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Fundamentals

Encrypted Collection Management

On this page

  • Overview
  • Metadata Collections
  • Dropping Encrypted Collections
  • Storage Costs
  • Write Costs
  • Insert Operations
  • Update Operations
  • Delete Operations
  • Metadata Collection Compaction

It is important that you understand the performance and storage costs of field level encryption. Each encrypted field:

  • Adds writes to insert and update operations.

  • Requires additional storage, because MongoDB maintains an encrypted field index.

This section lists the writes per operation and explains how to compact encrypted collection indexes so that you can minimize write and storage costs.

Overview

Queryable Encryption introduces the ability to encrypt sensitive fields in your documents using randomized encryption, while still being able to query the encrypted fields.

With Queryable Encryption, a given plaintext value always encrypts to a different ciphertext, while still remaining queryable. To enable this functionality, Queryable Encryption uses three data structures:

  • Two metadata collections

  • A field in every document in the encrypted collection called __safeContent__

Important

It is critical that these data structures are not modified or deleted, or query results will be incorrect.

Metadata Collections

When you create an encrypted collection, MongoDB creates two metadata collections:

  • enxcol_.<collectionName>.esc, referred to as ESC

  • enxcol_.<collectionName>.ecoc, referred to as ECOC

Example

If you create a collection called "patients", MongoDB creates the following metadata collections:

  • enxcol_.patients.esc

  • enxcol_.patients.ecoc

When you insert documents with a queryable encrypted field, MongoDB updates the metadata collections to maintain an index that enables querying. The field becomes an "indexed field". This comes at a cost in storage and write speed for every such field.

Dropping Encrypted Collections

When you drop an encrypted collection, drop the associated metadata collections enxcol_.<collectionName>.esc and enxcol_.<collectionName>.ecoc immediately afterwards. Otherwise, re-creating the collection with the same name puts the metadata collections in a conflicted state that consumes excess storage space and degrades CRUD performance.

Storage Costs

Storage and write costs increase based on the number of indexed fields per document.

Important

Expect a Queryable Encryption collection to have 2-3 times the storage requirements of the documents, to account for metadata collections. For example, a 1 GB collection may have a storage requirement of 2-3 GB.

Write Costs

Insert Operations

When inserting a document, each indexed field requires two additional writes to metadata collections.

  • One write to ESC

  • One write to ECOC

Example

Inserting a document with two indexed fields requires:

  • One write to the encrypted collection.

  • Four writes to the metadata collections.

Update Operations

When updating a document, each indexed field requires two additional writes to metadata collections.

  • One write to ESC

  • One write to ECOC

Example

Updating a document with two indexed fields requires:

  • One write to the encrypted collection.

  • Four writes to the metadata collections.

Delete Operations

When deleting a document, indexed fields do not require any additional writes.

Metadata Collection Compaction

As you insert or update documents, the metadata collections change and grow. Metadata collection compaction prunes the metadata collections and reduces their size.

Important

You must manually run metadata collection compaction. Compaction only works on clients configured for Queryable Encryption.

Run compaction when the size of ECOC exceeds 1 GB.

You can check the size of your collections using mongosh and issuing the db.collection.totalSize() command.

Example

In this example, the encrypted collection is named "patients".

db.enxcol_.patients.ecoc.totalSize()
1407960328

To run metadata collection compaction, use mongosh and run the db.collection.compactStructuredEncryptionData() command to reduce the size of the metadata collections.

Example

const eDB = "encryption"
const eKV = "__keyVault"
const secretDB = "records"
const secretCollection = "patients"
const localKey = fs.readFileSync("master-key.txt")
const localKeyProvider = { key: localKey }
const queryableEncryptionOpts = {
  kmsProviders: { local: localKeyProvider },
  keyVaultNamespace: `${eDB}.${eKV}`,
}
const encryptedClient = Mongo("localhost:27017", queryableEncryptionOpts)
const encryptedDB = encryptedClient.getDB(secretDB)
const encryptedCollection = encryptedDB.getCollection(secretCollection)
encryptedCollection.compactStructuredEncryptionData()
{
  "stats": {
     ...
  },
  "ok": 1,
  ...
}

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