Azure SQL Database serverless

• 04/16/2021
• 16 minutes to read

In this article

APPLIES TO: Azure SQL Database

Serverless is a compute tier for single databases in Azure SQL Database that automatically scales compute based on workload demand and bills for the amount of compute used per second. The serverless compute tier also automatically pauses databases during inactive periods when only storage is billed and automatically resumes databases when activity returns.

Serverless compute tier

The serverless compute tier for single databases in Azure SQL Database is parameterized by a compute autoscaling range and an auto-pause delay. The configuration of these parameters shapes the database performance experience and compute cost.

Performance configuration

• The minimum vCores and maximum vCores are configurable parameters that define the range of compute capacity available for the database. Memory and IO limits are proportional to the vCore range specified. 
• The auto-pause delay is a configurable parameter that defines the period of time the database must be inactive before it is automatically paused. The database is automatically resumed when the next login or other activity occurs. Alternatively, automatic pausing can be disabled.

• The cost for a serverless database is the summation of the compute cost and storage cost.
• When compute usage is between the min and max limits configured, the compute cost is based on vCore and memory used.
• When compute usage is below the min limits configured, the compute cost is based on the min vCores and min memory configured.
• When the database is paused, the compute cost is zero and only storage costs are incurred.
• The storage cost is determined in the same way as in the provisioned compute tier.

For more cost details, see Billing.

Scenarios

Serverless is price-performance optimized for single databases with intermittent, unpredictable usage patterns that can afford some delay in compute warm-up after idle usage periods. In contrast, the provisioned compute tier is price-performance optimized for single databases or multiple databases in elastic pools with higher average usage that cannot afford any delay in compute warm-up.

Scenarios well suited for serverless compute

• Single databases with intermittent, unpredictable usage patterns interspersed with periods of inactivity and lower average compute utilization over time.
• Single databases in the provisioned compute tier that are frequently rescaled and customers who prefer to delegate compute rescaling to the service.
• New single databases without usage history where compute sizing is difficult or not possible to estimate prior to deployment in SQL Database.

Scenarios well suited for provisioned compute

• Single databases with more regular, predictable usage patterns and higher average compute utilization over time.
• Databases that cannot tolerate performance trade-offs resulting from more frequent memory trimming or delays in resuming from a paused state.
• Multiple databases with intermittent, unpredictable usage patterns that can be consolidated into elastic pools for better price-performance optimization.

Comparison with provisioned compute tier

The following table summarizes distinctions between the serverless compute tier and the provisioned compute tier:

Purchasing model and service tier

SQL Database serverless is currently only supported in the General Purpose tier on Generation 5 hardware in the vCore purchasing model.

Autoscaling

Scaling responsiveness

In general, serverless databases are run on a machine with sufficient capacity to satisfy resource demand without interruption for any amount of compute requested within limits set by the max vCores value. Occasionally, load balancing automatically occurs if the machine is unable to satisfy resource demand within a few minutes. For example, if the resource demand is 4 vCores, but only 2 vCores are available, then it may take up to a few minutes to load balance before 4 vCores are provided. The database remains online during load balancing except for a brief period at the end of the operation when connections are dropped.

Memory management

Memory for serverless databases is reclaimed more frequently than for provisioned compute databases. This behavior is important to control costs in serverless and can impact performance.

Cache reclamation

Unlike provisioned compute databases, memory from the SQL cache is reclaimed from a serverless database when CPU or active cache utilization is low.

• Active cache utilization is considered low when the total size of the most recently used cache entries falls below a threshold for a period of time.
• When cache reclamation is triggered, the target cache size is reduced incrementally to a fraction of its previous size and reclaiming only continues if usage remains low.
• When cache reclamation occurs, the policy for selecting cache entries to evict is the same selection policy as for provisioned compute databases when memory pressure is high.
• The cache size is never reduced below the min memory limit as defined by min vCores, that can be configured.

In both serverless and provisioned compute databases, cache entries may be evicted if all available memory is used.

Note that when CPU utilization is low, active cache utilization can remain high depending on the usage pattern and prevent memory reclamation. Also, there can be additional delays after user activity stops before memory reclamation occurs due to periodic background processes responding to prior user activity. For example, delete operations and QDS cleanup tasks generate ghost records that are marked for deletion, but are not physically deleted until the ghost cleanup process runs that can involve reading data pages into cache.

Cache hydration

The SQL cache grows as data is fetched from disk in the same way and with the same speed as for provisioned databases. When the database is busy, the cache is allowed to grow unconstrained up to the max memory limit.

Auto-pausing and auto-resuming

Auto-pausing

Auto-pausing is triggered if all of the following conditions are true for the duration of the auto-pause delay:

• Number sessions = 0
• CPU = 0 for user workload running in the user pool

An option is provided to disable auto-pausing if desired.

