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 (opens new window) 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 (opens new window) is written this way. Please refer to Writing a built-in Tendermint Core application in Go 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 (opens new window).

Verify that you have the latest version of Go installed:

Copy $ go version go version go1.18.x darwin/amd64

1.2 Creating a new Go project

We'll start by creating a new Go project. Initialize the folder with go mod init. Running this command should create the go.mod file.

Copy $ mkdir kvstore $ cd kvstore $ go mod init github.com/<username>/kvstore go: creating new go.mod: module github.com/<username>/kvstore

Inside the project 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 (opens new window). 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) 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, we will go through each method and explain when it is executed while adding required business logic.

1.3.1 Key-value store setup

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

Copy $ go get github.com/dgraph-io/badger/v3 go: added github.com/dgraph-io/badger/v3 v3.2103.2 Copy import "github.com/dgraph-io/badger/v3" type KVStoreApplication struct { db *badger.DB currentBatch *badger.Txn } func NewKVStoreApplication(db *badger.DB) *KVStoreApplication { return &KVStoreApplication{ db: db, } }

1.3.2 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" (opens new window).

1.3.3 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.

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{}} }

1.3.4 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 (opens new window)) 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 (opens new window).

1.4 Starting an application and a Tendermint Core instances

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

Copy package main import ( "flag" "fmt" "os" "os/signal" "syscall" "github.com/dgraph-io/badger/v3" abciserver "github.com/tendermint/tendermint/abci/server" "github.com/tendermint/tendermint/libs/log" ) var socketAddr string func init() { flag.StringVar(&socketAddr, "socket-addr", "unix://kvstore.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:

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 (opens new window). 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 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.

Copy 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

Make sure to enable Go modules (opens new window). Run go mod tidy to download and add dependencies in go.mod file.

Copy $ go mod tidy ...

Let's make sure we're using the latest version of Tendermint (currently v0.35.8).

Copy $ go get github.com/tendermint/tendermint@latest ...

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

Copy module github.com/<username>/kvstore go 1.18 require ( github.com/dgraph-io/badger/v3 v3.2103.2 github.com/tendermint/tendermint v0.35.8 ... )

Now, we can build the binary:

Copy $ 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 (opens new window). 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.

Copy $ rm -rf /tmp/kvstore /tmp/badger $ TMHOME="/tmp/kvstore" tendermint init validator 2022-07-20T17:04:41+08:00 INFO Generated private validator keyFile=/tmp/kvstore/config/priv_validator_key.json module=main stateFile=/tmp/kvstore/data/priv_validator_state.json 2022-07-20T17:04:41+08:00 INFO Generated node key module=main path=/tmp/kvstore/config/node_key.json 2022-07-20T17:04:41+08:00 INFO Generated genesis file module=main path=/tmp/kvstore/config/genesis.json 2022-07-20T17:04:41+08:00 INFO Generated config mode=validator module=main

Feel free to explore the generated files, which can be found at /tmp/kvstore/config directory. Documentation on the config can be found here (opens new window).

We are ready to start our application:

Copy $ rm kvstore.sock $ ./kvstore badger 2022/07/20 17:07:17 INFO: All 1 tables opened in 9ms badger 2022/07/20 17:07:17 INFO: Replaying file id: 0 at offset: 256 badger 2022/07/20 17:07:17 INFO: Replay took: 9.077µs badger 2022/07/20 17:07:17 DEBUG: Value log discard stats empty 2022-07-20T17:07:17+08:00 INFO starting service impl=ABCIServer service=ABCIServer 2022-07-20T17:07:17+08:00 INFO Waiting for new connection...

Then, we need to start Tendermint Core and point it to our application. Staying within the project directory, open another terminal and execute the command below:

Copy $ TMHOME="/tmp/kvstore" tendermint node --proxy-app=unix://kvstore.sock 2022-07-20T17:10:22+08:00 INFO starting service impl=multiAppConn module=proxy service=multiAppConn 2022-07-20T17:10:22+08:00 INFO starting service connection=query impl=socketClient module=abci-client service=socketClient 2022-07-20T17:10:22+08:00 INFO starting service connection=snapshot impl=socketClient module=abci-client service=socketClient 2022-07-20T17:10:22+08:00 INFO starting service connection=mempool impl=socketClient module=abci-client service=socketClient 2022-07-20T17:10:22+08:00 INFO starting service connection=consensus impl=socketClient module=abci-client service=socketClient 2022-07-20T17:10:22+08:00 INFO starting service impl=EventBus module=events service=EventBus 2022-07-20T17:10:22+08:00 INFO starting service impl=PubSub module=pubsub service=PubSub 2022-07-20T17:10:22+08:00 INFO starting service impl=IndexerService module=txindex service=IndexerService ... 2022-07-20T17:10:22+08:00 INFO starting service impl=Node module=main service=Node 2022-07-20T17:10:22+08:00 INFO Starting RPC HTTP server on 127.0.0.1:26657 module=rpc-server 2022-07-20T17:10:22+08:00 INFO p2p service legacy_enabled=false module=main 2022-07-20T17:10:22+08:00 INFO starting service impl=router module=p2p service=router 2022-07-20T17:10:22+08:00 INFO starting router channels=402021222330386061626300 listen_addr=tcp://0.0.0.0:26656 module=p2p net_addr={"id":"715727499e94f8fcaef1763192ebcc8460f44666","ip":"0.0.0.0","port":26656} node_id=715727499e94f8fcaef1763192ebcc8460f44666 ...

This should start the full node and connect to our ABCI application.

Copy 2022-07-20T17:09:55+08:00 INFO Waiting for new connection... 2022-07-20T17:10:22+08:00 INFO Accepted a new connection 2022-07-20T17:10:22+08:00 INFO Waiting for new connection... 2022-07-20T17:10:22+08:00 INFO Accepted a new connection 2022-07-20T17:10:22+08:00 INFO Waiting for new connection... 2022-07-20T17:10:22+08:00 INFO Accepted a new connection

Let's try sending a transaction. Open another terminal and execute the below command.

Copy $ curl -s 'localhost:26657/broadcast_tx_commit?tx="tendermint=rocks"' { ... "result": { "check_tx": { ... "gas_wanted": "1", ... }, "deliver_tx": { ... }, "hash": "1B3C5A1093DB952C331B1749A21DCCBB0F6C7F4E0055CD04D16346472FC60EC6", "height": "15" } }

Response should contain the height where this transaction was committed.

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

Copy $ curl -s 'localhost:26657/abci_query?data="tendermint"' { ... "result": { "response": { "code": 0, "log": "exists", "info": "", "index": "0", "key": "dGVuZGVybWludA==", "value": "cm9ja3M=", "proofOps": null, "height": "0", "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 (opens new window). To dig deeper, read the docs (opens new window).