@Override
    public LogEntry getEntry(long index) {
        this.readLock.lock();
        try {
            checkState();
            if (this.hasLoadFirstLogIndex && index < this.firstLogIndex) {
                return null;
            }
            DatabaseEntry logEntry = new DatabaseEntry();
            OperationStatus operationStatus = this.defaultTable.get(null, getKeyDatabaseEntry(index), logEntry,
                LockMode.DEFAULT);
            if (isSuccessOperation(operationStatus)) {
                return toLogEntry(logEntry);
            }
        } catch (DatabaseException e) {
            LOG.error("Fail to get log entry at index {}.", index, e);
        } finally {
            this.readLock.unlock();
        }
        return null;
    }
First log index and last log index key in configuration column family.
jraft-extension/bdb-log-storage-impl/src/main/java/com/alipay/sofa/jraft/storage/impl/BDBLogStorage.java