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## Hyperscale service tier availability
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The Hyperscale service tier architecture is described in [Distributed functions architecture](https://docs.microsoft.com/en-us/azure/sql-database/sql-database-service-tier-hyperscale#distributed-functions-architecture).
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The Hyperscale service tier architecture is described in [Distributed functions architecture](https://docs.microsoft.com/azure/sql-database/sql-database-service-tier-hyperscale#distributed-functions-architecture).
The availability model in Hyperscale includes four layers:
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- A stateless compute layer that runs the `sqlservr.exe` processes and contains only transient and cached data, such as non-covering RBPEX cache, TempDB, model database, etc. on the attached SSD, and plan cache, buffer pool, and columnstore pool in memory. This stateless layer includes the primary compute replica and optionally a number of secondary compute replicas that can serve as failover targets.
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- A stateless storage layer formed by page servers. This layer is the distributed storage engine for the `sqlservr.exe` processes running on the compute replicas. Each page server contains only transient and cached data, such as covering RBPEX cache on the attached SSD, and data pages cached in memory. Each page server has a paired page server in an active-active configuration to provide load balancing, redundancy, and high availability.
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- A stateful transaction log storage layer formed by the compute node running the Log service process, the transaction log landing zone, and transaction log long term storage. Landing zone and long term storage use Azure Storage, which provides availability and [redundancy](https://docs.microsoft.com/en-us/azure/storage/common/storage-redundancy) for transaction log, ensuring data durability for committed transactions.
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- A stateful data storage layer with the database files (.mdf/.ndf) that are stored in Azure Storage and are updated by page servers. This layer uses data availability and [redundancy](https://docs.microsoft.com/en-us/azure/storage/common/storage-redundancy) features of Azure Storage. It guarantees that every page in a data file will be preserved even if processes in other layers of Hyperscale architecture crash, or if compute nodes fail.
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- A stateful transaction log storage layer formed by the compute node running the Log service process, the transaction log landing zone, and transaction log long term storage. Landing zone and long term storage use Azure Storage, which provides availability and [redundancy](https://docs.microsoft.com/azure/storage/common/storage-redundancy) for transaction log, ensuring data durability for committed transactions.
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- A stateful data storage layer with the database files (.mdf/.ndf) that are stored in Azure Storage and are updated by page servers. This layer uses data availability and [redundancy](https://docs.microsoft.com/azure/storage/common/storage-redundancy) features of Azure Storage. It guarantees that every page in a data file will be preserved even if processes in other layers of Hyperscale architecture crash, or if compute nodes fail.
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Compute nodes in all Hyperscale layers run on Azure Service Fabric, which controls health of each node and performs failovers to available healthy nodes as necessary.
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For more information on high availability in Hyperscale, see [Database High Availability in Hyperscale](https://docs.microsoft.com/en-us/azure/sql-database/sql-database-service-tier-hyperscale#database-high-availability-in-hyperscale).
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For more information on high availability in Hyperscale, see [Database High Availability in Hyperscale](https://docs.microsoft.com/azure/sql-database/sql-database-service-tier-hyperscale#database-high-availability-in-hyperscale).
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