<!---
order: 2
--->

# Creating a built-in application in Go

## Guide assumptions

This guide is designed for beginners who want to get started with a Tendermint
Core application from scratch. It does not assume that you have any prior
experience with Tendermint Core.

Tendermint Core is Byzantine Fault Tolerant (BFT) middleware that takes a state
transition machine - written in any programming language - and securely
replicates it on many machines.

Although Tendermint Core is written in the Golang programming language, prior
knowledge of it is not required for this guide. You can learn it as we go due
to it's simplicity. However, you may want to go through [Learn X in Y minutes
Where X=Go](https://learnxinyminutes.com/docs/go/) first to familiarize
yourself with the syntax.

By following along with this guide, you'll create a Tendermint Core project
called kvstore, a (very) simple distributed BFT key-value store.

> Note: please use a released version of Tendermint with this guide. The guides will work with the latest version. Please, do not use master.

## Built-in app vs external app

Running your application inside the same process as Tendermint Core will give
you the best possible performance.

For other languages, your application have to communicate with Tendermint Core
through a TCP, Unix domain socket or gRPC.

## 1.1 Installing Go

Please refer to [the official guide for installing
Go](https://golang.org/doc/install).

Verify that you have the latest version of Go installed:

```bash
$ go version
go version go1.16.x darwin/amd64
```

## 1.2 Creating a new Go project

We'll start by creating a new Go project.

```bash
mkdir kvstore
cd kvstore
go mod init github.com/<github_username>/<repo_name>
```

Inside the example directory create a `main.go` file with the following content:

> Note: there is no need to clone or fork Tendermint in this tutorial.

```go
package main

import (
 "fmt"
)

func main() {
 fmt.Println("Hello, Tendermint Core")
}
```

When run, this should print "Hello, Tendermint Core" to the standard output.

```bash
$ go run main.go
Hello, Tendermint Core
```

## 1.3 Writing a Tendermint Core application

Tendermint Core communicates with the application through the Application
BlockChain Interface (ABCI). All message types are defined in the [protobuf
file](https://github.com/tendermint/tendermint/blob/master/proto/tendermint/abci/types.proto).
This allows Tendermint Core to run applications written in any programming
language.

Create a file called `app.go` with the following content:

```go
package main

import (
 abcitypes "github.com/tendermint/tendermint/abci/types"
)

type KVStoreApplication struct {}

var _ abcitypes.Application = (*KVStoreApplication)(nil)

func NewKVStoreApplication() *KVStoreApplication {
 return &KVStoreApplication{}
}

func (KVStoreApplication) Info(req abcitypes.RequestInfo) abcitypes.ResponseInfo {
 return abcitypes.ResponseInfo{}
}

func (KVStoreApplication) DeliverTx(req abcitypes.RequestDeliverTx) abcitypes.ResponseDeliverTx {
 return abcitypes.ResponseDeliverTx{Code: 0}
}

func (KVStoreApplication) CheckTx(req abcitypes.RequestCheckTx) abcitypes.ResponseCheckTx {
 return abcitypes.ResponseCheckTx{Code: 0}
}

func (KVStoreApplication) Commit() abcitypes.ResponseCommit {
 return abcitypes.ResponseCommit{}
}

func (KVStoreApplication) Query(req abcitypes.RequestQuery) abcitypes.ResponseQuery {
 return abcitypes.ResponseQuery{Code: 0}
}

func (KVStoreApplication) InitChain(req abcitypes.RequestInitChain) abcitypes.ResponseInitChain {
 return abcitypes.ResponseInitChain{}
}

func (KVStoreApplication) BeginBlock(req abcitypes.RequestBeginBlock) abcitypes.ResponseBeginBlock {
 return abcitypes.ResponseBeginBlock{}
}

func (KVStoreApplication) EndBlock(req abcitypes.RequestEndBlock) abcitypes.ResponseEndBlock {
 return abcitypes.ResponseEndBlock{}
}

func (KVStoreApplication) ListSnapshots(abcitypes.RequestListSnapshots) abcitypes.ResponseListSnapshots {
 return abcitypes.ResponseListSnapshots{}
}

func (KVStoreApplication) OfferSnapshot(abcitypes.RequestOfferSnapshot) abcitypes.ResponseOfferSnapshot {
 return abcitypes.ResponseOfferSnapshot{}
}

func (KVStoreApplication) LoadSnapshotChunk(abcitypes.RequestLoadSnapshotChunk) abcitypes.ResponseLoadSnapshotChunk {
 return abcitypes.ResponseLoadSnapshotChunk{}
}

func (KVStoreApplication) ApplySnapshotChunk(abcitypes.RequestApplySnapshotChunk) abcitypes.ResponseApplySnapshotChunk {
 return abcitypes.ResponseApplySnapshotChunk{}
}
```

