public Translog.Location add(BytesReference data) throws IOException {
        rwl.writeLock().lock();//獲取讀寫鎖，每個(gè)文件的寫入都是順序的。
        try {
            operationCounter++;
            long position = lastPosition;
            if (data.length() >= buffer.length) {
                flushBuffer();
                // we use the channel to write, since on windows, writing to the RAF might not be reflected
                // when reading through the channel
                data.writeTo(raf.channel());//寫入數(shù)據(jù)
                lastWrittenPosition += data.length();
                lastPosition += data.length();//記錄位置
                return new Translog.Location(id, position, data.length());//返回由id，位置及長(zhǎng)度確定的操作位置信息。
            }
            if (data.length() > buffer.length - bufferCount) {
                flushBuffer();
            }
            data.writeTo(bufferOs);
            lastPosition += data.length();
            return new Translog.Location(id, position, data.length());
        } finally {
            rwl.writeLock().unlock();
        }
    }

public FsChannelSnapshot snapshot() throws TranslogException {
        if (raf.increaseRefCount()) {
            boolean success = false;
            try {
                rwl.writeLock().lock();
                try {
                    FsChannelSnapshot snapshot = new FsChannelSnapshot(this.id, raf, lastWrittenPosition, operationCounter);
                    snapshot.seekTo(this.headsuccess = true;
                    returnerSize);
                     snapshot;
                } finally {
                    rwl.writeLock().unlock();
                }
            } catch (FileNotFoundException e) {
                throw new TranslogException(shardId, "failed to create snapshot", e);
            } finally {
                if (!success) {
                    raf.decreaseRefCount(false);
                }
            }
        }
        return null;
    }

public void revertTransient() {
        FsTranslogFile tmpTransient;
        rwl.writeLock().lock();
        try {
            tmpTransient = trans;//交換
            this.trans = null;
        } finally {
            rwl.writeLock().unlock();
        }
        logger.trace("revert transient {}", tmpTransient);
        // previous transient might be null because it was failed on its creation
        // for example
        if (tmpTransient != null) {
            tmpTransient.close(true);
        }
    }

static interface Operation extends Streamable {
        static enum Type {
            CREATE((byte) 1),
            SAVE((byte) 2),
            DELETE((byte) 3),
            DELETE_BY_QUERY((byte) 4);
            private final byte id;
            private Type(byte id) {
                this.id = id;
            }
            public byte id() {
                return this.id;
            }
            public static Type fromId(byte id) {
                switch (id) {
                    case 1:
                        return CREATE;
                    case 2:
                        return SAVE;
                    case 3:
                        return DELETE;
                    case 4:
                        return DELETE_BY_QUERY;
                    default:
                        throw new ElasticsearchIllegalArgumentException("No type mapped for [" + id + "]");
                }
            }
        }
        Type opType();
        long estimateSize();
        Source getSource();
    }