MicroRaft

https://microraft.io/docs/
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MicroRaft is an embeddable Java Raft consensus library designed for building strongly consistent...
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### YAML configuration example

Source: https://microraft.io/docs/configuration

Example YAML configuration string showing the raft block with all supported settings and their default values.

```yaml
raft:
 leader-election-timeout-millis: 1000
 leader-heartbeat-timeout-secs: 10
 leader-heartbeat-period-secs: 2
 max-pending-log-entry-count: 5000
 append-entries-request-batch-size: 1000
 commit-count-to-take-snapshot: 50000
 transfer-snapshots-from-followers-enabled: true
 raft-node-report-publish-period-secs: 10
```

--------------------------------

### HOCON configuration example

Source: https://microraft.io/docs/configuration

Example HOCON configuration string showing the raft block with all supported settings and their default values.

```hocon
raft {
  leader-election-timeout-millis: 1000
  leader-heartbeat-timeout-secs: 10
  leader-heartbeat-period-secs: 2
  max-pending-log-entry-count: 5000
  append-entries-request-batch-size: 1000
  commit-count-to-take-snapshot: 50000
  transfer-snapshots-from-followers-enabled: true
  raft-node-report-publish-period-secs: 10
}
```

--------------------------------

### SnapshotInstallationTest.java helper methods

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

Helper methods for the snapshot installation test: waiting for leader election, retrieving the leader endpoint, disconnecting/connecting nodes, and polling until an assertion passes. The eventually() helper retries for up to 30 seconds with 200 ms sleeps and rethrows the last AssertionError if all attempts fail.

```java
  MILLISECONDS.sleep(100);  
  } catch (InterruptedException e) {  
  Thread.currentThread().interrupt();  
  }  
  }  
   
  throw new AssertionError("Could not elect a leader on time!");  
  }  
   
  private RaftEndpoint getLeaderEndpoint() {  
  RaftEndpoint leaderEndpoint = null;  
  int leaderTerm = 0;  
  for (RaftNode raftNode : raftNodes) {  
  if (raftNode.getStatus() == RaftNodeStatus.TERMINATED) {  
  continue;  
  }  
   
  RaftTerm term = raftNode.getTerm();  
  if (term.getLeaderEndpoint() != null) {  
  if (leaderEndpoint == null) {  
  leaderEndpoint = term.getLeaderEndpoint();  
  leaderTerm = term.getTerm();  
  } else if (!(leaderEndpoint.equals(term.getLeaderEndpoint()) && leaderTerm == term.getTerm())) {  
  leaderEndpoint = null;  
  break;  
  }  
  } else {  
  leaderEndpoint = null;  
  break;  
  }  
  }  
   
  return leaderEndpoint;  
  }  
   
  private RaftNode getAnyNodeExcept(RaftEndpoint endpoint) {  
  requireNonNull(endpoint);  
   
  return raftNodes.stream().filter(raftNode -> !raftNode.getLocalEndpoint().equals(endpoint)).findFirst()  
  .orElseThrow(IllegalArgumentException::new);  
  }  
   
  private void disconnect(RaftEndpoint endpoint1, RaftEndpoint endpoint2) {  
  requireNonNull(endpoint1);  
  requireNonNull(endpoint2);  
   
  getTransport(endpoint1).undiscoverNode(getNode(endpoint2));  
  getTransport(endpoint2).undiscoverNode(getNode(endpoint1));  
  }  
   
  private void connect(RaftEndpoint endpoint1, RaftEndpoint endpoint2) {  
  requireNonNull(endpoint1);  
  requireNonNull(endpoint2);  
   
  getTransport(endpoint1).discoverNode(getNode(endpoint2));  
  getTransport(endpoint2).discoverNode(getNode(endpoint1));  
  }  
   
  private RaftNode getNode(RaftEndpoint endpoint) {  
  requireNonNull(endpoint);  
   
  return raftNodes.stream().filter(raftNode -> raftNode.getLocalEndpoint().equals(endpoint)).findFirst()  
  .orElseThrow(IllegalArgumentException::new);  
  }  
   
  private LocalTransport getTransport(RaftEndpoint endpoint) {  
  requireNonNull(endpoint);  
   
  return transports.stream().filter(transport -> transport.getLocalEndpoint().equals(endpoint)).findFirst()  
  .orElseThrow(IllegalArgumentException::new);  
  }  
   
  private RaftLogStats getRaftLogStats(RaftNode leader) {  
  return leader.getReport().join().getResult().getLog();  
  }  
   
  private void eventually(AssertTask task) {  
  AssertionError error = null;  
  long timeoutSeconds = 30;  
  long sleepMillis = 200;  
  long iterations = TimeUnit.SECONDS.toMillis(timeoutSeconds) / sleepMillis;  
  for (int i = 0; i < iterations; i++) {  
  try {  
  try {  
  task.run();  
  return;  
  } catch (Exception e) {  
  throw new RuntimeException(e);  
  }  
  } catch (AssertionError e) {  
  error = e;  
  }  
   
  try {  
  MILLISECONDS.sleep(sleepMillis);  
  } catch (InterruptedException e) {  
  Thread.currentThread().interrupt();  
  }  
  }  
   
  if (error != null) {  
  throw error;  
  }  
   
  fail("eventually() failed without AssertionError!");  
  }  
   
  @FunctionalInterface  
  public interface AssertTask {  
  void run()  
  throws Exception;  
  }  
   
 }  
```

