/
StateMachine.java
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/
StateMachine.java
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/*
* Copyright 2015 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package net.kuujo.copycat.raft.server;
import net.kuujo.copycat.raft.ApplicationException;
import net.kuujo.copycat.raft.Command;
import net.kuujo.copycat.raft.Operation;
import net.kuujo.copycat.raft.Session;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.CompletableFuture;
import java.util.function.Function;
/**
* Base class for user-provided Raft state machines.
* <p>
* Users should extend this class to create a state machine for use within a {@link RaftServer}.
* <p>
* State machines are responsible for handling {@link net.kuujo.copycat.raft.Operation operations} submitted to the Raft
* cluster and filtering {@link Commit committed} operations out of the Raft log. The most
* important rule of state machines is that <em>state machines must be deterministic</em> in order to maintain Copycat's
* consistency guarantees. That is, state machines must not change their behavior based on external influences and have
* no side effects. Users should <em>never</em> use {@code System} time to control behavior within a state machine.
* <p>
* When {@link net.kuujo.copycat.raft.Command commands} and {@link net.kuujo.copycat.raft.Query queries} are submitted
* to the Raft cluster, the {@link RaftServer} will log and replicate them as necessary
* and, once complete, apply them to the configured state machine.
* <p>
* State machine commands and queries are defined by annotating public or protected methods with the
* {@link Apply} annotation:
* <pre>
* {@code
* public class SetStateMachine extends StateMachine {
* private final Set<String> values = new HashSet<>();
*
* protected boolean applyContains(Commit<ContainsQuery> commit) {
* return values.contains(commit.operation().value());
* }
*
* }
* }
* </pre>
* Operations are applied to {@link Apply} annotated methods wrapped in a
* {@link Commit} object. The {@code Commit} object contains information about how the
* operation was committed including the {@link Commit#index()} at which it was logged in the Raft log, the
* {@link Commit#timestamp()} at which it was logged, and the {@link net.kuujo.copycat.raft.Session}
* that created the commit.
* <p>
* Operation methods can return either a synchronous result or an asynchronous result via {@link java.util.concurrent.CompletableFuture}.
* When the state machine is constructed, the {@link Apply} annotated methods will be
* evaluated for return types, so asynchronous methods <em>must specify a {@link java.util.concurrent.CompletableFuture}
* return type</em>. Once an operation is applied to the state machine, the value returned by the operation will be returned
* to the client that submitted the operation.
*
* @see Apply
* @see Commit
*
* @author <a href="http://github.com/kuujo">Jordan Halterman</a>
*/
public abstract class StateMachine implements AutoCloseable {
private final Logger LOGGER = LoggerFactory.getLogger(getClass());
private final Map<Class<? extends Operation>, OperationExecutor> operations = new HashMap<>();
private OperationExecutor allOperation;
private long time;
protected StateMachine() {
init();
}
/**
* Initializes the state machine.
*/
private void init() {
init(getClass());
}
/**
* Initializes the state machine.
*/
private void init(Class<?> clazz) {
while (clazz != null && clazz != Object.class) {
for (Method method : clazz.getDeclaredMethods()) {
declareOperations(method);
}
for (Class<?> iface : clazz.getInterfaces()) {
init(iface);
}
clazz = clazz.getSuperclass();
}
}
/**
* Declares any operations defined by the given method.
*/
private void declareOperations(Method method) {
Apply apply = method.getAnnotation(Apply.class);
if (apply != null) {
method.setAccessible(true);
for (Class<? extends Operation> operation : apply.value()) {
if (operation == Apply.All.class) {
allOperation = new OperationExecutor(method);
} else if (!operations.containsKey(operation)) {
operations.put(operation, new OperationExecutor(method));
}
}
}
}
/**
* Finds the operation method for the given operation.
*/
private OperationExecutor findOperation(Class<? extends Operation> type) {
OperationExecutor operation = operations.computeIfAbsent(type, t -> {
for (Map.Entry<Class<? extends Operation>, OperationExecutor> entry : operations.entrySet()) {
if (entry.getKey().isAssignableFrom(type)) {
return entry.getValue();
}
}
return allOperation;
});
if (operation == null) {
throw new IllegalArgumentException("unknown operation type: " + type);
}
return operation;
}
/**
* Operation executor.
*/
private class OperationExecutor implements Function<Commit<?>, CompletableFuture<Object>> {
private final Method method;
private final boolean async;
private OperationExecutor(Method method) {
this.method = method;
async = method.getReturnType() == CompletableFuture.class;
}
@Override
@SuppressWarnings("unchecked")
public CompletableFuture<Object> apply(Commit<?> commit) {
if (Command.class.isAssignableFrom(commit.type())) {
setTime(commit.timestamp());
}
try {
return async ? (CompletableFuture<Object>) method.invoke(StateMachine.this, commit) : CompletableFuture.completedFuture(method.invoke(StateMachine.this, commit));
} catch (IllegalAccessException | InvocationTargetException e) {
throw new ApplicationException("failed to invoke operation", e);
}
}
}
/**
* Sets the state machine time.
*/
private void setTime(long time) {
this.time = time;
}
/**
* Returns the current state machine time.
*
* @return The current state machine time.
*/
protected long getTime() {
return time;
}
/**
* Called when a new session is registered.
*
* @param session The session that was registered.
*/
public void register(Session session) {
}
/**
* Applies an operation to the state machine.
*
* @param commit The commit to apply.
* @return The operation result.
*/
public CompletableFuture<Object> apply(Commit<? extends Operation> commit) {
LOGGER.debug("Applying {}", commit);
return findOperation(commit.type()).apply(commit);
}
/**
* Called when a session is expired.
*
* @param session The expired session.
*/
public void expire(Session session) {
}
/**
* Called when a session is closed.
*
* @param session The session that was closed.
*/
public void close(Session session) {
}
/**
* Closes the state machine.
*/
@Override
public void close() {
}
}