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# Transport Internals
## Ingress Networking
Ingress networking refers to when a MongoDB process receives incoming requests
from a client.
When a client wants to interact with a server, they must first establish a
connection. The server listens on a single port for incoming client connections.
Once a client connection is received over an ephermeral port, a client session
is created out of this connection and commands begin to be processed on a new
dedicated thread.
The session takes in commands received from the client and hands them off to a
[ServiceEntryPointImpl]. This entry point will spawn threads to perform specific
tasks via a [ServiceExecutor], which is then used to initialize a
[ServiceStateMachine]. The ServiceStateMachine is a state machine that manages
the life cycle of the client connection.
These are the valid state transitions:
* *Source -> SourceWait -> {ProcessStandard,ProcessExhaust,ProcessMoreToCome}*
* *ProcessStandard -> SinkStandard -> SinkWait -> Source (standard RPC)*
* *ProcessExhaust -> SinkMoreToCome-> SinkWait -> ProcessExhaust*
* *ProcessExhaust -> SinkFinal -> SinkWait -> Source*
* *ProcessMoreToCome -> Source*
*Source* - When the server is in the state it requests a new message from the
network to handle.
*SourceWait* - This simply the waiting state for that requested message to arrive.
The message that is received will also dictate which mode the *Process* state will
be transitioned into.
*Process* - The message enters through the [ServiceEntryPoint] and will be run
through the database in this state. This state may operate in different modes
(*Standard*, *Exhaust*, or *MoreToCome*) which dictate the next state transition. The
*MoreToCome* mode denotes that another message should be received before any other
action, and so the server returns to the *Source* state when *MoreToCome* mode is
seen by the *Process* state on the request message. Meanwhile the *Exhaust* mode
indicates that multiple messages must be sent back, and so it will continue to
send more response messages until this state stops setting *MoreToCome* mode.
*SinkWait* - Marks the period when the server waits for the database result to be
sent over the network This state is usually preceded by a *Sink* state that
follows three modes similar to *Process* (*Standard*, *MoreToCome*, *Final*). *Standard*
and *Final* will simply proceed to the *SinkWait* stage to await the database result
and then return to *Source*. If the *MoreToCome* mode is enabled, however, the
server will go on to *ProcessExhaust* following the standard *SinkWait* state. When
this happens the server will not transition back to *Source* until a message is
received with *MoreToCome* disabled.
*EndSession* - Denotes the end of a session.
Should an error occur throughout the lifecycle, the state will transition to the
*EndSession* state.
In order to return the results to the user whether it be a document or a response
code, MongoDB uses the [ReplyBuilderInterface]. This interface helps to build
message bodies for replying to commands or even returning documents.
This builder interface includes a standard BodyBuilder that builds reply
messages in serialized-BSON format.
A Document body builder ([DocSequenceBuilder]) is also defined to help build a
reply that can be used to build a response centered around a document.
The various builders supplied in the ReplyBuilderInterface can be appended
together to generate responses containing document bodies, error codes, and
other appropriate response types.
This interface acts as a cursor to build a response message to be sent out back
to the client.
## See Also
For details on egress networking, see [this document][egress_networking]. For
details on command dispatch, see [this document][command_dispatch]. For details
on *NetworkingBaton* and *BatonASIO*, see [this document][baton].
[ServiceExecutor]: service_executor.h
[ServiceStateMachine]: service_state_machine.h
[ServiceEntryPoint]: service_entry_point.h
[ServiceEntryPointImpl]: service_entry_point_impl.h
[ReplyBuilderInterface]: ../rpc/reply_builder_interface.h
[DocSequenceBuilder]: ../rpc/op_msg.h
[egress_networking]: ../../../docs/egress_networking.md
[command_dispatch]: ../../../docs/command_dispatch.md
[baton]: ../../../docs/baton.md
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