Mongosh client

• To print out commands in a json file to the mongoclient

  • cat FILE.json | mongosh

Administration

• Launch mongo as: mongod –config mongod.conf OR mongod -f mongo.conf

• To shutdown mongodb internally, issue the db.shutdownServer() operation against the admin database.

Roles and accounts

• https://www.mongodb.com/docs/manual/reference/built-in-roles/

• https://www.mongodb.com/docs/v5.0/tutorial/manage-users-and-roles/

Collections

• Capped : To a certain size and last in first out

Query

• https://docs.mongodb.com/manual/reference/operator/query/

  • https://www.mongodb.com/docs/manual/tutorial/query-embedded-documents/

  • https://www.mongodb.com/docs/manual/tutorial/query-arrays/

  • https://www.mongodb.com/docs/manual/tutorial/query-array-of-documents/

  • https://www.mongodb.com/docs/manual/tutorial/project-fields-from-query-results/

  • https://www.mongodb.com/docs/manual/tutorial/query-for-null-fields/

  • https://www.mongodb.com/docs/manual/reference/method/db.collection.find/#std-label-find-projection

• Query

  • A collection is named

  • A command is given where it can have sub commands

  • You can also have multiple commands where the output is fed to the next command

    • Such as : .db.DOC.find(name : 'mark').project( { name : 1)

    • project can have an aggregate command

  • Querying is one of formats

    • Against a value : Ex; db.people.find({ name : 'mark')}

      • against a value

    • Against against subdocuments. Ex: a docment called person, containing a subdocument 'contact', with field called 'name' and value 'mark'

      • db.person.find({ contact.name : 'mark')}

    • Against an array in the documents. “names” is an array.

      • Ex: db.person.find({ contact.names :[ 'mark', 'marcus'])}

    • Against an array of documents

      • Ex: Person is the document. Address is an array. In the array is a document with one of the fields named :”street”,

      • ex: db.person.find( { address : {street : 'main street'}})

  • For updates:

    • https://www.mongodb.com/docs/manual/reference/operator/update/positional/

    • https://www.mongodb.com/docs/manual/reference/operator/update/positional-all/#mongodb-update-up.---

    • The $ operator represents the first entry in a array if there are many entries in the array. It only updates the first match.

      • Ex: db.students.updateOne( { _id: 4, "grades.grade": 85 },{ $set: { "grades.$.std" : 6 } })

    • To update every match of documents for all elemts in the array

      • ex: db.students ( {grade :1}, { $inc : {test6.$[] : 10}

      • This adds 10 points to all students in 1^st grade for test 6.

• Multiple Insert with BulkWrite

  • db.people.bulkWrite([
    { insertOne : {person : “Mark”}}, {insertOne : {person : “Mark”}}
    ]

• Update fields inc, many

  • db.people.update({person : “MARK”}, {$inc : {money : 100 }, {multi : 1}}

  • db.people.updateMany({person : “MARK”}, {$inc : {money : 100 }}

• hint index

  • Use hint to override the index choice
    https://docs.mongodb.com/manual/reference/method/cursor.hint/

• Documents missing fields in a search are sorted last

• To update a specific entry of mutiple entries
  db.people.updateOne(
  { _id: ObjectId("123"), "people.name": "Mark" },
  { $set: { "people.$.children": 20 } }
  )

• To do a text search : db.collection.find({$text: {$search: "search_query"}})

Aggregation

• Links

  • https://www.mongodb.com/docs/upcoming/aggregation/

  • Tutorial : https://studio3t.com/knowledge-base/articles/mongodb-aggregation-framework/

• Aggregation

  • Is a list of commands where a commands output is fed to the next command in line

• To limit fields in an aggregation pipeline, use projection.

• $all : compares array to given array, if the array has all elements of given array

• $in : If array contains an element in given array

• $elematch : returns document if field matches. Works with subdocuments.

  • Ex: db.people.find( {children : { $elematch : { type : “son” } } } )

• Aggregation Framework each individual pipeline stage has a limit of 200 MB

• TODO: Make article showing aggregation pipeline with simple data and commands.