The following features do not support auto-pausing, but do support auto-scaling. If any of the following features are used, then auto-pausing must be disabled and the database will remain online regardless of the duration of database inactivity:

• Geo-replication (active geo-replication and auto-failover groups).
• Long-term backup retention (LTR).
• The sync database used in SQL data sync. Unlike sync databases, hub and member databases support auto-pausing.
• DNS aliasing
• The job database used in Elastic Jobs (preview).

Auto-pausing is temporarily prevented during the deployment of some service updates which require the database be online. In such cases, auto-pausing becomes allowed again once the service update completes.

Auto-resuming

Auto-resuming is triggered if any of the following conditions are true at any time:

Monitoring, management, or other solutions performing any of the operations listed above will trigger auto-resuming.

Auto-resuming is also triggered during the deployment of some service updates which require the database be online.

Connectivity

If a serverless database is paused, then the first login will resume the database and return an error stating that the database is unavailable with error code 40613. Once the database is resumed, the login must be retried to establish connectivity. Database clients with connection retry logic should not need to be modified.

Latency

The latency to auto-resume and auto-pause a serverless database is generally order of 1 minute to auto-resume and 1-10 minutes to auto-pause.

Customer managed transparent data encryption (BYOK)

If using customer managed transparent data encryption (BYOK) and the serverless database is auto-paused when key deletion or revocation occurs, then the database remains in the auto-paused state. In this case, after the database is next resumed, the database becomes inaccessible within approximately 10 minutes. Once the database becomes inaccessible, the recovery process is the same as for provisioned compute databases. If the serverless database is online when key deletion or revocation occurs, then the database also becomes inaccessible within approximately 10 minutes in the same way as with provisioned compute databases.

Onboarding into serverless compute tier

Creating a new database or moving an existing database into a serverless compute tier follows the same pattern as creating a new database in provisioned compute tier and involves the following two steps.

1. Specify the service objective. The service objective prescribes the service tier, hardware generation, and max vCores. For service objective options, see serverless resource limits

2. Optionally, specify the min vCores and autopause delay to change their default values. The following table shows the available values for these parameters.

   Table 3 Parameter Value choices Default value Min vCores Depends on max vCores configured - see resource limits. 0.5 vCores Autopause delay Minimum: 60 minutes (1 hour)
   Maximum: 10080 minutes (7 days)
   Increments: 10 minutes
   Disable autopause: -1 60 minutes

Create a new database in the serverless compute tier

The following examples create a new database in the serverless compute tier.

Use the Azure portal

See Quickstart: Create a single database in Azure SQL Database using the Azure portal.

Use PowerShell

New-AzSqlDatabase -ResourceGroupName $resourceGroupName -ServerName $serverName -DatabaseName $databaseName `
  -ComputeModel Serverless -Edition GeneralPurpose -ComputeGeneration Gen5 `
  -MinVcore 0.5 -MaxVcore 2 -AutoPauseDelayInMinutes 720

Use the Azure CLI

az sql db create -g $resourceGroupName -s $serverName -n $databaseName `
  -e GeneralPurpose -f Gen5 --min-capacity 0.5 -c 2 --compute-model Serverless --auto-pause-delay 720

Use Transact-SQL (T-SQL)

When using T-SQL, default values are applied for the min vcores and autopause delay.

CREATE DATABASE testdb
( EDITION = 'GeneralPurpose', SERVICE_OBJECTIVE = 'GP_S_Gen5_1' ) ;

For details, see CREATE DATABASE.

Move a database from the provisioned compute tier into the serverless compute tier

The following examples move a database from the provisioned compute tier into the serverless compute tier.

Use PowerShell

Set-AzSqlDatabase -ResourceGroupName $resourceGroupName -ServerName $serverName -DatabaseName $databaseName `
  -Edition GeneralPurpose -ComputeModel Serverless -ComputeGeneration Gen5 `
  -MinVcore 1 -MaxVcore 4 -AutoPauseDelayInMinutes 1440

Use the Azure CLI

az sql db update -g $resourceGroupName -s $serverName -n $databaseName `
  --edition GeneralPurpose --min-capacity 1 --capacity 4 --family Gen5 --compute-model Serverless --auto-pause-delay 1440

Use Transact-SQL (T-SQL)

When using T-SQL, default values are applied for the min vcores and auto-pause delay.

ALTER DATABASE testdb 
MODIFY ( SERVICE_OBJECTIVE = 'GP_S_Gen5_1') ;

For details, see ALTER DATABASE.

Move a database from the serverless compute tier into the provisioned compute tier

A serverless database can be moved into a provisioned compute tier in the same way as moving a provisioned compute database into a serverless compute tier.

Modifying serverless configuration

Use PowerShell

Modifying the maximum or minimum vCores, and autopause delay, is performed by using the Set-AzSqlDatabase command in PowerShell using the MaxVcore, MinVcore, and AutoPauseDelayInMinutes arguments.

