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SDK Application Architecture

Parts of a SDK blockchain

A blockchain application is just a replicated deterministic state machine. As a developer, you just have to define the state machine (i.e. what the state, a starting state and messages that trigger state transitions), and Tendermint will handle replication over the network for you.

Tendermint is an application-agnostic engine that is responsible for handling the networking and consensus layers of your blockchain. In practice, this means that Tendermint is reponsible for propagating and ordering transaction bytes. Tendermint Core relies on an eponymous Byzantine-Fault-Tolerant (BFT) algorithm to reach consensus on the order of transactions. For more on Tendermint, click here.

Tendermint passes transactions from the network to the application through an interface called the [ABCI](https://github.com/tendermint/tendermint/tree/master/abci. If you look at the architecture of the blockchain node you are building, it looks like the following:

+---------------------+
|                     |
|     Application     |
|                     |
+--------+---+--------+
         ^   |
         |   | ABCI
         |   v
+--------+---+--------+
|                     |
|                     |
|     Tendermint      |
|                     |
|                     |
+---------------------+

Fortunately, you do not have to implement the ABCI interface. The Cosmos SDK provides a boilerplate implementation of it in the form of baseapp.

BaseApp

Implements an ABCI App using a MultiStore for persistence and a Router to handle transactions. The goal is to provide a secure interface between the store and the extensible state machine while defining as little about that state machine as possible (staying true to the ABCI).

For more on baseapp, please go to the baseapp reference.

Modules

The power of the SDK lies in its modularity. SDK blockchains are built out of customizable and interoperable modules. These modules are contained in the x/ folder.

In addition to the already existing modules in x/, that anyone can use in their app, the SDK lets you build your own custom modules. In other words, building a SDK blockchain consists in importing some modules and building others (the ones you need that do not exist yet!).

Some core modules include:

x/auth: Used to manage accounts and signatures.
x/bank: Used to enable tokens and token transfers.
x/staking + x/slashing: Used to build Proof-Of-Stake blockchains.

Basecoin

Basecoin is the first complete application in the stack. Complete applications require extensions to the core modules of the SDK to actually implement handler functionality.

Basecoin implements a BaseApp state machine using the x/auth and x/bank modules, which define how transaction signers are authenticated and how coins are transferred. It should also use x/ibc and probably a simple staking extension.

Basecoin and the native x/ extensions use go-amino for all serialization needs, including for transactions and accounts.

SDK tutorial

If you want to learn more about how to build an SDK application and get a deepeer understanding of the concepts presented above in the process, please check out the SDK Application Tutorial.

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