Thrift版本管理

对于健壮的服务，其中的数据类型必须一种机制来对其进行版本管理，尤其是它可以在不中断服务（或者更坏的情况，出现段错误）的前提下，增加或删除一个对象中的字段，或者改变一个函数的参数列表。

字段标识符

Thrift的版本管理是通过字段标识符来实现的。对于每个被Thrift编码的结构的域头，都有一个唯一的字段标识符，这个字段标识符和它的类型说明符构成了对这个字段独一无二的识别。

Thrift定义语言支持字段标识符的自动分配，但是好的程序实践中是明确指出字段标识符。字段标识符使用如下方式指定：

struct Example {
  1:i32 number = 0,
  2:i64 bigNumber,
  3:double decimals,
  4:string name = "thrifty"
}

为了避免人工和自动分配的标识符冲突，忽略了标识符的字段自动从-1递减分配字段标识符，并且Thrift定义语言只支持人工分配正的标识符。

当数据正在被反序列化的时候，产生的代码能够使用这些字段标识符来恰当的识别字段，并判断这个标识符是否在它的定义文件中和一个字段对齐。如果一个字段标识符不被识别，产生的代码可以用类型说明符去跳过这个不可知的字段而不产生任何错误。同样，这个也可以归结为这样一个事实：所有的数据类型都是自划界的。

字段标识符也能够在函数列表中被指定。事实上，参数列表在后端不仅作为结构被呈现，而是在编译器前段分享了相同的代码。这是为了允许安全的修改方法的参数。

service StringCache {
  void set(1:i32 key, 2:string value),
  string get(1:i32 key) throws(1:KeyNotFound knf),
  void delete(1:i32 key)
}

对于每个结构，都选择指定字段标识符的语法。结构可以看成一个字典，标识符是主键，强类型名的字段是值。

Isset

当遇到一个未期望的字段，能够被安全的忽略和丢弃。当一个预期的字段未被找到，必须有一些方法来告知开发者该字段未出现。这是通过内部结构isset来实现的，这个结构位于定义对象内部。本质上说，每个thrift结构内部的isset对象为每个字段包含了一个布尔值，以此指示这个字段是否出现在结构中。当一个阅读器接受一个结构，它应该在直接操作它之前检查这个是否置位。

以上面Example为例，每个字段都有一个isset方法，生成的Java代码如下：

/**
 * Autogenerated by Thrift Compiler (0.11.0)
 *
 * DO NOT EDIT UNLESS YOU ARE SURE THAT YOU KNOW WHAT YOU ARE DOING
 *  @generated
 */
@SuppressWarnings({"cast", "rawtypes", "serial", "unchecked", "unused"})
@javax.annotation.Generated(value = "Autogenerated by Thrift Compiler (0.11.0)", date = "2018-06-18")
public class Example implements org.apache.thrift.TBase<Example, Example._Fields>, java.io.Serializable, Cloneable, Comparable<Example> {
  private static final org.apache.thrift.protocol.TStruct STRUCT_DESC = new org.apache.thrift.protocol.TStruct("Example");

  private static final org.apache.thrift.protocol.TField NUMBER_FIELD_DESC = new org.apache.thrift.protocol.TField("number", org.apache.thrift.protocol.TType.I32, (short)1);
  private static final org.apache.thrift.protocol.TField BIG_NUMBER_FIELD_DESC = new org.apache.thrift.protocol.TField("bigNumber", org.apache.thrift.protocol.TType.I64, (short)2);
  private static final org.apache.thrift.protocol.TField DECIMALS_FIELD_DESC = new org.apache.thrift.protocol.TField("decimals", org.apache.thrift.protocol.TType.DOUBLE, (short)3);
  private static final org.apache.thrift.protocol.TField NAME_FIELD_DESC = new org.apache.thrift.protocol.TField("name", org.apache.thrift.protocol.TType.STRING, (short)4);

  private static final org.apache.thrift.scheme.SchemeFactory STANDARD_SCHEME_FACTORY = new ExampleStandardSchemeFactory();
  private static final org.apache.thrift.scheme.SchemeFactory TUPLE_SCHEME_FACTORY = new ExampleTupleSchemeFactory();

  public int number; // required
  public long bigNumber; // required
  public double decimals; // required
  public java.lang.String name; // required

