Feed 元数据（旧版）

警告： nonce - 确保该字段的值在上传的 Feed 中具有唯一性。常见的一种做法是借助时间戳生成该值，以确保其唯一性。Feed 中所有碎片上的该值必须相同，以确保可同时正确处理跨多个碎片的完整 Feed。

message FeedMetadata {
  enum ProcessingInstruction {
    // Do not use.
    PROCESS_UNKNOWN = 0;

    // This Feed message is one shard of a complete feed. Anything previously
    // supplied by this partner will be deleted; the contents of this feed
    // represent the entire state of the world.
    PROCESS_AS_COMPLETE = 1;

    // Do not use. This value is deprecated.
    // Feed message is one shard of an incremental feed.
    // Existing entities will be left untouched except as modified in this feed.
    PROCESS_AS_INCREMENTAL = 2;
  }

  // Instructs us how to process the feed: either as a shard of a complete feed,
  // or as a shard of an incremental update. (required)
  ProcessingInstruction processing_instruction = 1;

  // The current shard and total number of shards for this feed.
  //
  // Shard number is assumed to be zero-based.
  //
  // There does not need to be any relationship to the file name.
  //
  // Shards do not need to be transferred in order, and they may not be
  // processed in order. (both required)
  int32 shard_number = 2;
  int32 total_shards = 3;

  // An identifier that must be consistent across all shards in a feed.
  // This value must be globally unique across each feed type.
  //
  // This value ensures that complete feeds spanning multiple shards are
  // processed together correctly.
  //
  // Clients only need to set this value when the processing_instruction is set
  // to PROCESS_AS_COMPLETE and the feed spans multiple shards (defined by
  // total_shards).
  //
  // Feeds that span multiple shards must set this nonce to the same value.
  // (required if total shards > 1)
  uint64 nonce = 5;

  // The timestamp at which this feed shard was generated.
  //
  // In Unix time format (seconds since the epoch). (required)
  int64 generation_timestamp = 4;
}