Now I will go through each method explaining when it's called and adding
required business logic.

### 1.3.1 CheckTx

When a new transaction is added to the Tendermint Core, it will ask the
application to check it (validate the format, signatures, etc.).

```go
import "bytes"

func (app *KVStoreApplication) isValid(tx []byte) (code uint32) {
 // check format
 parts := bytes.Split(tx, []byte("="))
 if len(parts) != 2 {
  return 1
 }

 key, value := parts[0], parts[1]

 // check if the same key=value already exists
 err := app.db.View(func(txn *badger.Txn) error {
  item, err := txn.Get(key)
  if err != nil && err != badger.ErrKeyNotFound {
   return err
  }
  if err == nil {
   return item.Value(func(val []byte) error {
    if bytes.Equal(val, value) {
     code = 2
    }
    return nil
   })
  }
  return nil
 })
 if err != nil {
  panic(err)
 }

 return code
}

func (app *KVStoreApplication) CheckTx(req abcitypes.RequestCheckTx) abcitypes.ResponseCheckTx {
 code := app.isValid(req.Tx)
 return abcitypes.ResponseCheckTx{Code: code, GasWanted: 1}
}
```

Don't worry if this does not compile yet.

If the transaction does not have a form of `{bytes}={bytes}`, we return `1`
code. When the same key=value already exist (same key and value), we return `2`
code. For others, we return a zero code indicating that they are valid.

Note that anything with non-zero code will be considered invalid (`-1`, `100`,
etc.) by Tendermint Core.

Valid transactions will eventually be committed given they are not too big and
have enough gas. To learn more about gas, check out ["the
specification"](https://docs.tendermint.com/master/spec/abci/apps.html#gas).

For the underlying key-value store we'll use
[badger](https://github.com/dgraph-io/badger), which is an embeddable,
persistent and fast key-value (KV) database.

```go
import "github.com/dgraph-io/badger"

type KVStoreApplication struct {
 db           *badger.DB
 currentBatch *badger.Txn
}

func NewKVStoreApplication(db *badger.DB) *KVStoreApplication {
 return &KVStoreApplication{
  db: db,
 }
}
```

### 1.3.2 BeginBlock -> DeliverTx -> EndBlock -> Commit

When Tendermint Core has decided on the block, it's transfered to the
application in 3 parts: `BeginBlock`, one `DeliverTx` per transaction and
`EndBlock` in the end. DeliverTx are being transfered asynchronously, but the
responses are expected to come in order.

```go
func (app *KVStoreApplication) BeginBlock(req abcitypes.RequestBeginBlock) abcitypes.ResponseBeginBlock {
 app.currentBatch = app.db.NewTransaction(true)
 return abcitypes.ResponseBeginBlock{}
}

```

Here we create a batch, which will store block's transactions.

```go
func (app *KVStoreApplication) DeliverTx(req abcitypes.RequestDeliverTx) abcitypes.ResponseDeliverTx {
 code := app.isValid(req.Tx)
 if code != 0 {
  return abcitypes.ResponseDeliverTx{Code: code}
 }

 parts := bytes.Split(req.Tx, []byte("="))
 key, value := parts[0], parts[1]

 err := app.currentBatch.Set(key, value)
 if err != nil {
  panic(err)
 }

 return abcitypes.ResponseDeliverTx{Code: 0}
}
```

If the transaction is badly formatted or the same key=value already exist, we
again return the non-zero code. Otherwise, we add it to the current batch.

