When you create a function app in Azure, you must choose a hosting plan for your app. There are three basic hosting plans available for Azure Functions: Consumption plan, Premium plan, and Dedicated (App Service) plan. All hosting plans are generally available (GA) on both Linux and Windows virtual machines.

The hosting plan you choose dictates the following behaviors:

• How your function app is scaled.
• The resources available to each function app instance.
• Support for advanced features, such as Azure Virtual Network connectivity.

With equal amounts of memory in both slots, memory can be interleaved so that successive chunks of memory alternate slots. That way, memory access gets distributed to both slots almost perfectly evenly, allowing their bandwidth to combine. With uneven amounts, memory cannot be interleaved and has to be mapped first to one stick and then the other. Most memory sticks sold today support dual-channel at the very least, which means there are two lanes (buses) between one memory slot and the CPU’s memory controller on the motherboard. To find the amount of memory per slot, divide the maximum amount of memory that the computer can hold by the number of memory slots on the motherboard. Usually, this is the maximum amount for each memory socket. For example, if the computer is capable of holding a maximum of 4096 MB (4 GB) of memory and the motherboard has four memory slots.

Both Consumption and Premium plans automatically add compute power when your code is running. Your app is scaled out when needed to handle load, and scaled in when code stops running. For the Consumption plan, you also don't have to pay for idle VMs or reserve capacity in advance.

Premium plan provides additional features, such as premium compute instances, the ability to keep instances warm indefinitely, and VNet connectivity.

App Service plan allows you to take advantage of dedicated infrastructure, which you manage. Your function app doesn't scale based on events, which means it never scales in to zero. (Requires that Always on is enabled.)

For a detailed comparison between the various hosting plans (including Kubernetes-based hosting), see the Hosting plans comparison section.

Consumption plan

When you're using the Consumption plan, instances of the Azure Functions host are dynamically added and removed based on the number of incoming events. This serverless plan scales automatically, and you're charged for compute resources only when your functions are running. On a Consumption plan, a function execution times out after a configurable period of time.

Billing is based on number of executions, execution time, and memory used. Usage is aggregated across all functions within a function app. For more information, see the Azure Functions pricing page.

Memory Slot Function

The Consumption plan is the default hosting plan and offers the following benefits:

• Pay only when your functions are running
• Scale out automatically, even during periods of high load

Function apps in the same region can be assigned to the same Consumption plan. There's no downside or impact to having multiple apps running in the same Consumption plan. Assigning multiple apps to the same Consumption plan has no impact on resilience, scalability, or reliability of each app.

To learn more about how to estimate costs when running in a Consumption plan, see Understanding Consumption plan costs.

Premium plan

When you're using the Premium plan, instances of the Azure Functions host are added and removed based on the number of incoming events just like the Consumption plan. Premium plan supports the following features:

• Perpetually warm instances to avoid any cold start
• VNet connectivity
• Unlimited execution duration (60 minutes guaranteed)
• Premium instance sizes (one core, two core, and four core instances)
• More predictable pricing
• High-density app allocation for plans with multiple function apps

To learn how you can create a function app in a Premium plan, see Azure Functions Premium plan.

Instead of billing per execution and memory consumed, billing for the Premium plan is based on the number of core seconds and memory allocated across instances. There is no execution charge with the Premium plan. At least one instance must be allocated at all times per plan. This results in a minimum monthly cost per active plan, regardless if the function is active or idle. Keep in mind that all function apps in a Premium plan share allocated instances.

Consider the Azure Functions Premium plan in the following situations:

• Your function apps run continuously, or nearly continuously.
• You have a high number of small executions and have a high execution bill but low GB second bill in the Consumption plan.
• You need more CPU or memory options than what is provided by the Consumption plan.
• Your code needs to run longer than the maximum execution time allowed on the Consumption plan.
• You require features that are only available on a Premium plan, such as virtual network connectivity.

Dedicated (App Service) plan

Your function apps can also run on the same dedicated VMs as other App Service apps (Basic, Standard, Premium, and Isolated SKUs).

Consider an App Service plan in the following situations:

• You have existing, underutilized VMs that are already running other App Service instances.
• You want to provide a custom image on which to run your functions.

You pay the same for function apps in an App Service Plan as you would for other App Service resources, like web apps. For details about how the App Service plan works, see the Azure App Service plans in-depth overview.

