# Creating an application in Kotlin ## 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 (your application) - written in any programming language - and securely replicates it on many machines. By following along with this guide, you'll create a Tendermint Core project called kvstore, a (very) simple distributed BFT key-value store. The application (which should implementing the blockchain interface (ABCI)) will be written in Kotlin. This guide assumes that you are not new to JVM world. If you are new please see [JVM Minimal Survival Guide](https://hadihariri.com/2013/12/29/jvm-minimal-survival-guide-for-the-dotnet-developer/#java-the-language-java-the-ecosystem-java-the-jvm) and [Gradle Docs](https://docs.gradle.org/current/userguide/userguide.html). ## Built-in app vs external app If you use Golang, you can run your app and Tendermint Core in the same process to get maximum performance. [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. If you choose another language, like we did in this guide, you have to write a separate app, which will communicate with Tendermint Core via a socket (UNIX or TCP) or gRPC. This guide will show you how to build external application using RPC server. 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 Java and Gradle Please refer to [the Oracle's guide for installing JDK](https://www.oracle.com/technetwork/java/javase/downloads/index.html). Verify that you have installed Java successfully: ```sh $ java -version java version "1.8.0_162" Java(TM) SE Runtime Environment (build 1.8.0_162-b12) Java HotSpot(TM) 64-Bit Server VM (build 25.162-b12, mixed mode) ``` You can choose any version of Java higher or equal to 8. In my case it is Java SE Development Kit 8. Make sure you have `$JAVA_HOME` environment variable set: ```sh $ echo $JAVA_HOME /Library/Java/JavaVirtualMachines/jdk1.8.0_162.jdk/Contents/Home ``` For Gradle installation, please refer to [their official guide](https://gradle.org/install/). ## 1.2 Creating a new Kotlin project We'll start by creating a new Gradle project. ```sh $ export KVSTORE_HOME=~/kvstore $ mkdir $KVSTORE_HOME $ cd $KVSTORE_HOME ``` Inside the example directory run: ```sh gradle init --dsl groovy --package io.example --project-name example --type kotlin-application ``` This will create a new project for you. The tree of files should look like: ```sh $ tree . |-- build.gradle |-- gradle | `-- wrapper | |-- gradle-wrapper.jar | `-- gradle-wrapper.properties |-- gradlew |-- gradlew.bat |-- settings.gradle `-- src |-- main | |-- kotlin | | `-- io | | `-- example | | `-- App.kt | `-- resources `-- test |-- kotlin | `-- io | `-- example | `-- AppTest.kt `-- resources ``` When run, this should print "Hello world." to the standard output. ```sh $ ./gradlew run > Task :run Hello world. ``` ## 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/abci/types/types.proto). This allows Tendermint Core to run applications written in any programming language. ### 1.3.1 Compile .proto files Add the following piece to the top of the `build.gradle`: ```groovy buildscript { repositories { mavenCentral() } dependencies { classpath 'com.google.protobuf:protobuf-gradle-plugin:0.8.8' } } ``` Enable the protobuf plugin in the `plugins` section of the `build.gradle`: ```groovy plugins { id 'com.google.protobuf' version '0.8.8' } ``` Add the following code to `build.gradle`: ```groovy protobuf { protoc { artifact = "com.google.protobuf:protoc:3.7.1" } plugins { grpc { artifact = 'io.grpc:protoc-gen-grpc-java:1.22.1' } } generateProtoTasks { all()*.plugins { grpc {} } } } ``` Now we should be ready to compile the `*.proto` files. Copy the necessary `.proto` files to your project: ```sh mkdir -p \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/abci/types \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/crypto/merkle \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/libs/kv \ $KVSTORE_HOME/src/main/proto/github.com/gogo/protobuf/gogoproto cp $GOPATH/src/github.com/tendermint/tendermint/abci/types/types.proto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/abci/types/types.proto cp $GOPATH/src/github.com/tendermint/tendermint/crypto/merkle/merkle.proto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/crypto/merkle/merkle.proto cp $GOPATH/src/github.com/tendermint/tendermint/libs/kv/types.proto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/libs/kv/types.proto cp $GOPATH/src/github.com/gogo/protobuf/gogoproto/gogo.proto \ $KVSTORE_HOME/src/main/proto/github.com/gogo/protobuf/gogoproto/gogo.proto ``` Add these dependencies to `build.gradle`: ```groovy dependencies { implementation 'io.grpc:grpc-protobuf:1.22.1' implementation 'io.grpc:grpc-netty-shaded:1.22.1' implementation 'io.grpc:grpc-stub:1.22.1' } ``` To generate all protobuf-type classes run: ```sh ./gradlew generateProto ``` To verify that everything went smoothly, you can inspect the `build/generated/` directory: ```sh $ tree build/generated/ build/generated/ `-- source `-- proto `-- main |-- grpc | `-- types | `-- ABCIApplicationGrpc.java `-- java |-- com | `-- google | `-- protobuf | `-- GoGoProtos.java |-- common | `-- Types.java |-- merkle | `-- Merkle.java `-- types `-- Types.java ``` ### 1.3.2 Implementing ABCI The resulting `$KVSTORE_HOME/build/generated/source/proto/main/grpc/types/ABCIApplicationGrpc.java` file contains the abstract class `ABCIApplicationImplBase`, which is an interface we'll need to implement. Create `$KVSTORE_HOME/src/main/kotlin/io/example/KVStoreApp.kt` file with the following content: ```kotlin package io.example import io.grpc.stub.StreamObserver import types.ABCIApplicationGrpc import types.Types.* class KVStoreApp : ABCIApplicationGrpc.ABCIApplicationImplBase() { // methods implementation } ``` Now I will go through each method of `ABCIApplicationImplBase` explaining when it's called and adding required business logic. ### 1.3.3 CheckTx When a new transaction is added to the Tendermint Core, it will ask the application to check it (validate the format, signatures, etc.). ```kotlin override fun checkTx(req: RequestCheckTx, responseObserver: StreamObserver) { val code = req.tx.validate() val resp = ResponseCheckTx.newBuilder() .setCode(code) .setGasWanted(1) .build() responseObserver.onNext(resp) responseObserver.onCompleted() } private fun ByteString.validate(): Int { val parts = this.split('=') if (parts.size != 2) { return 1 } val key = parts[0] val value = parts[1] // check if the same key=value already exists val stored = getPersistedValue(key) if (stored != null && stored.contentEquals(value)) { return 2 } return 0 } private fun ByteString.split(separator: Char): List { val arr = this.toByteArray() val i = (0 until this.size()).firstOrNull { arr[it] == separator.toByte() } ?: return emptyList() return listOf( this.substring(0, i).toByteArray(), this.substring(i + 1).toByteArray() ) } ``` 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 [JetBrains Xodus](https://github.com/JetBrains/xodus), which is a transactional schema-less embedded high-performance database written in Java. `build.gradle`: ```groovy dependencies { implementation 'org.jetbrains.xodus:xodus-environment:1.3.91' } ``` ```kotlin ... import jetbrains.exodus.ArrayByteIterable import jetbrains.exodus.env.Environment import jetbrains.exodus.env.Store import jetbrains.exodus.env.StoreConfig import jetbrains.exodus.env.Transaction class KVStoreApp( private val env: Environment ) : ABCIApplicationGrpc.ABCIApplicationImplBase() { private var txn: Transaction? = null private var store: Store? = null ... private fun getPersistedValue(k: ByteArray): ByteArray? { return env.computeInReadonlyTransaction { txn -> val store = env.openStore("store", StoreConfig.WITHOUT_DUPLICATES, txn) store.get(txn, ArrayByteIterable(k))?.bytesUnsafe } } } ``` ### 1.3.4 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. ```kotlin override fun beginBlock(req: RequestBeginBlock, responseObserver: StreamObserver) { txn = env.beginTransaction() store = env.openStore("store", StoreConfig.WITHOUT_DUPLICATES, txn!!) val resp = ResponseBeginBlock.newBuilder().build() responseObserver.onNext(resp) responseObserver.onCompleted() } ``` Here we begin a new transaction, which will accumulate the block's transactions and open the corresponding store. ```kotlin override fun deliverTx(req: RequestDeliverTx, responseObserver: StreamObserver) { val code = req.tx.validate() if (code == 0) { val parts = req.tx.split('=') val key = ArrayByteIterable(parts[0]) val value = ArrayByteIterable(parts[1]) store!!.put(txn!!, key, value) } val resp = ResponseDeliverTx.newBuilder() .setCode(code) .build() responseObserver.onNext(resp) responseObserver.onCompleted() } ``` 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 store. 