MongoDB can’t find data directory after upgrading to Mac OS 10.15 (Catalina)

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This is the main error:

exception in initAndListen: NonExistentPath: Data directory /data/db not found., terminating

Catalina has a surprise change: it won’t allow changes to the root directory (this was discussed in a forum thread as well):

% sudo mkdir -p /data/db
mkdir: /data/db: Read-only file system

Unfortunately, this is not spelled out explicitly in Apple’s Catalina release notes, other than a brief mention in Catalina features:

macOS Catalina runs in a dedicated, read-only system volume

Since the directory /data/db is coded as MongoDB default, a workaround is to specify a different dbpath that is not located on the root directory. For example:

mongod --dbpath ~/data/db

This will place MongoDB’s data in your home directory. Just make sure that the path ~/data/db actually exists.

Alternative method

An alternative method is to follow the instructions at Install MongoDB Community Edition on macOS by leveraging brew:

brew tap mongodb/brew
brew install mongodb-community

This will create some additional files by default:

  • the configuration file (/usr/local/etc/mongod.conf)
  • the log directory path (/usr/local/var/log/mongodb)
  • the data directory path (/usr/local/var/mongodb)

To run mongod you can either:

  • Run the command manually from the command line (this can be aliased for convenience):

    mongod --config /usr/local/etc/mongod.conf

  • Run MongoDB as a service using brew services. Note that this will run MongoDB as a standalone node (not a replica set), so features that depends on the oplog e.g. changestreams will not work unless you modify the mongod configuration file:

    brew services start mongodb-community

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