  /** The set of fields this struct contains, along with convenience methods for finding and manipulating them. */
  public enum _Fields implements org.apache.thrift.TFieldIdEnum {
    NUMBER((short)1, "number"),
    BIG_NUMBER((short)2, "bigNumber"),
    DECIMALS((short)3, "decimals"),
    NAME((short)4, "name");

    private static final java.util.Map<java.lang.String, _Fields> byName = new java.util.HashMap<java.lang.String, _Fields>();

    static {
      for (_Fields field : java.util.EnumSet.allOf(_Fields.class)) {
        byName.put(field.getFieldName(), field);
      }
    }

    /**
     * Find the _Fields constant that matches fieldId, or null if its not found.
     */
    public static _Fields findByThriftId(int fieldId) {
      switch(fieldId) {
        case 1: // NUMBER
          return NUMBER;
        case 2: // BIG_NUMBER
          return BIG_NUMBER;
        case 3: // DECIMALS
          return DECIMALS;
        case 4: // NAME
          return NAME;
        default:
          return null;
      }
    }

    /**
     * Find the _Fields constant that matches fieldId, throwing an exception
     * if it is not found.
     */
    public static _Fields findByThriftIdOrThrow(int fieldId) {
      _Fields fields = findByThriftId(fieldId);
      if (fields == null) throw new java.lang.IllegalArgumentException("Field " + fieldId + " doesn't exist!");
      return fields;
    }

    /**
     * Find the _Fields constant that matches name, or null if its not found.
     */
    public static _Fields findByName(java.lang.String name) {
      return byName.get(name);
    }

    private final short _thriftId;
    private final java.lang.String _fieldName;

    _Fields(short thriftId, java.lang.String fieldName) {
      _thriftId = thriftId;
      _fieldName = fieldName;
    }

    public short getThriftFieldId() {
      return _thriftId;
    }

    public java.lang.String getFieldName() {
      return _fieldName;
    }
  }

  // isset id assignments
  private static final int __NUMBER_ISSET_ID = 0;
  private static final int __BIGNUMBER_ISSET_ID = 1;
  private static final int __DECIMALS_ISSET_ID = 2;
  private byte __isset_bitfield = 0;
  public static final java.util.Map<_Fields, org.apache.thrift.meta_data.FieldMetaData> metaDataMap;
  static {
    java.util.Map<_Fields, org.apache.thrift.meta_data.FieldMetaData> tmpMap = new java.util.EnumMap<_Fields, org.apache.thrift.meta_data.FieldMetaData>(_Fields.class);
    tmpMap.put(_Fields.NUMBER, new org.apache.thrift.meta_data.FieldMetaData("number", org.apache.thrift.TFieldRequirementType.DEFAULT, 
        new org.apache.thrift.meta_data.FieldValueMetaData(org.apache.thrift.protocol.TType.I32)));
    tmpMap.put(_Fields.BIG_NUMBER, new org.apache.thrift.meta_data.FieldMetaData("bigNumber", org.apache.thrift.TFieldRequirementType.DEFAULT, 
        new org.apache.thrift.meta_data.FieldValueMetaData(org.apache.thrift.protocol.TType.I64)));
    tmpMap.put(_Fields.DECIMALS, new org.apache.thrift.meta_data.FieldMetaData("decimals", org.apache.thrift.TFieldRequirementType.DEFAULT, 
        new org.apache.thrift.meta_data.FieldValueMetaData(org.apache.thrift.protocol.TType.DOUBLE)));
    tmpMap.put(_Fields.NAME, new org.apache.thrift.meta_data.FieldMetaData("name", org.apache.thrift.TFieldRequirementType.DEFAULT, 
        new org.apache.thrift.meta_data.FieldValueMetaData(org.apache.thrift.protocol.TType.STRING)));
    metaDataMap = java.util.Collections.unmodifiableMap(tmpMap);
    org.apache.thrift.meta_data.FieldMetaData.addStructMetaDataMap(Example.class, metaDataMap);
  }

  public Example() {
    this.number = 0;

    this.name = "thrifty";

  }

  public Example(
    int number,
    long bigNumber,
    double decimals,
    java.lang.String name)
  {
    this();
    this.number = number;
    setNumberIsSet(true);
    this.bigNumber = bigNumber;
    setBigNumberIsSet(true);
    this.decimals = decimals;
    setDecimalsIsSet(true);
    this.name = name;
  }

