PragmaTwice commented on code in PR #1413: URL: https://github.com/apache/incubator-fury/pull/1413#discussion_r1537329225
########## docs/protocols/xlang_object_graph_spec.md: ########## @@ -0,0 +1,635 @@ +# Cross language object graph serialization + +Fury xlang serialization is an automatic object serialization framework that supports reference and polymorphism. +Fury will convert an object from/to fury xlang serialization binary format. +Fury has two core concepts for xlang serialization: + +- **Fury xlang binary format** +- **Framework implemented in different languages to convert object to/from Fury xlang binary format** + +The serialization format is a dynamic binary format. The dynamics and reference/polymorphism support make Fury flexible, +much more easy to use, but +also introduce more complexities compared to static serialization frameworks. So the format will be more complex. + +## Type Systems + +### Data Types + +- bool: a boolean value (true or false). +- int8: a 8-bit signed integer. +- int16: a 16-bit signed integer. +- int32: a 32-bit signed integer. +- int64: a 64-bit signed integer. +- float16: a 16-bit floating point number. +- float32: a 32-bit floating point number. +- float64: a 64-bit floating point number including NaN and Infinity. +- string: a text string encoded using Latin1/UTF16/UTF-8 encoding. +- enum: a data type consisting of a set of named values. Rust enum with non-predefined field values are not supported as + an enum. +- list: a sequence of objects. +- set: an unordered set of unique elements. +- map: a map of key-value pairs. +- time types: + - duration: an absolute length of time, independent of any calendar/timezone, as a count of nanoseconds. + - timestamp: a point in time, independent of any calendar/timezone, as a count of nanoseconds. The count is relative + to an epoch at UTC midnight on January 1, 1970. +- decimal: exact decimal value represented as an integer value in two's complement. +- binary: an variable-length array of bytes. +- array type: only allow numeric component. Other arrays will be taken as List. The implementation should support the + interoperability between array and list. + - array: multidimensional array which every sub-array can have different size but all have same type. + - bool_array: one dimension int16 array. + - int16_array: one dimension int16 array. + - int32_array: one dimension int32 array. + - int64_array: one dimension int64 array. + - float16_array: one dimension half_float_16 array. + - float32_array: one dimension float32 array. + - float64_array: one dimension float64 array. +- tensor: a multidimensional dense array of fixed-size values such as a NumPy ndarray. +- sparse tensor: a multidimensional array whose elements are almost all zeros. +- arrow record batch: an arrow [record batch](https://arrow.apache.org/docs/cpp/tables.html#record-batches) object. +- arrow table: an arrow [table](https://arrow.apache.org/docs/cpp/tables.html#tables) object. + +### Type disambiguation + +Due to differences between type systems of languages, those types can't be mapped one-to-one between languages. When +deserializing, Fury use the target data structure type and the data type in the data jointly to determine how to +deserialize and populate the target data structure. For example: + +```java +class Foo { + int[] intArray; + Object[] objects; + List<Object> objectList; +} + +class Foo2 { + int[] intArray; + List<Object> objects; + List<Object> objectList; +} +``` + +`intArray` has `int32_array` type. But both `objects` and `objectList` field in the serialize data have `list` data +type. When deserializing, the implementation will create an `Object` array for `objects`, but create a `ArrayList` +for `objectList` to populate it's elements. And the serialized data of `Foo` can be deserialized into `Foo2` too. + +Users can also provide meta hint for fields of a type, or the type whole. Here is an example in java which use +annotation to provide such information. + +```java + +@TypeInfo(fieldsNullable = false, trackingRef = false, polymorphic = false) +class Foo { + @FieldInfo(trackingRef = false) + int[] intArray; + @FieldInfo(polymorphic = true) + Object object; + @FieldInfo(tagId = 1, nullable = true) + List<Object> objectList; +} +``` + +Such information can be provided in other languages too: + +- cpp: use macro and template. +- golang: use struct tag. +- python: use typehint. +- rust: use macro. + +### Type ID + +All internal data types are expressed using a ID in range `-64~-1`. Users can use `0~32703` for representing their +types. At runtime, all type ids are added by `64`, and then encoded as an unsigned varint. + +### Type mapping + +See [Type mapping](../guide/xlang_type_mapping.md) + +## Spec overview + +Here is the overall format: + +``` +| fury header | object ref meta | object type meta | object value data | +``` + +The data are serialized using little endian byte order overall. If bytes swap is costly for some object, +Fury will write the byte order for that object into the data instead of converting it to little endian. + +## Fury header + +Fury header consists starts one byte: + +``` +| 4 bits | 1 bit | 1 bit | 1 bit | 1 bit | optional 4 bytes | ++---------------+-------+-------+--------+-------+------------------------------------+ +| reserved bits | oob | xlang | endian | null | unsigned int for meta start offset | +``` + +- null flag: 1 when object is null, 0 otherwise. If an object is null, other bits won't be set. +- endian flag: 1 when data is encoded by little endian, 0 for big endian. +- xlang flag: 1 when serialization uses xlang format, 0 when serialization uses Fury java format. +- oob flag: 1 when passed `BufferCallback` is not null, 0 otherwise. + +If meta share mode is enabled, an uncompressed unsigned int is appended to indicate the start offset of metadata. + +## Reference Meta + +Reference tracking handles whether the object is null, and whether to track reference for the object by writing +corresponding flags and maintaining internal state. + +Reference flags: + +| Flag | Byte Value | Description | +|---------------------|------------|---------------------------------------------------------------------------------------------------------------------------------------------------------| +| NULL FLAG | `-3` | This flag indicates the object is a null value. We don't use another byte to indicate REF, so that we can save one byte. | +| REF FLAG | `-2` | This flag indicates the object is already serialized previously, and fury will write a ref id with unsigned varint format instead of serialize it again | +| NOT_NULL VALUE FLAG | `-1` | This flag indicates the object is a non-null value and fury doesn't track ref for this type of object. | +| REF VALUE FLAG | `0` | This flag indicates the object is referencable and the first time to serialize. | + +When reference tracking is disabled globally or for specific types, or for certain types within a particular +context(e.g., a field of a type), only the `NULL` and `NOT_NULL VALUE` flags will be used for reference meta. + +For languages which doesn't support reference such as rust, reference tracking must be disabled for correct +deserialization by fury rust implementation. + +For languages whose object values are not null by default: + +- In rust, Fury takes `Option:None` as a null value +- In c++, Fury takes `std::nullopt` as a null value +- In golang, Fury takes `null interface/pointer` as a null value + +If one want to deserialize in languages like `Java/Python/JavaScript`, he should mark the type with all fields +not-null by default, or using schema-evolution mode to carry the not-null fields info in the data. + +## Type Meta + +For every type to be serialized, it must be registered with an optional ID first. The registered type will have a +user-provided or an auto-growing unsigned int i.e. `type_id`. The registration can be used for security check and type +identification. The id of user registered type will be added by `64` to make space for Fury internal data types. + +Depending on whether meta share mode and registration is enabled for current type, Fury will write type meta +differently. + +### Schema consistent + +If schema consistent mode is enabled globally or enabled for current type, type meta will be written as a fury unsigned +varint of `type_id`. + +### Schema evolution + +If schema evolution mode is enabled globally or enabled for current type, type meta will be written as follows: + +- If meta share mode is not enabled, type meta will be written as schema consistent mode. Additionally, field meta such + as field type and name will be written with the field value using a key-value like layout. +- If meta share mode is enabled, type meta will be written as a meta-share encoded binary if type hasn't been written + before, otherwise an unsigned varint id which references to previous written type meta will be written. + +## Meta share + +> This mode will forbid streaming writing since it needs to look back for update the start offset after the whole object +> graph +> writing and meta collecting is finished. Only in this way we can ensure deserialization failure doesn't lost shared +> meta. +> Meta streamline will be supported in the future for enclosed meta sharing which doesn't cross multiple serializations +> of different objects. + +For Schema consistent mode, type will be encoded as an enumerated string by full type name. Here we mainly describe +the meta layout for schema evolution mode: + +``` +| 8 bytes meta header | variable bytes | variable bytes | variable bytes | ++-------------------------------+--------------------+-------------------+----------------+ +| 7 bytes hash + 1 bytes header | current type meta | parent type meta | ... | +``` + +Type meta are encoded from parent type to leaf type, only type with serializable fields will be encoded. + +### Meta header + +Meta header is a 64 bits number value encoded in little endian order. + +- Lowest 4 digits `0b0000~0b1110` are used to record num classes. `0b1111` is preserved to indicate that Fury need to + read more bytes for length using Fury unsigned int encoding. If current type doesn't has parent type, or parent + type doesn't have fields to serialize, or we're in a context which serialize fields of current type + only, num classes will be 1. +- 5rd bit is used to indicate whether this type needs schema evolution. +- Other 56 bits is used to store the unique hash of `flags + all layers type meta`. +- Inheritance: Fury + - For languages + +### Single layer type meta + +``` +| unsigned varint | var uint | field info: variable bytes | variable bytes | ... | ++-----------------+----------+-------------------------------+-----------------+-----+ +| num_fields | type id | header + type id + field name | next field info | ... | +``` + +- num fields: encode `num fields` as unsigned varint. + - If current type is schema consistent, then num_fields will be `0` to flag it. + - If current type isn't schema consistent, then num_fields will be the number of compatible fields. For example, + users can use tag id to mark some field as compatible field in schema consistent context. In such cases, schema + consistent fields will be serialized first, then compatible fields will be serialized next. At deserialization, + Fury will use fields info of those fields which aren't annotated by tag id for deserializing schema consistent + fields, then use fields info in meta for deserializing compatible fields. +- Field info: + - Header(8 bits): + - Format: + - `reserved 1 bit + 3 bits field name encoding + polymorphism flag + nullability flag + ref tracking flag + tag id flag`. + - Users can use annotation to provide those info. + - tag id: when set to 1, field name will be written by an unsigned varint tag id. + - ref tracking: when set to 0, ref tracking will be disabled for this field. + - nullability: when set to 0, this field won't be null. + - polymorphism: when set to 1, the actual type of field will be the declared field type even the type if + not `final`. + - 3 bits field name encoding will be set to meta string encoding flags when tag id is not set. + - Type id: + - For registered type-consistent classes, it will be the registered type id. + - Otherwise it will be encoded as `OBJECT_ID` if it isn't `final` and `FINAL_OBJECT_ID` if it's `final`. The + meta + for such types is written separately instead of inlining here is to reduce meta space cost if object of this + type is serialized in current object graph multiple times, and the field value may be null too. + - List Type Info: this type will have an extra byte for elements info. + Users can use annotation to provide those info. + - elements type same + - elements tracking ref + - elements nullability + - elements declared type + - Map Type Info: this type will have an extra byte for kv items info. + Users can use annotation to provide those info. + - keys type same + - keys tracking ref + - keys nullability + - keys declared type + - values type same + - values tracking ref + - values nullability + - values declared type + - Field name: If tag id is set, tag id will be used instead. Otherwise meta string encoding length and data will + be written instead. + +Field order are left as implementation details, which is not exposed to specification, the deserialization need to +resort fields based on Fury field comparator. In this way, fury can compute statistics for field names or types and +using a more compact encoding. + +### Other layers type meta + +Same encoding algorithm as the previous layer. + +## Meta String + +Meta string is mainly used to encode meta strings such as field names. + +### Encoding Algorithms + +String binary encoding algorithm: + +| Algorithm | Pattern | Description | +|---------------------------|----------------|--------------------------------------------------------------------------------------------------------------------------------------------------| +| LOWER_SPECIAL | `a-z._$\|` | every char is written using 5 bits, `a-z`: `0b00000~0b11001`, `._$\|`: `0b11010~0b11101` | +| LOWER_UPPER_DIGIT_SPECIAL | `a-zA-Z0~9._$` | every char is written using 6 bits, `a-z`: `0b00000~0b11110`, `A-Z`: `0b11010~0b110011`, `0~9`: `0b110100~0b111101`, `._$`: `0b111110~0b1000000` | +| UTF-8 | any chars | UTF-8 encoding | + +Encoding flags: + +| Encoding Flag | Pattern | Encoding Algorithm | +|---------------------------|-----------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------| +| LOWER_SPECIAL | every char is in `a-z._$\|` | `LOWER_SPECIAL` | +| REP_FIRST_LOWER_SPECIAL | every char is in `a-z._$` except first char is upper case | replace first upper case char to lower case, then use `LOWER_SPECIAL` | +| REP_MUL_LOWER_SPECIAL | every char is in `a-zA-Z._$` | replace every upper case char by `\|` + `lower case`, then use `LOWER_SPECIAL`, use this encoding if it's smaller than Encoding `3` | +| LOWER_UPPER_DIGIT_SPECIAL | every char is in `a-zA-Z._$` | use `LOWER_UPPER_DIGIT_SPECIAL` encoding if it's smaller than Encoding `2` | +| UTF8 | any utf-8 char | use `UTF-8` encoding | +| Compression | any utf-8 char | lossless compression | + +Depending on cases, one can choose encoding `flags + data` jointly, uses 3 bits of first byte for flags and other bytes +for data. + +## Value Format + +### Basic types + +#### Bool + +- size: 1 byte +- format: 0 for `false`, 1 for `true` + +#### Byte + +- size: 1 byte +- format: write as pure byte. + +#### Short Review Comment: How about unsigned short? -- This is an automated message from the Apache Git Service. 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