Moving from govendor to dep, updated dependencies (#48)
* Moving from govendor to dep. * Making the pull request template more friendly. * Fixing akward space in PR template. * goimports run on whole project using ` goimports -w $(find . -type f -name '*.go' -not -path "./vendor/*" -not -path "./gen-go/*")` source of command: https://gist.github.com/bgentry/fd1ffef7dbde01857f66
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Thrift Compact protocol encoding
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================================
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<!--
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--------------------------------------------------------------------
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Licensed to the Apache Software Foundation (ASF) under one
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or more contributor license agreements. See the NOTICE file
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distributed with this work for additional information
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regarding copyright ownership. The ASF licenses this file
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to you under the Apache License, Version 2.0 (the
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"License"); you may not use this file except in compliance
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with the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing,
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software distributed under the License is distributed on an
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"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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KIND, either express or implied. See the License for the
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specific language governing permissions and limitations
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under the License.
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--------------------------------------------------------------------
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-->
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This documents describes the wire encoding for RPC using the Thrift *compact protocol*.
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The information here is _mostly_ based on the Java implementation in the Apache thrift library (version 0.9.1) and
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[THRIFT-110 A more compact format](https://issues.apache.org/jira/browse/THRIFT-110). Other implementation however,
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should behave the same.
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For background on Thrift see the [Thrift whitepaper (pdf)](https://thrift.apache.org/static/files/thrift-20070401.pdf).
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# Contents
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* Compact protocol
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* Base types
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* Message
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* Struct
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* List and Set
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* Map
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* BNF notation used in this document
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# Compact protocol
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## Base types
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### Integer encoding
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The _compact protocol_ uses multiple encodings for ints: the _zigzag int_, and the _var int_.
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Values of type `int32` and `int64` are first transformed to a *zigzag int*. A zigzag int folds positive and negative
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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,
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-2 or 2 respectively. Here are the (Scala) formulas to convert from int32/int64 to a zigzag int and back:
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```scala
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def intToZigZag(n: Int): Int = (n << 1) ^ (n >> 31)
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def zigzagToInt(n: Int): Int = (n >>> 1) ^ - (n & 1)
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def longToZigZag(n: Long): Long = (n << 1) ^ (n >> 63)
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def zigzagToLong(n: Long): Long = (n >>> 1) ^ - (n & 1)
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```
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The zigzag int is then encoded as a *var int*. Var ints take 1 to 5 bytes (int32) or 1 to 10 bytes (int64). The most
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significant bit of each byte indicates if more bytes follow. The concatenation of the least significant 7 bits from each
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byte form the number, where the first byte has the most significant bits (so they are in big endian or network order).
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Var ints are sometimes used directly inside the compact protocol to represent positive numbers.
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To encode an `int16` as zigzag int, it is first converted to an `int32` and then encoded as such. The type `int8` simply
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uses a single byte as in the binary protocol.
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### Enum encoding
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The generated code encodes `Enum`s by taking the ordinal value and then encoding that as an int32.
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### Binary encoding
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Binary is sent as follows:
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```
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Binary protocol, binary data, 1+ bytes:
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+--------+...+--------+--------+...+--------+
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| byte length | bytes |
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+--------+...+--------+--------+...+--------+
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```
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Where:
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* `byte length` is the length of the byte array, using var int encoding (must be >= 0).
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* `bytes` are the bytes of the byte array.
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### String encoding
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*String*s are first encoded to UTF-8, and then send as binary.
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### Double encoding
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Values of type `double` are first converted to an int64 according to the IEEE 754 floating-point "double format" bit
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layout. Most run-times provide a library to make this conversion. Both the binary protocol as the compact protocol then
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encode the int64 in 8 bytes in big endian order.
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### Boolean encoding
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Booleans are encoded differently depending on whether it is a field value (in a struct) or an element value (in a set,
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list or map). Field values are encoded directly in the field header. Element values of type `bool` are sent as an int8;
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true as `1` and false as `0`.
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## Message
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A `Message` on the wire looks as follows:
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```
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Compact protocol Message (4+ bytes):
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+--------+--------+--------+...+--------+--------+...+--------+--------+...+--------+
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|pppppppp|mmmvvvvv| seq id | name length | name |
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+--------+--------+--------+...+--------+--------+...+--------+--------+...+--------+
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```
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Where:
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* `pppppppp` is the protocol id, fixed to `1000 0010`, 0x82.
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* `mmm` is the message type, an unsigned 3 bit integer.
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* `vvvvv` is the version, an unsigned 5 bit integer, fixed to `00001`.
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* `seq id` is the sequence id, a signed 32 bit integer encoded as a var int.
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* `name length` is the byte length of the name field, a signed 32 bit integer encoded as a var int (must be >= 0).
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* `name` is the method name to invoke, a UTF-8 encoded string.
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Message types are encoded with the following values:
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* _Call_: 1
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* _Reply_: 2
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* _Exception_: 3
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* _Oneway_: 4
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### Struct
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A *Struct* is a sequence of zero or more fields, followed by a stop field. Each field starts with a field header and
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is followed by the encoded field value. The encoding can be summarized by the following BNF:
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```
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struct ::= ( field-header field-value )* stop-field
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field-header ::= field-type field-id
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```
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Because each field header contains the field-id (as defined by the Thrift IDL file), the fields can be encoded in any
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order. Thrift's type system is not extensible; you can only encode the primitive types and structs. Therefore is also
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possible to handle unknown fields while decoding; these are simply ignored. While decoding the field type can be used to
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determine how to decode the field value.