  /**
   * Performs a deep copy on <i>other</i>.
   */
  public Example(Example other) {
    __isset_bitfield = other.__isset_bitfield;
    this.number = other.number;
    this.bigNumber = other.bigNumber;
    this.decimals = other.decimals;
    if (other.isSetName()) {
      this.name = other.name;
    }
  }

  public Example deepCopy() {
    return new Example(this);
  }

  @Override
  public void clear() {
    this.number = 0;

    setBigNumberIsSet(false);
    this.bigNumber = 0;
    setDecimalsIsSet(false);
    this.decimals = 0.0;
    this.name = "thrifty";

  }

  public int getNumber() {
    return this.number;
  }

  public Example setNumber(int number) {
    this.number = number;
    setNumberIsSet(true);
    return this;
  }

  public void unsetNumber() {
    __isset_bitfield = org.apache.thrift.EncodingUtils.clearBit(__isset_bitfield, __NUMBER_ISSET_ID);
  }

  /** Returns true if field number is set (has been assigned a value) and false otherwise */
  public boolean isSetNumber() {
    return org.apache.thrift.EncodingUtils.testBit(__isset_bitfield, __NUMBER_ISSET_ID);
  }

  public void setNumberIsSet(boolean value) {
    __isset_bitfield = org.apache.thrift.EncodingUtils.setBit(__isset_bitfield, __NUMBER_ISSET_ID, value);
  }

  public long getBigNumber() {
    return this.bigNumber;
  }

  public Example setBigNumber(long bigNumber) {
    this.bigNumber = bigNumber;
    setBigNumberIsSet(true);
    return this;
  }

  public void unsetBigNumber() {
    __isset_bitfield = org.apache.thrift.EncodingUtils.clearBit(__isset_bitfield, __BIGNUMBER_ISSET_ID);
  }

  /** Returns true if field bigNumber is set (has been assigned a value) and false otherwise */
  public boolean isSetBigNumber() {
    return org.apache.thrift.EncodingUtils.testBit(__isset_bitfield, __BIGNUMBER_ISSET_ID);
  }

  public void setBigNumberIsSet(boolean value) {
    __isset_bitfield = org.apache.thrift.EncodingUtils.setBit(__isset_bitfield, __BIGNUMBER_ISSET_ID, value);
  }

  public double getDecimals() {
    return this.decimals;
  }

  public Example setDecimals(double decimals) {
    this.decimals = decimals;
    setDecimalsIsSet(true);
    return this;
  }

  public void unsetDecimals() {
    __isset_bitfield = org.apache.thrift.EncodingUtils.clearBit(__isset_bitfield, __DECIMALS_ISSET_ID);
  }

  /** Returns true if field decimals is set (has been assigned a value) and false otherwise */
  public boolean isSetDecimals() {
    return org.apache.thrift.EncodingUtils.testBit(__isset_bitfield, __DECIMALS_ISSET_ID);
  }

  public void setDecimalsIsSet(boolean value) {
    __isset_bitfield = org.apache.thrift.EncodingUtils.setBit(__isset_bitfield, __DECIMALS_ISSET_ID, value);
  }

  public java.lang.String getName() {
    return this.name;
  }

  public Example setName(java.lang.String name) {
    this.name = name;
    return this;
  }

  public void unsetName() {
    this.name = null;
  }

  /** Returns true if field name is set (has been assigned a value) and false otherwise */
  public boolean isSetName() {
    return this.name != null;
  }

  public void setNameIsSet(boolean value) {
    if (!value) {
      this.name = null;
    }
  }

  public void setFieldValue(_Fields field, java.lang.Object value) {
    switch (field) {
    case NUMBER:
      if (value == null) {
        unsetNumber();
      } else {
        setNumber((java.lang.Integer)value);
      }
      break;

    case BIG_NUMBER:
      if (value == null) {
        unsetBigNumber();
      } else {
        setBigNumber((java.lang.Long)value);
      }
      break;

    case DECIMALS:
      if (value == null) {
        unsetDecimals();
      } else {
        setDecimals((java.lang.Double)value);
      }
      break;

    case NAME:
      if (value == null) {
        unsetName();
      } else {
        setName((java.lang.String)value);
      }
      break;