--------------------------------

### SnapshotInstallationTest: verify snapshot installation and follower catch-up

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

Test class for snapshot installation in a MicroRaft group. It creates a three-node Raft group, disconnects a follower, replicates enough operations to trigger a snapshot, reconnects, and verifies the follower installs the snapshot and catches up to the leader's commit index. Requires the tutorial module and the SnapshotableAtomicRegister state machine.

```java
import io.microraft.Ordered;  
import io.microraft.QueryPolicy;  
import io.microraft.RaftConfig;  
import io.microraft.RaftEndpoint;  
import io.microraft.RaftNode;  
import io.microraft.RaftNodeStatus;  
import io.microraft.report.RaftLogStats;  
import io.microraft.report.RaftTerm;  
import io.microraft.statemachine.StateMachine;  
import io.microraft.tutorial.atomicregister.SnapshotableAtomicRegister;  
import org.junit.After;  
import org.junit.Before;  
import org.junit.Test;  
 
import java.util.ArrayList;  
import java.util.Arrays;  
import java.util.List;  
import java.util.concurrent.TimeUnit;  
 
import static java.util.Objects.requireNonNull;  
import static java.util.concurrent.TimeUnit.MILLISECONDS;  
import static org.assertj.core.api.Assertions.assertThat;  
import static org.junit.Assert.fail;  
 
/*  
 
 TO RUN THIS TEST ON YOUR MACHINE:  
 
 $ gh repo clone MicroRaft/MicroRaft  
 $ cd MicroRaft && ./mvnw clean test -Dtest=io.microraft.tutorial.SnapshotInstallationTest -DfailIfNoTests=false -Ptutorial  
 
 YOU CAN SEE THIS CLASS AT:  
 
 https://github.com/MicroRaft/MicroRaft/blob/master/microraft-tutorial/src/test/java/io/microraft/tutorial/SnapshotInstallationTest.java  
 
 */  
public class SnapshotInstallationTest {  
 
 private static final int COMMIT_COUNT_TO_TAKE_SNAPSHOT = 100;  
 
 
 private List<RaftEndpoint> initialMembers = Arrays  
 .asList(LocalRaftEndpoint.newEndpoint(), LocalRaftEndpoint.newEndpoint(), LocalRaftEndpoint.newEndpoint());  
 private List<LocalTransport> transports = new ArrayList<>();  
 private List<RaftNode> raftNodes = new ArrayList<>();  
 
 
 @Before  
 public void startRaftGroup() {  
 for (RaftEndpoint endpoint : initialMembers) {  
 RaftNode raftNode = createRaftNode(endpoint);  
 raftNode.start();  
 }  
 }  
 
 @After  
 public void terminateRaftGroup() {  
 raftNodes.forEach(RaftNode::terminate);  
 }  
 
 @Test  
 public void testSnapshotInstallation() {  
 RaftNode leader = waitUntilLeaderElected();  
 RaftNode follower = getAnyNodeExcept(leader.getLocalEndpoint());  
 
