The queue server coordinates the processing steps that take place during indexing. It collects incoming document, triggers preprocessing by the preprocessor, and further processing by the index server.
The queue server enables documents to be indexed asynchronously. This has the advantage that you can control the time of indexing. For example, you can schedule indexing for times when the system load is lower because there are fewer search queries.
In addition, the queue server can trigger index replication and integration of the delta index in the main index.