Glossary¶

Definitions of core Azure Functions terms used throughout this guide. Terms are grouped by domain; within each group they are ordered alphabetically.

Runtime and Hosting¶

Term	Definition
Function	A single, independently triggered unit of code deployed within a function app.
Function app	The deployment and management unit that hosts one or more functions sharing the same configuration, runtime, and hosting plan.
Functions host	The runtime process that loads the function app, manages the language worker, and dispatches trigger events to functions.
Language worker	A separate process (Python, Node.js, Java, or .NET isolated) that runs your function code and communicates with the Functions host over gRPC.
In-process model	Legacy .NET model where function code runs inside the Functions host process. Superseded by the isolated worker model.
Isolated worker model	The current .NET model where function code runs in a separate worker process, decoupled from the host runtime version.
Consumption plan (Y1)	Serverless, event-driven hosting that scales to zero and bills per execution. Legacy default for new workloads.
Flex Consumption plan (FC1)	Serverless plan with fast/large scale-out, VNet integration, and per-instance concurrency control. Recommended default for most new workloads.
Premium plan (EP)	Plan with always-ready (pre-warmed) instances, VNet integration, and no cold start for warm instances.
Dedicated plan (App Service plan)	Fixed-capacity hosting on an App Service plan, suitable for reusing an existing App Service estate.

Term	Definition
Trigger	The event that causes a function to run (for example an HTTP request, a queue message, or a timer schedule). Each function has exactly one trigger.
Binding	A declarative connection to data or a service. Input bindings supply data to a function; output bindings send data from a function.
Binding extension	A NuGet/npm/pip package that provides a specific trigger or binding type (for example the Service Bus or Event Hubs extension).
Extension bundle	A versioned set of binding extensions declared in `host.json` so non-.NET apps do not manage individual extension packages.
Poison message	A message that repeatedly fails processing and is moved to a poison/dead-letter location after the maximum dequeue/delivery count is reached.

Term	Definition
Scale controller	The Azure component that monitors trigger activity and decides when to add or remove function app instances.
Target-based scaling	A scaling model that computes desired instance count from queue/stream backlog and per-instance target, allowing faster scale-out than incremental scaling.
Always-ready instance	A pre-provisioned Premium/Flex instance kept warm to eliminate cold start for a baseline of concurrent executions.
Per-instance concurrency	The number of concurrent invocations a single instance handles before the platform scales out (configurable on Flex Consumption).
Cold start	The added latency when a request is served by a newly allocated instance that must initialize the host, worker, and dependencies.
Scale to zero	The behavior of serverless plans that remove all instances when there is no work, eliminating idle cost.

Term	Definition
AzureWebJobsStorage	The storage account connection setting the Functions runtime uses for internal state such as triggers, leases, and logs.
Content share	The Azure Files share that stores the deployed application content for Consumption and Premium plans.
Deployment package	The zipped application artifact. On Flex Consumption it is stored in a blob container and mounted at runtime.
Lease	A storage-based lock used to coordinate singleton behavior and trigger ownership across instances.

Term	Definition
VNet integration	A feature that lets a function app make outbound calls into an Azure virtual network.
Private endpoint	A network interface that exposes an Azure service (including a function app or its storage) with a private IP inside a VNet.
Service endpoint	A VNet feature that secures an Azure service to specific subnets over the Azure backbone without a private IP.
Fixed outbound IP	A predictable egress IP for a function app, typically achieved with VNet integration and a NAT gateway.
Private DNS zone	A DNS zone that resolves a service's hostname to its private endpoint IP inside a VNet.

Term	Definition
Managed identity	An Azure-managed Microsoft Entra identity assigned to a function app for passwordless authentication to other Azure services.
System-assigned identity	A managed identity whose lifecycle is tied to a single function app.
User-assigned identity	A standalone managed identity that can be shared across multiple resources.
Function access key	A shared secret that authorizes calls to an HTTP-triggered function or the app's admin endpoints.
Authorization level	The access requirement for an HTTP trigger: `anonymous`, `function`, or `admin`.

Term	Definition
Deployment slot	A separate, addressable instance of a function app used for staging and zero-downtime swaps.
Zip deploy / run-from-package	Deployment mechanisms that publish the app as a package, with run-from-package mounting it read-only.
Kudu / SCM	The advanced deployment and diagnostics site for App Service and Functions apps (not available on Flex Consumption).
Application setting	A key/value configuration entry exposed to the app as an environment variable.

Term	Definition
Application Insights	The Azure Monitor telemetry service that collects requests, dependencies, traces, exceptions, and metrics from a function app.
Live Metrics	A near-real-time stream of request, dependency, and performance metrics in Application Insights.
Distributed tracing	Correlation of a single logical operation across functions, bindings, and downstream dependencies using trace context.
Sampling	A telemetry-volume control that records a representative subset of events to manage cost.