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Functions lifecycle hooks - AWS Elemental MediaTailor
OverviewInput field reference

Functions lifecycle hooks

A lifecycle hook defines when MediaTailor runs your function during playback. This page is a complete reference for input fields, output namespaces, and the rules that govern data flow at each hook.

Overview

MediaTailor supports two lifecycle hooks:

  • PRE_SESSION_INITIALIZATION fires once when a viewer starts a new session. Use it for one-time setup work such as fetching audience segments. At this point, no ad break has occurred, so ad break context is not available.

  • PRE_ADS_REQUEST fires before every ad decision server (ADS) request — once per ad break in the stream. Use it to customize the ADS request with targeting data, modify the ADS URL, or add headers.

The key difference is timing: PRE_SESSION_INITIALIZATION runs once and sets up data that persists for the entire session, while PRE_ADS_REQUEST runs repeatedly and can tailor each ADS request to the specific ad break.

Input field reference

Field Type PRE_SESSION_INITIALIZATION PRE_ADS_REQUEST
session.id Long ✓ ✓
session.uuid String ✓ ✓
session.client_ip String ✓ ✓
session.user_agent String ✓ ✓
session.referer* String ✓ ✓
session.avail_duration_secs Long ✗ ✓
session.avail_duration_ms Long ✗ ✓
player_params.* String ✓ ✓
event.id String ✓ ✓
event.hook String ✓ ✓
event.timestamp String ✓ ✓
avail.index Int ✗ ✓
avail.random Long ✗ ✓
avail.source_content_time_epoch_ms Long ✗ ✓
scte.event_id Int ✗ ✓
scte.avail_num Int ✗ ✓
scte.segmentation_event_id Int ✗ ✓
scte.segmentation_type_id Int ✗ ✓
scte.segmentation_upid String ✗ ✓
scte.segmentation_upid.assetId String ✗ ✓
scte.segmentation_upid.cueData.key String ✗ ✓
scte.segmentation_upid.cueData.value String ✗ ✓
scte.unique_program_id Int ✗ ✓
scte.archive_allowed_flag Boolean ✗ ✓
scte.delivery_not_restricted_flag Boolean ✗ ✓
scte.device_restrictions Int ✗ ✓
scte.no_regional_blackout_flag Boolean ✗ ✓
scte.segment_num Int ✗ ✓
scte.segments_expected Int ✗ ✓
scte.sub_segment_num Int ✗ ✓
scte.sub_segments_expected Int ✗ ✓
scte.avails_expected Long ✗ ✓
adsRequest.url String ✗ ✓
adsRequest.method String ✗ ✓
adsRequest.headers.<key> String ✗ ✓
adsRequest.body String ✗ ✓

* session.referer is only present when a Referer header is included in the session initialization request. Use $exists(session.referer) to check before accessing.