Compatibility¶

SecantusDB’s conformance target is pymongo: a pymongo client should not be able to tell SecantusDB apart from a real mongod for the operations it supports. This page lists the divergences that exist anyway.

Stubs¶

These commands accept the request and return a wire-valid response, but the response is fabricated.

Command	Behaviour
`serverStatus`	Version + zeroed metrics (uptime, connections)
`connectionStatus`	Real `authenticatedUsers` (from SCRAM-SHA-256). `authenticatedUserRoles` / `authenticatedUserPrivileges` are empty (RBAC not enforced — see Authentication)
`hostInfo` / `whatsmyuri` / `buildInfo`	Hardcoded values; `buildInfo.version` is `"7.0.0"`
`getLog`	Empty log array
`startSession` / `endSessions` / `refreshSessions`	`startSession` returns a fresh UUID; the others are no-ops. No session state is tracked, so cross-session correlation isn’t enforced
`abortTransaction` / `commitTransaction`	Return `{ok: 1}` but do not roll back. Operations inside a transaction take effect immediately. Tests that depend on real transactional rollback need a real `mongod`

dbStats and collStats return real count, size, storageSize, avgObjSize, indexSize, indexSizes, totalIndexSize, and totalSize computed from the WT tables.

explain reports IXSCAN when an index would be used and COLLSCAN otherwise, with indexName, keyPattern, and direction populated.

Stopgaps (functional but with limitations)¶

`$lookup` doesn’t use storage indexes¶

Joins are O(N+M) via an in-memory hash table built once over the foreign collection (covers array-valued local/foreign fields correctly via element expansion). Both simple (localField/foreignField) and let/pipeline forms are accelerated.

A true index-driven join would skip materialising the foreign collection but needs multikey-index support to stay correct for array-valued foreign fields.

`_id` numeric type bridge¶

Works for finite int / float / Decimal128 (they collide on equal value). bool is deliberately not numeric. NaN and infinity _id values fall through to the BSON-blob path; behaviour is unspecified.

Date format strings¶

$dateFromString and $dateToString use Python’s strptime / strftime codes plus the %L extension for milliseconds. The timezone argument is supported (IANA, UTC offsets, GMT/UTC).

Still missing: full MongoDB format spec (%G / %V ISO-week, %j day-of-year edge cases) and the MongoDB-specific format tokens.

`$merge` `whenMatched: "merge"`¶

Recursive sub-document merge implemented (matches MongoDB). Arrays are replaced as a whole on overlapping keys.

`renameCollection`¶

Atomic per the storage RLock, but no protection against concurrent writers across processes. Tests are single-process so this is fine.

`createIndexes` options¶

Option	Status
`unique`	Honoured
`sparse`	Honoured
`expireAfterSeconds`	Honoured via `Storage.prune_ttl` (opt-in; no background sweeper)
`partialFilterExpression`	Honoured at write time and at picker time
`collation`	Accepted but ignored (Python compares with default locale)

Cursor TTL¶

Cursors idle longer than 600s are pruned opportunistically (matches MongoDB’s 10-minute cursor TTL). The clock is injectable via time_func for deterministic tests.

Deferred¶

$densify with month / quarter / year units — rejected; numeric ranges and fixed-duration date units (week / day / hour / minute / second / millisecond) are implemented.
Per-index collation — the per-query infrastructure honours collation for find / count / distinct / findAndModify, but index entries are written in BSON codepoint order; queries carrying collation fall through to COLLSCAN by design.

Out of scope¶

These are explicit non-goals:

Replica sets / sharding — depend on multi-node cluster topology. SecantusDB is single-process. (Change streams are supported — oplog-backed and single-node — see Change streams. The oplog is queryable at local.oplog.rs like real mongod.)
Authentication mechanisms beyond SCRAM-SHA-256 — x509, LDAP, Kerberos, GSSAPI, MONGODB-AWS, MONGODB-OIDC. SCRAM-SHA-256 itself is implemented; SCRAM-SHA-1 is not advertised (modern drivers default to SHA-256). See Authentication.
Authorization (RBAC) — createUser accepts a roles array but no command consults it. An authenticated principal is treated as fully privileged.
TLS / SSL. SCRAM credentials therefore travel in plaintext — use only on a trusted network.
OP_COMPRESSED — compression negotiation. Clients can be told the server doesn’t support compression.
Text search ($text, $meta: "textScore", text indexes) — would need a full-text index implementation.
$where / $function / $accumulator / mapReduce — all four evaluate user-supplied JavaScript and would need an embedded JS engine + sandbox + BSON↔JS shim layer. mapReduce is also explicitly deprecated by MongoDB; the canonical emit(this.<field>, 1) + values.length count pattern is recognised and translated to $group, but anything else needs real mongod.
Real transaction rollback — commitTransaction / abortTransaction return {ok: 1} but operations take effect immediately. Logical sessions ARE tracked end-to-end.

What HAS shipped that’s worth calling out (was previously listed as “deferred” or “out of scope”): geo operators + 2d / 2dsphere indexes; capped collections (create capped: true) with FIFO eviction; profiling (profile command + <db>.system.profile); SCRAM-SHA-256 authentication; the oplog as a queryable local.oplog.rs collection. See the changelog for the full inventory and aggregation / indexes / change streams for the detail.

Known edge cases¶

$sample uses random.sample without a fixed seed. Deterministic only if the test does random.seed(...) first.
$type: "number" in queries handles int, float, Decimal128, but the int32-vs-int64 distinction depends on the Python value range, not the original BSON type tag (which is dropped on decode). A doc inserted as Int64(5) reads back as a small Python int and matches $type: "int", not "long".
$lookup simple-form-plus-pipeline — when both localField / foreignField and pipeline are present, we pre-filter by the simple form and then run the pipeline. Real MongoDB does this in modern versions; documentation isn’t crystal clear on the order. If a test breaks here, this is the place to look.
Aggregation $group stable order — group buckets are emitted in first-seen order, not sorted. Matches unsharded MongoDB; sharded behaviour isn’t modelled.