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add hacking.md for notes on internal structure

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Jeff Becker 8 years ago
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# Hacking on I2PD
This document contains notes compiled from hacking on i2pd
## prerequisites
This guide assumes:
* a decent understanding of c++
* basic understanding of how i2p works at i2np level and up
## general structure
Notes on multithreading
* every compontent runs in its own thread
* each component (usually) has a public function `GetService()` which can be used to obtain the `boost::asio::io_service` that it uses.
* when talking between components/threads, **always** use `GetService().post()` and be mindfull of stack allocated memory.
### NetDb
#### NetDb.h
The `i2p::data::netdb` is a `i2p::data::NetDb` instance processes and dispatches *inbound* i2np messages passed in from transports.
global singleton at `i2p::data::netdb` as of 2.10.1
#### NetDbRequests.h
For Pending RouterInfo/LeaseSet lookup and store requests
### ClientContext
#### ClientContext.h
`i2p::client::ClientContext` spawns all destinations used by the i2p router including the shared local destination.
global singleton at `i2p::client::context` as of 2.10.1
### Daemon
File: Daemon.cpp
`i2p::util::Daemon_Singleton_Private` subclasses implement the daemon start-up and tear-down, creates Http Webui and i2p control server.
### Destinations
#### Destination.h
each destination runs in its own thread
##### i2p::client::LeaseSetDestination
Base for `i2p::client::ClientDestination`
##### i2p::client::ClientDestination
Destination capable of creating (tcp/i2p) streams and datagram sessions.
#### Streaming.h
##### i2p::stream::StreamingDestination
Does not implement any destination related members, the name is a bit misleading.
Owns a `i2p::client::ClientDestination` and runs in the destination thread.
Anyone creating or using streams outside of the destination thread **MUST** be aware of the consequences of multithreaded c++ :^)
If you use streaming please consider running all code within the destination thread using `ClientDestination::GetService().post()`
#### Garlic.h
Provides Inter-Destination routing primatives.
##### i2p::garlic::GarlicDestination
sublcass of `i2p::client::LeaseSetDestination` for sending messages down shared routing paths.
##### i2p::garlic::GarlicRoutingSession
a point to point conversation between us and 1 other destination.
##### i2p::garlic::GarlicRoutingPath
A routing path currently used by a routing session. specifies which outbound tunnel to use and which remote lease set to use for `OBEP` to `IBGW` inter tunnel communication.
members:
* outboundTunnel (OBEP)
* remoteLease (IBGW)
* rtt (round trip time)
* updatedTime (last time this path's IBGW/OBEP was updated)
* numTimesUsesd (number of times this path was used)
### Transports
each transport runs in its own thread
#### Transports.h
`i2p::transport::Transports` contains NTCP and SSU transport instances
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