codeconsole opened a new pull request, #15744:
URL: https://github.com/apache/grails-core/pull/15744
## What
Adds **opt-in MongoDB multi-document transactions** to GORM for MongoDB.
Previously GORM for MongoDB never used a `com.mongodb.client.ClientSession`:
every driver write was issued session-less and auto-committed, so a GORM
transaction was only a client-side flush boundary — writes already flushed
within it were **not** rolled back on failure (no server-side atomicity).
With `grails.mongodb.transactional = true`, a GORM transaction now starts
and drives a real `ClientSession` transaction, so all reads and writes commit
or roll back atomically:
```groovy
Person.withTransaction {
new Person(name: "Fred").save()
new Person(name: "Wilma").save()
// both commit together, or neither if an exception is thrown
}
```
## How
- New `MongoTransaction` (replaces the flush-only `SessionOnlyTransaction`
when enabled): `commit()` flushes then `commitTransaction()` (with bounded
retry on `UnknownTransactionCommitResult`), `rollback()` aborts; both close the
session. On commit failure the GORM session cache is cleared.
- `AbstractMongoSession` holds the active `ClientSession`, starts it in
`beginTransactionInternal()`, and routes every read/write through small helpers
that pass the session when a transaction is active and stay session-less
otherwise.
- The session is threaded through both session engines, both persisters,
`MongoQuery`, and the `MongoStaticApi`/`MongoEntity` surface.
- Core `DatastoreTransactionManager` is unchanged — it already orchestrates
flush/commit/rollback; this just supplies a `Transaction` that drives a server
transaction.
## Opt-in and fallback
- Default is **off** (`grails.mongodb.transactional` defaults to `false`) —
no behavior change for existing apps.
- Requires a replica set or sharded cluster. If a standalone topology is
detected, the feature is disabled with a one-time warning and GORM falls back
to the legacy flush behavior.
## Boundaries
- Identifier generation (the native `Long` counter) is intentionally left
non-transactional, mirroring database sequence semantics.
- Like GORM's transaction manager generally, a single flat transaction
(`PROPAGATION_REQUIRED`) is supported; `REQUIRES_NEW`/`NESTED` are not.
## Tests
- `MongoTransactionSpec` — commit persists multiple docs; rollback discards
them on the server; read-your-writes within a transaction; cross-collection
atomic rollback; `findOneAndDelete` participates in the transaction; nested
GORM `REQUIRES_NEW`.
- `MongoTransactionDisabledSpec` — default-off keeps the legacy flush
behavior.
Targets `8.0.x`. Independent of #15743; this is also the prerequisite for a
follow-up Spring Data MongoDB interop module.
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