Monitor Slow Queries

The $currentOp aggregation stage provides information on all operations currently running on MongoDB. If your application is experiencing performance issues, you can build an aggregation pipeline around this stage to monitor for slow queries and similar issues.

About This Task

Database Profilers

This task uses $currentOp to identify slow queries currently running on your application. To find all slow queries within a specified period, consider using a profiler.

Profiler	Description
Atlas Query Profiler	Provides a scatterplot chart to Atlas customers, making it easy to identify slow queries and performance outliers.
Database Profiler	Stores query performance information in a collection, allowing you to query MongoDB for queries with specific performance issues.

Both the Atlas Query Profiler and the database profiler can affect server performance, use up disk space, and expose query metadata around encrypted fields. Consider the performance and security implications before enabling them.

Explain Queries

This task identifies queries with performance issues. If you already know which queries have performance issues, see Explain Slow Queries to troubleshoot them.

Steps

Retrieve current operations.

Use the $currentOp aggregation stage to retrieve current operations from MongoDB:

db.getSiblingDB("admin").aggregate( [
   { $currentOp: { allUsers: true } },
   { $match: { secs_running: { $gt: 2 } } },
   { $sort: { secs_running: 1 } }
] )

[
   {
      "opid": "12345",
      "secs_running": 5,
      "active": true,
      "ns": "sample_mflix.movies",
      "command": {
         "find": "movies",
         "filter": { "title": { "$regex": "The" } }
      },
      "planSummary": "COLLSCAN",
      "locks": { ... },
      "client": "203.0.113.25:43210"
   }
]

This aggregation pipeline retrieves all current operations in the cluster. The $match aggregation stage then filters the operations to those that have been running for more than two seconds. This allows you to filter queries that run within a specified period. Adjust the value to match your application and database needs.

The $sort stage sorts the results in ascending operation time order.

Check for activity.

View the currentOp.active field. If currentOp.active is true, MongoDB indicates that the operation is currently running.

To stop a long running operation, use the db.killOp() method to stop the given opid.

Check for locks.

View the currentOp.waitingForLock field. If currentOp.waitingForLock is true, another operation running on the server or cluster is blocking the query.

To stop a blocked operation, use the db.killOp() method to stop the given opid.

Check the plan summary.

Check the value in the currentOp.planSummary field.

IXSCAN: Indicates the query performed an index scan.
COLLSCAN: Indicates the query performed a full collection scan. To correct this, Create an Index.

Explain the query.

If the $currentOp aggregation stage returns a query that requires further investigation, use the explain() method to analyze the query plan and execution statistics.

For details, see Explain Slow Queries.

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Find Slow Queries

Block Slow Queries