Background & Motivation

The current expectations of the "explain" command differ between csfle and mongocryptd.

mongocryptd expects "encryptionInformation" as a sibling of "explain":

{
    "explain": {
        "find": "foo"
    },
    "encryptionInformation": { "fields": [] },
    "$db": "db" // Appended by driver.
}

csfle expects "encryptionInformation" nested in "explain":

{
    "explain": {
        "find": "foo",
        "encryptionInformation": { "fields": [] },
        "$db": "db"
    },
    "$db": "db"
}

mongocryptd and csfle return "encryptionInformation" nested in "explain". Neither csfle nor mongocryptd return "$db" in the reply.

csfle requires a "$db" field be nested in the "explain" document matching the top-level "$db".

mongod and mongos expect "encryptionInformation" nested in "explain".

This is not urgent. libmongocrypt works around this in MONGOCRYPT-427. The consistent behavior would help simplify libmongocrypt auto encryption logic.

Proposed Scope

• csfle should parse "encryptionInformation" as either a sibling of "explain" or nested inside "explain". Allowing both supports the old behavior and new behavior.
• csfle should not require "$db" nested inside "explain".