Use the Azure CLI

Modifying the maximum or minimum vCores, and autopause delay, is performed by using the az sql db update command in Azure CLI using the capacity, min-capacity, and auto-pause-delay arguments.

Monitoring

Resources used and billed

The resources of a serverless database are encapsulated by app package, SQL instance, and user resource pool entities.

App package

The app package is the outer most resource management boundary for a database, regardless of whether the database is in a serverless or provisioned compute tier. The app package contains the SQL instance and external services like full-text search that all together scope all user and system resources used by a database in SQL Database. The SQL instance generally dominates the overall resource utilization across the app package.

User resource pool

The user resource pool is the inner most resource management boundary for a database, regardless of whether the database is in a serverless or provisioned compute tier. The user resource pool scopes CPU and IO for user workload generated by DDL queries such as CREATE and ALTER and DML queries such as SELECT, INSERT, UPDATE, and DELETE. These queries generally represent the most substantial proportion of utilization within the app package.

Metrics

Metrics for monitoring the resource usage of the app package and user pool of a serverless database are listed in the following table:

Pause and resume status

In the Azure portal, the database status is displayed in the overview pane of the server that lists the databases it contains. The database status is also displayed in the overview pane for the database.

Using the following commands to query the pause and resume status of a database:

Use PowerShell

Get-AzSqlDatabase -ResourceGroupName $resourcegroupname -ServerName $servername -DatabaseName $databasename `
  | Select -ExpandProperty "Status"

Use the Azure CLI

az sql db show --name $databasename --resource-group $resourcegroupname --server $servername --query 'status' -o json

Resource limits

For resource limits, see serverless compute tier.

Billing

The amount of compute billed is the maximum of CPU used and memory used each second. If the amount of CPU used and memory used is less than the minimum amount provisioned for each, then the provisioned amount is billed. In order to compare CPU with memory for billing purposes, memory is normalized into units of vCores by rescaling the amount of memory in GB by 3 GB per vCore.

• Resource billed: CPU and memory
• Amount billed: vCore unit price * max (min vCores, vCores used, min memory GB * 1/3, memory GB used * 1/3)
• Billing frequency: Per second

The vCore unit price is the cost per vCore per second. Refer to the Azure SQL Database pricing page for specific unit prices in a given region.

The amount of compute billed is exposed by the following metric:

• Metric: app_cpu_billed (vCore seconds)
• Definition: max (min vCores, vCores used, min memory GB * 1/3, memory GB used * 1/3)
• Reporting frequency: Per minute

This quantity is calculated each second and aggregated over 1 minute.

Minimum compute bill

If a serverless database is paused, then the compute bill is zero. If a serverless database is not paused, then the minimum compute bill is no less than the amount of vCores based on max (min vCores, min memory GB * 1/3).

Examples:

• Suppose a serverless database is not paused and configured with 8 max vCores and 1 min vCore corresponding to 3.0 GB min memory. Then the minimum compute bill is based on max (1 vCore, 3.0 GB * 1 vCore / 3 GB) = 1 vCore.
• Suppose a serverless database is not paused and configured with 4 max vCores and 0.5 min vCores corresponding to 2.1 GB min memory. Then the minimum compute bill is based on max (0.5 vCores, 2.1 GB * 1 vCore / 3 GB) = 0.7 vCores.

The Azure SQL Database pricing calculator for serverless can be used to determine the min memory configurable based on the number of max and min vCores configured. As a rule, if the min vCores configured is greater than 0.5 vCores, then the minimum compute bill is independent of the min memory configured and based only on the number of min vCores configured.

Example scenario

Consider a serverless database configured with 1 min vCore and 4 max vCores. This corresponds to around 3 GB min memory and 12-GB max memory. Suppose the auto-pause delay is set to 6 hours and the database workload is active during the first 2 hours of a 24-hour period and otherwise inactive.

In this case, the database is billed for compute and storage during the first 8 hours. Even though the database is inactive starting after the second hour, it is still billed for compute in the subsequent 6 hours based on the minimum compute provisioned while the database is online. Only storage is billed during the remainder of the 24-hour period while the database is paused.

More precisely, the compute bill in this example is calculated as follows:

Suppose the compute unit price is $0.000145/vCore/second. Then the compute billed for this 24-hour period is the product of the compute unit price and vCore seconds billed: $0.000145/vCore/second * 50400 vCore seconds ~ $7.31

Azure Hybrid Benefit and reserved capacity

Azure Hybrid Benefit (AHB) and reserved capacity discounts do not apply to the serverless compute tier.

Available regions

The serverless compute tier is available worldwide except the following regions: China East, China North, Germany Central, Germany Northeast, and US Gov Central (Iowa).

Next steps

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