# Creating an application in Java

# 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 Java.

This guide assumes that you are not new to JVM world. If you are new please see JVM Minimal Survival Guide and Gradle Docs.

# 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 is written this way. Please refer to Writing a built-in Tendermint Core application in Go 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.

Verify that you have installed Java successfully:

Copy $ java -version java version "12.0.2" 2019-07-16 Java(TM) SE Runtime Environment (build 12.0.2+10) Java HotSpot(TM) 64-Bit Server VM (build 12.0.2+10, mixed mode, sharing)

You can choose any version of Java higher or equal to 8. This guide is written using Java SE Development Kit 12.

Make sure you have $JAVA_HOME environment variable set:

Copy $ echo $JAVA_HOME /Library/Java/JavaVirtualMachines/jdk-12.0.2.jdk/Contents/Home

For Gradle installation, please refer to their official guide.

# 1.2 Creating a new Java project

We'll start by creating a new Gradle project.

Copy export KVSTORE_HOME=~/kvstore mkdir $KVSTORE_HOME cd $KVSTORE_HOME

Inside the example directory run:

Copy gradle init --dsl groovy --package io.example --project-name example --type java-application --test-framework junit

This will create a new project for you. The tree of files should look like:

Copy $ tree . |-- build.gradle |-- gradle | `-- wrapper | |-- gradle-wrapper.jar | `-- gradle-wrapper.properties |-- gradlew |-- gradlew.bat |-- settings.gradle `-- src |-- main | |-- java | | `-- io | | `-- example | | `-- App.java | `-- resources `-- test |-- java | `-- io | `-- example | `-- AppTest.java `-- resources

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

Copy $ ./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. 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:

Copy 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:

Copy plugins { id 'com.google.protobuf' version '0.8.8' }

Add the following code to build.gradle:

Copy 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:

Copy mkdir -p \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/abci \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/version \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/types \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/crypto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/libs \ $KVSTORE_HOME/src/main/proto/github.com/gogo/protobuf/gogoproto cp $GOPATH/src/github.com/tendermint/tendermint/proto/tendermint/abci/types.proto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/abci/types.proto cp $GOPATH/src/github.com/tendermint/tendermint/proto/tendermint/version/version.proto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/version/version.proto cp $GOPATH/src/github.com/tendermint/tendermint/proto/tendermint/types/types.proto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/types/types.proto cp $GOPATH/src/github.com/tendermint/tendermint/proto/tendermint/types/evidence.proto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/types/evidence.proto cp $GOPATH/src/github.com/tendermint/tendermint/proto/tendermint/types/params.proto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/types/params.proto cp $GOPATH/src/github.com/tendermint/tendermint/proto/tendermint/crypto/merkle.proto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/crypto/merkle.proto cp $GOPATH/src/github.com/tendermint/tendermint/proto/tendermint/crypto/keys.proto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/crypto/keys.proto cp $GOPATH/src/github.com/tendermint/tendermint/proto/tendermint/libs/types.proto \ $KVSTORE_HOME/src/main/proto/github.com/tendermint/tendermint/proto/tendermint/libs/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:

Copy 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:

Copy ./gradlew generateProto

To verify that everything went smoothly, you can inspect the build/generated/ directory:

Copy $ 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/java/io/example/KVStoreApp.java file with the following content:

Copy package io.example; import io.grpc.stub.StreamObserver; import types.ABCIApplicationGrpc; import types.Types.*; class KVStoreApp extends 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.).

Copy @Override public void checkTx(RequestCheckTx req, StreamObserver<ResponseCheckTx> responseObserver) { var tx = req.getTx(); int code = validate(tx); var resp = ResponseCheckTx.newBuilder() .setCode(code) .setGasWanted(1) .build(); responseObserver.onNext(resp); responseObserver.onCompleted(); } private int validate(ByteString tx) { List<byte[]> parts = split(tx, '='); if (parts.size() != 2) { return 1; } byte[] key = parts.get(0); byte[] value = parts.get(1); // check if the same key=value already exists var stored = getPersistedValue(key); if (stored != null && Arrays.equals(stored, value)) { return 2; } return 0; } private List<byte[]> split(ByteString tx, char separator) { var arr = tx.toByteArray(); int i; for (i = 0; i < tx.size(); i++) { if (arr[i] == (byte)separator) { break; } } if (i == tx.size()) { return Collections.emptyList(); } return List.of( tx.substring(0, i).toByteArray(), tx.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".

