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JGroups RAFT implementation
There is a newer version: 1.0.14.Final
package org.jgroups.protocols.raft;

import org.jgroups.Address;
import org.jgroups.EmptyMessage;
import org.jgroups.Message;

/**
 * Base class for the different roles a RAFT node can have (follower, candidate, leader)
 * @author Bela Ban
 * @since  0.1
 */
public abstract class RaftImpl {
    protected RAFT raft; // a ref to the enclosing RAFT protocol

    public RaftImpl(RAFT raft) {this.raft=raft;}

    public RAFT     raft()       {return raft;}
    public RaftImpl raft(RAFT r) {this.raft=r; return this;}

    /** Called right after instantiation */
    public void init() {
    }

    /** Called before getting destroyed (on a role change) */
    public void destroy() {}


    /**
     * Called (on a follower) when an AppendEntries request is received
     * @param data The data (command to be appended to the log)
     * @param offset The offset
     * @param length The length
     * @param leader The leader's address (= the sender)
     * @param prev_log_index The index of the previous log entry
     * @param prev_log_term The term of the previous log entry
     * @param entry_term The term of the entry
     * @param leader_commit The leader's commit_index
     * @param internal True if the command is an internal command
     * @return AppendResult A result (true or false), or null if the request was ignored (e.g. due to lower term)
     */
    protected AppendResult handleAppendEntriesRequest(byte[] data, int offset, int length, Address leader,
                                                      int prev_log_index, int prev_log_term, int entry_term,
                                                      int leader_commit, boolean internal) {
        // todo: synchronize
        raft.leader(leader);

        if(data == null || length == 0) { // we got an empty AppendEntries message containing only leader_commit
            handleCommitRequest(leader, leader_commit);
            return null;
        }

        LogEntry prev=raft.log_impl.get(prev_log_index);
        int curr_index=prev_log_index+1;

        if(prev == null && prev_log_index > 0) // didn't find entry
            return new AppendResult(false, raft.lastAppended());

        // if the term at prev_log_index != prev_term -> return false and the start index of the conflicting term
        if(prev_log_index == 0 || prev.term == prev_log_term) {
            LogEntry existing=raft.log_impl.get(curr_index);
            if(existing != null && existing.term != entry_term) {
                // delete this and all subsequent entries and overwrite with received entry
                raft.deleteAllLogEntriesStartingFrom(curr_index);
            }
            raft.append(entry_term, curr_index, data, offset, length, internal).commitLogTo(leader_commit);
            if(internal)
                raft.executeInternalCommand(null, data, offset, length);
            return new AppendResult(true, curr_index).commitIndex(raft.commitIndex());
        }
        return new AppendResult(false, getFirstIndexOfConflictingTerm(prev_log_index, prev.term), prev.term);
    }


    protected void handleAppendEntriesResponse(Address sender, int term, AppendResult result) {
    }

    protected void handleInstallSnapshotRequest(Message msg, int term, Address leader,
                                                int last_included_index, int last_included_term) {

    }


    /** Finds the first index at which conflicting_term starts, going back from start_index towards the head of the log */
    protected int getFirstIndexOfConflictingTerm(int start_index, int conflicting_term) {
        Log log=raft.log_impl;
        int first=Math.max(1, log.firstAppended()), last=log.lastAppended();
        int retval=Math.min(start_index, last);
        for(int i=retval; i >= first; i--) {
            LogEntry entry=log.get(i);
            if(entry == null || entry.term != conflicting_term)
                break;
            retval=i;
        }
        return retval;
    }

    protected void handleCommitRequest(Address sender, int leader_commit) {
        raft.commitLogTo(leader_commit);
        AppendResult result=new AppendResult(true, raft.lastAppended()).commitIndex(raft.commitIndex());
        Message msg=new EmptyMessage(sender).putHeader(raft.getId(), new AppendEntriesResponse(raft.currentTerm(), result));
        raft.getDownProtocol().down(msg);
    }

}