Project: Apache Thrift (Orchestration & Management / Remote Procedure Call)
File: thrift-compact-protocol.md

Thrift Compact protocol encoding ================================ <!-- -- Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to you under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. -- --> This documents describes the wire encoding for RPC using the Thrift compact protocol. The information here is mostly based on the Java implementation in the Apache thrift library (version 0.9.1) and . Other implementation however, should behave the same. For background on Thrift see the . Compact protocol Base types Message Struct List and Set Map BNF notation used in this document The compact protocol uses multiple encodings for ints: the zigzag int, and the var int. Values of type `int32` and `int64` are first transformed to a zigzag int. A zigzag int folds positive and negative numbers into the positive number space. When we read 0, 1, 2, 3, 4 or 5 from the wire, this is translated to 0, -1, 1, -2 or 2 respectively. Here are the (Scala) formulas to convert from int32/int64 to a zigzag int and back: ```scala def intToZigZag(n: Int): Int = (n << 1) ^ (n >> 31) def zigzagToInt(n: Int): Int = (n >>> 1) ^ - (n & 1) def longToZigZag(n: Long): Long = (n << 1) ^ (n >> 63) def zigzagToLong(n: Long): Long = (n >>> 1) ^ - (n & 1) ``` The zigzag int is then encoded as a var int, also known as Unsigned LEB128. Var ints take 1 to 5 bytes (int32) or 1 to 10 bytes (int64). The process consists in taking a Big Endian unsigned integer, left-padding the bit-string to make it a multiple of 7 bits, splitting it into 7-bit groups, prefixing the most-significant 7-bit group with the 0 bit, prefixing the remaining 7-bit groups with the 1 bit and encoding the resulting bit-string in Little Endian. For example, the integer 50399 is encoded as follows: ``` 50399 = 1100 0100 1101 1111 (Big Endian representation) = 00000 1100 0100 1101 1111 (Left-padding) = 0000011 0001001 1011111 (7-bit groups) = 00000011 10001001 11011111 (Most-significant bit prefixes) = 11011111 10001001 00000011 (Little Endian representation) = 0xDF 0x89 0x03 ``` Var ints are sometimes used directly inside the compact protocol to represent positive numbers. To encode an `int16` as zigzag int, it is first converted to an `int32` and then encoded as such. The type `int8` simply uses a single byte as in the binary protocol. The generated code encodes `Enum`s by taking the ordinal value and then encoding that as an int32. Binary is sent as follows: ``` Binary protocol, binary data, 1+ bytes: +--+...+--+--+...+--+ | byte length | bytes | +--+...+--+--+...+--+ ``` Where: `byte length` is the length of the byte array, using var int encoding (must be >= 0). `bytes` are the bytes of the byte array. Strings are first encoded to UTF-8, and then send as binary. They do not include a NUL delimiter. Values of type `double` are first converted to an int64 according to the IEEE 754 floating-point "double format" bit

Most run-times provide a library to make this conversion. But while the binary protocol encodes the int64 in 8 bytes in big endian order, the compact protocol encodes it in little endian order - this is due to an early implementation bug that finally became the de-facto standard. Booleans are encoded differently depending on whether it is a field value (in a struct) or an element value (in a set, list or map). Field values are encoded directly in the field header. Element values of type `bool` are sent as an int8; true as `1` and false as `0`. Values of `uuid` type are expected as 16-byte binary in big endian (or "network") order. Byte order conversion might be necessary on certain platforms, e.g. Windows holds GUIDs in a complex record-like structure whose memory layout differs. Note: Since the length is fixed, no `byte length` prefix is necessary and the field is always 16 bytes long. A `Message` on the wire looks as follows: ``` Compact protocol Message (4+ bytes): +--+--+--+...+--+--+...+--+--+...+--+ |pppppppp|mmmvvvvv| seq id | name length | name | +--+--+--+...+--+--+...+--+--+...+--+ ``` Where: `pppppppp` is the protocol id, fixed to `1000 0010`, 0x82. `mmm` is the message type, an unsigned 3 bit integer. `vvvvv` is the version, an unsigned 5 bit integer, fixed to `00001`. `seq id` is the sequence id, a signed 32 bit integer encoded as a var int. `name length` is the byte length of the name field, a signed 32 bit integer encoded as a var int (must be >= 0). `name` is the method name to invoke, a UTF-8 encoded string. Message types are encoded with the following values: Call: 1 Reply: 2 Exception: 3 Oneway: 4 A Struct is a sequence of zero or more fields, followed by a stop field. Each field starts with a field header and is followed by the encoded field value. The encoding can be summarized by the following BNF: ``` struct ::= ( field-header field-value )* stop-field field-header ::= field-type field-id ``` Because each field header contains the field-id (as defined by the Thrift IDL file), the fields can be encoded in any order. Thrift's type system is not extensible; you can only encode the primitive types and structs. Therefore is also possible to handle unknown fields while decoding; these are simply ignored. While decoding the field type can be used to determine how to decode the field value. Note that the field name is not encoded so field renames in the IDL do not affect forward and backward compatibility. The default Java implementation (Apache Thrift 0.9.1) has undefined behavior when it tries to decode a field that has another field-type than what is expected. Theoretically this could be detected at the cost of some additional checking. Other implementation may perform this check and then either ignore the field, or return a protocol exception. A Union is encoded exactly the same as a struct with the additional restriction that at most 1 field may be encoded. An Exception is encoded exactly the same as a struct. ``` Compact protocol field header (short form) and field value: +--+--+...+--+ |ddddtttt| field value | +--+--+...+--+ Compact protocol field header (1 to 3 bytes, long form) and field value: +--+--+...+--+--+...+--+ |0000tttt| field id | field value | +--+--+...+--+--+...+--+ Compact protocol stop field: +--+ |00000000| +--+ ``` Where: `dddd` is the field id delta, an unsigned 4 bits integer, strictly positive. `tttt` is field-type id, an unsigned 4 bit integer. `field id` the field id, a signed 16 bit integer encoded as zigzag

