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Chapter 4. Using DB_CONFIG to configure the Berkeley DB SQL interface
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## Chapter 4. Using DB_CONFIG to configure the Berkeley DB SQL interface

In almost all cases, there is no need for you to directly configure Berkeley DB resources; instead, you can use the same configuration techniques that you always use for SQLite, or use the Berkeley DB PRAGMAs describe Berkeley DB SQL: The Absolute Basics, and Replication PRAGMAs. The Berkeley DB SQL interface will take care of the rest.

However, there are a few configuration activities that some unusually large or busy installations might need to make and for which there is no SQLite equivalent. This chapter describes those activities.

## Introduction to Environments

Before continuing with this section, it is necessary for you to have a high-level understanding of Berkeley DB's environments.

In order to manage its resources (data, shared cache, locks, and transaction logs), Berkeley DB often uses a directory that is called the Berkeley DB environment. As used with the BDB SQL interface, environments contain log files and the information required to implement a shared cache and fine-grained locking. This environment is placed in a directory that appears on the surface to be a SQLite rollback file.

That is, if you use BDB SQL interface to create a database called `mydb.db`, then a directory is created alongside of it called `mydb.db-journal`. Normally, SQLite creates a journal file only when a transaction is underway, and deletes this file once the transaction is committed or rolled back. However, that is not what is happening here. The BDB SQL interface journal directory contains important Berkeley DB environment information that is meant to persist between transactions and even between process runtimes. So it is very important that you do not delete the contents of your Berkeley DB journal directory. Doing so will cause improper operation and could lead to data loss.

Note that the environment directory is also where you put your `DB_CONFIG` file. This file can be used to configure additional tuning parameters of Berkeley DB, if its default behavior is not appropriate for your application. For more information on the `DB_CONFIG` file, see the next section.

### Note

Experienced users of Berkeley DB should be aware that neither `DB_USE_ENVIRON` nor `DB_USE_ENVIRON_ROOT` are specified to `DB_ENV->open()`. As a result, the `DB_HOME` environment variable is ignored. This means that the BDB SQL interface will always create a database in the location defined by the database name given to the BDB SQL interface.

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