For the underlying key-value store we'll use JetBrains Xodus, which is a transactional schema-less embedded high-performance database written in Java.

build.gradle:

Copy dependencies { implementation 'org.jetbrains.xodus:xodus-environment:1.3.91' } Copy ... import jetbrains.exodus.ArrayByteIterable; import jetbrains.exodus.ByteIterable; import jetbrains.exodus.env.Environment; import jetbrains.exodus.env.Store; import jetbrains.exodus.env.StoreConfig; import jetbrains.exodus.env.Transaction; class KVStoreApp extends ABCIApplicationGrpc.ABCIApplicationImplBase { private Environment env; private Transaction txn = null; private Store store = null; KVStoreApp(Environment env) { this.env = env; } ... private byte[] getPersistedValue(byte[] k) { return env.computeInReadonlyTransaction(txn -> { var store = env.openStore("store", StoreConfig.WITHOUT_DUPLICATES, txn); ByteIterable byteIterable = store.get(txn, new ArrayByteIterable(k)); if (byteIterable == null) { return null; } return byteIterable.getBytesUnsafe(); }); } }

# 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.

Copy @Override public void beginBlock(RequestBeginBlock req, StreamObserver<ResponseBeginBlock> responseObserver) { txn = env.beginTransaction(); store = env.openStore("store", StoreConfig.WITHOUT_DUPLICATES, txn); var 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.

Copy @Override public void deliverTx(RequestDeliverTx req, StreamObserver<ResponseDeliverTx> responseObserver) { var tx = req.getTx(); int code = validate(tx); if (code == 0) { List<byte[]> parts = split(tx, '='); var key = new ArrayByteIterable(parts.get(0)); var value = new ArrayByteIterable(parts.get(1)); store.put(txn, key, value); } var 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.

Copy @Override public void commit(RequestCommit req, StreamObserver<ResponseCommit> responseObserver) { txn.commit(); var resp = ResponseCommit.newBuilder() .setData(ByteString.copyFrom(new byte[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) 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 @Override public void query(RequestQuery req, StreamObserver<ResponseQuery> responseObserver) { var k = req.getData().toByteArray(); var v = getPersistedValue(k); var builder = ResponseQuery.newBuilder(); if (v == null) { builder.setLog("does not exist"); } else { builder.setLog("exists"); builder.setKey(ByteString.copyFrom(k)); builder.setValue(ByteString.copyFrom(v)); } responseObserver.onNext(builder.build()); responseObserver.onCompleted(); }

The complete specification can be found here.

# 1.4 Starting an application and a Tendermint Core instances

Put the following code into the $KVSTORE_HOME/src/main/java/io/example/App.java file:

Copy package io.example; import jetbrains.exodus.env.Environment; import jetbrains.exodus.env.Environments; import java.io.IOException; public class App { public static void main(String[] args) throws IOException, InterruptedException { try (Environment env = Environments.newInstance("tmp/storage")) { var app = new KVStoreApp(env); var server = new 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 the $KVSTORE_HOME/src/main/java/io/example/GrpcServer.java file with the following content:

Copy package io.example; import io.grpc.BindableService; import io.grpc.Server; import io.grpc.ServerBuilder; import java.io.IOException; class GrpcServer { private Server server; GrpcServer(BindableService service, int port) { this.server = ServerBuilder.forPort(port) .addService(service) .build(); } void start() throws IOException { server.start(); System.out.println("gRPC server started, listening on $port"); Runtime.getRuntime().addShutdownHook(new Thread(() -> { System.out.println("shutting down gRPC server since JVM is shutting down"); GrpcServer.this.stop(); System.out.println("server shut down"); })); } private void stop() { server.shutdown(); } /** * Await termination on the main thread since the grpc library uses daemon threads. */ void blockUntilShutdown() throws InterruptedException { 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.

Copy $ 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.

We are ready to start our application:

Copy ./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:

Copy $ 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:

Copy $ 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:

Copy $ 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. To dig deeper, read the docs.

The full source code of this example project can be found here.