# Book API

### Overview <a href="#bookapi-overview" id="bookapi-overview"></a>

UltraDirect supports session control for all booking transactions. This means the hotel CRS (Central Reservation System) will temporarily hold booking, modification, and cancellation requests until a subsequent request to complete or ignore the action is received and processed.

Affiliate partners (demand partners integrating with RateGain) can choose between **active** or **passive** participation in session control:

1. **Active Participation**:
   * The affiliate sends two messages per transaction: one to initiate the booking, modification, or cancellation and another to complete (commit) or ignore the transaction.
   * A transaction is only finalized when it is committed. Consequently, the confirmation number or cancellation number is returned only in response to a commit transaction.
2. **Passive Participation**:
   * RateGain automatically sends the commit or ignore message to the CRS on behalf of the affiliate.
   * The affiliate sends only one request message per transaction and receives the confirmation or cancellation number directly in the response.

**Note**:\
This document focuses exclusively on the **passive mode** of session control. In the case of RateGain Pull connectivity, RateGain handles the commit or ignore message to the CRS on behalf of the affiliate. Therefore, affiliates only need to send a single request message per transaction to receive the confirmation or cancellation number.


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