## Replica Set Read Preference 

For replica sets and sharded clusters with replica set components, applications specify read preferences (page 588). A read preference determines how the client directs read operations to the set. 

## Write Concern 

Applications can also control the behavior of write operations using write concern (page 78). Particularly useful for deployments with replica sets, the write concern semantics allow clients to specify the assurance that MongoDB provides when reporting on the success of a write operation. 

## Aggregation 

In addition to the basic queries, MongoDB provides several data aggregation features. For example, MongoDB can return counts of the number of documents that match a query, or return the number of distinct values for a field, or process a collection of documents using a versatile stage-based data processing pipeline or map-reduce operations. 

## 3.2 MongoDB CRUD Concepts 

The Read Operations (page 60) and Write Operations (page 73) documents introduce the behavior and operations of read and write operations for MongoDB deployments. 

Read Operations (page 60) Queries are the core operations that return data in MongoDB. Introduces queries, their behavior, and performances. 

Cursors (page 64) Queries return iterable objects, called cursors, that hold the full result set. 

Query Optimization (page 65) Analyze and improve query performance. 

Distributed Queries (page 69) Describes how sharded clusters and replica sets affect the performance of read operations. 

Write Operations (page 73) Write operations insert, update, or remove documents in MongoDB. Introduces data create and modify operations, their behavior, and performances. 

Write Concern (page 78) Describes the kind of guarantee MongoDB provides when reporting on the success of a write operation. 

Distributed Write Operations (page 82) Describes how MongoDB directs write operations on sharded clusters and replica sets and the performance characteristics of these operations. 

Continue reading from Write Operations (page 73) for additional background on the behavior of data modifica- tion operations in MongoDB. 

## 3.2.1 Read Operations 

The following documents describe read operations: 

Read Operations Overview (page 61) A high level overview of queries and projections in MongoDB, including a discussion of syntax and behavior. 

Cursors (page 64) Queries return iterable objects, called cursors, that hold the full result set. 

Query Optimization (page 65) Analyze and improve query performance. 

Query Plans (page 68) MongoDB executes queries using optimal plans . 