 disconnect(leader.getLocalEndpoint(), follower.getLocalEndpoint());  
 
 for (int i = 0; i < COMMIT_COUNT_TO_TAKE_SNAPSHOT; i++) {  
 leader.replicate(SnapshotableAtomicRegister.newSetOperation("value" + i)).join();  
 }  
 
 assertThat(getRaftLogStats(leader).getTakeSnapshotCount()).isEqualTo(1);  
 
 connect(leader.getLocalEndpoint(), follower.getLocalEndpoint());  
 
 eventually(() -> {  
 RaftLogStats logStats = getRaftLogStats(follower);  
 assertThat(logStats.getInstallSnapshotCount()).isEqualTo(1);  
 assertThat(logStats.getCommitIndex()).isEqualTo(getRaftLogStats(leader).getCommitIndex());  
 });  
 
 eventually(() -> assertThat(getRaftLogStats(follower).getInstallSnapshotCount()).isEqualTo(1));  
 
 Ordered<String> leaderQueryResult = leader.<String>query(SnapshotableAtomicRegister.newGetOperation(),  
 QueryPolicy.EVENTUAL_CONSISTENCY, 0).join();  
 
 Ordered<String> followerQueryResult = follower.<String>query(SnapshotableAtomicRegister.newGetOperation(),  
 QueryPolicy.EVENTUAL_CONSISTENCY, 0).join();  
 
 assertThat(followerQueryResult.getCommitIndex()).isEqualTo(leaderQueryResult.getCommitIndex());  
 assertThat(followerQueryResult.getResult()).isEqualTo(leaderQueryResult.getResult());  
 }  
 
 private RaftNode createRaftNode(RaftEndpoint endpoint) {  
 RaftConfig config = RaftConfig.newBuilder().setCommitCountToTakeSnapshot(COMMIT_COUNT_TO_TAKE_SNAPSHOT).build();  
 LocalTransport transport = new LocalTransport(endpoint);  
 StateMachine stateMachine = new SnapshotableAtomicRegister();  
 RaftNode raftNode = RaftNode.newBuilder().setGroupId("default").setLocalEndpoint(endpoint)  
 .setInitialGroupMembers(initialMembers).setConfig(config).setTransport(transport)  
 .setStateMachine(stateMachine).build();  
 
 raftNodes.add(raftNode);  
 transports.add(transport);  
 enableDiscovery(raftNode, transport);  
 
 return raftNode;  
 }  
 
 private void enableDiscovery(RaftNode raftNode, LocalTransport transport) {  
 for (int i = 0; i < raftNodes.size(); i++) {  
 RaftNode otherNode = raftNodes.get(i);  
 if (otherNode != raftNode) {  
 transports.get(i).discoverNode(raftNode);  
 transport.discoverNode(otherNode);  
 }  
 }  
 }  
 
 private RaftNode waitUntilLeaderElected() {  
 long deadline = System.currentTimeMillis() + TimeUnit.SECONDS.toMillis(60);  
 while (System.currentTimeMillis() < deadline) {  
 RaftEndpoint leaderEndpoint = getLeaderEndpoint();  
 if (leaderEndpoint != null) {  
 return raftNodes.stream().filter(node -> node.getLocalEndpoint().equals(leaderEndpoint)).findFirst()  
 .orElseThrow(IllegalStateException::new);  
 }  
 
 try {  

