Concurrency in Azure Functions

This article describes the concurrency behaviors of event-driven triggers in Azure Functions. It also describes a new dynamic model for optimizing concurrency behaviors.

Note

The dynamic concurrency model is currently in preview. Support for dynamic concurrency is limited to specific binding extensions, which are also in preview.

The hosting model for Functions allows multiple function invocations to run concurrently on a single compute instance. For example, consider a case where you have three different functions in your function app, which is scaled out and running on multiple instances. In this scenario, each function processes invocations on each VM instance on which your function app is running. The function invocations on a single instance share the same VM compute resources, such as memory, CPU, and connections. When your app is hosted in a dynamic plan (Consumption or Premium), the platform scales the number of function app instances up or down based on the number of incoming events. To learn more, see Event Driven Scaling). When you host your functions in a Dedicated (App Service) plan, you manually configure your instances or set up an autoscale scheme.

Because multiple function invocations can run on each instance concurrently, each function needs to have a way to throttle how many concurrent invocations it's processing at any given time.

Static concurrency

Many of the triggers support a host-level static configuration model, which is used to specify per-instance concurrency for that trigger type. For example, the Service Bus trigger provides both a MaxConcurrentCalls and a MaxConcurrentSessions setting in the host.json file. These settings together control the maximum number of messages each function processes concurrently on each instance. Other trigger types have built-in mechanisms for load-balancing invocations across instances. For example, Event Hubs and Azure Cosmos DB both use a partition-based scheme.

For trigger types that support concurrency configuration, there's a default behavior, which you can choose to override in the host.json file for your function app. These settings, which apply to all running instances, allow you to control the maximum concurrency for your functions on each instance. For example, when your function is CPU or resource-intensive, you may choose to limit concurrency to keep instances healthy. Similarly, when your function is making requests to a downstream service that is being throttled, you should also consider limiting concurrency.

While such concurrency configurations give you control of certain trigger behaviors such as throttling your functions, it can be difficult to determine the optimal values for these settings. Generally, you have to arrive at acceptable values via a trial and error process of load testing. Even when you determine a set of values that are working for a particular load profile, the number of events arriving from your connected services may change from day to day. This variability means your app often may run with suboptimal values. For example, your function app may process particularly demanding message payloads on the last day of the week, which requires you to throttle concurrency down. However, during the rest of the week the message payloads are simpler, which means you could use a higher concurrency level the rest of the week.

Ideally, we want the system to allow instances to process as much work as they can while keeping each instance healthy and latencies low, which is what dynamic concurrency is designed to do.

Dynamic concurrency (preview)

Functions now provides a dynamic concurrency model that simplifies configuring concurrency for all function apps running in the same plan.

Note

Dynamic concurrency is currently only supported for the Service Bus trigger and requires you to use version 5.0.0-beta.6 (or higher) of the Microsoft.Azure.WebJobs.Extensions.ServiceBus extension.

Benefits

Using dynamic concurrency provides the following benefits:

  • Simplified configuration: You no longer have to manually determine per-trigger concurrency settings. The system learns the optimal values for your workload over time.
  • Dynamic adjustments: Concurrency is adjusted up or down dynamically in real time, which allows the system to adapt to changing load patterns over time.
  • Instance health protection: The runtime limits concurrency to levels a function app instance can comfortably handle. This protects the app from overloading itself by taking on more work than it should.
  • Improved throughput: Overall throughput is improved because individual instances aren't pulling more work than they can quickly process. This allows work to be load-balanced more effectively across instances. For functions that can handle higher loads, concurrency can be increased to values beyond the default config values, which yields higher throughput.

Dynamic concurrency configuration

Dynamic concurrency can be enabled at the host level in the host.json file. When, enabled any binding extensions used by your function app that support dynamic concurrency will adjust concurrency dynamically as needed. Dynamic concurrency settings override any manually configured concurrency settings for triggers that support dynamic concurrency.

By default, dynamic concurrency is disabled. With dynamic concurrency enabled, concurrency starts at 1 for each function, and is adjusted up to an optimal value, which is determined by the host.

You can enable dynamic concurrency in your function app by adding the following settings in your host.json file:

    { 
        "version": "2.0", 
        "concurrency": { 
            "dynamicConcurrencyEnabled": true, 
            "snapshotPersistenceEnabled": true 
        } 
    } 

When SnapshotPersistenceEnabled is true, which is the default, the learned concurrency values are periodically persisted to storage so new instances start from those values instead of starting from 1 and having to redo the learning.

Concurrency manager

Behind the scenes, when dynamic concurrency is enabled there's a concurrency manager process running in the background. This manager constantly monitors instance health metrics, like CPU and thread utilization, and changes throttles as needed. When one or more throttles are enabled, function concurrency is adjusted down until the host is healthy again. When throttles are disabled, concurrency is allowed to increase. Various heuristics are used to intelligently adjust concurrency up or down as needed based on these throttles. Over time, concurrency for each function stabilizes to a particular level.

Concurrency levels are managed for each individual function. As such, the system balances between resource-intensive functions that require a low level of concurrency and more lightweight functions that can handle higher concurrency. The balance of concurrency for each function helps to maintain overall health of the function app instance.

When dynamic concurrency is enabled, you'll see dynamic concurrency decisions in your logs. For example, you'll see logs when various throttles are enabled, and whenever concurrency is adjusted up or down for each function. These logs are written under the Host.Concurrency log category in the traces table.

Extension support

Dynamic concurrency is enabled for a function app at the host level, and any extensions that support dynamic concurrency run in that mode. Dynamic concurrency requires collaboration between the host and individual trigger extensions. For preview, only the latest (preview) versions of the following extensions support dynamic concurrency.

Service Bus

The Service Bus trigger currently supports three execution models. Dynamic concurrency affects these execution models as follows:

  • Single dispatch topic/queue processing: Each invocation of your function processes a single message. When using static config, concurrency is governed by the MaxConcurrentCalls config option. When using dynamic concurrency, that config value is ignored, and concurrency is adjusted dynamically.
  • Session based single dispatch topic/queue processing: Each invocation of your function processes a single message. Depending on the number of active sessions for your topic/queue, each instance leases one or more sessions. Messages in each session are processed serially, to guarantee ordering in a session. When not using dynamic concurrency, concurrency is governed by the MaxConcurrentSessions setting. With dynamic concurrency enabled, MaxConcurrentSessions is ignored and the number of sessions each instance is processing is dynamically adjusted.
  • Batch processing: Each invocation of your function processes a batch of messages, governed by the MaxMessageCount setting. Because batch invocations are serial, concurrency for your batch-triggered function is always one and dynamic concurrency doesn't apply.

To enable your Service Bus trigger to use dynamic concurrency, you must use version 5.0.0-beta.6 (or higher) of the Microsoft.Azure.WebJobs.Extensions.ServiceBus extension.

Next steps

For more information, see the following resources: