Improve the performance and reliability of Azure Functions

This article provides guidance to improve the performance and reliability of your serverless function apps. For a more general set of Azure Functions best practices, see Azure Functions best practices.

The following are best practices in how you build and architect your serverless solutions using Azure Functions.

Avoid long running functions

Large, long-running functions can cause unexpected timeout issues. To learn more about the timeouts for a given hosting plan, see function app timeout duration.

A function can become large because of many Node.js dependencies. Importing dependencies can also cause increased load times that result in unexpected timeouts. Dependencies are loaded both explicitly and implicitly. A single module loaded by your code may load its own additional modules.

Whenever possible, refactor large functions into smaller function sets that work together and return responses fast. For example, a webhook or HTTP trigger function might require an acknowledgment response within a certain time limit; it's common for webhooks to require an immediate response. You can pass the HTTP trigger payload into a queue to be processed by a queue trigger function. This approach lets you defer the actual work and return an immediate response.

Make sure background tasks complete

When your function starts any tasks, callbacks, threads, processes, they must complete before your function code returns. Because Functions doesn't track these background threads, site shutdown can occur regardless of background thread status, which can cause unintended behavior in your functions.

For example, if a function starts a background task and returns a successful response before the task completes, the Functions runtime considers the execution as having completed successfully, regardless of the result of the background task. If this background task is performing essential work, it may be preempted by site shutdown, leaving that work in an unknown state.

Cross function communication

Durable Functions and Azure Logic Apps are built to manage state transitions and communication between multiple functions.

If not using Durable Functions or Logic Apps to integrate with multiple functions, it's best to use storage queues for cross-function communication. The main reason is that storage queues are cheaper and much easier to provision than other storage options.

Individual messages in a storage queue are limited in size to 64 KB. If you need to pass larger messages between functions, an Azure Service Bus queue could be used to support message sizes up to 256 KB in the Standard tier, and up to 100 MB in the Premium tier.

Service Bus topics are useful if you require message filtering before processing.

Event hubs are useful to support high volume communications.

Write functions to be stateless

Functions should be stateless and idempotent if possible. Associate any required state information with your data. For example, an order being processed would likely have an associated state member. A function could process an order based on that state while the function itself remains stateless.

Idempotent functions are especially recommended with timer triggers. For example, if you have something that absolutely must run once a day, write it so it can run anytime during the day with the same results. The function can exit when there's no work for a particular day. Also if a previous run failed to complete, the next run should pick up where it left off. This is particularly important for message-based bindings that retry on failure. For more information, see Designing Azure Functions for identical input.

Write defensive functions

Assume your function could encounter an exception at any time. Design your functions with the ability to continue from a previous fail point during the next execution. Consider a scenario that requires the following actions:

1. Query for 10,000 rows in a database.
2. Create a queue message for each of those rows to process further down the line.

Depending on how complex your system is, you may have: involved downstream services behaving badly, networking outages, or quota limits reached, etc. All of these can affect your function at any time. You need to design your functions to be prepared for it.

How does your code react if a failure occurs after inserting 5,000 of those items into a queue for processing? Track items in a set that you’ve completed. Otherwise, you might insert them again next time. This double-insertion can have a serious impact on your work flow, so make your functions idempotent.

If a queue item was already processed, allow your function to be a no-op.

Take advantage of defensive measures already provided for components you use in the Azure Functions platform. For example, see Handling poison queue messages in the documentation for Azure Storage Queue triggers and bindings.

For HTTP based functions consider API versioning strategies with Azure API Management. For example, if you have to update your HTTP based function app, deploy the new update to a separate function app and use API Management revisions or versions to direct clients to the new version or revision. Once all clients are using the version or revision and no more executions are left on the previous function app, you can deprovision the previous function app.

Function organization best practices

As part of your solution, you may develop and publish multiple functions. These functions are often combined into a single function app, but they can also run in separate function apps. In Premium and dedicated (App Service) hosting plans, multiple function apps can also share the same resources by running in the same plan. How you group your functions and function apps can impact the performance, scaling, configuration, deployment, and security of your overall solution. There aren't rules that apply to every scenario, so consider the information in this section when planning and developing your functions.

Organize functions for performance and scaling

Each function that you create has a memory footprint. While this footprint is usually small, having too many functions within a function app can lead to slower startup of your app on new instances. It also means that the overall memory usage of your function app might be higher. It's hard to say how many functions should be in a single app, which depends on your particular workload. However, if your function stores a lot of data in memory, consider having fewer functions in a single app.