```

--------------------------------

### Run the benchmark suite

Source: https://microraft.io/docs/benchmarks

Executes the full JMH benchmark suite for the core module. Requires Java 11 to be installed before running locally.

```bash
./gradlew benchmark
```

--------------------------------

### Run LeaderElectionTest with Gradle

Source: https://microraft.io/docs/troubleshooting

Run the LeaderElectionTest to isolate cluster formation when no leader is elected locally. Execute from the repository root and check logs for local transport setup failures before assuming a protocol issue.

```bash
./gradlew :microraft-tutorial:test \
  --tests io.microraft.tutorial.LeaderElectionTest
```

--------------------------------

### Output of get operation

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

Output of the final `get` operation, which reads the current value of the atomic register and returns value3.

```text
5th operation commit index: 6, result: value3

```

--------------------------------

### Run SnapshotInstallationTest with Gradle

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

Run the SnapshotInstallationTest from the microraft-tutorial module using Gradle. This command executes only the specified test class.

```bash
$ ./gradlew :microraft-tutorial:test --tests io.microraft.tutorial.SnapshotInstallationTest

```

--------------------------------

### Run MicroRaft tutorial test with Gradle

Source: https://microraft.io

Run a single local test to verify leader election, quorum, and commit behavior. Execute from the repository root. The test target is io.microraft.tutorial.OperationCommitTest.

```bash
./gradlew :microraft-tutorial:test \
  --tests io.microraft.tutorial.OperationCommitTest
```

--------------------------------

### ChangeRaftGroupMemberListTest: add members to Raft group

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

Test class for changing the Raft group member list. It starts a 3-member Raft group, replicates a value, then adds a new endpoint (endpoint4) using changeMembership() with commit index 0, starts the new Raft node, replicates another value, and begins adding a fifth endpoint. The comment notes that the commit index of the current member list is needed for adding endpoint5.

```java
package io.microraft.tutorial;  

import io.microraft.MembershipChangeMode;  
import io.microraft.Ordered;  
import io.microraft.QueryPolicy;  
import io.microraft.RaftEndpoint;  
import io.microraft.RaftNode;  
import io.microraft.RaftNodeStatus;  
import io.microraft.report.RaftGroupMembers;  
import io.microraft.report.RaftTerm;  
import io.microraft.statemachine.StateMachine;  
import io.microraft.tutorial.atomicregister.OperableAtomicRegister;  
import io.microraft.tutorial.atomicregister.SnapshotableAtomicRegister;  
import org.junit.After;  
import org.junit.Before;  
import org.junit.Test;  

import java.util.ArrayList;  
import java.util.Arrays;  
import java.util.List;  
import java.util.concurrent.TimeUnit;  

import static java.util.concurrent.TimeUnit.MILLISECONDS;  
import static org.assertj.core.api.Assertions.assertThat;  
import static org.junit.Assert.fail;  

/*  

 TO RUN THIS TEST ON YOUR MACHINE:  

 $ gh repo clone MicroRaft/MicroRaft  
 $ cd MicroRaft && ./mvnw clean test -Dtest=io.microraft.tutorial.ChangeRaftGroupMemberListTest -DfailIfNoTests=false -Ptutorial  

 YOU CAN SEE THIS CLASS AT:  

 https://github.com/MicroRaft/MicroRaft/blob/master/microraft-tutorial/src/test/java/io/microraft/tutorial/ChangeRaftGroupMemberListTest.java  

 */  
public class ChangeRaftGroupMemberListTest {  

 private List<RaftEndpoint> initialMembers = Arrays  
 .asList(LocalRaftEndpoint.newEndpoint(), LocalRaftEndpoint.newEndpoint(), LocalRaftEndpoint.newEndpoint());  
 private List<LocalTransport> transports = new ArrayList<>();  
 private List<RaftNode> raftNodes = new ArrayList<>();  

 @Before  
 public void startRaftGroup() {  
 for (RaftEndpoint endpoint : initialMembers) {  
 RaftNode raftNode = createRaftNode(endpoint);  
 raftNode.start();  
 }  
 }  

 @After  
 public void terminateRaftGroup() {  
 raftNodes.forEach(RaftNode::terminate);  
 }  

 @Test  
 public void testChangeMemberListOfRaftGroup() {  
 RaftNode leader = waitUntilLeaderElected();  

 String value1 = "value1";  
 leader.replicate(OperableAtomicRegister.newSetOperation(value1)).join();  

 // when we start a new Raft node, we should first add it to the Raft  
 // group, then start its RaftNode instance.  
 RaftEndpoint endpoint4 = LocalRaftEndpoint.newEndpoint();  
 // group members commit index of the initial Raft group members is 0.  
 RaftGroupMembers newMemberList1 = leader.changeMembership(endpoint4, MembershipChangeMode.ADD_OR_PROMOTE_TO_FOLLOWER, 0).join().getResult();  
 System.out.println("New member list: " + newMemberList1.getMembers() + ", majority: " + newMemberList1.getMajority()  
 + ", commit index: " + newMemberList1.getLogIndex());  

 // endpoint4 is now part of the member list. Let's start its Raft node  
 RaftNode raftNode4 = createRaftNode(endpoint4);  
 raftNode4.start();  

 String value2 = "value2";  
 leader.replicate(OperableAtomicRegister.newSetOperation(value2)).join();  

 RaftEndpoint endpoint5 = LocalRaftEndpoint.newEndpoint();  
 // we need the commit index of the current Raft group member list for  
 // adding endpoint5. We can get it either from  