In the current design, a block can include incorrect transactions (those who
passed CheckTx, but failed DeliverTx or transactions included by the proposer
directly). This is done for performance reasons.

Note we can't commit transactions inside the `DeliverTx` because in such case
`Query`, which may be called in parallel, will return inconsistent data (i.e.
it will report that some value already exist even when the actual block was not
yet committed).

`Commit` instructs the application to persist the new state.

```go
func (app *KVStoreApplication) Commit() abcitypes.ResponseCommit {
 app.currentBatch.Commit()
 return abcitypes.ResponseCommit{Data: []byte{}}
}
```

### 1.3.3 Query

Now, when the client wants to know whenever a particular key/value exist, it
will call Tendermint Core RPC `/abci_query` endpoint, which in turn will call
the application's `Query` method.

Applications are free to provide their own APIs. But by using Tendermint Core
as a proxy, clients (including [light client
package](https://godoc.org/github.com/tendermint/tendermint/light)) can leverage
the unified API across different applications. Plus they won't have to call the
otherwise separate Tendermint Core API for additional proofs.

Note we don't include a proof here.

```go
func (app *KVStoreApplication) Query(reqQuery abcitypes.RequestQuery) (resQuery abcitypes.ResponseQuery) {
 resQuery.Key = reqQuery.Data
 err := app.db.View(func(txn *badger.Txn) error {
  item, err := txn.Get(reqQuery.Data)
  if err != nil && err != badger.ErrKeyNotFound {
   return err
  }
  if err == badger.ErrKeyNotFound {
   resQuery.Log = "does not exist"
  } else {
   return item.Value(func(val []byte) error {
    resQuery.Log = "exists"
    resQuery.Value = val
    return nil
   })
  }
  return nil
 })
 if err != nil {
  panic(err)
 }
 return
}
```

The complete specification can be found
[here](https://docs.tendermint.com/master/spec/abci/).

## 1.4 Starting an application and a Tendermint Core instance in the same process

Put the following code into the "main.go" file:

```go
package main

import (
 "flag"
 "fmt"
 "os"
 "os/signal"
 "path/filepath"
 "syscall"

 "github.com/dgraph-io/badger"
 "github.com/spf13/viper"

 abci "github.com/tendermint/tendermint/abci/types"
 cfg "github.com/tendermint/tendermint/config"
 tmflags "github.com/tendermint/tendermint/libs/cli/flags"
 "github.com/tendermint/tendermint/libs/log"
 nm "github.com/tendermint/tendermint/node"
 "github.com/tendermint/tendermint/internal/p2p"
 "github.com/tendermint/tendermint/privval"
 "github.com/tendermint/tendermint/proxy"
)

var configFile string

func init() {
 flag.StringVar(&configFile, "config", "$HOME/.tendermint/config/config.toml", "Path to config.toml")
}

func main() {
 db, err := badger.Open(badger.DefaultOptions("/tmp/badger"))
 if err != nil {
  fmt.Fprintf(os.Stderr, "failed to open badger db: %v", err)
  os.Exit(1)
 }
 defer db.Close()
 app := NewKVStoreApplication(db)

 flag.Parse()

 node, err := newTendermint(app, configFile)
 if err != nil {
  fmt.Fprintf(os.Stderr, "%v", err)
  os.Exit(2)
 }

 node.Start()
 defer func() {
  node.Stop()
  node.Wait()
 }()

 c := make(chan os.Signal, 1)
 signal.Notify(c, os.Interrupt, syscall.SIGTERM)
 <-c
}

func newTendermint(app abci.Application, configFile string) (*nm.Node, error) {
 // read config
 config := cfg.DefaultValidatorConfig()
 config.RootDir = filepath.Dir(filepath.Dir(configFile))
 viper.SetConfigFile(configFile)
 if err := viper.ReadInConfig(); err != nil {
  return nil, fmt.Errorf("viper failed to read config file: %w", err)
 }
 if err := viper.Unmarshal(config); err != nil {
  return nil, fmt.Errorf("viper failed to unmarshal config: %w", err)
 }
 if err := config.ValidateBasic(); err != nil {
  return nil, fmt.Errorf("config is invalid: %w", err)
 }

 // create logger
 logger := log.NewTMLogger(log.NewSyncWriter(os.Stdout))
 var err error
 logger, err = tmflags.ParseLogLevel(config.LogLevel, logger, cfg.DefaultLogLevel)
 if err != nil {
  return nil, fmt.Errorf("failed to parse log level: %w", err)
 }

 // read private validator
 pv := privval.LoadFilePV(
  config.PrivValidatorKeyFile(),
  config.PrivValidatorStateFile(),
 )

 // read node key
 nodeKey, err := p2p.LoadNodeKey(config.NodeKeyFile())
 if err != nil {
  return nil, fmt.Errorf("failed to load node's key: %w", err)
 }

 // create node
 node, err := nm.NewNode(
  config,
  pv,
  nodeKey,
  abcicli.NewLocalClientCreator(app),
  nm.DefaultGenesisDocProviderFunc(config),
  nm.DefaultDBProvider,
  nm.DefaultMetricsProvider(config.Instrumentation),
  logger)
 if err != nil {
  return nil, fmt.Errorf("failed to create new Tendermint node: %w", err)
 }

 return node, nil
}
```

