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`	Empty `authInfo` (no auth implemented)
`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¶

Aggregation expressions: $function (out of scope — needs JS).
Aggregation stages: $fill (planned), $densify with unit: for date ranges (planned; numeric $densify is implemented).
mapReduce — deprecated by MongoDB; not implemented.

Out of scope¶

These are explicit non-goals:

Replica sets / sharding / change streams — depend on cluster topology or oplog. SecantusDB is single-process.
Authentication (SCRAM-SHA-256, x509, LDAP, Kerberos).
TLS / SSL.
OP_COMPRESSED — compression negotiation. Clients can be told the server doesn’t support compression.
Text search ($text, $meta: "textScore", text indexes).
Geo ($near, $nearSphere, $geoWithin, $geoIntersects, 2d / 2dsphere indexes).
$where — runs JavaScript. We don’t ship a JS runtime.
Capped collections — fixed-size, FIFO collections.
Profiling (setProfilingLevel, system.profile collection).
Tailable / awaitData cursors — depend on oplog or capped collections.

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.