Using an App Service plan, you can manually scale out by adding more VM instances. You can also enable autoscale, though autoscale will be slower than the elastic scale of the Premium plan. For more information, see Scale instance count manually or automatically. You can also scale up by choosing a different App Service plan. For more information, see Scale up an app in Azure.

When running JavaScript functions on an App Service plan, you should choose a plan that has fewer vCPUs. For more information, see Choose single-core App Service plans.

Running in an App Service Environment (ASE) lets you fully isolate your functions and take advantage of higher number of instances than an App Service Plan.

Always On

If you run on an App Service plan, you should enable the Always on setting so that your function app runs correctly. On an App Service plan, the functions runtime goes idle after a few minutes of inactivity, so only HTTP triggers will 'wake up' your functions. Always on is available only on an App Service plan. On a Consumption plan, the platform activates function apps automatically.

Function app timeout duration

The timeout duration of a function app is defined by the functionTimeout property in the host.json project file. The following table shows the default and maximum values in minutes for both plans and the different runtime versions:

Note

Regardless of the function app timeout setting, 230 seconds is the maximum amount of time that an HTTP triggered function can take to respond to a request. This is because of the default idle timeout of Azure Load Balancer. For longer processing times, consider using the Durable Functions async pattern or defer the actual work and return an immediate response.

Even with Always On enabled, the execution timeout for individual functions is controlled by the functionTimeout setting in the host.json project file.

Determine the hosting plan of an existing application

To determine the hosting plan used by your function app, see App Service plan in the Overview tab for the function app in the Azure portal. To see the pricing tier, select the name of the App Service Plan, and then select Properties from the left pane.

You can also use the Azure CLI to determine the plan, as follows:

When the output from this command is dynamic, your function app is in the Consumption plan. When the output from this command is ElasticPremium, your function app is in the Premium plan. All other values indicate different tiers of an App Service plan.

Storage account requirements

On any plan, a function app requires a general Azure Storage account, which supports Azure Blob, Queue, Files, and Table storage. This is because Azure Functions relies on Azure Storage for operations such as managing triggers and logging function executions, but some storage accounts don't support queues and tables. These accounts, which include blob-only storage accounts (including premium storage) and general-purpose storage accounts with zone-redundant storage replication, are filtered-out from your existing Storage Account selections when you create a function app.

The same storage account used by your function app can also be used by your triggers and bindings to store your application data. However, for storage-intensive operations, you should use a separate storage account.

It's possible for multiple function apps to share the same storage account without any issues. (A good example of this is when you develop multiple apps in your local environment using the Azure Storage Emulator, which acts like one storage account.)

To learn more about storage account types, see Introducing the Azure Storage services.

In Region Data Residency

When necessary for all customer data to remain within a single region, the storage account associated with the function app must be one with in region redundancy. An in-region redundant storage account would also need to be used with Azure Durable Functions for Durable Functions.

Other platform-managed customer data will only be stored within the region when hosting in an Internal Load Balancer App Service Environment (or ILB ASE). Details can be found in ASE zone redundancy.

How the Consumption and Premium plans work

In the Consumption and Premium plans, the Azure Functions infrastructure scales CPU and memory resources by adding additional instances of the Functions host, based on the number of events that its functions are triggered on. Each instance of the Functions host in the Consumption plan is limited to 1.5 GB of memory and one CPU. An instance of the host is the entire function app, meaning all functions within a function app share resource within an instance and scale at the same time. Function apps that share the same Consumption plan are scaled independently. In the Premium plan, your plan size will determine the available memory and CPU for all apps in that plan on that instance.

Function code files are stored on Azure Files shares on the function's main storage account. When you delete the main storage account of the function app, the function code files are deleted and cannot be recovered.

Runtime scaling

Azure Functions uses a component called the scale controller to monitor the rate of events and determine whether to scale out or scale in. The scale controller uses heuristics for each trigger type. For example, when you're using an Azure Queue storage trigger, it scales based on the queue length and the age of the oldest queue message.

Function Of Memory Slot

The unit of scale for Azure Functions is the function app. When the function app is scaled out, additional resources are allocated to run multiple instances of the Azure Functions host. Conversely, as compute demand is reduced, the scale controller removes function host instances. The number of instances is eventually scaled in to zero when no functions are running within a function app.