This is a huge blob of code, so let's break it down into pieces.

First, we initialize the Badger database and create an app instance:

```go
db, err := badger.Open(badger.DefaultOptions("/tmp/badger"))
if err != nil {
 fmt.Fprintf(os.Stderr, "failed to open badger db: %v", err)
 os.Exit(1)
}
defer db.Close()
app := NewKVStoreApplication(db)
```

For **Windows** users, restarting this app will make badger throw an error as it requires value log to be truncated. For more information on this, visit [here](https://github.com/dgraph-io/badger/issues/744).
This can be avoided by setting the truncate option to true, like this:

```go
db, err := badger.Open(badger.DefaultOptions("/tmp/badger").WithTruncate(true))
```

Then we use it to create a Tendermint Core `Node` instance:

```go
flag.Parse()

node, err := newTendermint(app, configFile)
if err != nil {
 fmt.Fprintf(os.Stderr, "%v", err)
 os.Exit(2)
}

...

// create node
node, err := nm.NewNode(
 config,
 pv,
 nodeKey,
 abcicli.NewLocalClientCreator(app),
 nm.DefaultGenesisDocProviderFunc(config),
 nm.DefaultDBProvider,
 nm.DefaultMetricsProvider(config.Instrumentation),
 logger)
if err != nil {
 return nil, fmt.Errorf("failed to create new Tendermint node: %w", err)
}
```

`NewNode` requires a few things including a configuration file, a private
validator, a node key and a few others in order to construct the full node.

Note we use `abcicli.NewLocalClientCreator` here to create a local client instead
of one communicating through a socket or gRPC.

[viper](https://github.com/spf13/viper) is being used for reading the config,
which we will generate later using the `tendermint init` command.

```go
config := cfg.DefaultValidatorConfig()
config.RootDir = filepath.Dir(filepath.Dir(configFile))
viper.SetConfigFile(configFile)
if err := viper.ReadInConfig(); err != nil {
 return nil, fmt.Errorf("viper failed to read config file: %w", err)
}
if err := viper.Unmarshal(config); err != nil {
 return nil, fmt.Errorf("viper failed to unmarshal config: %w", err)
}
if err := config.ValidateBasic(); err != nil {
 return nil, fmt.Errorf("config is invalid: %w", err)
}
```

We use `FilePV`, which is a private validator (i.e. thing which signs consensus
messages). Normally, you would use `SignerRemote` to connect to an external
[HSM](https://kb.certus.one/hsm.html).

```go
pv := privval.LoadFilePV(
 config.PrivValidatorKeyFile(),
 config.PrivValidatorStateFile(),
)

```

`nodeKey` is needed to identify the node in a p2p network.

```go
nodeKey, err := p2p.LoadNodeKey(config.NodeKeyFile())
if err != nil {
 return nil, fmt.Errorf("failed to load node's key: %w", err)
}
```

As for the logger, we use the build-in library, which provides a nice
abstraction over [go-kit's
logger](https://github.com/go-kit/kit/tree/master/log).

```go
logger := log.NewTMLogger(log.NewSyncWriter(os.Stdout))
var err error
logger, err = tmflags.ParseLogLevel(config.LogLevel, logger, cfg.DefaultLogLevel())
if err != nil {
 return nil, fmt.Errorf("failed to parse log level: %w", err)
}
```

