11.10 Deleting documents in MongoDB: Recovering deleted documents

MongoDB is a document-oriented NoSQL database system that offers high performance, high availability and easy scalability. It works with the concept of collections and documents, instead of the traditional relational model of tables and records. In this section, we will discuss how to delete documents in a MongoDB database and how to recover these deleted documents.

Deleting documents in MongoDB

To delete documents in a MongoDB database, we use the remove() method. This method removes documents from a collection. The basic syntax of remove() is db.COLLECTION_NAME.remove(DELLETION_CRITTERIA).

Let's consider the following collection 'students'

{
   "_id" : ObjectId(5983548781331adf45ec5),
   "name" : "John Doe",
   "age": 22,
   "subjects" : ["Math", "Science", "English"]
}

To remove this document, we can use the following command:

db.students.remove({"name":"John Doe"})

This command removes the document where the 'name' field is 'John Doe'. If there were multiple documents with the name 'John Doe' they would all be removed as we did not specify any criteria for the _id.

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Recovery of deleted documents

Once a document is removed from MongoDB, it is permanently deleted and cannot be recovered. Therefore, it is important to make sure you really want to delete a document before doing so.

However, there are some strategies you can use to recover deleted documents:

1. Regular backups

The most effective way to protect yourself against data loss is to make regular backups of your database. MongoDB offers several backup strategies, including mongodump, file snapshot backups, and cloud backups. By taking regular backups, you can restore your database to a previous state and recover deleted documents.

2. Journaling

MongoDB supports journaling, which writes data change operations to disk before they are applied to the database. If a document is deleted, you may be able to recover it using journal. However, this is only possible if the document was recently deleted and the journal has not yet been overwritten.

3. Soft Delete

Another strategy is to use a 'soft delete', where you don't actually delete the document, but mark it as deleted. For example, you can add a 'deleted' field to the document and set its value to 'true' when the document is deleted. To recover the document, you simply reset the value of 'deleted' to 'false'.

In conclusion, deleting documents in MongoDB is a simple operation, but recovering them can be complicated. This is why it is so important to have an effective backup strategy and consider using techniques such as journaling and soft delete.