MapReduce in MongoDB is used to process large amounts of data and perform specific operations on it. The MapReduce operation consists of two main functions “map”, which processes each document and emits one or more objects for each, and “reduce”, which combines all the emitted objects from the map function.
Here is a step-by-step process on how to use MapReduce in MongoDB:
1. Define the Map Function:
This function will be applied to each document in the data set and it will emit any data needed for the operation.
For example:
```
var mapFunction1 = function() {
emit(this.key, this.value);
};
```
1. Define the Reduce Function:
This function takes the output from the map operation and reduces it to the operation’s required form.
For example:
```
var reduceFunction1 = function(key, values) {
return Array.sum(values);
};
```
1. Execute MapReduce:
You use run the MapReduce operation using the `mapReduce` function. Specify the collection you want to apply the MapReduce operation on. Also integrate the map, reduce functions defined, and also specify an output collection.
For example:
```
db.collection.mapReduce(
mapFunction1,
reduceFunction1,
{ out: “map_reduce_example” }
)
```
1. Check the Output:
You can then check your MapReduce results in the output collection you specified.
For example:
```
db.map_reduce_example.find()
```
Note: MapReduce can be replaced with the aggregation framework which can be faster and more capable for many common use cases. However, there are specific situations where MapReduce can be necessary or more efficient.