Finally, we start the node and add some signal handling to gracefully stop it
upon receiving SIGTERM or Ctrl-C.

```go
node.Start()
defer func() {
 node.Stop()
 node.Wait()
}()

c := make(chan os.Signal, 1)
signal.Notify(c, os.Interrupt, syscall.SIGTERM)
<-c
```

## 1.5 Getting Up and Running

We are going to use [Go modules](https://github.com/golang/go/wiki/Modules) for
dependency management.

```bash
export GO111MODULE=on
go mod init github.com/me/example
```

This should create a `go.mod` file. The current tutorial only works with
the master branch of Tendermint. so let's make sure we're using the latest version:

```sh
go get github.com/tendermint/tendermint@master
```

This will populate the `go.mod` with a release number followed by a hash for Tendermint.

```go
module github.com/me/example

go 1.15

require (
 github.com/dgraph-io/badger v1.6.2
 github.com/tendermint/tendermint <vX>
)
```

Now we can build the binary:

```bash
go build
```

To create a default configuration, nodeKey and private validator files, let's
execute `tendermint init validator`. But before we do that, we will need to install
Tendermint Core. Please refer to [the official
guide](https://docs.tendermint.com/master/introduction/install.html). If you're
installing from source, don't forget to checkout the latest release (`git
checkout vX.Y.Z`). Don't forget to check that the application uses the same
major version.

```bash
$ rm -rf /tmp/example
$ TMHOME="/tmp/example" tendermint init validator

I[2019-07-16|18:40:36.480] Generated private validator                  module=main keyFile=/tmp/example/config/priv_validator_key.json stateFile=/tmp/example2/data/priv_validator_state.json
I[2019-07-16|18:40:36.481] Generated node key                           module=main path=/tmp/example/config/node_key.json
I[2019-07-16|18:40:36.482] Generated genesis file                       module=main path=/tmp/example/config/genesis.json
I[2019-07-16|18:40:36.483] Generated config                             module=main mode=validator
```

We are ready to start our application:

```bash
$ ./example -config "/tmp/example/config/config.toml"

badger 2019/07/16 18:42:25 INFO: All 0 tables opened in 0s
badger 2019/07/16 18:42:25 INFO: Replaying file id: 0 at offset: 0
badger 2019/07/16 18:42:25 INFO: Replay took: 695.227s
E[2019-07-16|18:42:25.818] Couldn't connect to any seeds                module=p2p
I[2019-07-16|18:42:26.853] Executed block                               module=state height=1 validTxs=0 invalidTxs=0
I[2019-07-16|18:42:26.865] Committed state                              module=state height=1 txs=0 appHash=
```

Now open another tab in your terminal and try sending a transaction:

```bash
$ curl -s 'localhost:26657/broadcast_tx_commit?tx="tendermint=rocks"'
{
  "check_tx": {
    "gasWanted": "1",
    ...
  },
  "deliver_tx": { ... },
  "hash": "1B3C5A1093DB952C331B1749A21DCCBB0F6C7F4E0055CD04D16346472FC60EC6",
  "height": "128"
}
```

Response should contain the height where this transaction was committed.

Now let's check if the given key now exists and its value:

```json
$ curl -s 'localhost:26657/abci_query?data="tendermint"'
{
  "response": {
    "code": 0,
    "log": "exists",
    "info": "",
    "index": "0",
    "key": "dGVuZGVybWludA==",
    "value": "cm9ja3M=",
    "proofOps": null,
    "height": "6",
    "codespace": ""
  }
}
```

"dGVuZGVybWludA==" and "cm9ja3M=" are the base64-encoding of the ASCII of
"tendermint" and "rocks" accordingly.

## Outro

I hope everything went smoothly and your first, but hopefully not the last,
Tendermint Core application is up and running. If not, please [open an issue on
Github](https://github.com/tendermint/tendermint/issues/new/choose). To dig
deeper, read [the docs](https://docs.tendermint.com/master/).