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Replication Mechanisms
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## Replication Mechanisms

There are three ways that you can choose to implement replication in your transactional application. The first, and preferred, mechanism is to use the pre-packaged Replication Manager that comes with the DB distribution. This framework should be sufficient for most customers.

For applications with simple data and transaction models, Replication Manager provides automatic write forwarding as a configurable option.

If for some reason using Replication Manager or write forwarding does not meet your application's technical requirements, you will have to use the Replication Base APIs available through the Berkeley DB library to write your own custom replication framework.

These approaches are described in slightly greater detail in this section. The bulk of the chapters later in this book are dedicated to these two replication implementation mechanisms.

### Replication Manager Overview

DB's pre-packaged Replication Manager exists as a layer on top of the DB library. The Replication Manager is a multi-threaded implementation that allows you to easily add replication to your existing transactional application. You access and manage the Replication Manager using methods that are available off the `DB_ENV` class.

The Replication Manager:

- Provides a multi-threaded communications layer using pthreads (on Unix-style systems and similar derivatives such as Mac OS X), or Windows threads on Microsoft Windows systems.

- Uses TCP/IP sockets. Network traffic is handled via threads that handle inbound and outbound messages. However, each process uses a single socket that is shared using `select()`.

  Note that for this reason, the Replication Manager is limited to a maximum of 60 replicas (on Windows) and approximately 1000 replicas (on Unix and related systems), depending on how your system is configured.

- Upon application startup, a master can be selected either manually or via elections. After startup time, however, during the course of normal operations it is possible for the replication group to need to locate a new master (due to network or other hardware related problems, for example).

If your application has technical requirements that do not conform to the implementation provided by the Replication Manager, you must write implement replication using the DB Replication Base APIs. See the next section for introductory details.

### Write Forwarding Overview

By default, write operations cannot be performed on a replication client site. Replication Manager provides a configurable option that allows forwarding of simple client put and delete operations to the master site for processing. These operations must use an implicit NULL transaction ID to be forwarded. Any other write operation that specifies a non-NULL transaction or uses a cursor returns an error. This option is turned off by default.

The master must have an open database handle for the database on which a forwarded write operation is being performed. All sites in the replication group should have the same value for this configuration option.

For more information, see Configuring for Write Forwarding.

### Replication Base API Overview

The Replication Base API is a series of Berkeley DB library classes and methods that you can use to build your own replication infrastructure. You should use the Base API only if the Replication Manager does not meet your application's technical requirements.

To make use of the Base API, you must write your own networking code. This frees you from the technical constraints imposed by the Replication Manager. For example, by writing your own framework, you can:

- Use a threading package other than pthreads (Unix) or Windows threads (Microsoft Windows). This might be interesting to you if you are using a platform whose preferred threading package is something other than (for example) pthreads, such as is the case for Sun Microsystem's Solaris operating systems.

- Implement your own sockets. The Replication Manager uses TCP/IP sockets. While this should be acceptable for the majority of applications, sometimes UDP or even raw sockets might be desired.

For information on writing a replicated application using the Berkeley DB Replication Base APIs, see the Berkeley DB Programmer's Reference Guide.

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