Cold Start

After your function app has been idle for a number of minutes, the platform may scale the number of instances on which your app runs down to zero. The next request has the added latency of scaling from zero to one. This latency is referred to as a cold start. The number of dependencies that must be loaded by your function app can impact the cold start time. Cold start is more of an issue for synchronous operations, such as HTTP triggers that must return a response. If cold starts are impacting your functions, consider running in a Premium plan or in a Dedicated plan with Always on enabled.

Understanding scaling behaviors

Ddr Memory Slot Function

Scaling can vary on a number of factors, and scale differently based on the trigger and language selected. There are a few intricacies of scaling behaviors to be aware of:

• A single function app only scales out to a maximum of 200 instances. A single instance may process more than one message or request at a time though, so there isn't a set limit on number of concurrent executions. You can specify a lower maximum to throttle scale as required.
• For HTTP triggers, new instances are allocated, at most, once per second.
• For non-HTTP triggers, new instances are allocated, at most, once every 30 seconds. Scaling is faster when running in a Premium plan.
• For Service Bus triggers, use Manage rights on resources for the most efficient scaling. With Listen rights, scaling isn't as accurate because the queue length can't be used to inform scaling decisions. To learn more about setting rights in Service Bus access policies, see Shared Access Authorization Policy.
• For Event Hub triggers, see the scaling guidance in the reference article.

Limit scale out

You may wish to restrict the number of instances an app scales out to. This is most common for cases where a downstream component like a database has limited throughput. By default, consumption plan functions will scale out to as many as 200 instances, and premium plan functions will scale out to as many as 100 instances. You can specify a lower maximum for a specific app by modifying the functionAppScaleLimit value. The functionAppScaleLimit can be set to 0 or null for unrestricted, or a valid value between 1 and the app maximum.

Best practices and patterns for scalable apps

There are many aspects of a function app that will impact how well it will scale, including host configuration, runtime footprint, and resource efficiency. For more information, see the scalability section of the performance considerations article. You should also be aware of how connections behave as your function app scales. For more information, see How to manage connections in Azure Functions.

For more information on scaling in Python and Node.js, see Azure Functions Python developer guide - Scaling and concurrency and Azure Functions Node.js developer guide - Scaling and concurrency.

Billing model

Billing for the different plans is described in detail on the Azure Functions pricing page. Usage is aggregated at the function app level and counts only the time that function code is executed. The following are units for billing:

• Resource consumption in gigabyte-seconds (GB-s). Computed as a combination of memory size and execution time for all functions within a function app.
• Executions. Counted each time a function is executed in response to an event trigger.

Useful queries and information on how to understand your consumption bill can be found on the billing FAQ.

Hosting plans comparison

The following comparison table shows all important aspects to help the decision of Azure Functions App hosting plan choice:

Plan summary

¹ For specific limits for the various App Service plan options, see the App Service plan limits.

Operating system/runtime

¹Linux is the only supported operating system for the Python runtime stack.
²Windows is the only supported operating system for the PowerShell runtime stack.
³Linux is the only supported operating system for Docker containers.⁴ For specific limits for the various App Service plan options, see the App Service plan limits.

Scale

¹ For specific limits for the various App Service plan options, see the App Service plan limits.

Cold start behavior

¹ For specific limits for the various App Service plan options, see the App Service plan limits.

Service limits

¹ By default, the timeout for the Functions 1.x runtime in an App Service plan is unbounded.
² Requires the App Service plan be set to Always On. Pay at standard rates.
³ These limits are set in the host.
⁴ The actual number of function apps that you can host depends on the activity of the apps, the size of the machine instances, and the corresponding resource utilization.
⁵ The storage limit is the total content size in temporary storage across all apps in the same App Service plan. Consumption plan uses Azure Files for temporary storage.
⁶ When your function app is hosted in a Consumption plan, only the CNAME option is supported. For function apps in a Premium plan or an App Service plan, you can map a custom domain using either a CNAME or an A record.
⁷ Guaranteed for up to 60 minutes.
⁸ Workers are roles that host customer apps. Workers are available in three fixed sizes: One vCPU/3.5 GB RAM; Two vCPU/7 GB RAM; Four vCPU/14 GB RAM.

Networking features

Billing

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¹ For specific limits for the various App Service plan options, see the App Service plan limits.

Memory Slot Function

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