```

--------------------------------

### Build MicroRaft from source (gh repo clone, ./gradlew build)

Source: https://microraft.io/docs/setup

Clone the MicroRaft repository and build it with the Gradle wrapper. Built artifacts land under microraft/build/libs, microraft-hocon/build/libs, and microraft-yaml/build/libs.

```bash
gh repo clone MicroRaft/MicroRaft
cd MicroRaft
./gradlew build
```

--------------------------------

### OperableAtomicRegister.java - Atomic register state machine with set/get/cas operations

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

Defines the atomic register state machine with set, get, and compare-and-set operations. It implements the StateMachine interface but throws UnsupportedOperationException for snapshot methods, so it is not snapshot-capable. Use this class as the state machine for Raft nodes in the tutorial.

```java
package io.microraft.tutorial.atomicregister;

import io.microraft.integration.StateMachine;

import javax.annotation.Nonnull;
import java.util.List;
import java.util.Objects;
import java.util.function.Consumer;

/**
 * Implements operations on the atomic register state machine.
 * <p>
 * This class does not implement the snapshotting logic.
 *
 * YOU CAN SEE THIS CLASS AT:
 *
 * https://github.com/MicroRaft/MicroRaft/blob/master/microraft-tutorial/src/main/java/io/microraft/tutorial/atomicregister/OperableAtomicRegister.java
 *
 */
public class OperableAtomicRegister
        extends AtomicRegister
        implements StateMachine {

    /**
     * Marker interface for operations to be executed on the atomic register
     * state machine.
     */
    public interface AtomicRegisterOperation {
    }

    /**
     * Returns a new operation to update the atomic register with the given
     * value. The operation returns the previous value as response.
     *
     * @param value
     *        to update the atomic register
     *
     * @return the operation to update the atomic register value
     */
    public static AtomicRegisterOperation newSetOperation(String value) {
        return new SetOperation(value);
    }

    /**
     * Returns a new operation to get the current value of the atomic register.
     *
     * @return the operation to get the current value of the atomic register
     */
    public static AtomicRegisterOperation newGetOperation() {
        return new GetOperation();
    }

    /**
     * Returns a compare-and-swap operation to update the atomic register with
     * the given new value of only if its current value matches with the given
     * current value. The operations returns true if the compare-and-swap
     * operation is successful.
     *
     * @param currentValue
     *        the current value of the atomic register for cas
     * @param newValue
     *        the new value for cas
     *
     * @return the operation to perform cas on the atomic register
     */
    public static AtomicRegisterOperation newCasOperation(Object currentValue, Object newValue) {
        return new CasOperation(currentValue, newValue);
    }


    protected Object value;

    @Override
    public Object runOperation(long commitIndex, @Nonnull Object operation) {
        if (operation instanceof SetOperation) {
            Object prev = this.value;
            this.value = ((SetOperation) operation).value;
            return prev;
        } else if (operation instanceof CasOperation) {
