“Dashboard in the pilot's cockpit” by Mitchel Boot on Unsplash Know the operations currently executing on your MongoDB server inside out. Introduction Have you ever been asked — . If you did wonder, then you are thinking in right direction. This is one of the many articles in multi-part series, , solely created for you to master by learning . ‘Why is your MongoDB database server running slow?’ ‘Which operations are making it slow?’ Mastering MongoDB — One tip a day MongoDB ‘one tip a day’ In a few series of articles, I would like to give various tips to help you answer the above question. This article discusses command — its applications, use case scenarios and finally use case scenarios and finally some hands-on lab exercises. currentOp Mastering — currentOp What is currentOp The is an administrative command that provides information about the current operations executing on the MongoDB server. currentOp The must run against the admin database. When the authorization is turned on, the current user must either have ‘ privilege on database to view all operations or must use ‘ to view their own operations. currentOp inprog’ admin $ownOps’ How to use currentOp You could run the currentOp in mongo shell using either the adminCommand or the currentOp wrapper functions as show below. db.adminCommand({ : }) db.adminCommand({ : , : }) "currentOp" 1 // or "currentOp" 1 "$ownOps" true The MongoDB currentOp command via db.adminCommand An alternative wrapper function for the above adminCommand db.currentOp() db.currentOp({ : }) // or "$ownOps" true The MongoDB currentOp command via a db.currentOp() wrapper function. Use cases for the currentOp The use case for the command is to gathering information about operations currently executing on the MongoDB server. The command helps you find currentOp currentOp Queries not using any index Operations with high num YieldsOperations waiting for a lock Long running operations Queries not using any index The below example will help you find operations using no index query { “planSummary”: “COLLSCAN” } db.adminCommand({ : , : , : }) "currentOp" true "op" "query" "planSummary" "COLLSCAN" The MongoDB currentOp command to help find queries not using any index Operations with high numYields The below example will help you find all operations with greater than or 100 on database. Notice that the regular expression is also used to further filter the operations for database. numYields guidebook “ns”: /^guidebook./ guidebook db.adminCommand({ : , : , : { : } }) "currentOp" true "ns" /^guidebook\./ "numYields" "$gte" 100 The MongoDB currentOp command to help you find database operations with number of yields greater than or equal to 100 Operations waiting for a lock The following example returns information on all the write operations that are waiting for a lock. db.adminCommand({ : , : , : [ { : { : [ , , ] } }, { : { : } } ] }) "currentOp" true "waitingForLock" true "$or" "op" "$in" "insert" "update" "remove" "query.findandmodify" "$exists" true The MongoDB currentOp command to help you find all the write operations that are waiting for a lock. Long running query The following example returns information on all the operations running longer than 300 milliseconds. db.adminCommand({ : , : { : } }) "currentOp" true "microsecs_running" "$gte" 300000 /* // output for the above command { "inprog": [{ "host": "shyam-macbook.local:27017", "desc": "conn", "threadId": "0x70000a805000", "connectionId": 3807, "client": "127.0.0.1:52818", "appName": "MongoDB Shell", "clientMetadata": { "application": { "name": "MongoDB Shell" }, "driver": { "name": "MongoDB Internal Client", "version": "3.6.2" }, "os": { "type": "Darwin", "name": "Mac OS X", "architecture": "x86_64", "version": "17.5.0" } }, "active": true, "currentOpTime": "2018-05-16T15:20:22.253-0500", "opid": 835646, "secs_running": NumberLong(0), "microsecs_running": NumberLong(343179), "op": "update", "ns": "guidebook.restaurants", "command": { "q": { }, "u": { "$inc": { "visitor_count": 1 } }, "multi": true, "upsert": false }, "planSummary": "COLLSCAN", "numYields": 400, "locks": { "Global": "w", "Database": "w", "Collection": "w" }, "waitingForLock": false, "lockStats": { "Global": { "acquireCount": { "r": NumberLong(401), "w": NumberLong(401) } }, "Database": { "acquireCount": { "w": NumberLong(401) } }, "Collection": { "acquireCount": { "w": NumberLong(401) } } } }], "ok": 1 } */ The MongoDB currentOp command to help you find all the operations running longer than 300 milliseconds. Hands-On lab exercises This lab exercise makes use of the sample dataset from MongoDB. Please follow the below instructions to import the data, generate some load to help you learn the concepts discussed in here. restaurants restaurants # download and the restaurant sample data set wget https: mongoimport --db guidebook --collection restaurants --type json --file ./primer-dataset.json # download and run the load generation script wget https: mongo guidebook -- > >& & # open mongo shell and practice all the above use cases currentOp mongo guidebook #!/bin/sh import //raw.githubusercontent.com/mongodb/docs-assets/primer-dataset/primer-dataset.json //goo.gl/BXHH2d -O labs-performance-restaurant.js eval "load('./labs-performance-restaurant.js');r.performReads();" /dev/ null 2 1 for A bash script to help you download, import and generate load on the sample restaurant database. Summary The command provides means to find all the queries that are currently running on your MongoDB server. The optional fields can be further leveraged to help filter the results to specific criteria. db.currentOp() I want to reiterate a very important point — “the returns the information on ”. db.currentOp() in-progress operations If the queries of your interest execute very fast (a good thing), it may not even show up in output (most frustrating), if you ran the command a tad bit early or late. currentOp db.currentOp() To ensure you capture all your operations over certain period of time, you may execute the command in a loop as shown below. db.currentOp() while i = ; (i++ var 0 while < { ( ()); // ( ) } 10) printjson db.currentOp sleep for 10 ms if required. sleep 10 A JavaScript function to execute currentOp in a while loop to capture operations over a period of time. However, to capture operations over long period of time and consolidating the output for further processing would be become quite challenging. To help you properly capture all the queries over the period of time, you would need use . But that’s a topic for another day. Profiling Hopefully, you learned something new today on you scale the path to “ ”. Mastering MongoDB — One tip a day
