section 4.3 describes single-pass in-memory indexing, an algorithm that has
even better scaling properties because it does not hold the vocabulary in
memory. For very large collections like the web, indexing has to be distributed over computer clusters with hundreds or thousands of machines.
We discuss this in Section 4.4. Collections with frequent changes require dynamic indexing introduced in Section 4.5 so that changes in the collection are
immediately reflected in the index. Finally, we cover some complicating is sues that can arise in indexing – such as security and indexes for ranked
retrieval – in Section 4.6.