This documents describes the 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 THRIFT-110 A more compact format. Other implementation however, should behave the same.

For background on Thrift see the Thrift whitepaper (pdf).

Contents

• Compact protocol
  • ZigZag encoding
  • Varint encoding
  • Base types
  • Message
  • Struct
  • List and Set
  • Map
• BNF notation used in this document

Compact protocol

ZigZag encoding

Integers that might be negative are transformed using ZigZag encoding. Zigzag encoding maps signed integers to another domain, one where the sign bit is encoded in the least significant bit (LSB). For example 0 maps to 0, -1 to 1, 1 to 2, -2 to 3, etc., hence the term zigzag. Mapping the sign bit to the LSB is important for compactness when the absolute value of the value is small.

Here are the (Scala) formulas to convert from int64 to a zigzag int64 and back:

def ToZigzag(n: Long): Long = (n << 1) ^ (n >> 63)
def FromZigzag(n: Long): Long = (n >>> 1) ^ - (n & 1)

Varint encoding

Integers larger than a single byte are encoded using varint (aka ULEB128) encoding.

A ULEB128 value is encoded 7 bits at a time, starting from the LSB. Each 7 bits are encoded as 8 bits with the top bit set if it is not the last byte.

For example, the integer 50399 is encoded as follows:

50399 =          11000100 11011111  (LSB)
      =  0000011  0001001  1011111  (7-bit groups)
      = 00000011 10001001 11011111  (add continuation bits)
      =     0x03     0x89     0xDF  (hex)
→ 0xDF 0x89 0x03 (write to ram LSB first)

Base types

Integer encoding

Values of type i8 and byte are encoded as one byte. Values of type i16, i32 and i64 are first transformed to using ZigZag and that result is then varint encoded before being written.

Enum encoding

enums are encoded as their integer value as defined by the Thrift IDL.

Binary encoding

binary is sent as follows:

Binary protocol, binary data, 1+ bytes:
+--------+...+--------+--------+...+--------+
| byte length         | bytes               |
+--------+...+--------+--------+...+--------+

Where:

• byte length is the length of the byte array (non-negative), encoded as a varint (not ZigZag'ed).
• bytes are the bytes of the byte array.

String encoding

strings are written as binary and must be encoded as UTF-8. Strings do not include a NULL(0) terminator.

Double encoding

Values of type double are written in IEEE 754 format (the standard representation on almost all systems) in little-endian byte order (8 bytes).

Boolean encoding

bools are encoded differently depending on whether 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 i8; true as 1 and false as 2.

Universal unique identifier encoding

Values of uuid type are not encoded. That is, the 128 bits that make up the source uuid are written without modification. UUIDs not stored in the standard big-endian format should be converted to such before encoding with the compact protocol.

Message

A Message on the wire looks as follows:

Compact protocol Message (4+ bytes):
+--------+--------+--------+...+--------+--------+...+--------+
|pppppppp|mmmvvvvv| seq id              | name                |
+--------+--------+--------+...+--------+--------+...+--------+

Where:

• pppppppp is the protocol id, fixed to 1000 0010, 0x82.
• mmm is the message type, an unsigned 3-bit value.
• vvvvv is the version, an unsigned 5-bit integer, fixed to 00001.
• seq id is the sequence id, a signed 32-bit value encoded as a varint. The value is not ZigZag'ed.
• name is the method name to invoke, encoded as a string.

Message types are encoded with the following values:

• Call: 1
• Reply: 2
• Exception: 3
• Oneway: 4

Struct

A struct is a sequence of zero or more fields, followed by a stop field. Each field other than a stop 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 it is also possible to handle unknown fields while decoding by ignoring them. The field type is used to determine how to decode field values.

Note that the fields are identified by their integer value and not their name.

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 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.

Struct encoding

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-bit integer.
• tttt is field-type, an unsigned 4-bit integer (see below).
• field id the field id (i16), ZigZag'ed varint encoded.
• field-value is 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 Set

lists 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 values 0 - 14
• tttt is the element-type, a 4-bit unsigned value
• size is the size, an unsigned 32-bit varint, 15 or higher (not ZigZag encoded).
• 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 1 below):

• BOOL, encoded as 1 or 2 (see note 2 below)
• 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:

1. 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.
2. For historical and compatibility reasons, a reader should be capable to deal with both cases. The only valid value in the original spec was 2, but due to an widespread implementation bug the defacto standard across large parts of the library became 1 instead. As a result, both values are now allowed.

The maximum list/set size is configurable. By default there is no limit (meaning the limit is the maximum signed 32-bit integer value: 2147483647).

Map

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 32-bit unsigned size, varint encoded (not ZigZag'ed).
• kkkk is the key element-type, a 4-bit unsigned value
• vvvv is the value element-type, a 4-bit unsigned value
• 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 32-bit value: 2147483647).

BNF notation used in this document

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