[[mongo.sessions]]
= MongoDB Sessions

As of version 3.6, MongoDB supports the concept of sessions. The use of sessions enables MongoDB's https://docs.mongodb.com/manual/core/read-isolation-consistency-recency/#causal-consistency[Causal Consistency] model, which guarantees running operations in an order that respects their causal relationships. Those are split into `ServerSession` instances and `ClientSession` instances. In this section, when we speak of a session, we refer to `ClientSession`.

WARNING: Operations within a client session are not isolated from operations outside the session.

Both `MongoOperations` and `ReactiveMongoOperations` provide gateway methods for tying a `ClientSession` to the operations. `MongoCollection` and `MongoDatabase` use session proxy objects that implement MongoDB's collection and database interfaces, so you need not add a session on each call. This means that a potential call to `MongoCollection#find()` is delegated to `MongoCollection#find(ClientSession)`.

NOTE: Methods such as `(Reactive)MongoOperations#getCollection` return native MongoDB Java Driver gateway objects (such as `MongoCollection`) that themselves offer dedicated methods for `ClientSession`. These methods are *NOT* session-proxied. You should provide the `ClientSession` where needed when interacting directly with a `MongoCollection` or `MongoDatabase` and not through one of the `#execute` callbacks on `MongoOperations`.

[[mongo.sessions.sync]]
== Synchronous `ClientSession` support.

The following example shows the usage of a session:

.`ClientSession` with `MongoOperations`
====
[source,java]
----
ClientSessionOptions sessionOptions = ClientSessionOptions.builder()
    .causallyConsistent(true)
    .build();

ClientSession session = client.startSession(sessionOptions); <1>

template.withSession(() -> session)
    .execute(action -> {

        Query query = query(where("name").is("Durzo Blint"));
        Person durzo = action.findOne(query, Person.class);  <2>

        Person azoth = new Person("Kylar Stern");
        azoth.setMaster(durzo);

        action.insert(azoth);                                <3>

        return azoth;
    });

session.close()                                              <4>
----
<1> Obtain a new session from the server.
<2> Use `MongoOperation` methods as before. The `ClientSession` gets applied automatically.
<3> Make sure to close the `ClientSession`.
<4> Close the session.
====

WARNING: When dealing with `DBRef` instances, especially lazily loaded ones, it is essential to *not* close the `ClientSession` before all data is loaded. Otherwise, lazy fetch fails.

[[mongo.sessions.reactive]]
== Reactive `ClientSession` support

The reactive counterpart uses the same building blocks as the imperative one, as the following example shows:

.ClientSession with `ReactiveMongoOperations`
====
[source,java]
----
ClientSessionOptions sessionOptions = ClientSessionOptions.builder()
    .causallyConsistent(true)
    .build();

Publisher<ClientSession> session = client.startSession(sessionOptions); <1>

template.withSession(session)
    .execute(action -> {

        Query query = query(where("name").is("Durzo Blint"));
        return action.findOne(query, Person.class)
            .flatMap(durzo -> {

                Person azoth = new Person("Kylar Stern");
                azoth.setMaster(durzo);

                return action.insert(azoth);                            <2>
            });
    }, ClientSession::close)                                            <3>
    .subscribe();                                                       <4>
----
<1> Obtain a `Publisher` for new session retrieval.
<2> Use `ReactiveMongoOperation` methods as before. The `ClientSession` is obtained and applied automatically.
<3> Make sure to close the `ClientSession`.
<4> Nothing happens until you subscribe. See https://projectreactor.io/docs/core/release/reference/#reactive.subscribe[the Project Reactor Reference Guide] for details.
====

By using a `Publisher` that provides the actual session, you can defer session acquisition to the point of actual subscription.
Still, you need to close the session when done, so as to not pollute the server with stale sessions. Use the `doFinally` hook on `execute` to call `ClientSession#close()` when you no longer need the session.
If you prefer having more control over the session itself, you can obtain the `ClientSession` through the driver and provide it through a `Supplier`.

NOTE: Reactive use of `ClientSession` is limited to Template API usage. There's currently no session integration with reactive repositories.

[[mongo.transactions]]
= MongoDB Transactions

As of version 4, MongoDB supports https://www.mongodb.com/transactions[Transactions]. Transactions are built on top of <<mongo.sessions,Sessions>> and, consequently, require an active `ClientSession`.

NOTE: Unless you specify a `MongoTransactionManager` within your application context, transaction support is *DISABLED*. You can use `setSessionSynchronization(ALWAYS)` to participate in ongoing non-native MongoDB transactions.

To get full programmatic control over transactions, you may want to use the session callback on `MongoOperations`.