    }
  }

  public java.lang.Object getFieldValue(_Fields field) {
    switch (field) {
    case NUMBER:
      return getNumber();

    case BIG_NUMBER:
      return getBigNumber();

    case DECIMALS:
      return getDecimals();

    case NAME:
      return getName();

    }
    throw new java.lang.IllegalStateException();
  }

  /** Returns true if field corresponding to fieldID is set (has been assigned a value) and false otherwise */
  public boolean isSet(_Fields field) {
    if (field == null) {
      throw new java.lang.IllegalArgumentException();
    }

    switch (field) {
    case NUMBER:
      return isSetNumber();
    case BIG_NUMBER:
      return isSetBigNumber();
    case DECIMALS:
      return isSetDecimals();
    case NAME:
      return isSetName();
    }
    throw new java.lang.IllegalStateException();
  }

  @Override
  public boolean equals(java.lang.Object that) {
    if (that == null)
      return false;
    if (that instanceof Example)
      return this.equals((Example)that);
    return false;
  }

  public boolean equals(Example that) {
    if (that == null)
      return false;
    if (this == that)
      return true;

    boolean this_present_number = true;
    boolean that_present_number = true;
    if (this_present_number || that_present_number) {
      if (!(this_present_number && that_present_number))
        return false;
      if (this.number != that.number)
        return false;
    }

    boolean this_present_bigNumber = true;
    boolean that_present_bigNumber = true;
    if (this_present_bigNumber || that_present_bigNumber) {
      if (!(this_present_bigNumber && that_present_bigNumber))
        return false;
      if (this.bigNumber != that.bigNumber)
        return false;
    }

    boolean this_present_decimals = true;
    boolean that_present_decimals = true;
    if (this_present_decimals || that_present_decimals) {
      if (!(this_present_decimals && that_present_decimals))
        return false;
      if (this.decimals != that.decimals)
        return false;
    }

    boolean this_present_name = true && this.isSetName();
    boolean that_present_name = true && that.isSetName();
    if (this_present_name || that_present_name) {
      if (!(this_present_name && that_present_name))
        return false;
      if (!this.name.equals(that.name))
        return false;
    }

    return true;
  }

  @Override
  public int hashCode() {
    int hashCode = 1;

    hashCode = hashCode * 8191 + number;

    hashCode = hashCode * 8191 + org.apache.thrift.TBaseHelper.hashCode(bigNumber);

    hashCode = hashCode * 8191 + org.apache.thrift.TBaseHelper.hashCode(decimals);

    hashCode = hashCode * 8191 + ((isSetName()) ? 131071 : 524287);
    if (isSetName())
      hashCode = hashCode * 8191 + name.hashCode();

    return hashCode;
  }

  @Override
  public int compareTo(Example other) {
    if (!getClass().equals(other.getClass())) {
      return getClass().getName().compareTo(other.getClass().getName());
    }

    int lastComparison = 0;

    lastComparison = java.lang.Boolean.valueOf(isSetNumber()).compareTo(other.isSetNumber());
    if (lastComparison != 0) {
      return lastComparison;
    }
    if (isSetNumber()) {
      lastComparison = org.apache.thrift.TBaseHelper.compareTo(this.number, other.number);
      if (lastComparison != 0) {
        return lastComparison;
      }
    }
    lastComparison = java.lang.Boolean.valueOf(isSetBigNumber()).compareTo(other.isSetBigNumber());
    if (lastComparison != 0) {
      return lastComparison;
    }
    if (isSetBigNumber()) {
      lastComparison = org.apache.thrift.TBaseHelper.compareTo(this.bigNumber, other.bigNumber);
      if (lastComparison != 0) {
        return lastComparison;
      }
    }
    lastComparison = java.lang.Boolean.valueOf(isSetDecimals()).compareTo(other.isSetDecimals());
    if (lastComparison != 0) {
      return lastComparison;
    }
    if (isSetDecimals()) {
      lastComparison = org.apache.thrift.TBaseHelper.compareTo(this.decimals, other.decimals);
      if (lastComparison != 0) {
        return lastComparison;
      }
    }
    lastComparison = java.lang.Boolean.valueOf(isSetName()).compareTo(other.isSetName());
    if (lastComparison != 0) {
      return lastComparison;
    }
    if (isSetName()) {
      lastComparison = org.apache.thrift.TBaseHelper.compareTo(this.name, other.name);
      if (lastComparison != 0) {
        return lastComparison;
      }
    }
    return 0;
  }

  public _Fields fieldForId(int fieldId) {
    return _Fields.findByThriftId(fieldId);
  }

  public void read(org.apache.thrift.protocol.TProtocol iprot) throws org.apache.thrift.TException {
    scheme(iprot).read(iprot, this);
  }

  public void write(org.apache.thrift.protocol.TProtocol oprot) throws org.apache.thrift.TException {
    scheme(oprot).write(oprot, this);
  }

