第4-4 章：ZKDatabase

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李延 LV6 2021-09-08

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# 作用
前面我们分析DataTree、SnapShot、FileSnap 3个接口，在ZKDatabase中将这3个接口进行封装。

# 初始化

```java

/**
     * 创建zkDatabase
     * 其中：创建dataTree
     * 并且传如快照日志路径对象FeleTxnSnapLog
     *
     * 并初始化快照规则
     * 1、当日志大小大于快照的1/3 时，开启快照
     * 2、设置日志字段快照的行数，默认大于500
     *
     * 在启动的时候加载了日志：具体方法{@linkplain ZKDatabase#loadDataBase()}
     *
     * the filetxnsnaplog that this zk database
     * maps to. There is a one to one relationship
     * between a filetxnsnaplog and zkdatabase.
     * @param snapLog the FileTxnSnapLog mapping this zkdatabase
     */
    public ZKDatabase(FileTxnSnapLog snapLog) {
        dataTree = createDataTree();
        sessionsWithTimeouts = new ConcurrentHashMap<Long, Integer>();
        this.snapLog = snapLog;

try {
            snapshotSizeFactor = Double.parseDouble(
                    System.getProperty(SNAPSHOT_SIZE_FACTOR,
                            Double.toString(DEFAULT_SNAPSHOT_SIZE_FACTOR)));
            if (snapshotSizeFactor > 1) {
                snapshotSizeFactor = DEFAULT_SNAPSHOT_SIZE_FACTOR;
                LOG.warn(
                    "The configured {} is invalid, going to use the default {}",
                    SNAPSHOT_SIZE_FACTOR,
                    DEFAULT_SNAPSHOT_SIZE_FACTOR);
            }
        } catch (NumberFormatException e) {
            LOG.error(
                "Error parsing {}, using default value {}",
                SNAPSHOT_SIZE_FACTOR,
                DEFAULT_SNAPSHOT_SIZE_FACTOR);
            snapshotSizeFactor = DEFAULT_SNAPSHOT_SIZE_FACTOR;
        }

LOG.info("{} = {}", SNAPSHOT_SIZE_FACTOR, snapshotSizeFactor);

try {
            commitLogCount = Integer.parseInt(
                    System.getProperty(COMMIT_LOG_COUNT,
                            Integer.toString(DEFAULT_COMMIT_LOG_COUNT)));
            if (commitLogCount < DEFAULT_COMMIT_LOG_COUNT) {
                commitLogCount = DEFAULT_COMMIT_LOG_COUNT;
                LOG.warn(
                    "The configured commitLogCount {} is less than the recommended {}, going to use the recommended one",
                    COMMIT_LOG_COUNT,
                    DEFAULT_COMMIT_LOG_COUNT);
            }
        } catch (NumberFormatException e) {
            LOG.error(
                "Error parsing {} - use default value {}",
                COMMIT_LOG_COUNT,
                DEFAULT_COMMIT_LOG_COUNT);
            commitLogCount = DEFAULT_COMMIT_LOG_COUNT;
        }
        LOG.info("{}={}", COMMIT_LOG_COUNT, commitLogCount);
    }

```

# 主要方法

我们主要看一下在启动的时候加载了日志：具体方法。其他方法基本都是直接调用的我们之前分析的接口来实现的

```java

public long loadDataBase() throws IOException {
        long startTime = Time.currentElapsedTime();
        long zxid = snapLog.restore(dataTree, sessionsWithTimeouts, commitProposalPlaybackListener);
        initialized = true;
        long loadTime = Time.currentElapsedTime() - startTime;
        ServerMetrics.getMetrics().DB_INIT_TIME.add(loadTime);
        LOG.info("Snapshot loaded in {} ms, highest zxid is 0x{}, digest is {}",
                loadTime, Long.toHexString(zxid), dataTree.getTreeDigest());
        return zxid;
    }

```

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