If you run multiple function apps in a single Premium plan or dedicated (App Service) plan, these apps are all sharing the same resources allocated to the plan. If you have one function app that has a much higher memory requirement than the others, it uses a disproportionate amount of memory resources on each instance to which the app is deployed. Because this could leave less memory available for the other apps on each instance, you might want to run a high-memory-using function app like this in its own separate hosting plan.

Consider whether you want to group functions with different load profiles. For example, if you have a function that processes many thousands of queue messages, and another that is only called occasionally but has high memory requirements, you might want to deploy them in separate function apps so they get their own sets of resources and they scale independently of each other.

Organize functions for configuration and deployment

Function apps have a host.json file, which is used to configure advanced behavior of function triggers and the Azure Functions runtime. Changes to the host.json file apply to all functions within the app. If you have some functions that need custom configurations, consider moving them into their own function app.

All functions in your local project are deployed together as a set of files to your function app in Azure. You might need to deploy individual functions separately or use features like deployment slots for some functions and not others. In such cases, you should deploy these functions (in separate code projects) to different function apps.

Organize functions by privilege

Connection strings and other credentials stored in application settings gives all of the functions in the function app the same set of permissions in the associated resource. Consider minimizing the number of functions with access to specific credentials by moving functions that don't use those credentials to a separate function app. You can always use techniques such as function chaining to pass data between functions in different function apps.

Scalability best practices

There are a number of factors that impact how instances of your function app scale. The details are provided in the documentation for function scaling. The following are some best practices to ensure optimal scalability of a function app.

Share and manage connections

Reuse connections to external resources whenever possible. See how to manage connections in Azure Functions.

Avoid sharing storage accounts

When you create a function app, you must associate it with a storage account. The storage account connection is maintained in the AzureWebJobsStorage application setting.

To maximize performance, use a separate storage account for each function app. This is particularly important when you have Durable Functions or Event Hub triggered functions, which both generate a high volume of storage transactions. When your application logic interacts with Azure Storage, either directly (using the Storage SDK) or through one of the storage bindings, you should use a dedicated storage account. For example, if you have an Event Hub-triggered function writing some data to blob storage, use two storage accounts—one for the function app and another for the blobs being stored by the function.

Don't mix test and production code in the same function app

Functions within a function app share resources. For example, memory is shared. If you're using a function app in production, don't add test-related functions and resources to it. It can cause unexpected overhead during production code execution.

Be careful what you load in your production function apps. Memory is averaged across each function in the app.

If you have a shared assembly referenced in multiple .NET functions, put it in a common shared folder. Otherwise, you could accidentally deploy multiple versions of the same binary that behave differently between functions.

Don't use verbose logging in production code, which has a negative performance impact.

Use async code but avoid blocking calls

Asynchronous programming is a recommended best practice, especially when blocking I/O operations are involved.

In C#, always avoid referencing the Result property or calling Wait method on a Task instance. This approach can lead to thread exhaustion.

Use multiple worker processes

By default, any host instance for Functions uses a single worker process. To improve performance, especially with single-threaded runtimes like Python, use the FUNCTIONS_WORKER_PROCESS_COUNT to increase the number of worker processes per host (up to 10). Azure Functions then tries to evenly distribute simultaneous function invocations across these workers.

The FUNCTIONS_WORKER_PROCESS_COUNT applies to each host that Functions creates when scaling out your application to meet demand.

Receive messages in batch whenever possible

Some triggers like Event Hub enable receiving a batch of messages on a single invocation. Batching messages has much better performance. You can configure the max batch size in the host.json file as detailed in the host.json reference documentation

For C# functions, you can change the type to a strongly-typed array. For example, instead of EventData sensorEvent the method signature could be EventData[] sensorEvent. For other languages, you'll need to explicitly set the cardinality property in your function.json to many in order to enable batching as shown here.

Configure host behaviors to better handle concurrency

The host.json file in the function app allows for configuration of host runtime and trigger behaviors. In addition to batching behaviors, you can manage concurrency for a number of triggers. Often adjusting the values in these options can help each instance scale appropriately for the demands of the invoked functions.

Settings in the host.json file apply across all functions within the app, within a single instance of the function. For example, if you had a function app with two HTTP functions and maxConcurrentRequests requests set to 25, a request to either HTTP trigger would count towards the shared 25 concurrent requests. When that function app is scaled to 10 instances, the ten functions effectively allow 250 concurrent requests (10 instances * 25 concurrent requests per instance).

Other host configuration options are found in the host.json configuration article.

Next steps

For more information, see the following resources:

• How to manage connections in Azure Functions
• Azure App Service best practices