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# Main Components of the Cosmos SDK

The Cosmos SDK is a framework that facilitates the development of secure state-machines on top of Tendermint. At its core, the SDK is a boilerplate implementation of the [ABCI](./sdk-app-architecture.md#abci) in Golang. It comes with a [`multistore`](../core/store.md#multistore) to persist data and a [`router`](../core/baseapp.md#routing) to handle transactions.

Here is a simplified view of how transactions are handled by an application built on top of the Cosmos SDK when transferred from Tendermint via `DeliverTx`:

1. Decode `transactions` received from the Tendermint consensus engine (remember that Tendermint only deals with `[]bytes`).
2. Extract `messages` from `transactions` and do basic sanity checks.
3. Route each message to the appropriate module so that it can be processed.
4. Commit state changes.

## `baseapp`

`baseapp` is the boilerplate implementation of a Cosmos SDK application. It comes with an implementation of the ABCI to handle the connexion with the underlying consensus engine. Typically, a Cosmos SDK application extends `baseapp` by embedding it in [`app.go`](../basics/app-anatomy.md#core-application-file). See an example of this from the SDK application tutorial:

+++ https://github.com/cosmos/sdk-tutorials/blob/c6754a1e313eb1ed973c5c91dcc606f2fd288811/app.go#L72-L92

The goal of `baseapp` is to provide a secure interface between the store and the extensible state machine while defining as little about the state machine as possible (staying true to the ABCI).

For more on `baseapp`, please click [here](../core/baseapp.md).

## Multistore

The Cosmos SDK provides a [`multistore`](../core/store.md#multisotre) for persisting state. The multistore allows developers to declare any number of [`KVStores`](../core/store.md#base-layer-kvstores). These `KVStores` only accept the `[]byte` type as value and therefore any custom structure needs to be marshalled using [a codec](../core/encoding.md) before being stored.

The multistore abstraction is used to divide the state in distinct compartments, each managed by its own module. For more on the multistore, click [here](../core/store.md#multistore)

## Modules

The power of the Cosmos SDK lies in its modularity. SDK applications are built by aggregating a collection of interoperable modules. Each module defines a subset of the state and contains its own message/transaction processor, while the SDK is responsible for routing each message to its respective module.

Here is a simplified view of how a transaction is processed by the application of each full-node when it is received in a valid block:

```
                                      +
                                      |
                                      |  Transaction relayed from the full-node's Tendermint engine
                                      |  to the node's application via DeliverTx
                                      |
                                      |
                                      |
                +---------------------v--------------------------+
                |                 APPLICATION                    |
                |                                                |
                |     Using baseapp's methods: Decode the Tx,    |
                |     extract and route the message(s)           |
                |                                                |
                +---------------------+--------------------------+
                                      |
                                      |
                                      |
                                      +---------------------------+
                                                                  |
                                                                  |
                                                                  |
                                                                  |  Message routed to the correct
                                                                  |  module to be processed
                                                                  |
                                                                  |
+----------------+  +---------------+  +----------------+  +------v----------+
|                |  |               |  |                |  |                 |
|  AUTH MODULE   |  |  BANK MODULE  |  | STAKING MODULE |  |   GOV MODULE    |
|                |  |               |  |                |  |                 |
|                |  |               |  |                |  | Handles message,|
|                |  |               |  |                |  | Updates state   |
|                |  |               |  |                |  |                 |
+----------------+  +---------------+  +----------------+  +------+----------+
                                                                  |
                                                                  |
                                                                  |
                                                                  |
                                       +--------------------------+
                                       |
                                       | Return result to Tendermint
                                       | (0=Ok, 1=Err)
                                       v
```

Each module can be seen as a little state-machine. Developers need to define the subset of the state handled by the module, as well as custom message types that modify the state (*Note:* `messages` are extracted from `transactions` by `baseapp`). In general, each module declares its own `KVStore` in the `multistore` to persist the subset of the state it defines. Most developers will need to access other 3rd party modules when building their own modules. Given that the Cosmos-SDK is an open framework, some of the modules may be malicious, which means there is a need for security principles to reason about inter-module interactions. These principles are based on [object-capabilities](../core/ocap.md). In practice, this means that instead of having each module keep an access control list for other modules, each module implements special objects called `keepers` that can be passed to other modules to grant a pre-defined set of capabilities. 

SDK modules are defined in the `x/` folder of the SDK. 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.

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. You can check an [example of that in the tutorial](https://cosmos.network/docs/tutorial/keeper.html).

## Next {hide}

Learn more about the [anatomy of an SDK application](../basics/app-anatomy.md) {hide}