Go Conference 2019 Spring https://gocon.connpass.com/event/124530/ オリジナルのGopherはRenée Frenchによってデザインされ、CC BY 3.0ライセンスが適用されています。

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Dark side of the reflect
Masaki Hitonishi
Common Support Services Dept.
DeNA Co., Ltd.

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About me
 Masaki Hitonishi
 Software Engineer
⁃ Golang Enthusiast
⁃ Contributor to Gin the framework
⁃ HTML5 Game Developer
 twitter.com/@sairoutine
 github.com/sairoutine

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DeNA.go will be held!
 https://dena.connpass.com/event/128034/
 Join registration is acceptable until 5/21 (Tue) !

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Today’s topic
 Introduction to reflect
 Building ORM by using reflect
 reflect or code generation?

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What is reflect?

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golang’s reflect
 Treats any types at runtime
⁃ Examines types
⁃ Creates and modifies it’s value
 makes us enable to do 2 things
⁃ Finding variables types
⁃ Getting field information

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reflect package
 Provides the way to use reflect
 Defines the important 2 types
⁃ reflect.Type
⁃ reflect.Value

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reflect.Type
 A Type represents golang’s type
 An interface with methods to distinguish
among types
⁃ Map? Struct? Slice? Function?
⁃ String? Int? Ptr?

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reflect.Value
 A value represents golang’s all value
 It can hold a value of any type

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Reason to use reflect
 To manipulate dynamic types and values
⁃ Works with variables at runtime using
information that didn’t exist when the
program was written.
 To get meta information through struct tag

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Example of dynamic types and values

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Example of struct tag information

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Example of use case in the real world
 ORM like gorm, xorm, gorp and so on...
⁃ Object Relational Mapper
⁃ Gets and sets the structure the user
defined
 JSON Encoder/Decoder
⁃ Parse any value to JSON value
 etc...

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Writing ORM with reflect

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DeNA Game dept.
 Using golang in some products
 Example
⁃ Game server framework
⁃ Development workflow platform

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Game server framework
 Next generation server framework under
development
 Golang + GCP
 Using Google Cloud Spanner as a backend
database
 Original ORM “Span” to treat Spanner

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Development workflow platform
 Named “Oyakata”
 Written in golang
 Using Noms as a backend database
 Original ORM “Norm” to use Noms database

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Noms
 Decentralized database
 https://github.com/attic-labs/noms
 No-SQL
 With Git like version control system
 Commit/Merge/Diff and so on in database
layer

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Norm
 ORM for Noms.
⁃ ORM is a library to convert data between
incompatible type systems to do object-
oriented programming.
 Users can define their own model struct.
 Maps their original struct to Noms data.
 Create/Read/Update/Delete.

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Norm

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Norm Internal
 Create/Update
⁃ Any struct to any data
 Read
⁃ Any data to struct

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Norm Internal

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Norm Internal

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Norm Internal

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Norm Internal

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Continue to use reflect?

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Span
 O/R Mapper for Google Spanner.
 Users can define their own model struct.
 Create/Read/Update/Delete.
 Currently, It’s implemented by code generator,
not by reflect.

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Why stop using reflect?
 Difficult to read source code
⁃ The reflect code tends to be complexity.
 Performance issue
⁃ Reflect is slow
 Now, Span generates CRUD code by parsing
definition the users defines.

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Pros/Cons
 Pros
⁃ Even if struct field is changed, there is no
need to regenerate code
 Cons
⁃ The library code tends to be complexity
⁃ Reflect is slow
 Norm is still using reflect approach, Span has
changed to use code generation approach

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Conclusions
 There are some cases the golang’s reflect is
efficient.
 However, code with reflect is a little difficult to
read.
 Consider code generating approach by your
use cases.
 Compare user comfortable and code
maintainability to decide either.