Real World Haskell

Chapter 21. Using Databases

Everything from web forums to podcatchers or even backup programs frequently use databases for persistent storage. SQL-based databases are often quite convenient: they are fast, can scale from tiny to massive sizes, can operate over the network, often help handle locking and transactions, and can even provide failover and redundancy improvements for applications. Databases come in many different shapes: the large commercial databases such as Oracle, open source engines such as PostgreSQL or MySQL, and even embeddable engines such as Sqlite.

Because databases are so important, Haskell support for them is important as well. In this chapter, we will introduce you to one of the Haskell frameworks for working with databases. We will also use this framework to begin building a podcast downloader, which we will further develop in Chapter 22.

Overview of HDBC

At the bottom of the database stack is the database engine, which is responsible for actually storing data on disk. Well-known database engines include PostgreSQL, MySQL, and Oracle.

Most modern database engines support the Structured Query Language (SQL) as a standard way of getting data into and out of relational databases. This book will not provide a tutorial on SQL or relational database management.^[51]

Once you have a database engine that supports SQL, you need a way to communicate with it. Each database has its own protocol. Since SQL is reasonably constant across databases, it is possible to make a generic ...