MongoDB: Failed to unlink socket file /tmp/mongodb-27017.sock Operation not permitted

Overview

When working with MongoDB on Unix-like systems such as Linux or macOS, you might encounter the error Failed to unlink socket file /tmp/mongodb-27017.sock Operation not permitted. This cryptic message can be quite daunting for those unfamiliar with it. In this tutorial, we’ll decode this message, understand its causes and present step-by-step solutions to address this error.

Understanding the MongoDB Socket File

Before diving into the solution, one should understand what a socket file is. In Unix and Unix-like systems, a socket file is used for inter-process communication. MongoDB typically uses a socket file located in the /tmp directory to listen for connections. The number ‘27017’ is the default port for MongoDB, hence the name of the socket file.

Now, the error message indicates that MongoDB cannot remove (unlink) an existing socket file when starting. This situation typically arises due to one or more of the following reasons:

• A previous instance of the service did not close properly and left the socket file behind.
• A new instance of MongoDB is being started with a user lacking the needed permissions to remove the socket file.
• File system corruption or permission changes due to a system crash or forced reboot.
• The directory /tmp has inappropriate permissions, being restricted from write access by the MongoDB user.

Basic Resolution Steps

Let’s begin by addressing this issue with basic troubleshooting steps before moving onto more advanced techniques.

Step 1: Check if MongoDB is already running

ps aux | grep mongod

This command line utility checks if the MongoDB daemon (mongod) is active. If you see an active mongod process, consider stopping it with:

sudo systemctl stop mongod

If your system uses a different service manager, you might need to adjust the above command accordingly.

Step 2: Remove the Socket File Manually

sudo rm /tmp/mongodb-27017.sock

If no active process was found or stopping it didn’t remove the socket file, you can try to remove it manually with the command above.

Step 3: Verify and Fix Directory Permissions

ls -ld /tmp

This command will show you the current permissions for the /tmp directory. It should be writable by all users:

drwxrwxrwt ...

If the permissions are incorrect, change them using:

sudo chmod 1777 /tmp

Once the correct permissions are set, attempt to restart MongoDB:

sudo systemctl start mongod

Advanced Troubleshooting Techniques

If the simple methods above do not fix the issue, let’s explore some advanced techniques that can be used to resolve it.

Verify MongoDB Configuration

Inspect the MongoDB configuration file (usually /etc/mongod.conf or /etc/mongodb.conf) and make sure the path for the socket file and its permissions are set correctly:

net:
  unixDomainSocket:
    enabled: true
    pathPrefix: /tmp
    filePermissions: 0755

If changes are made, restart the MongoDB service.

Use fuser or lsof

To investigate whether another process is bound to the file, use one of these commands:

sudo fuser /tmp/mongodb-27017.sock
sudo lsof /tmp/mongodb-27017.sock

If another process is using the file, these tools will output the PID. You can then decide to kill the process:

sudo kill -9 [PID]

Then attempt to remove the socket file and restart MongoDB.

Automatic Start on Boot

Configure MongoDB to start automatically when the system boots, which can mitigate the chances of this issue by having a controlled startup and shutdown process:

sudo systemctl enable mongod

Repair MongoDB

If the error still persists, consider repairing the MongoDB instance. This step should not be taken lightly, as it may lead to data loss, so it’s essential to back up your data before proceeding:

mongod --repair

Conclusion

Dealing with the Failed to unlink socket file error can be frustrating, but by following the step-by-step guide, users can get their MongoDB server back up and running in no time. For most situations, carefully stopping the MongoDB service, cleaning up leftover files, and making sure proper permissions are enabled should resolve the issue. As with all troubleshooting, ensure that you approach each step with caution and back up your data where appropriate.