MongoDB · January 2, 2024

How to fix MongoDB Error Code - 221 - IndexModified

How to Fix MongoDB Error Code - 221 - IndexModified

MongoDB is a popular NoSQL database that offers high performance, scalability, and flexibility. However, like any other software, it can encounter errors that need to be resolved. One such error is MongoDB Error Code - 221 - IndexModified. In this article, we will explore the causes of this error and provide step-by-step instructions on how to fix it.

Understanding MongoDB Error Code - 221 - IndexModified

MongoDB Error Code - 221 - IndexModified occurs when an attempt is made to modify an index that is currently being used by an active operation. This error typically arises when you try to drop or modify an index while it is being used by a query or an ongoing operation.

When this error occurs, MongoDB throws an exception with the error code 221. The error message usually looks like this:

Cannot drop index with namespace <namespace> because it is being used by an active operation.

Causes of MongoDB Error Code - 221 - IndexModified

There are several reasons why you might encounter MongoDB Error Code - 221 - IndexModified:

  • Concurrent Operations: If there are multiple operations running simultaneously on your MongoDB database, it is possible that one operation is using the index you are trying to modify or drop.
  • Long-Running Queries: If you have long-running queries that are still executing, they might be using the index you want to modify or drop.
  • Replica Set Syncing: If you are using a replica set, the syncing process might be using the index you are trying to modify or drop.

Fixing MongoDB Error Code - 221 - IndexModified

To fix MongoDB Error Code - 221 - IndexModified, follow these steps:

Step 1: Identify the Active Operations

First, you need to identify the active operations that are using the index you want to modify or drop. You can use the following command in the MongoDB shell to get a list of active operations:

db.currentOp()

This command will return a list of operations along with their details. Look for any operations that are using the index in question.

Step 2: Stop or Kill the Active Operations

Once you have identified the active operations, you have two options:

  • Stop the Operations: If the operations are not critical and can be stopped without causing any issues, you can stop them using the following command:
db.killOp(opid)

Replace opid with the ID of the operation you want to stop.

  • Kill the Operations: If stopping the operations is not an option or if they are unresponsive, you can kill them using the following command:
db.killOp(opid, "noassert")

Again, replace opid with the ID of the operation you want to kill.

Step 3: Modify or Drop the Index

Once the active operations are stopped or killed, you can proceed with modifying or dropping the index. Use the appropriate MongoDB commands to perform the desired action:

  • Drop Index: Use the db.collection.dropIndex() command to drop the index. Make sure to specify the correct index name.
  • Modify Index: Use the db.collection.reIndex() command to modify the index. This command rebuilds the index without locking the collection.

Summary

MongoDB Error Code - 221 - IndexModified occurs when an attempt is made to modify or drop an index that is being used by an active operation. To fix this error, you need to identify the active operations, stop or kill them, and then proceed with modifying or dropping the index. If you encounter this error, follow the steps outlined in this article to resolve it.

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