Memory resident database systems (MMDB's) store their data in main physical memory and provide very high-speed access. Conventional database systems are optimized for the particular characteristics of disk storage mechanisms. Memory resident systems, on the other hand, use different optimizations to structure and organize data, as well as to make it reliable. This paper surveys the major memory residence optimizations and briefly discusses some of the memory resident systems that have been designed or implemented.

Long lived transactions (LLTs) hold on to database resources for relatively long periods of time, slgmficantly delaymg the termmatlon of shorter and more common transactionsTo alleviate these problems we propose the notion of a saga A LLT 1s a saga if it can be written as a sequence of transactions that can be interleaved with other transactionsThe database management system guarantees that either all the transactions m a saga are successfully completed or compensatmg transactions are run to amend a partial execution Both the concept of saga and its lmplementatlon are relatively simple, but they have the potential to improve performance slgmficantlyWe analyze the various lmplementatron issues related to sagas, including how they can be run on an exlstmg system that does not directly support themWe also discuss techniques for database and LLT design that make it feasible to break up LLTs mto sagas

Long lived transactions (LLTs) hold on to database resources for relatively long periods of time, slgmficantly delaymg the termmatlon of shorter and more common transactionsTo alleviate these problems we propose the notion of a saga A LLT 1s a saga if it can be written as a sequence of transactions that can be interleaved with other transactionsThe database management system guarantees that either all the transactions m a saga are successfully completed or compensatmg transactions are run to amend a partial execution Both the concept of saga and its lmplementatlon are relatively simple, but they have the potential to improve performance slgmficantlyWe analyze the various lmplementatron issues related to sagas, including how they can be run on an exlstmg system that does not directly support themWe also discuss techniques for database and LLT design that make it feasible to break up LLTs mto sagas

Data centers are often under-utilized due to over-provisioning as well as time-varying resource demands of typical enterprise applications. One approach to increase resource utilization is to consolidate applications in a shared infrastructure using virtualization. Meeting application-level quality of service (QoS) goals becomes a challenge in a consolidated environment as application resource needs differ. Furthermore, for multi-tier applications, the amount of resources needed to achieve their QoS goals might be different at each tier and may also depend on availability of resources in other tiers. In this paper, we develop an adaptive resource control system that dynamically adjusts the resource shares to individual tiers in order to meet application-level QoS goals while achieving high resource utilization in the data center. Our control system is developed using classical control theory, and we used a black-box system modeling approach to overcome the absence of first principle models for complex enterprise applications and systems. To evaluate our controllers, we built a testbed simulating a virtual data center using Xen virtual machines. We experimented with two multi-tier applications in this virtual data center: a twotier implementation of RUBiS, an online auction site, and a two-tier Java implementation of TPC-W. Our results indicate that the proposed control system is able to maintain high resource utilization and meets QoS goals in spite of varying resource demands from the applications.

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