# 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 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.

# 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.

Verify that you have the latest version of Go installed:

Copy $ go version go version go1.13.1 darwin/amd64

Make sure you have $GOPATH environment variable set:

Copy $ echo $GOPATH /Users/melekes/go

# 1.2 Creating a new Go project

We'll start by creating a new Go project.

Copy $ mkdir kvstore $ cd kvstore

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

Copy package main import ( "fmt" ) func main() { fmt.Println("Hello, Tendermint Core") }

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

Copy $ 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. This allows Tendermint Core to run applications written in any programming language.

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

Copy 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) SetOption(req abcitypes.RequestSetOption) abcitypes.ResponseSetOption { return abcitypes.ResponseSetOption{} } 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{} }

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.).

Copy 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".

For the underlying key-value store we'll use badger, which is an embeddable, persistent and fast key-value (KV) database.

Copy 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.

Copy 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.

Copy 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.

Copy 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) 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.

Copy 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.

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

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

Copy package main import ( "errors" "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/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 os.Exit(0) } func newTendermint(app abci.Application, configFile string) (*nm.Node, error) { // read config config := cfg.DefaultConfig() 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, proxy.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:

Copy 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. This can be avoided by setting the truncate option to true, like this:

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

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

Copy 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, proxy.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 proxy.NewLocalClientCreator here to create a local client instead of one communicating through a socket or gRPC.

viper is being used for reading the config, which we will generate later using the tendermint init command.

Copy config := cfg.DefaultConfig() 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.

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

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

Copy 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.

Copy 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.

Copy node.Start() defer func() { node.Stop() node.Wait() }() c := make(chan os.Signal, 1) signal.Notify(c, os.Interrupt, syscall.SIGTERM) <-c os.Exit(0)

# 1.5 Getting Up and Running

We are going to use Go modules for dependency management.

Copy $ go mod init github.com/me/example $ go build

This should build the binary.

To create a default configuration, nodeKey and private validator files, let's execute tendermint init. But before we do that, we will need to install Tendermint Core. Please refer to the official guide. If you're installing from source, don't forget to checkout the latest release (git checkout vX.Y.Z).

Copy $ rm -rf /tmp/example $ TMHOME="/tmp/example" tendermint init 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

We are ready to start our application:

Copy $ ./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:

Copy $ curl -s 'localhost:26657/broadcast_tx_commit?tx="tendermint=rocks"' { "jsonrpc": "2.0", "id": "", "result": { "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:

Copy $ curl -s 'localhost:26657/abci_query?data="tendermint"' { "jsonrpc": "2.0", "id": "", "result": { "response": { "log": "exists", "key": "dGVuZGVybWludA==", "value": "cm9ja3M=" } } }

"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. To dig deeper, read the docs.