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. ```kotlin override fun commit(req: RequestCommit, responseObserver: StreamObserver) { txn!!.commit() val resp = ResponseCommit.newBuilder() .setData(ByteString.copyFrom(ByteArray(8))) .build() responseObserver.onNext(resp) responseObserver.onCompleted() } ``` ### 1.3.5 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. ```kotlin override fun query(req: RequestQuery, responseObserver: StreamObserver) { val k = req.data.toByteArray() val v = getPersistedValue(k) val builder = ResponseQuery.newBuilder() if (v == null) { builder.log = "does not exist" } else { builder.log = "exists" builder.key = ByteString.copyFrom(k) builder.value = ByteString.copyFrom(v) } responseObserver.onNext(builder.build()) responseObserver.onCompleted() } ``` 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 `$KVSTORE_HOME/src/main/kotlin/io/example/App.kt` file: ```kotlin package io.example import jetbrains.exodus.env.Environments fun main() { Environments.newInstance("tmp/storage").use { env -> val app = KVStoreApp(env) val server = GrpcServer(app, 26658) server.start() server.blockUntilShutdown() } } ``` It is the entry point of the application. Here we create a special object `Environment`, which knows where to store the application state. Then we create and start the gRPC server to handle Tendermint Core requests. Create `$KVSTORE_HOME/src/main/kotlin/io/example/GrpcServer.kt` file with the following content: ```kotlin package io.example import io.grpc.BindableService import io.grpc.ServerBuilder class GrpcServer( private val service: BindableService, private val port: Int ) { private val server = ServerBuilder .forPort(port) .addService(service) .build() fun start() { server.start() println("gRPC server started, listening on $port") Runtime.getRuntime().addShutdownHook(object : Thread() { override fun run() { println("shutting down gRPC server since JVM is shutting down") this@GrpcServer.stop() println("server shut down") } }) } fun stop() { server.shutdown() } /** * Await termination on the main thread since the grpc library uses daemon threads. */ fun blockUntilShutdown() { server.awaitTermination() } } ``` ## 1.5 Getting Up and Running 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. ```sh $ rm -rf /tmp/example $ cd $GOPATH/src/github.com/tendermint/tendermint $ make install $ TMHOME="/tmp/example" tendermint init 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 ``` 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: ```sh ./gradlew run gRPC server started, listening on 26658 ``` Then we need to start Tendermint Core and point it to our application. Staying within the application directory execute: ```sh $ TMHOME="/tmp/example" tendermint node --abci grpc --proxy_app tcp://127.0.0.1:26658 I[2019-07-28|15:44:53.632] Version info module=main software=0.32.1 block=10 p2p=7 I[2019-07-28|15:44:53.677] Starting Node module=main impl=Node I[2019-07-28|15:44:53.681] Started node module=main nodeInfo="{ProtocolVersion:{P2P:7 Block:10 App:0} ID_:7639e2841ccd47d5ae0f5aad3011b14049d3f452 ListenAddr:tcp://0.0.0.0:26656 Network:test-chain-Nhl3zk Version:0.32.1 Channels:4020212223303800 Moniker:Ivans-MacBook-Pro.local Other:{TxIndex:on RPCAddress:tcp://127.0.0.1:26657}}" I[2019-07-28|15:44:54.801] Executed block module=state height=8 validTxs=0 invalidTxs=0 I[2019-07-28|15:44:54.814] Committed state module=state height=8 txs=0 appHash=0000000000000000 ``` Now open another tab in your terminal and try sending a transaction: ```sh $ curl -s 'localhost:26657/broadcast_tx_commit?tx="tendermint=rocks"' { "jsonrpc": "2.0", "id": "", "result": { "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: ```sh $ curl -s 'localhost:26657/abci_query?data="tendermint"' { "jsonrpc": "2.0", "id": "", "result": { "response": { "log": "exists", "key": "dGVuZGVybWludA==", "value": "cm9ja3My" } } } ``` `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/). The full source code of this example project can be found [here](https://github.com/climber73/tendermint-abci-grpc-kotlin).