• Operations that process data sets and return calculated results are called aggregations.
• MongoDB provides data aggregations that examine data sets and perform calculations on them.
• Aggregation is run on the mongod instance to simplify application codes and limit resource requirements.

Aggregation

• Similar to queries, aggregation operations in MongoDB use collections of documents as an input and return results in the form of one or more documents.

• The aggregation framework in MongoDB is based on data processing pipelines.

• Documents pass through multi-stage pipelines and get transformed into an aggregated result.

Aggregation stages Reference

• The most basic pipeline stage in the aggregation framework provides filters that function like queries.

• It also provides document transformations that modify the output document.

• The pipeline operations group and sort documents by defined field or fields. In addition, they perform aggregation on arrays.

• Pipeline stages can use operators to perform tasks such as calculate the average or concatenate a string.

• The pipeline uses native operations within MongoDB to allow efficient data aggregation and is the favored method for data aggregation.

• The aggregate command in MongoDB functions on a single collection and logically passes the collection through the aggregation pipeline.

• You can optimise the operation and avoid scanning the entire collection by using the $match, $limit, and $kip stages.
• You may require only a subset of data from a collection to perform an aggregation operation.
• Therefore, use the $match, $limit, and $skip stages to filter the documents.
• When placed at the beginning of a pipeline, the $match operation scans and selects only the matching documents in a collection.

Which of the following is true about pipelines and the Aggregation Framework?

1. Pipelines must consist of at least two stages.
2. Stages cannot be configured to produce our desired output.
3. The Aggregation Framework provides us many stages to filter and transform our data
4. Documents flow through the pipeline, passing from one stage to the next

• Pipeline stages appear in an array.

• Documents are passed through the pipeline stages in a proper order one after the other.

• Barring $out and $geoNear, all stages of the pipeline can appear multiple times.

• The db.collection.aggregate() method provides access to the aggregation pipeline and returns a cursor and result sets of any size.

AGGREGATE PIPELINE STAGES

$project:

• This stage adds new fields or removes existing fields and thus restructure each document in the stream.
• This stage returns one output document for each input document provided.

$match:

• It filters the document stream and allows only matching documents to pass into the next stage without any modification.
• $match uses the standard MongoDB queries.
• For each input document, it returns either one output document if there is a match or zero documents when there is no match.

Which of the following is/are true of the $match stage?

• $match can only filter documents on one field.
• $match can use both query operators and aggregation expressions.
• It uses the familiar MongoDB query language.
• It should come very early in an aggregation pipeline.

Which of the following statements are true of the $project stage?

• Once we specify a field to retain or perform some computation in a $project stage, we must specify all fields we wish to retain. The only exception to this is the _id field.
• Beyond simply removing and retaining fields, $project lets us add new fields.
• $project can only be used once within an Aggregation pipeline.
• $project cannot be used to assign new values to existing fields.

$group:

• This stage groups documents based on the specified identifier expression and applies logic known as accumulator expression to compute the output document.
   

$sort:

• This stage rearranges the order of the document stream using specified sort keys.
• The documents remain unaltered even though the order changes.
• This stage provides one output document for each input document.

$skip:

• This stage skips the first n documents where n is the specified skip number.
• It passes the remaining documents without any modifications to the pipeline.
• For each input document, it returns either zero documents for the first n documents or one document.

$limit:

• It passes the first n number of documents without any modifications to the pipeline.
• For each input document, this stage returns either one document for the first n documents or zero documents after the first n documents.

$unwind:

• It deconstructs an array field in the input documents to return a document for each element.
• Each output document replaces the array with an element value.
• For each input document, it returns n documents where n is the number of array elements and can be zero for an empty array.

Which of the following statements is true?

1. Only one expression per stage can be used.
2. An aggregation pipeline is an array of stages.
3. Some expressions can only be used in certain stages.

Which of the following statements apply to $graphLookup operator? 

• $graphLookup depends on $lookup operator. Cannot be used without $lookup
• $lookup and $graphLookup stages require the exact same fields in their specification.
• $graphLookup is a new stage of the aggregation pipeline introduced in MongoDB 3.2
• Provides MongoDB with graph or graph-like capabilities
• $graphLookup provides MongoDB a transitive closure implementation

$group stage does three things:

• Groups the documents of the zip code collection under the state field
• Calculates thetotalPop (read as the total population) field for each state
• Returns an output document for each unique state.

• The aggregation operation given below returns all states with the total population greater than 10 million.
• This example depicts that the aggregation pipeline contains the $group stage followed by the $match stage.

• This aggregation operation shown returns usernames sorted by the month of their joining.
• This kind of aggregation could help generate membership renewal notices.

• Here in this command, the aggregate pipeline is used.
• The $sort stage orders those documents and $group stage applies the sum operation on the amount fields of those documents.

Replication

• The primary task of a MongoDB administrator is to set up replication and ensure that it is functioning correctly.
• The replication functionality is recommended for use in the production setting.
• Replication in MongoDB increases data availability by creating data redundancy.
• MongoDB replication stores multiple copies of data across different databases in multiple locations, and thus protects data when the database suffers any loss.

