Maintaining derived state
Batch processing has a quite strong functional flavor (even if the code is not written in a functional programming language): it encourages deterministic, pure functions whose output depends only on the input and which have no side effects other than the explicit outputs, treating inputs as immutable and outputs as append-only. Stream processing is similar, but it extends operators to allow managed, fault-tolerant state (see “Rebuilding state after a failure” on page 478).
The principle of deterministic functions with well-defined inputs and outputs is not only good for fault tolerance (see “Idempotence” on page 478), but also simplifies reasoning about the dataflows in an organization . No matter whether the derived data is a search index, a statistical model, or a cache, it is helpful to think in terms of data pipelines that derive one thing from another, pushing state changes in one system through functional application code and applying the effects to derived systems.
In principle, derived data systems could be maintained synchronously, just like a relational database updates secondary indexes synchronously within the same transaction as writes to the table being indexed. However, asynchrony is what makes systems based on event logs robust: it allows a fault in one part of the system to be contained locally, whereas distributed transactions abort if any one participant fails, so they tend to amplify failures by spreading them to the rest of the system (see “Limitations of distributed transactions” on page 363).
We saw in “Partitioning and Secondary Indexes” on page 206 that secondary indexes often cross partition boundaries. A partitioned system with secondary indexes either needs to send writes to multiple partitions (if the index is term-partitioned) or send reads to all partitions (if the index is document-partitioned). Such cross-partition communication is also most reliable and scalable if the index is maintained asynchronously  (see also “Multi-partition data processing” on page 514).