            CasOperation cas = (CasOperation) operation;
            boolean success = Objects.equals(this.value, cas.currentValue);
            if (success) {
                this.value = cas.newValue;
            }

            return success;
        } else if (operation instanceof GetOperation) {
            return value;
        }

        return super.runOperation(commitIndex, operation);
    }

    @Override
    public void takeSnapshot(long commitIndex, Consumer<Object> snapshotChunkConsumer) {
        throw new UnsupportedOperationException();
    }

    @Override
    public void installSnapshot(long commitIndex, @Nonnull List<Object> snapshotChunks) {
        throw new UnsupportedOperationException();
    }

    private static class SetOperation implements AtomicRegisterOperation {
        final Object value;

        SetOperation(Object value) {
            this.value = value;
        }

        @Override
        public String toString() {
            return "SetOperation{" + "value='" + value + '\'' + '}';
        }
    }

    private static class GetOperation implements AtomicRegisterOperation {
        @Override
        public String toString() {
            return "GetOperation{}";
        }
    }

    private static class CasOperation implements AtomicRegisterOperation {
        final Object currentValue;
        final Object newValue;

        CasOperation(Object currentValue, Object newValue) {
            this.currentValue = currentValue;
            this.newValue = newValue;
        }

        @Override
        public String toString() {
            return "CasOperation{" + "currentValue='" + currentValue + '\'' + ", newValue='" + newValue + '\'' + '}';
        }
    }

}
```

--------------------------------

### Run NetworkPartitionTest with Gradle

Source: https://microraft.io/docs/resiliency-and-fault-tolerance

Command to run the NetworkPartitionTest using Gradle. Executes the test class io.microraft.faulttolerance.NetworkPartitionTest in the microraft module.

```shell
$ ./gradlew :microraft:test --tests io.microraft.faulttolerance.NetworkPartitionTest
```

--------------------------------

### Run LeaderElectionTest with Gradle

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

Command to run the LeaderElectionTest on your machine. It uses Gradle to execute the specific test class in the microraft-tutorial module, with the tutorial profile enabled.

```bash
$ ./gradlew :microraft-tutorial:test --tests io.microraft.tutorial.LeaderElectionTest -DfailIfNoTests=false -Ptutorial

```

--------------------------------

### Run LinearizableQueryTest with Gradle

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

Command to run the LinearizableQueryTest on your machine using Gradle. It executes the test class io.microraft.tutorial.LinearizableQueryTest in the microraft-tutorial module.

```bash
$ ./gradlew :microraft-tutorial:test --tests io.microraft.tutorial.LinearizableQueryTest
```

--------------------------------

### Expected output of LinearizableQueryTest

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

Expected output from the test's sysout lines. It shows the set operation commit index and the get operation commit index and result. Both commit indices are the same because MicroRaft executes linearizable queries at the last committed log index without appending a new entry.

```text
set operation commit index: 2
get operation commit index: 2, result: value
```