  @Override
  public java.lang.String toString() {
    java.lang.StringBuilder sb = new java.lang.StringBuilder("Example(");
    boolean first = true;

    sb.append("number:");
    sb.append(this.number);
    first = false;
    if (!first) sb.append(", ");
    sb.append("bigNumber:");
    sb.append(this.bigNumber);
    first = false;
    if (!first) sb.append(", ");
    sb.append("decimals:");
    sb.append(this.decimals);
    first = false;
    if (!first) sb.append(", ");
    sb.append("name:");
    if (this.name == null) {
      sb.append("null");
    } else {
      sb.append(this.name);
    }
    first = false;
    sb.append(")");
    return sb.toString();
  }

  public void validate() throws org.apache.thrift.TException {
    // check for required fields
    // check for sub-struct validity
  }

  private void writeObject(java.io.ObjectOutputStream out) throws java.io.IOException {
    try {
      write(new org.apache.thrift.protocol.TCompactProtocol(new org.apache.thrift.transport.TIOStreamTransport(out)));
    } catch (org.apache.thrift.TException te) {
      throw new java.io.IOException(te);
    }
  }

  private void readObject(java.io.ObjectInputStream in) throws java.io.IOException, java.lang.ClassNotFoundException {
    try {
      // it doesn't seem like you should have to do this, but java serialization is wacky, and doesn't call the default constructor.
      __isset_bitfield = 0;
      read(new org.apache.thrift.protocol.TCompactProtocol(new org.apache.thrift.transport.TIOStreamTransport(in)));
    } catch (org.apache.thrift.TException te) {
      throw new java.io.IOException(te);
    }
  }

  private static class ExampleStandardSchemeFactory implements org.apache.thrift.scheme.SchemeFactory {
    public ExampleStandardScheme getScheme() {
      return new ExampleStandardScheme();
    }
  }

  private static class ExampleStandardScheme extends org.apache.thrift.scheme.StandardScheme<Example> {

    public void read(org.apache.thrift.protocol.TProtocol iprot, Example struct) throws org.apache.thrift.TException {
      org.apache.thrift.protocol.TField schemeField;
      iprot.readStructBegin();
      while (true)
      {
        schemeField = iprot.readFieldBegin();
        if (schemeField.type == org.apache.thrift.protocol.TType.STOP) { 
          break;
        }
        switch (schemeField.id) {
          case 1: // NUMBER
            if (schemeField.type == org.apache.thrift.protocol.TType.I32) {
              struct.number = iprot.readI32();
              struct.setNumberIsSet(true);
            } else { 
              org.apache.thrift.protocol.TProtocolUtil.skip(iprot, schemeField.type);
            }
            break;
          case 2: // BIG_NUMBER
            if (schemeField.type == org.apache.thrift.protocol.TType.I64) {
              struct.bigNumber = iprot.readI64();
              struct.setBigNumberIsSet(true);
            } else { 
              org.apache.thrift.protocol.TProtocolUtil.skip(iprot, schemeField.type);
            }
            break;
          case 3: // DECIMALS
            if (schemeField.type == org.apache.thrift.protocol.TType.DOUBLE) {
              struct.decimals = iprot.readDouble();
              struct.setDecimalsIsSet(true);
            } else { 
              org.apache.thrift.protocol.TProtocolUtil.skip(iprot, schemeField.type);
            }
            break;
          case 4: // NAME
            if (schemeField.type == org.apache.thrift.protocol.TType.STRING) {
              struct.name = iprot.readString();
              struct.setNameIsSet(true);
            } else { 
              org.apache.thrift.protocol.TProtocolUtil.skip(iprot, schemeField.type);
            }
            break;
          default:
            org.apache.thrift.protocol.TProtocolUtil.skip(iprot, schemeField.type);
        }
        iprot.readFieldEnd();
      }
      iprot.readStructEnd();