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Note that the field name is not encoded so field renames in the IDL do not affect forward and backward compatibility.
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The default Java implementation (Apache Thrift 0.9.1) has undefined behavior when it tries to decode a field that has
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another field-type then what is expected. Theoretically this could be detected at the cost of some additional checking.
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Other implementation may perform this check and then either ignore the field, or return a protocol exception.
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A *Union* is encoded exactly the same as a struct with the additional restriction that at most 1 field may be encoded.
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An *Exception* is encoded exactly the same as a struct.
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### Struct encoding
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```
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Compact protocol field header (short form) and field value:
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+--------+--------+...+--------+
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|ddddtttt| field value |
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+--------+--------+...+--------+
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Compact protocol field header (1 to 3 bytes, long form) and field value:
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+--------+--------+...+--------+--------+...+--------+
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|0000tttt| field id | field value |
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+--------+--------+...+--------+--------+...+--------+
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Compact protocol stop field:
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+--------+
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|00000000|
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+--------+
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```
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Where:
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* `dddd` is the field id delta, an unsigned 4 bits integer, strictly positive.
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* `tttt` is field-type id, an unsigned 4 bit integer.
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* `field id` the field id, a signed 16 bit integer encoded as zigzag int.
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* `field-value` the encoded field value.
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The field id delta can be computed by `current-field-id - previous-field-id`, or just `current-field-id` if this is the
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first of the struct. The short form should be used when the field id delta is in the range 1 - 15 (inclusive).
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The following field-types can be encoded:
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* `BOOLEAN_TRUE`, encoded as `1`
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* `BOOLEAN_FALSE`, encoded as `2`
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* `BYTE`, encoded as `3`
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* `I16`, encoded as `4`
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* `I32`, encoded as `5`
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* `I64`, encoded as `6`
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* `DOUBLE`, encoded as `7`
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* `BINARY`, used for binary and string fields, encoded as `8`
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* `LIST`, encoded as `9`
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* `SET`, encoded as `10`
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* `MAP`, encoded as `11`
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* `STRUCT`, used for both structs and union fields, encoded as `12`
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Note that because there are 2 specific field types for the boolean values, the encoding of a boolean field value has no
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length (0 bytes).
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## List and Set
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List and sets are encoded the same: a header indicating the size and the element-type of the elements, followed by the
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encoded elements.
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```
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Compact protocol list header (1 byte, short form) and elements:
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+--------+--------+...+--------+
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|sssstttt| elements |
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+--------+--------+...+--------+
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Compact protocol list header (2+ bytes, long form) and elements:
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+--------+--------+...+--------+--------+...+--------+
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|1111tttt| size | elements |
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+--------+--------+...+--------+--------+...+--------+
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```
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Where:
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* `ssss` is the size, 4 bit unsigned int, values `0` - `14`
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* `tttt` is the element-type, a 4 bit unsigned int
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* `size` is the size, a var int (int32), positive values `15` or higher
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* `elements` are the encoded elements
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The short form should be used when the length is in the range 0 - 14 (inclusive).
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The following element-types are used (note that these are _different_ from the field-types):
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* `BOOL`, encoded as `2`
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* `BYTE`, encoded as `3`
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* `DOUBLE`, encoded as `4`
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* `I16`, encoded as `6`
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* `I32`, encoded as `8`
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* `I64`, encoded as `10`
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* `STRING`, used for binary and string fields, encoded as `11`
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* `STRUCT`, used for structs and union fields, encoded as `12`
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* `MAP`, encoded as `13`
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* `SET`, encoded as `14`
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* `LIST`, encoded as `15`
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The maximum list/set size is configurable. By default there is no limit (meaning the limit is the maximum int32 value:
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2147483647).
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## Map
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Maps are encoded with a header indicating the size, the type of the keys and the element-type of the elements, followed
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by the encoded elements. The encoding follows this BNF:
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```
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map ::= empty-map | non-empty-map
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empty-map ::= `0`
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non-empty-map ::= size key-element-type value-element-type (key value)+
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```
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```
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Compact protocol map header (1 byte, empty map):
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+--------+
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|00000000|
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+--------+
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Compact protocol map header (2+ bytes, non empty map) and key value pairs:
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+--------+...+--------+--------+--------+...+--------+
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| size |kkkkvvvv| key value pairs |
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+--------+...+--------+--------+--------+...+--------+
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```
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Where:
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* `size` is the size, a var int (int32), strictly positive values
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* `kkkk` is the key element-type, a 4 bit unsigned int
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* `vvvv` is the value element-type, a 4 bit unsigned int
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* `key value pairs` are the encoded keys and values
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The element-types are the same as for lists. The full list is included in the 'List and set' section.
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The maximum map size is configurable. By default there is no limit (meaning the limit is the maximum int32 value:
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2147483647).
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# BNF notation used in this document
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The following BNF notation is used:
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* a plus `+` appended to an item represents repetition; the item is repeated 1 or more times
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* a star `*` appended to an item represents optional repetition; the item is repeated 0 or more times
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* a pipe `|` between items represents choice, the first matching item is selected
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* parenthesis `(` and `)` are used for grouping multiple items
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