--------------------------------

### MonotonicLocalQueryTest JUnit test for monotonic local reads

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

JUnit test class for verifying monotonic local reads. It sets up a three-node Raft group, starts and terminates nodes, and includes a test method that blocks communication between leader and follower to demonstrate LaggingCommitIndexException. Run with the provided Maven command.

```java
package io.microraft.tutorial;  
  
import io.microraft.Ordered;  
import io.microraft.QueryPolicy;  
import io.microraft.RaftEndpoint;  
import io.microraft.RaftNode;  
import io.microraft.RaftNodeStatus;  
import io.microraft.exception.LaggingCommitIndexException;  
import io.microraft.report.RaftTerm;  
import io.microraft.statemachine.StateMachine;  
import io.microraft.tutorial.atomicregister.OperableAtomicRegister;  
import org.junit.After;  
import org.junit.Before;  
import org.junit.Test;  
  
import java.util.ArrayList;  
import java.util.Arrays;  
import java.util.List;  
import java.util.concurrent.CompletionException;  
import java.util.concurrent.TimeUnit;  
  
import static java.util.Objects.requireNonNull;  
import static java.util.concurrent.TimeUnit.MILLISECONDS;  
import static org.assertj.core.api.Assertions.assertThat;  
import static org.assertj.core.api.Assertions.fail;  
  
/*  
  
 TO RUN THIS TEST ON YOUR MACHINE:  
  
 $ gh repo clone MicroRaft/MicroRaft  
 $ cd MicroRaft && ./mvnw clean test -Dtest=io.microraft.tutorial.MonotonicLocalQueryTest -DfailIfNoTests=false -Ptutorial  
 YOU CAN SEE THIS CLASS AT:  
  
 https://github.com/MicroRaft/MicroRaft/blob/master/microraft-tutorial/src/test/java/io/microraft/examples/MonotonicLocalQueryTest.java  
  
 */  
public class MonotonicLocalQueryTest {  
  
 private List<RaftEndpoint> initialMembers = Arrays  
 .asList(LocalRaftEndpoint.newEndpoint(), LocalRaftEndpoint.newEndpoint(), LocalRaftEndpoint.newEndpoint());  
 private List<LocalTransport> transports = new ArrayList<>();  
 private List<RaftNode> raftNodes = new ArrayList<>();  
  
 @Before  
 public void startRaftGroup() {  
 for (RaftEndpoint endpoint : initialMembers) {  
 RaftNode raftNode = createRaftNode(endpoint);  
 raftNode.start();  
 }  
 }  
  
 @After  
 public void terminateRaftGroup() {  
 raftNodes.forEach(RaftNode::terminate);  
 }  
  
 @Test  
 public void testMonotonicLocalQuery() {  
 RaftNode leader = waitUntilLeaderElected();  
 RaftNode follower = getAnyNodeExcept(leader.getLocalEndpoint());  
 