      // check for required fields of primitive type, which can't be checked in the validate method
      struct.validate();
    }

    public void write(org.apache.thrift.protocol.TProtocol oprot, Example struct) throws org.apache.thrift.TException {
      struct.validate();

      oprot.writeStructBegin(STRUCT_DESC);
      oprot.writeFieldBegin(NUMBER_FIELD_DESC);
      oprot.writeI32(struct.number);
      oprot.writeFieldEnd();
      oprot.writeFieldBegin(BIG_NUMBER_FIELD_DESC);
      oprot.writeI64(struct.bigNumber);
      oprot.writeFieldEnd();
      oprot.writeFieldBegin(DECIMALS_FIELD_DESC);
      oprot.writeDouble(struct.decimals);
      oprot.writeFieldEnd();
      if (struct.name != null) {
        oprot.writeFieldBegin(NAME_FIELD_DESC);
        oprot.writeString(struct.name);
        oprot.writeFieldEnd();
      }
      oprot.writeFieldStop();
      oprot.writeStructEnd();
    }

  }

  private static class ExampleTupleSchemeFactory implements org.apache.thrift.scheme.SchemeFactory {
    public ExampleTupleScheme getScheme() {
      return new ExampleTupleScheme();
    }
  }

  private static class ExampleTupleScheme extends org.apache.thrift.scheme.TupleScheme<Example> {

    @Override
    public void write(org.apache.thrift.protocol.TProtocol prot, Example struct) throws org.apache.thrift.TException {
      org.apache.thrift.protocol.TTupleProtocol oprot = (org.apache.thrift.protocol.TTupleProtocol) prot;
      java.util.BitSet optionals = new java.util.BitSet();
      if (struct.isSetNumber()) {
        optionals.set(0);
      }
      if (struct.isSetBigNumber()) {
        optionals.set(1);
      }
      if (struct.isSetDecimals()) {
        optionals.set(2);
      }
      if (struct.isSetName()) {
        optionals.set(3);
      }
      oprot.writeBitSet(optionals, 4);
      if (struct.isSetNumber()) {
        oprot.writeI32(struct.number);
      }
      if (struct.isSetBigNumber()) {
        oprot.writeI64(struct.bigNumber);
      }
      if (struct.isSetDecimals()) {
        oprot.writeDouble(struct.decimals);
      }
      if (struct.isSetName()) {
        oprot.writeString(struct.name);
      }
    }

    @Override
    public void read(org.apache.thrift.protocol.TProtocol prot, Example struct) throws org.apache.thrift.TException {
      org.apache.thrift.protocol.TTupleProtocol iprot = (org.apache.thrift.protocol.TTupleProtocol) prot;
      java.util.BitSet incoming = iprot.readBitSet(4);
      if (incoming.get(0)) {
        struct.number = iprot.readI32();
        struct.setNumberIsSet(true);
      }
      if (incoming.get(1)) {
        struct.bigNumber = iprot.readI64();
        struct.setBigNumberIsSet(true);
      }
      if (incoming.get(2)) {
        struct.decimals = iprot.readDouble();
        struct.setDecimalsIsSet(true);
      }
      if (incoming.get(3)) {
        struct.name = iprot.readString();
        struct.setNameIsSet(true);
      }
    }
  }

  private static <S extends org.apache.thrift.scheme.IScheme> S scheme(org.apache.thrift.protocol.TProtocol proto) {
    return (org.apache.thrift.scheme.StandardScheme.class.equals(proto.getScheme()) ? STANDARD_SCHEME_FACTORY : TUPLE_SCHEME_FACTORY).getScheme();
  }
}

案例分析：

版本不匹配可能发生在如下四种情况中：

1.已添加字段，旧客户端，新服务器

旧客户端没有发送新字段，新服务器识别到那个新字段未置位，执行对于旧数据请求的默认操作。

2.已删除字段，旧客户端，新服务器

旧客户端发送已被删除的字段，新服务器简单忽略这个字段。

3.已添加字段，新客户端，旧服务器

新客户端发送了旧服务器不能识别的字段，旧服务器简单的忽略，并按正常请求处理。

4.已删除字段，新客户端，旧服务器

最危险的情况，对于丢失的字段，旧服务器不大可能有默认的合适动作执行。这种情况，推荐在升级客户端之前升级服务器端。