# Creating an 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. ## Built-in app vs external app To get maximum performance it is better to run your application alongside Tendermint Core. [Cosmos SDK](https://github.com/cosmos/cosmos-sdk) is written this way. Please refer to [Writing a built-in Tendermint Core application in Go](./go-built-in.md) guide for details. Having a separate application might give you better security guarantees as two processes would be communicating via established binary protocol. Tendermint Core will not have access to application's state. ## 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 ``` Inside the example directory create a `main.go` file with the following content: ```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 transferred to the application in 3 parts: `BeginBlock`, one `DeliverTx` per transaction and `EndBlock` in the end. DeliverTx are being transferred 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 instances Put the following code into the "main.go" file: ```go package main import ( "flag" "fmt" "os" "os/signal" "syscall" "github.com/dgraph-io/badger" abciserver "github.com/tendermint/tendermint/abci/server" "github.com/tendermint/tendermint/libs/log" ) var socketAddr string func init() { flag.StringVar(&socketAddr, "socket-addr", "unix://example.sock", "Unix domain socket address") } 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() logger, err := log.NewDefaultLogger(log.LogFormatPlain, log.LogLevelInfo, false) if err != nil { fmt.Fprintf(os.Stderr, "failed to configure logger: %v", err) os.Exit(1) } server := abciserver.NewSocketServer(socketAddr, app) server.SetLogger(logger) if err := server.Start(); err != nil { fmt.Fprintf(os.Stderr, "error starting socket server: %v", err) os.Exit(1) } defer server.Stop() c := make(chan os.Signal, 1) signal.Notify(c, os.Interrupt, syscall.SIGTERM) <-c } ``` 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 start the ABCI server and add some signal handling to gracefully stop it upon receiving SIGTERM or Ctrl-C. Tendermint Core will act as a client, which connects to our server and send us transactions and other messages. ```go server := abciserver.NewSocketServer(socketAddr, app) server.SetLogger(logger) if err := server.Start(); err != nil { fmt.Fprintf(os.Stderr, "error starting socket server: %v", err) os.Exit(1) } defer server.Stop() 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@97a3e44e0724f2017079ce24d36433f03124c09e ``` This will populate the `go.mod` with a release number followed by a hash for Tendermint. ```go module github.com/me/example go 1.16 require ( github.com/dgraph-io/badger v1.6.2 github.com/tendermint/tendermint ) ``` 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|18:20:36.480] 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|18:20:36.481] Generated node key module=main path=/tmp/example/config/node_key.json I[2019-07-16|18:20:36.482] Generated genesis file module=main path=/tmp/example/config/genesis.json I[2019-07-16|18:20:36.483] Generated config module=main mode=validator ``` Feel free to explore the generated files, which can be found at `/tmp/example/config` directory. Documentation on the config can be found [here](https://docs.tendermint.com/master/tendermint-core/configuration.html). We are ready to start our application: ```bash rm example.sock ./example badger 2019/07/16 18:25:11 INFO: All 0 tables opened in 0s badger 2019/07/16 18:25:11 INFO: Replaying file id: 0 at offset: 0 badger 2019/07/16 18:25:11 INFO: Replay took: 300.4s I[2019-07-16|18:25:11.523] Starting ABCIServer impl=ABCIServ ``` Then we need to start Tendermint Core and point it to our application. Staying within the application directory execute: ```bash TMHOME="/tmp/example" tendermint node --proxy-app=unix://example.sock I[2019-07-16|18:26:20.362] Version info module=main software=0.32.1 block=10 p2p=7 I[2019-07-16|18:26:20.383] Starting Node module=main impl=Node E[2019-07-16|18:26:20.392] Couldn't connect to any seeds module=p2p I[2019-07-16|18:26:20.394] Started node module=main nodeInfo="{ProtocolVersion:{P2P:7 Block:10 App:0} ID_:8dab80770ae8e295d4ce905d86af78c4ff634b79 ListenAddr:tcp://0.0.0.0:26656 Network:test-chain-nIO96P Version:0.32.1 Channels:4020212223303800 Moniker:app48.fun-box.ru Other:{TxIndex:on RPCAddress:tcp://127.0.0.1:26657}}" I[2019-07-16|18:26:21.440] Executed block module=state height=1 validTxs=0 invalidTxs=0 I[2019-07-16|18:26:21.446] Committed state module=state height=1 txs=0 appHash= ``` This should start the full node and connect to our ABCI application. ```sh I[2019-07-16|18:25:11.525] Waiting for new connection... I[2019-07-16|18:26:20.329] Accepted a new connection I[2019-07-16|18:26:20.329] Waiting for new connection... I[2019-07-16|18:26:20.330] Accepted a new connection I[2019-07-16|18:26:20.330] Waiting for new connection... I[2019-07-16|18:26:20.330] Accepted a new connection ``` Now open another tab in your terminal and try sending a transaction: ```json $ curl -s 'localhost:26657/broadcast_tx_commit?tx="tendermint=rocks"' { "check_tx": { "gasWanted": "1", ... }, "deliver_tx": { ... }, "hash": "CDD3C6DFA0A08CAEDF546F9938A2EEC232209C24AA0E4201194E0AFB78A2C2BB", "height": "33" } ``` 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/).