```

--------------------------------

### Change Raft group membership with changeMembership and ADD_OR_PROMOTE_TO_FOLLOWER

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

Shows how to change the Raft group member list by adding a new endpoint with ADD_OR_PROMOTE_TO_FOLLOWER, then start the new node and replicate a value. Includes helper methods for querying, creating nodes, enabling discovery, waiting for leader election, and polling until an assertion passes.

```java
// newMemberList1.getCommitIndex() or leader.getCommittedMembers().
RaftGroupMembers newMemberList2 = leader
        .changeMembership(endpoint5, MembershipChangeMode.ADD_OR_PROMOTE_TO_FOLLOWER, newMemberList1.getLogIndex()).join().getResult();
System.out.println("New member list: " + newMemberList2.getMembers() + ", majority: " + newMemberList2.getMajority()
        + ", commit index: " + newMemberList2.getLogIndex());

// endpoint5 is also part of the member list now. Let's start its Raft node
RaftNode raftNode5 = createRaftNode(endpoint5);
raftNode5.start();

String value3 = "value3";
leader.replicate(OperableAtomicRegister.newSetOperation(value3)).join();

// wait until the new Raft nodes replicate the log entries.
eventually(() -> {
    assertThat(query(raftNode4).getResult()).isEqualTo(value3);
    assertThat(query(raftNode5).getResult()).isEqualTo(value3);
});

System.out.println(endpoint4.getId() + "'s local query result: " + query(raftNode4).getResult());
System.out.println(endpoint5.getId() + "'s local query result: " + query(raftNode5).getResult());
}

private Ordered<String> query(RaftNode raftNode) {
    return raftNode.<String>query(OperableAtomicRegister.newGetOperation(), QueryPolicy.ANY_LOCAL, 0).join();
}

private RaftNode createRaftNode(RaftEndpoint endpoint) {
    LocalTransport transport = new LocalTransport(endpoint);
    StateMachine stateMachine = new SnapshotableAtomicRegister();
    RaftNode raftNode = RaftNode.newBuilder().setGroupId("default").setLocalEndpoint(endpoint)
            .setInitialGroupMembers(initialMembers).setTransport(transport)
            .setStateMachine(stateMachine).build();

    raftNodes.add(raftNode);
    transports.add(transport);
    enableDiscovery(raftNode, transport);

    return raftNode;
}

private void enableDiscovery(RaftNode raftNode, LocalTransport transport) {
    for (int i = 0; i < raftNodes.size(); i++) {
        RaftNode otherNode = raftNodes.get(i);
        if (otherNode != raftNode) {
            transports.get(i).discoverNode(raftNode);
            transport.discoverNode(otherNode);
        }
    }
}

private RaftNode waitUntilLeaderElected() {
    long deadline = System.currentTimeMillis() + TimeUnit.SECONDS.toMillis(60);
    while (System.currentTimeMillis() < deadline) {
        RaftEndpoint leaderEndpoint = getLeaderEndpoint();
        if (leaderEndpoint != null) {
            return raftNodes.stream().filter(node -> node.getLocalEndpoint().equals(leaderEndpoint)).findFirst()
                    .orElseThrow(IllegalStateException::new);
        }

        try {
            MILLISECONDS.sleep(100);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }

    throw new AssertionError("Could not elect a leader on time!");
}

private RaftEndpoint getLeaderEndpoint() {
    RaftEndpoint leaderEndpoint = null;
    int leaderTerm = 0;
    for (RaftNode raftNode : raftNodes) {
        if (raftNode.getStatus() == RaftNodeStatus.TERMINATED) {
            continue;
        }

        RaftTerm term = raftNode.getTerm();
        if (term.getLeaderEndpoint() != null) {
            if (leaderEndpoint == null) {
                leaderEndpoint = term.getLeaderEndpoint();
                leaderTerm = term.getTerm();
            } else if (!(leaderEndpoint.equals(term.getLeaderEndpoint()) && leaderTerm == term.getTerm())) {
                leaderEndpoint = null;
                break;
            }
        } else {
            leaderEndpoint = null;
            break;
        }
    }

    return leaderEndpoint;
}

private void eventually(AssertTask task) {
    AssertionError error = null;
    long timeoutSeconds = 30;
    long sleepMillis = 200;
    long iterations = TimeUnit.SECONDS.toMillis(timeoutSeconds) / sleepMillis;
    for (int i = 0; i < iterations; i++) {
        try {
            try {
                task.run();
                return;
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        } catch (AssertionError e) {
            error = e;
        }

        try {
            MILLISECONDS.sleep(sleepMillis);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }

    if (error != null) {
        throw error;
    }

    fail("eventually() failed without AssertionError!");
}

@FunctionalInterface
public interface AssertTask {
    void run()
            throws Exception;
}

}
```

--------------------------------

### Run ChangeRaftGroupMemberListTest with Gradle

Source: https://microraft.io/docs/tutorial-building-an-atomic-register

Command to run the ChangeRaftGroupMemberListTest on your machine using Gradle.

```bash
$ ./gradlew :microraft-tutorial:test --tests io.microraft.tutorial.ChangeRaftGroupMemberListTest

```
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