# PRE\_SESSION\_INITIALIZATION
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## When it fires
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MediaTailor runs the function mapped to `PRE_SESSION_INITIALIZATION` once, at the start of a new playback session. The function runs before MediaTailor constructs the initial manifest response.

## Input
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`session.*`, `player_params.*`, and `event.*`. See the [Input field reference](monetization-functions-hooks.md#monetization-functions-hooks-input-ref) for the complete list.

## Output namespace allowed
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| Namespace | Accepted types | 
| --- | --- | 
| player\_params.\* | Strings, numbers, booleans | 

Values written to `player_params.*` are persisted to the session. They are available:
+ As input at the `PRE_ADS_REQUEST` lifecycle hook via `player_params.*`
+ In ADS request URLs through [MediaTailor dynamic ad variables for ADS requests](variables.md) (for example, `[player_params.deviceType]`)
+ For the lifetime of the session across all ad breaks

**Note**  
The total serialized size of all `player_params` output keys and values must not exceed 1,000 characters. If the total exceeds this limit, the function output is discarded. For more information, see [Functions limits](monetization-functions-limits.md).

## Typical use cases
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+ Fetch identity or audience data from an external service and store it in player parameters for use in later ADS requests.
+ Classify the device type based on the user agent and write the classification to a player parameter.
+ Set default player parameter values that downstream ad break processing relies on.
+ Store values in player parameters that are included in the ADS URL through [MediaTailor dynamic ad variables for ADS requests](variables.md).

## Failure behavior
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If a function attached to `PRE_SESSION_INITIALIZATION` fails for any reason, MediaTailor discards the function's output and proceeds as if no function were attached. The session starts normally without the function's player parameter values.