## 2 Background on CRM and Big Data in Hospitality  
The CRM focuses on the relationship with the client, implementing a client-centric process [25]. It is a client-oriented culture [26] that contributes to increase from guest satisfaction to loyalty and retention, by acting on the reduction of guest acquisition costs while focusing on increasing profits [7; 27; 28; 29]. Therefore, the purpose of the CRM is the client knowledge (needs, preferences and emotions), focusing on a single view of him [16], in order to develop an efficient relationship (commercial and experiential) in each interaction. This is increasingly possible thanks to the developments and advances in information technology (IT) applications [26]. The synergistic relationship between Big Data and CRM is relatively new and so far little studied [16; 30]. With regard to the hospitality industry, the scarcity of studies is even greater, and there are few works that have brought the two concepts together [31; 32]. However, the CRM is a strategy that has been implemented for over thirty years in the hotel sector [27], although its use and importance has increased in recent years [7]. The main reasons are mainly due to the greater power of the client, the increase in competition [33], the increase in client information due to the digital environments [30] and the technological development [34] that have caused the profitability of the hotel to be strongly linked to the capacity to satisfy clients in an effective and efficient way [33]. The CRM, therefore, plays a major role since it comprises the organizational procedures and strategies to better understand its clients and to use this knowledge for the production and marketing of hotel services [16]. On the other hand, the relationship between the hotel CRM and technology has been studied in several works [27]--initially, to analyze what these systems consisted of and what technologies they used; later, with the arrival of the Internet, to find out how hotels used multimedia channels (e-mail, channels, etc.) to create relationships with their clients [35; 36]; and, finally, to analyze their interaction with social media. Currently, in the hotel industry, digital technology has generated a vast amount of client data sources in very different formats, whose study represents a challenge due to their diversity. Specifically, we face structured data in conventional databases (from property management and CRM systems) as well as semi-structured and unstructured data (obtained from meta-search generated data, such as those given by Kayak, Trivago and Tripadvisor or extracted from social networks, such as Facebook, Twitter or LinkedIn) [12; 37; 38]. Big Data tools and techniques facilitate access, storage and analysis of a wide variety of client data that are captured and processed at high velocity and are quite effective [12; 30]. According to [PERSON] [16], \"Big Data brings a whole new inclusion of CRM strategies which can support customization of sales, personalization of services, and client interactions\". For this reason, the application of Big data tools and technology in the context of hotel CRM strategies is now indispensable and it is therefore pertinent to deepen the knowledge and usefulness of these tools. Thus, big data technology facilitates segmentation and better knowledge of the client [12] with the continuous updating of profiles, giving the hotel the possibility of interacting in real time with the client, as well as providing knowledge of the client's value and life cycle [31], adjusting the services to their needs, enabling the offer of personalized services and products and carrying out marketing campaigns aimed at each client segment, predicting client reaction to marketing messages and product offers or providing a higher quality of information [32], among many others. Data mining facilities are being increasingly implemented in the hotel CRM to help managers find hidden client information from available data, thus aiming to serve towards more effective and personalized CRM strategies [16].