  • Starting : aggregation.txt

• OLD not readable to newbies article : http://www.menprojects.com/public/mongo_queries.html

Indexes

• MongoDB recommends following the ESR (Equality, Search, Range)

  • equality, sort order, range filters

  • Indexes are B-tree

  • A field with numeric and non-numeric, it sorted lexicographical

  • Types: regular , spatial, text

Replica set

• Add replica : Connect to the primary member, add the new member to the replica set configuration, initiate a resync on the new member, and wait for the new member to become a secondary.

• Host Ids in replica set must be unique. And host:port as well.

• Write Concern is the no of secondaries needs to return write ack.

• To add a hidden member : rs.add(hostname, {priority: 0, hidden: true})

  • You want it hidden and no possibility of failover.

• Backup server : {“priority : 0, :hidden” : true , “buildIndexes” : false}

Shards

• Meta is stored on config servers

• Covered queries are in a single shard.

• To migrate data from one shard to another, use MongoDB's tool
  or https://www.mongodb.com/docs/manual/tutorial/migrate-sharded-cluster-to-new-hardware/

• Choose shard key that is unique and has a large no of distinct values. Shard key can be multi-keys.

Security

• For high security : Role based accounts, secure LDAP, full disk encryption, secure key management, and SSL

Explain

• db.people.find({age: {$gt: 30}}).explain("executionStats")

• https://www.mongodb.com/docs/manual/reference/explain-results/

• Terms :

  • COLLSCAN : full collection scan

  • IXSCAN : index scan

  • FETCH L : retrieving documents

  • GROUP : grouping documents

  • SHARD_MERGE

  • SHARDING_FILTER

  • BATCHED_DELETE

Slow Log

• Use Profile to Identify queries : https://www.mongodb.com/docs/manual/tutorial/manage-the-database-profiler/

• db.setProfilingLevel(1, { slowms: 20 })

• db.system.profile.find().limit(10).sort( { ts : -1 } ).pretty()

Location of uncommitted queries is replica failover, rollback:

• “The createRollbackDataFiles parameter controls whether or not rollback files are created during rollbacks.”

• After 4.4, its used the uid od the collection

• “<dbpath>/rollback/20f74796-d5ea-42f5-8c95-f79b39bad190/removed.2020-02-19T04-57-11.0.bson”

• on the former primary

Upgrades

• Check compatibility issues and if set
  db.adminCommand( { setFeatureCompatibilityVersion: "6.0" } )

• For replica sets and shardsUpgrade secondaries one at a time, wait for members to catch up or he primary, then upgrade the primary

• When upgrading the replica sets, make sure applications are compatible with the upgrade.

Text Searches

• You must have a text index.

• To perform text searches, use $text operator

• https://www.mongodb.com/docs/manual/reference/operator/query/text/

Schema Validation

• https://www.mongodb.com/docs/manual/core/schema-validation/

Status and stats commands

• db.collection.stats(<option>) returns statistics about the collection.

• db.stats()

• rs.status – replica status

• db.runCommand({ serverStatus: 1}) – server status. Includes replica status, and other things. Contains most information from status commands.
  Or db.serverStatus()

• show dbs

• show collections

• sh.status() - shard status

Changes

• Mongodb 6 : https://www.mongodb.com/docs/manual/release-notes/6.0/

• MongoDB 7 : https://www.mongodb.com/docs/v7.0/release-notes/7.0/

CheatSheet

• https://www.mongodb.com/developer/products/mongodb/cheat-sheet/

• Amazon : https://mongodb-devhub-cms.s3.us-west-1.amazonaws.com/Mongo_DB_Shell_Cheat_Sheet_1a0e3aa962.pdf

• Interviewbit : https://www.interviewbit.com/mongodb-cheat-sheet/

New Stuff

• Time Series : https://www.mongodb.com/developer/products/mongodb/new-time-series-collections/

Misc

• maximum size of a MongoDB document : 16 mb

Data Migration (TODO)

• AWS

• AzureComos

• GCP

Engines

• WiredTiger

  • document level concurrency.

  • With 4 shard ACID

  • compression with ZSTD starting in 4.2

• MMAPv1 was depreciated in 4.2