# Function types and composition
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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
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| 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
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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.