The following example shows programmatic transaction control within a `SessionCallback`:

.Programmatic transactions
====
[source,java]
----
ClientSession session = client.startSession(options);                   <1>

template.withSession(session)
    .execute(action -> {

        session.startTransaction();                                     <2>

        try {

            Step step = // ...;
            action.insert(step);

            process(step);

            action.update(Step.class).apply(Update.set("state", // ...

            session.commitTransaction();                                <3>

        } catch (RuntimeException e) {
            session.abortTransaction();                                 <4>
        }
    }, ClientSession::close)                                            <5>
----
<1> Obtain a new `ClientSession`.
<2> Start the transaction.
<3> If everything works out as expected, commit the changes.
<4> Something broke, so roll back everything.
<5> Do not forget to close the session when done.
====

The preceding example lets you have full control over transactional behavior while using the session scoped `MongoOperations` instance within the callback to ensure the session is passed on to every server call.
To avoid some of the overhead that comes with this approach, you can use a `TransactionTemplate` to take away some of the noise of manual transaction flow.

[[mongo.transactions.transaction-template]]
== Transactions with `TransactionTemplate`

Spring Data MongoDB transactions support a `TransactionTemplate`. The following example shows how to create and use a `TransactionTemplate`:

.Transactions with `TransactionTemplate`
====
[source,java]
----
template.setSessionSynchronization(ALWAYS);                                     <1>

// ...

TransactionTemplate txTemplate = new TransactionTemplate(anyTxManager);         <2>

txTemplate.execute(new TransactionCallbackWithoutResult() {

    @Override
    protected void doInTransactionWithoutResult(TransactionStatus status) {     <3>

        Step step = // ...;
        template.insert(step);

        process(step);

        template.update(Step.class).apply(Update.set("state", // ...
    };
});
----
<1> Enable transaction synchronization during Template API configuration.
<2> Create the `TransactionTemplate` using the provided `PlatformTransactionManager`.
<3> Within the callback the `ClientSession` and transaction are already registered.
====

CAUTION: Changing state of `MongoTemplate` during runtime (as you might think would be possible in item 1 of the preceding listing) can cause threading and visibility issues.

[[mongo.transactions.tx-manager]]
== Transactions with `MongoTransactionManager`

`MongoTransactionManager` is the gateway to the well known Spring transaction support. It lets applications use  https://docs.spring.io/spring/docs/{springVersion}/spring-framework-reference/html/transaction.html[the managed transaction features of Spring].
The `MongoTransactionManager` binds a `ClientSession` to the thread. `MongoTemplate` detects the session and operates on these resources which are associated with the transaction accordingly. `MongoTemplate` can also participate in other, ongoing transactions. The following example shows how to create and use transactions with a `MongoTransactionManager`:

.Transactions with `MongoTransactionManager`
====
[source,java]
----
@Configuration
static class Config extends AbstractMongoClientConfiguration {

    @Bean
    MongoTransactionManager transactionManager(MongoDatabaseFactory dbFactory) {  <1>
        return new MongoTransactionManager(dbFactory);
    }

    // ...
}

@Component
public class StateService {

    @Transactional
    void someBusinessFunction(Step step) {                                        <2>

        template.insert(step);

        process(step);

        template.update(Step.class).apply(Update.set("state", // ...
    };
});

----
<1> Register `MongoTransactionManager` in the application context.
<2> Mark methods as transactional.
====

NOTE: `@Transactional(readOnly = true)` advises `MongoTransactionManager` to also start a transaction that adds the
 `ClientSession` to outgoing requests.

[[mongo.transactions.reactive]]
== Reactive Transactions

Same as with the reactive `ClientSession` support, the `ReactiveMongoTemplate` offers dedicated methods for operating
within a transaction without having to worry about the committing or stopping actions depending on the operations outcome.

NOTE: Unless you specify a `ReactiveMongoTransactionManager` within your application context, transaction support is *DISABLED*. You can use `setSessionSynchronization(ALWAYS)` to participate in ongoing non-native MongoDB transactions.

Using the plain MongoDB reactive driver API a `delete` within a transactional flow may look like this.

.Native driver support
====
[source,java]
----
Mono<DeleteResult> result = Mono
    .from(client.startSession())                                                             <1>

    .flatMap(session -> {
        session.startTransaction();                                                          <2>

        return Mono.from(collection.deleteMany(session, ...))                                <3>

            .onErrorResume(e -> Mono.from(session.abortTransaction()).then(Mono.error(e)))   <4>

            .flatMap(val -> Mono.from(session.commitTransaction()).then(Mono.just(val)))     <5>

            .doFinally(signal -> session.close());                                           <6>
      });
----
<1> First we obviously need to initiate the session.
<2> Once we have the `ClientSession` at hand, start the transaction.
<3> Operate within the transaction by passing on the `ClientSession` to the operation.
<4> If the operations completes exceptionally, we need to stop the transaction and preserve the error.
<5> Or of course, commit the changes in case of success. Still preserving the operations result.
<6> Lastly, we need to make sure to close the session.
====

The culprit of the above operation is in keeping the main flows `DeleteResult` instead of the transaction outcome
published via either `commitTransaction()` or `abortTransaction()`, which leads to a rather complicated setup.

[[mongo.transactions.reactive-operator]]
== Transactions with `TransactionalOperator`

Spring Data MongoDB transactions support a `TransactionalOperator`. The following example shows how to create and use a `TransactionalOperator`:

.Transactions with `TransactionalOperator`
====
[source,java]
----
template.setSessionSynchronization(ALWAYS);                                          <1>

// ...

