# MediaTailor parameter session behavior and persistence
<a name="parameter-session-behavior"></a>

AWS Elemental MediaTailor processes parameters at session initialization and maintains them throughout the session lifecycle. Understanding session behavior is crucial for implementing dynamic parameter scenarios.

**Session initialization methods**  
MediaTailor supports multiple methods for session initialization with parameters:

1. **Implicit session initialization:** Parameters included in the initial manifest request

   ```
   GET /v1/master/123456789/originId/index.m3u8?manifest.auth_token=abc123&start=2024-08-26T10:00:00Z
   ```

1. **Explicit session initialization (POST):** Parameters provided in the request body

   ```
   POST /v1/session/123456789/originId/index.m3u8
   {
       "adsParams": {"param1": "value1"},
       "manifestParams": {"auth_token": "abc123"}
   }
   ```

1. **Explicit session initialization (GET):** Parameters provided as query parameters

   ```
   GET /v1/session/123456789/originId/index.m3u8?ads.param1=value1&manifestParams.auth_token=abc123
   ```

**Parameter persistence and immutability**  
MediaTailor parameter behavior follows these rules:
+ **One-time specification:** Parameters can only be specified once, at session initialization
+ **Session-wide persistence:** Parameters are maintained throughout the entire session
+ **Immutable after initialization:** Parameters cannot be modified after the session is created
+ **Configuration alias resolution:** Aliases are resolved to actual values before forwarding to destinations

**Parameter modification scenarios**  
To modify parameters during playback:
+ **Create new session:** Initialize a new session with updated parameter values
+ **Player transition:** Seamlessly transition the player to the new session
+ **Parameter inheritance:** Carry forward unchanged parameters to maintain consistency

**Example Modifying time-shift parameters**  
To change from a 1-hour window to a 2-hour window:  

1. Current session: `start=2024-08-26T10:00:00Z&end=2024-08-26T11:00:00Z`

1. Create new session: `start=2024-08-26T10:00:00Z&end=2024-08-26T12:00:00Z`

1. Transition player to new session URL

**Important**  
Multiple multivariant playlist requests for a single session do not update parameters after the first request. Parameters remain immutable for the session duration.