Ruminations on Multi-Tenant Databases

This is a position paper on multi-tenant databases. As motivation, it first describes the emerging marketplace of hosted enterprise services and the importance of using multi-tenancy to handle high traffic volumes at low cost. It then outlines the main requirements on multi-tenant databases: scale up by consolidating multiple tenants onto the same server and scale out by providing an administrative framework that manages a farm of such servers. Finally it describes three approaches to implementing multi-tenant databases and compares them based on some simple experiments. The main conclusion is that existing database vendors need to enhance their products to better support multi-tenancy. 1 Hosted Services and Multi-Tenancy In the hosted service model [GM02a, GM02b, Wa03], a service provider develops an application and operates the system that hosts it. Customers access the application over the Internet using industry-standard web browsers or Web Services clients. As the Internet has matured, hosted services have appeared for an increasingly wide variety of enterprise applications, including ones that manage sales, marketing, support, human resources, planning, manufacturing, inventory, financials, purchasing, and compliance [Th06]. While hosted services are attractive to all segments of the market, they are particularly appealing to smallto mediumsized businesses, which often lack the resources to maintain a complex data center. Hosted services are exploiting such “greenfield” opportunities to expand the overall size of the market. In comparison to traditional on-premises solutions, hosted services can reduce the total cost of ownership of an application by aggregating customers together and leveraging economy of scale. This principle applies to both capital expenditures, e.g., for hardware and software, and operational expenditures, e.g., for bandwidth and personnel. Because a hosted service is focused on one application, the infrastructure and the procedures for managing it can be highly optimized: well-known examples here include the Google File System [GGL03] and Hotmail [Sm05]. Such optimizations are essential to support large numbers of smallto medium-sized businesses, which would otherwise be prohibitively expensive. Multi-tenancy is an optimization for hosted services in which multiple customers are consolidated onto the same operational system, a technique pioneered by salesforce.com [Sa06, Co06]. Multi-tenancy allows pooling of resources, which improves utilization by eliminating the need to provision each customer for their maximum load. Multi-tenancy can also improve management efficiencies by providing a uniform framework for administering the system. A multi-tenant system should support both scale up and scale out [De99]: scale up by consolidating multiple customers onto the same server and scale out by having the administrative framework span a farm of such servers. Scale out is required because it is not cost effective to scale up a single server indefinitely. The administrative framework of a multi-tenant system should support the ability to migrate a customer from one server to another within the farm. For example, a customer might start out on a server that manages trial accounts, be moved to a server that manages small production accounts, and grow until it is moved to a dedicated server that manages only one account. Migration should also be possible between farms, for example, to allow customers to be moved from one data center to another. The administrative framework should also support rolling upgrade, where the servers in a farm are upgraded to a new version of the application one at a time. Since it is difficult to generate realistic Internet-scale workloads for testing, rolling upgrades are needed to gain experience with new versions before they are put widely into production.