TransactionalOperator rxtx = TransactionalOperator.create(anyTxManager,
                                   new DefaultTransactionDefinition());              <2>


Step step = // ...;
template.insert(step);

Mono<Void> process(step)
    .then(template.update(Step.class).apply(Update.set("state", …))
    .as(rxtx::transactional)                                                         <3>
    .then();
----
<1> Enable transaction synchronization for Transactional participation.
<2> Create the `TransactionalOperator` using the provided `ReactiveTransactionManager`.
<3> `TransactionalOperator.transactional(…)` provides transaction management for all upstream operations.
====

[[mongo.transactions.reactive-tx-manager]]
== Transactions with `ReactiveMongoTransactionManager`

`ReactiveMongoTransactionManager` is the gateway to the well known Spring transaction support.
It allows applications to leverage https://docs.spring.io/spring/docs/{springVersion}/spring-framework-reference/html/transaction.html[the managed transaction features of Spring].
The `ReactiveMongoTransactionManager` binds a `ClientSession` to the subscriber `Context`.
`ReactiveMongoTemplate` detects the session and operates on these resources which are associated with the transaction accordingly.
`ReactiveMongoTemplate` can also participate in other, ongoing transactions.
The following example shows how to create and use transactions with a `ReactiveMongoTransactionManager`:

.Transactions with `ReactiveMongoTransactionManager`
====
[source,java]
----
@Configuration
public class Config extends AbstractReactiveMongoConfiguration {

    @Bean
    ReactiveMongoTransactionManager transactionManager(ReactiveMongoDatabaseFactory factory) {  <1>
        return new ReactiveMongoTransactionManager(factory);
    }

    // ...
}

@Service
public class StateService {

    @Transactional
    Mono<UpdateResult> someBusinessFunction(Step step) {                                  <2>

        return template.insert(step)
            .then(process(step))
            .then(template.update(Step.class).apply(Update.set("state", …));
    };
});

----
<1> Register `ReactiveMongoTransactionManager` in the application context.
<2> Mark methods as transactional.
====

NOTE: `@Transactional(readOnly = true)` advises `ReactiveMongoTransactionManager` to also start a transaction that adds the `ClientSession` to outgoing requests.

[[mongo.transactions.behavior]]
== Special behavior inside transactions

Inside transactions, MongoDB server has a slightly different behavior.

*Connection Settings*

The MongoDB drivers offer a dedicated replica set name configuration option turing the driver into auto detection
mode. This option helps identifying the primary replica set nodes and command routing during a transaction.

NOTE: Make sure to add `replicaSet` to the MongoDB URI. Please refer to https://docs.mongodb.com/manual/reference/connection-string/#connections-connection-options[connection string options] for further details.

*Collection Operations*

MongoDB does *not* support collection operations, such as collection creation, within a transaction. This also
affects the on the fly collection creation that happens on first usage. Therefore make sure to have all required
structures in place.

*Transient Errors*

MongoDB can add special labels to errors raised during transactional operations. Those may indicate transient failures
that might vanish by merely retrying the operation.
We highly recommend https://github.com/spring-projects/spring-retry[Spring Retry] for those purposes. Nevertheless
one may override `MongoTransactionManager#doCommit(MongoTransactionObject)` to implement a https://docs.mongodb.com/manual/core/transactions/#retry-commit-operation[Retry Commit Operation]
behavior as outlined in the MongoDB reference manual.

*Count*

MongoDB `count` operates upon collection statistics which may not reflect the actual situation within a transaction.
The server responds with _error 50851_ when issuing a `count` command inside of a multi-document transaction.
Once `MongoTemplate` detects an active transaction, all exposed `count()` methods are converted and delegated to the
aggregation framework using `$match` and `$count` operators, preserving `Query` settings, such as `collation`.

Restrictions apply when using geo commands inside of the aggregation count helper. The following operators cannot be used and must be replaced with a different operator:

* `$where` -> `$expr`
* `$near` -> `$geoWithin` with `$center`
* `$nearSphere` -> `$geoWithin` with `$centerSphere`

Queries using `Criteria.near(…)` and `Criteria.nearSphere(…)` must be rewritten to `Criteria.within(…)` respective `Criteria.withinSphere(…)`. Same applies for the `near` query keyword in repository query methods that must be changed to `within`. See also MongoDB JIRA ticket https://jira.mongodb.org/browse/DRIVERS-518[DRIVERS-518] for further reference.

The following snippet shows `count` usage inside the session-bound closure:

====
[source,javascript]
----
session.startTransaction();

template.withSession(session)
    .execute(action -> {
        action.count(query(where("state").is("active")), Step.class)
        ...
----
====

The snippet above materializes in the following command:

====
[source,javascript]
----
db.collection.aggregate(
   [
      { $match: { state: "active" } },
      { $count: "totalEntityCount" }
   ]
)
----
====

instead of:

====
[source,javascript]
----
db.collection.find( { state: "active" } ).count()
----
====