• It helps you manage data in the event of hardware failure and any kind of service interruptions.

• Having multiple copies of data stored on various servers across various locales, you can perform tasks, such as disaster recovery, backup, or reporting with ease.

• You can also use replication to enhance read operations.

• Typically, clients send read and write operations to different servers.

• You can store copies of these operations in different data centers to increase the locality and availability of data for distributed applications.

• A replica set consists of a group of mongod instances that host the same data set.

• In a replica set, the primary mongod receives all write operations and the secondary mongod replicates the operations from the primary and thus both have the same data set.

• The primary node receives write operations from clients.

• A replica set can have only one primary and therefore only one member of the replica set can receive write operations.

• A replica set provides strict consistency for all read operations from the primary.

Replica Sets

• The primary logs any changes or updates to its data sets in its oplog(read as op log).
• The secondaries also replicate the oplog of the primary and apply all the operations to their data sets.
• When the primary becomes unavailable, the replica set nominates a secondary as the primary. 

• By default, clients read data from the primary.
• However, they can also specify read preferences and request read operations to send to the secondaries.
• Secondary members in a replica set asynchronously apply operations from the primary.
• By applying operations after the primary, replica sets can function without any secondary members.
• As a result, all secondary members may not return the updated data to clients. Reads from secondaries may return data that does not reflect the state of the primary.
• A primary can convert to a secondary or vice versa. However, an arbiter will remain unchanged.

• When the primary node of a replica set stops communicating with other members for more than 10 seconds or fails, the replica set selects another member as the new primary.
• The selection of the new primary happens through an election process and whichever secondary node gets majority of the votes becomes the primary.
• A replica set supports application needs in various ways. For example, you may deploy a replica set in multiple data centers, or manipulate the primary election by adjusting the priority of members.
• In addition, replica sets support dedicated members for functions, such as reporting, disaster recovery, or backup.

 AUTOMATIC FAILOVER IN MONGODB

• In addition to the primary and secondaries, a replica set can also have an arbiter.
• Unlike secondaries, arbiters do not replicate or store data.
• However, arbiters play a crucial role in selecting a secondary to take the place of the primary when the primary becomes unavailable.

• A replica set typically contains:

  • A primary node
  • A secondary node
  • An arbiter
• Typically, most of the replica set deployments keep three members that store data, one primary and two secondaries.
• A replica set in MongoDB version 3.0 can have up to 50 members with 7 voting members.

Arbiter

Priority 0

• A priority zero member in a replica set is a secondary member that cannot become the primary.
• These members can act as normal secondaries, but cannot trigger any election.
• The main functions of a priority are as follows:
  • Maintains data set copies
  • Accepts and performs read operations
  • Elects the primary node
• A priority zero member is particularly useful in multi-data center deployments.
• In a replica set containing three members, one data center hosts both, the primary and a secondary, and the second data center hosts one priority zero member.

• Typically, a priority zero member acts as a backup. For example, in some replica sets, a new member may not be able to add immediately.
• The backup or standby member stores the updated data and can immediately replace an unavailable member.

HIDDEN MONGODB REPLICA SET MEMBERS

• Hidden members of a replica set are invisible to the client applications.
• They store a copy of the primary node’s data and are good for workloads with different usage patterns from the other members in the replica set.
• Although the hidden replica set members are priority zero members and can never replace the primary, they may elect the primary.

• You can utilize the hidden members for dedicated functions like reporting and backups.

 DELAYED MONGODB REPLICA SET MEMBERS 

• Delayed replica set members are those secondaries that copy data from the primary node’s oplog file after a certain  delay.

• Delayed replica set members store their data set copies. However, they reflect a previous version, or a delayed state of the set.

• If the data shown in the primary is for 10 AM then the delayed member will show data for 9 AM. Delayed members perform a “roll backup” or run a “historical” snapshot of the dataset.

• Therefore, they help you manage various human errors and recover from errors, such as unsuccessful application upgrade, and dropped databases and collections.

 DELAYED MONGODB REPLICA SET MEMBERS 

• To be considered delayed member, a replica set member must:

  • Be a priority zero member
  • Be hidden and not visible to applications and
  • Participate in electing the primary 

• You can configure a delayed secondary member with the following settings:

  • Priority Value - Zero,
  • Hidden Value - True, and
  • SlaveDelay Value - Number of seconds to delay
• After a replica set reconfigures, the delayed secondary member cannot replace the primary and is hidden from applications.
• The slaveDelay value delays both replication and the member’s oplog by 3600 seconds or 1 hour. 

Write concern

• w: 0
• w:1
• w:2
• w: majority

• unacknowledged
• acknowledged

• Primary: This is the default read preference mode. All operations read from the current replica set are primary.
• PrimaryPreferred: Operations are mostly read from the primary member. However, when the primary is unavailable, operations are read from a secondary member.
• Secondary: All operations are read from secondary members of the replica set.
• SecondaryPreferred: In most situations, operations are read from a secondary, but when a
  secondary member is unavailable, operations are read from the primary.
• Nearest: Operations are read from a member of the replica set with the least network
  latency, irrespective of the member’s type. 

Read preference