`field-value` the encoded field value. The field id delta can be computed by `current-field-id - previous-field-id`, or just `current-field-id` if this is the first of the struct. The short form should be used when the field id delta is in the range 1 - 15 (inclusive). The following field-types can be encoded: `BOOLEAN_TRUE`, encoded as `1` `BOOLEAN_FALSE`, encoded as `2` `I8`, encoded as `3` `I16`, encoded as `4` `I32`, encoded as `5` `I64`, encoded as `6` `DOUBLE`, encoded as `7` `BINARY`, used for binary and string fields, encoded as `8` `LIST`, encoded as `9` `SET`, encoded as `10` `MAP`, encoded as `11` `STRUCT`, used for both structs and union fields, encoded as `12` `UUID`, encoded as `13` Note that because there are 2 specific field types for the boolean values, the encoding of a boolean field value has no length (0 bytes). List and sets are encoded the same: a header indicating the size and the element-type of the elements, followed by the encoded elements. ``` Compact protocol list header (1 byte, short form) and elements: +--+--+...+--+ |sssstttt| elements | +--+--+...+--+ Compact protocol list header (2+ bytes, long form) and elements: +--+--+...+--+--+...+--+ |1111tttt| size | elements | +--+--+...+--+--+...+--+ ``` Where: `ssss` is the size, 4 bit unsigned int, values `0` - `14` `tttt` is the element-type, a 4 bit unsigned int `size` is the size, a var int (int32), positive values `15` or higher `elements` are the encoded elements The short form should be used when the length is in the range 0 - 14 (inclusive). The following element-types are used (see note below): `BOOL`, encoded as `2` `I8`, encoded as `3` `I16`, encoded as `4` `I32`, encoded as `5` `I64`, encoded as `6` `DOUBLE`, encoded as `7` `BINARY`, used for binary and string fields, encoded as `8` `LIST`, encoded as `9` `SET`, encoded as `10` `MAP`, encoded as `11` `STRUCT`, used for structs and union fields, encoded as `12` `UUID`, encoded as `13` Note: Although field-types and element-types lists are currently very similar, there is no guarantee that this will remain true after new types are added. The maximum list/set size is configurable. By default there is no limit (meaning the limit is the maximum int32 value: 2147483647). Maps are encoded with a header indicating the size, the type of the keys and the element-type of the elements, followed by the encoded elements. The encoding follows this BNF: ``` map ::= empty-map | non-empty-map empty-map ::= `0` non-empty-map ::= size key-element-type value-element-type (key value)+ ``` ``` Compact protocol map header (1 byte, empty map): +--+ |00000000| +--+ Compact protocol map header (2+ bytes, non empty map) and key value pairs: +--+...+--+--+--+...+--+ | size |kkkkvvvv| key value pairs | +--+...+--+--+--+...+--+ ``` Where: `size` is the size, a var int (int32), strictly positive values `kkkk` is the key element-type, a 4 bit unsigned int `vvvv` is the value element-type, a 4 bit unsigned int `key value pairs` are the encoded keys and values The element-types are the same as for lists. The full list is included in the 'List and set' section. The maximum map size is configurable. By default there is no limit (meaning the limit is the maximum int32 value: 2147483647). The following BNF notation is used: a plus `+` appended to an item represents repetition; the item is repeated 1 or more times a star `` appended to an item represents optional repetition; the item is repeated 0 or more times a pipe `|` between items represents choice, the first matching item is selected parenthesis `(` and `)` are used for grouping multiple items