leveldb-Impl : SequenceNumber.java

SequenceNumber.java:leveldb/SequenceNumber.java at master · dain/leveldb · GitHub

public final class SequenceNumber
{
    // We leave eight bits empty at the bottom so a type and sequence#
    // can be packed together into 64-bits.
    public static final long MAX_SEQUENCE_NUMBER = ((0x1L << 56) - 1);

    private SequenceNumber()
    {
    }

    public static long packSequenceAndValueType(long sequence, ValueType valueType)
    {
        checkArgument(sequence <= MAX_SEQUENCE_NUMBER, "Sequence number is greater than MAX_SEQUENCE_NUMBER");
        requireNonNull(valueType, "valueType is null");

        return (sequence << 8) | valueType.getPersistentId();
    }

    public static ValueType unpackValueType(long num)
    {
        return ValueType.getValueTypeByPersistentId((byte) num);
    }

    public static long unpackSequenceNumber(long num)
    {
        return num >>> 8;
    }
}

sequenceNumber是一个64位的数。每次更新时，由SequenceNumber来标识，整个 db 有一个全局值保存着当前使用到的 SequnceNumber。SequnceNumber 在 leveldb 有重要的地位，key 的排序，compact 以及 snapshot 都依赖于它. SequnceNumber只占56bits,ValueType占用8bits

该类实现了利用7 byte的SequenceNumbers和1byte的valuetype构建完整的long，也实现了从long中拆出SequenceNumbers和valuetype的功能