Cannot obtain resource count from API ( _summary=count ) when using MongoDB if matched resources count is moderately large (roughly 170k+)

[JeremiahSanders]

Describe the bug
Spark returns an internal server error when a ?_summary=count request would find a moderately large number of resources. (Exact count is unknown but observed when the applicable resources were between 170,000 and 225,000.)

Captured Message from OperationOutcome:

Operation was unsuccessful, and returned status InternalServerError. OperationOutcome: Overall result: FAILURE (2 errors and 0 warnings)
[ERROR] (no details)(further diagnostics: BsonSerializationException: An error occurred while serializing the Keys property of class Spark.Engine.Core.Snapshot: Size 17094593 is larger than MaxDocumentSize 16777216.)
[ERROR] (no details)(further diagnostics: FormatException: Size 17094593 is larger than MaxDocumentSize 16777216.)
.

To Reproduce
Steps to reproduce the behavior:

1. Seed a Spark server using MongoDB with 225,000 Patient resources. (Exact count needed is unknown, but the minimum is > 150,000.)
2. Submit a FHIR GET request to obtain the Patient count at the server's Patient API (i.e., <fhirRootUri>/Patient?_summary=count)
3. Observe failure.

Expected behavior
Expected to receive a Bundle response containing only the count of Patient resources.

Spark version

• Version: 2.3.4

Operating system + Database

• OS: Linux
• Database: MongoDB

Container service / Cloud infrastructure:

• Container service: Elastic Container Service
• Cloud provider: AWS
• Cloud infrastructure: Docker container hosted on AWS EC2 in ECS
• Database as a service: AWS DocumentDB w/ MongoDB API

Additional context
This persistence error appears directly related to search snapshot persistence.
This error does not prevent inserting resources or preclude searches which return small result sets.