The preprocessors can run in the following different modes:
The preprocessor only preprocesses search queries. In this mode the preprocessor runs on the slave hosts by default.
The preprocessor only preprocesses documents. In this mode the preprocessor runs on the master hosts by default.
The preprocessor's tasks are not restricted.
The modes merely define preferences for the distribution of tasks for the preprocessors. If necessary, a preprocessor carries out all tasks regardless of its mode. For example, in certain circumstances a preprocessor that runs in index mode also processes search queries. This behavior increases the availability of the system, because in principle all preprocessors are able to carry out all tasks.