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Thrift Binary protocol encoding

This documents describes the wire encoding for RPC using the older Thrift binary protocol.

The information here is mostly based on the Java implementation in the Apache thrift library (version 0.9.1 and 0.9.3). Other implementation however, should behave the same.

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

Contents

  • Binary protocol
    • Base types
    • Message
    • Struct
    • List and Set
    • Map
  • BNF notation used in this document

Binary protocol

Base types

Integer encoding

In the binary protocol integers are encoded with the most significant byte first (big endian byte order, aka network order). An int8 needs 1 byte, an int16 2, an int32 4 and an int64 needs 8 bytes.

The CPP version has the option to use the binary protocol with little endian order. Little endian gives a small but noticeable performance boost because contemporary CPUs use little endian when storing integers to RAM.

Enum encoding

The generated code encodes Enums by taking the ordinal value and then encoding that as an int32.

Binary encoding

Binary is sent as follows:

Binary protocol, binary data, 4+ bytes:
+--------+--------+--------+--------+--------+...+--------+
| byte length                       | bytes                |
+--------+--------+--------+--------+--------+...+--------+

Where:

  • byte length is the length of the byte array, a signed 32 bit integer encoded in network (big endian) order (must be >= 0).
  • bytes are the bytes of the byte array.

String encoding

Strings are first encoded to UTF-8, and then send as binary.

Double encoding

Values of type double are first converted to an int64 according to the IEEE 754 floating-point "double format" bit layout. Most run-times provide a library to make this conversion. Both the binary protocol as the compact protocol then encode the int64 in 8 bytes in big endian order.

Boolean encoding

Values of bool type are first converted to an int8. True is converted to 1, false to 0.

Message

A Message can be encoded in two different ways:

Binary protocol Message, strict encoding, 12+ bytes:
+--------+--------+--------+--------+--------+--------+--------+--------+--------+...+--------+--------+--------+--------+--------+
|1vvvvvvv|vvvvvvvv|unused  |00000mmm| name length                       | name                | seq id                            |
+--------+--------+--------+--------+--------+--------+--------+--------+--------+...+--------+--------+--------+--------+--------+

Where:

  • vvvvvvvvvvvvvvv is the version, an unsigned 15 bit number fixed to 1 (in binary: 000 0000 0000 0001). The leading bit is 1.
  • unused is an ignored byte.
  • mmm is the message type, an unsigned 3 bit integer. The 5 leading bits must be 0 as some clients (checked for java in 0.9.1) take the whole byte.
  • name length is the byte length of the name field, a signed 32 bit integer encoded in network (big endian) order (must be >= 0).
  • name is the method name, a UTF-8 encoded string.
  • seq id is the sequence id, a signed 32 bit integer encoded in network (big endian) order.

The second, older encoding (aka non-strict) is:

Binary protocol Message, old encoding, 9+ bytes:
+--------+--------+--------+--------+--------+...+--------+--------+--------+--------+--------+--------+
| name length                       | name                |00000mmm| seq id                            |
+--------+--------+--------+--------+--------+...+--------+--------+--------+--------+--------+--------+

Where name length, name, mmm, seq id are as above.

Because name length must be positive (therefore the first bit is always 0), the first bit allows the receiver to see whether the strict format or the old format is used. Therefore a server and client using the different variants of the binary protocol can transparently talk with each other. However, when strict mode is enforced, the old format is rejected.

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 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 then 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.

Struct encoding

In the binary protocol field headers and the stop field are encoded as follows:

Binary protocol field header and field value:
+--------+--------+--------+--------+...+--------+
|tttttttt| field id        | field value         |
+--------+--------+--------+--------+...+--------+

Binary protocol stop field:
+--------+
|00000000|
+--------+

Where:

  • tttttttt the field-type, a signed 8 bit integer.
  • field id the field-id, a signed 16 bit integer in big endian order.
  • field-value the encoded field value.

The following field-types are used:

  • BOOL, encoded as 2
  • BYTE, encoded as 3
  • DOUBLE, encoded as 4
  • I16, encoded as 6
  • I32, encoded as 8
  • I64, encoded as 10
  • STRING, used for binary and string fields, encoded as 11
  • STRUCT, used for structs and union fields, encoded as 12
  • MAP, encoded as 13
  • SET, encoded as 14
  • LIST, encoded as 15

List and Set

List and sets are encoded the same: a header indicating the size and the element-type of the elements, followed by the encoded elements.

Binary protocol list (5+ bytes) and elements:
+--------+--------+--------+--------+--------+--------+...+--------+
|tttttttt| size                              | elements            |
+--------+--------+--------+--------+--------+--------+...+--------+

Where:

  • tttttttt is the element-type, encoded as an int8
  • size is the size, encoded as an int32, positive values only
  • elements the element values

The element-type values are the same as field-types. The full list is included in the struct section above.

The maximum list/set size is configurable. By default there is no limit (meaning the limit is the maximum int32 value: 2147483647).

Map

Maps are encoded with a header indicating the size, the element-type of the keys and the element-type of the elements, followed by the encoded elements. The encoding follows this BNF:

map  ::=  key-element-type value-element-type size ( key value )*

Binary protocol map (6+ bytes) and key value pairs:
+--------+--------+--------+--------+--------+--------+--------+...+--------+
|kkkkkkkk|vvvvvvvv| size                              | key value pairs     |
+--------+--------+--------+--------+--------+--------+--------+...+--------+

Where:

  • kkkkkkkk is the key element-type, encoded as an int8
  • vvvvvvvv is the value element-type, encoded as an int8
  • size is the size of the map, encoded as an int32, positive values only
  • key value pairs are the encoded keys and values

The element-type values are the same as field-types. The full list is included in the struct section above.

The maximum map size is configurable. By default there is no limit (meaning the limit is the maximum int32 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