Application Architecture Guide
==============================

Overview
--------

A blockchain application is more than the consensus engine and the
transaction logic (eg. smart contracts, business logic) as implemented
in the ABCI app. There are also (mobile, web, desktop) clients that will
need to connect and make use of the app. We will assume for now that you
have a well designed transactions and database model, but maybe this
will be the topic of another article. This article is more interested in
various ways of setting up the "plumbing" and connecting these pieces,
and demonstrating some evolving best practices.

Security
--------

A very important aspect when constructing a blockchain is security. The
consensus model can be DoSed (no consensus possible) by corrupting 1/3
of the validators and exploited (writing arbitrary blocks) by corrupting
2/3 of the validators. So, while the security is not that of the
"weakest link", you should take care that the "average link" is
sufficiently hardened.

One big attack surface on the validators is the communication between
the ABCI app and the tendermint core. This should be highly protected.
Ideally, the app and the core are running on the same machine, so no
external agent can target the communication channel. You can use unix
sockets (with permissions preventing access from other users), or even
compile the two apps into one binary if the ABCI app is also writen in
go. If you are unable to do that due to language support, then the ABCI
app should bind a TCP connection to localhost (127.0.0.1), which is less
efficient and secure, but still not reachable from outside. If you must
run the ABCI app and tendermint core on separate machines, make sure you
have a secure communication channel (ssh tunnel?)

Now assuming, you have linked together your app and the core securely,
you must also make sure no one can get on the machine it is hosted on.
At this point it is basic network security. Run on a secure operating
system (SELinux?). Limit who has access to the machine (user accounts,
but also where the physical machine is hosted). Turn off all services
except for ssh, which should only be accessible by some well-guarded
public/private key pairs (no password). And maybe even firewall off
access to the ports used by the validators, so only known validators can
connect.

There was also a suggestion on slack from @jhon about compiling
everything together with a unikernel for more security, such as
`Mirage <https://mirage.io>`__ or
`UNIK <https://github.com/emc-advanced-dev/unik>`__.

Connecting your client to the blockchain
----------------------------------------

Tendermint Core RPC
~~~~~~~~~~~~~~~~~~~

The concept is that the ABCI app is completely hidden from the outside
world and only communicated through a tested and secured `interface
exposed by the tendermint core <./rpc.html>`__. This interface
exposes a lot of data on the block header and consensus process, which
is quite useful for externally verifying the system. It also includes
3(!) methods to broadcast a transaction (propose it for the blockchain,
and possibly await a response). And one method to query app-specific
data from the ABCI application.

Pros:
* Server code already written
* Access to block headers to validate merkle proofs (nice for light clients)
* Basic read/write functionality is supported

Cons:
* Limited interface to app. All queries must be serialized into
[]byte (less expressive than JSON over HTTP) and there is no way to push
data from ABCI app to the client (eg. notify me if account X receives a
transaction)

Custom ABCI server
~~~~~~~~~~~~~~~~~~

This was proposed by @wolfposd on slack and demonstrated by
`TMChat <https://github.com/wolfposd/TMChat>`__, a sample app. The
concept is to write a custom server for your app (with typical REST
API/websockets/etc for easy use by a mobile app). This custom server is
in the same binary as the ABCI app and data store, so can easily react
to complex events there that involve understanding the data format (send
a message if my balance drops below 500). All "writes" sent to this
server are proxied via websocket/JSON-RPC to tendermint core. When they
come back as deliver\_tx over ABCI, they will be written to the data
store. For "reads", we can do any queries we wish that are supported by
our architecture, using any web technology that is useful. The general
architecture is shown in the following diagram:

Pros: \* Separates application logic from blockchain logic \* Allows
much richer, more flexible client-facing API \* Allows pub-sub, watching
certain fields, etc.

Cons: \* Access to ABCI app can be dangerous (be VERY careful not to
write unless it comes from the validator node) \* No direct access to
the blockchain headers to verify tx \* You must write your own API (but
maybe that's a pro...)

Hybrid solutions
~~~~~~~~~~~~~~~~

Likely the least secure but most versatile. The client can access both
the tendermint node for all blockchain info, as well as a custom app
server, for complex queries and pub-sub on the abci app.

Pros: All from both above solutions

Cons: Even more complexity; even more attack vectors (less
security)

Scalability
-----------

Read replica using non-validating nodes? They could forward transactions
to the validators (fewer connections, more security), and locally allow
all queries in any of the above configurations. Thus, while
transaction-processing speed is limited by the speed of the abci app and
the number of validators, one should be able to scale our read
performance to quite an extent (until the replication process drains too
many resources from the validator nodes).
