Function types and composition
AWS Elemental MediaTailor provides three function types, each designed for a different category of work. You choose a type when you create a function, and the type determines what the function can do at runtime. This page explains how each type executes, when to use it, and how to compose functions into multi-step pipelines.
Function type overview
| Type | Category | Purpose |
|---|---|---|
CUSTOM_OUTPUT |
Data transformation | Evaluate expressions against the current session state and produce outputs. No external calls. |
HTTP_REQUEST |
External integration | Make an HTTP call to an external service, then evaluate output expressions that can reference the response. |
SEQUENTIAL_EXECUTOR |
Orchestration | Run a sequence of functions in order, passing data between steps through temporary data. |
Each type serves a distinct role. Choose a type when you create a function — the type determines what the function can do at runtime.
Composition rules
MediaTailor enforces the following limits on function composition:
| Rule | Limit |
|---|---|
| Maximum nesting depth | 2 (a sequence can contain functions, but those children cannot be sequences themselves) |
| Steps per sequence | 1 to 10 |
| Total function executions per lifecycle hook | 20 |
| Circular references | Not allowed. A function cannot reference itself, directly or indirectly. |
| Function existence | All functions referenced in a FunctionList must exist
before you create or update the parent sequence. |
When you create a function, MediaTailor validates expression syntax, checks for restricted functions, verifies that all referenced functions exist, and detects circular references. When you attach a function to a playback configuration, MediaTailor additionally validates that all output keys across the entire function tree are compatible with the assigned lifecycle hook.
