the same endpoint using a different DNS name in the host header, such as a CNAME, the request is rejected.

Cognitive Search provides a multi-region deployment sample that uses Azure Traffic Manager for request redirection if the primary endpoint fails. This solution is useful when you route to a search-enabled client that only calls a search service in the same region.

Azure Traffic Manager is primarily used for routing network traffic across different endpoints based on specific routing methods (such as priority, performance, or geographic location). It acts at the DNS level to direct incoming requests to the appropriate endpoint. If an endpoint that Traffic Manager is servicing begins refusing requests, traffic is routed to another endpoint.

Traffic Manager doesn't provide an endpoint for a direct connection to Cognitive Search, which means you can't put a search service directly behind Traffic Manager. Instead, the assumption is that requests flow to Traffic Manager, then to a search-enabled web client, and finally to a search service on the backend. The client and service are located in the same region. If one search service goes down, the search client starts failing, and Traffic Manager redirects to the remaining client.

Search apps connecting through Azure Traffic Manager

About data residency in a multi-region deployment

When you deploy multiple search services in various geographic regions, your content is stored in the region you chose for each search service.

Azure Cognitive Search won't store data outside of your specified region without your authorization. Authorization is implicit when you use features that write to an Azure Storage resource: enrichment cache, debug session, knowledge store. In all cases, the storage account is one that you provide, in the region of your choice.

Note

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.

About service outages and catastrophic events

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, remember 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

A business continuity strategy for the data layer usually includes a restore-from-backup step. 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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