This document 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).
- Binary protocol
- Base types
- Message
- Struct
- List and Set
- Map
- BNF notation used in this document
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.
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, 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.
Strings are first encoded to UTF-8, and then send as binary.
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.
Values of bool
type are first converted to an int8. True is converted to 1
, false to 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
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 to1
(in binary:000 0000 0000 0001
). The leading bit is1
.unused
is an ignored byte.mmm
is the message type, an unsigned 3 bit integer. The 5 leading bits must be0
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
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.
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 as2
I8
, encoded as3
DOUBLE
, encoded as4
I16
, encoded as6
I32
, encoded as8
I64
, encoded as10
BINARY
, used for binary and string fields, encoded as11
STRUCT
, used for structs and union fields, encoded as12
MAP
, encoded as13
SET
, encoded as14
LIST
, encoded as15
UUID
, encoded as16
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 int8size
is the size, encoded as an int32, positive values onlyelements
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).
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 int8vvvvvvvv
is the value element-type, encoded as an int8size
is the size of the map, encoded as an int32, positive values onlykey 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).
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