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

Thrift Binary 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 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 . 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 and 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

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 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 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

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 as `2` `I8`, encoded as `3` `DOUBLE`, encoded as `4` `I16`, encoded as `6` `I32`, encoded as `8` `I64`, encoded as `10` `BINARY`, 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` `UUID`, encoded as `16` 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). 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). 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