1.3. The Datastore

Most useful web applications need to store information during the handling of a request for retrieval during a later request. A typical arrangement for a small website involves a single database server for the entire site, and one or more web servers that connect to the database to store or retrieve data. Using a single central database server makes it easy to have one canonical representation of the data, so multiple users accessing multiple web servers all see the same and most recent information. But a central server is difficult to scale once it reaches its capacity for simultaneous connections.

By far the most popular kind of data storage system for web applications in the past decade has been the relational database, with tables of rows and columns arranged for space efficiency and concision, and with indexes and raw computing power for performing queries, especially “join” queries that can treat multiple related records as a queryable unit. Other kinds of data storage systems include hierarchical datastores (filesystems, XML databases) and object databases. Each kind of database has pros and cons, and which type is best suited for an application depends on the nature of the application’s data and how it is accessed. And each kind of database has its own techniques for growing past the first server.