# 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