Consider better error handling for web apps

Go Conference 2017 Autumn
https://gocon.connpass.com/event/66615/

references
- Gocon Spring 2016 Keynote https://dave.cheney.net/paste/gocon-spring-2016.pdf
- Golangのエラー処理とpkg/errors | SOTA http://deeeet.com/writing/2016/04/25/go-pkg-errors/
- https://github.com/pkg/errors
- https://github.com/creasty/apperrors
- https://github.com/izumii5210/grpc-errors
- https://github.com/izumin5210-sandbox/grpc-and-gateway-sample-app-go

source
https://gist.github.com/izumin5210/c3f7615a786775a434acc6f3269cc9c9

Transcript

1. Consider better error handling for web apps
   Go Conference 2017 Autumn - by @izumin5210
   

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2. How do you handle errors in applications ?
   

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3. Ignore ?
   ```
   resp, _ := DoSomething()
   ```
   

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4. Panic ?
   ```
   resp, err := DoSomething()
   if err != nil {
   panic(err)
   }
   ```
   

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5. Wrap ?
   ```
   import (
   "github.com/pkg/errors"
   )
   resp, err := DoSomething()
   if err != nil {
   return errors.Wrap(err)
   }
   ```
   

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6. ### Why should we `Wrap` errors ?
   The errors package allows programmers to add context to
   the failure path in their code in a way that does not
   destroy the original value of the error.
   github.com/pkg/errors (emphasis mine)
   6
   

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7. ### `return nil, errors.Wrap(err)` , and what then?
   ```
   func GetProfile(w http.ResponseWriter, r *http.Request) {
   // ...
   s, err := store.GetProfileByID(userID)
   if err != nil {
   w.WriteHeader(500)
   return
   }
   // ...
   }
   ```
   7
   

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9. Error reporting
   

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10. ### e.g. Error reporting as a Service
    - Honeybadger.io
    - Sentry
    etc.
    10
    

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11. From where do we send error reports?
    11
    

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12. #### 1. レスポンス直前
    ```
    if err != nil {
    notifyError(err) // w.WriteHeader(500)
    return
    }
    // or middlewares, interceptors, etc.
    ```
    - expected errorとunexpected errorをどう区別する？
    - status codeをどうやって決定する？
    12
    

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13. #### 2. エラー出次第？
    ```
    q := "INSERT INTO working_histories (profile_id, company) VALUES ($1, $2)"
    result, err := conn.Exec(q, 1253477, "Wantedly, Inc.")
    if err != nil {
    errorreporter.NotifyError(err) // return nil, errors.Wrap(err, "failed to write new working history")
    }
    ```
    - あらゆるレイヤがerror reporterと密結合することになる
    - 「どこで `NotifyError` したか」わからなくなる
    13
    

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14. ### For appropriate error reporting
    - Adds context to errors
    - It's provided by `github.com/pkg/errors`
    - Decides whether errors is as expected or unexpected
    - Decides responses to users (e.g. HTTP Status Code, etc.)
    14
    

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15. ## Better error annotations for applications
    e.g. `github.com/creasty/apperrors`
    `apperrors` provides contextual metadata to errors.
    - Stack trace
    - Additional information
    - Status code (for a HTTP server)
    - Reportability (for an integration with error reporting
    service)
    15
    

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16. ```
    if isProfileExists(profileID) {
    // can annotate errors with error codes
    return nil, apperrors.WithStatusCode(
    apperrors.New("profile does not exist"),
    http.StatusNotFound,
    )
    }
    result, err := conn.Exec("INSERT INTO ...", profileID, company)
    if err != nil {
    // can annotate unexpected errors
    return nil, apperrors.WithReport(
    apperrors.WithMessage(err, "failed to write new working history"),
    )
    }
    ```
    16
    

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17. ```
    func HandleError(w http.ResponseWriter, err error) {
    appErr := apperrors.Unwrap(err)
    if appErr.Report {
    // We can send report only about unexpected errors
    go uploadAppError(appErr)
    }
    if appErr.StatusCode > 0 {
    w.WriteHeader(appErr.StatusCode)
    } else {
    w.WriteHeader(http.StatusInternalServerError)
    }
    }
    ```
    17
    

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18. ```
    err := store.CreateWorkingHistory(userID, company)
    if err != nil {
    HandleError(w, err)
    return
    }
    ```
    18
    

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19. ### 独自Error typeはいいのか？
    Gocon Spring 2016 Keynote
    - APIは `github.com/pkg/errors` + α
    - `WithReport()` と `WithStatusCode()` ぐらい
    - errorをmiddlewareで処理すれば依存は最小限？
    19
    

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20. ### `WithStatusCode` をDBに近いレイヤーで使うの？
    - これはあまりうれしくない
    - HTTP Status CodeはclientへのViewの一つでしかない
    - そのドメインでのエラーコード一覧をつくる？
    - エラー処理時にHTTP, gRPC等のStatus Codeへマップ
    20
    

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21. Sample application
    izumin5210-sandbox/grpc-and-gateway-sample-app-go
    on github
    

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22. ### `ErrorCode`
    ```
    // type/system/error.go
    type ErrorCode int
    const (
    ErrorUnknown ErrorCode = iota
    ErrorNotFound
    ErrorFailedToReadDB
    )
    ```
    22
    

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23. ### Functions for `ErrorCode`
    ```
    // type/system/error.go
    func (c ErrorCode) Wrap(err error) error {
    return apperrors.WithStatusCode(apperrors.Wrap(err), int(c))
    }
    func (c ErrorCode) WithReport(err error) error {
    return apperrors.WithReport(c.Wrap(err))
    }
    ```
    23
    

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24. ### Annotate errors
    ```
    // store/profile/store.go
    err := s.DB.Get(prof, "SELECT * FROM profiles WHERE user_id = $1", userID)
    if err != nil {
    if err == sql.ErrNoRows {
    err = errors.Wrap(err, "profile was not found")
    return nil, system.ErrorNotFound.Wrap(err)
    }
    err = errors.Wrap(err, "failed to read profile")
    return nil, system.ErrorFailedToReadDB.WithReport(err)
    }
    ```
    24
    

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25. ### Handle errors on interceptor
    （ `github.com/izumin5210/grpc-errors` : gRPCサーバ
    で `apperrors` をいい感じにやるやつ）
    25
    

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26. #### Convert status code
    ```
    // store/interceptor/errors.go
    var grpcCodeBySystemCode = map[system.ErrorCode]codes.Code{
    system.ErrorUnknown: codes.Unknown,
    system.ErrorNotFound: codes.NotFound,
    system.ErrorFailedToReadDB: codes.Internal,
    }
    ```
    26
    

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27. ```
    // store/interceptor/errors.go
    grpcerrors.WithStatusCodeMapper(func(code int) codes.Code {
    grpcCode, ok := grpcCodeBySystemCode[system.ErrorCode(code)]
    if !ok {
    return codes.Unknown
    }
    return grpcCode
    })
    ```
    27
    

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28. #### Send error reports
    ```
    // store/interceptor/errors.go
    grpcerrors.WithUnaryServerReportableErrorHandler(func(...) error {
    st := &raven.Stacktrace{}
    for _, t := range err.StackTrace {
    // create and append stacktrace frames
    }
    pckt := raven.NewPacket(err.Error(), st)
    // set contextual metadata to packet
    raven.Capture(pckt, map[string]string{"method": info.FullMethod})
    return err
    }),
    ```
    28
    

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29. #### Error report on sentry.io
    29
    

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31. ## Conclusion
    - アプリケーション改善には `pkg/errors` じゃ足りない？
    - error reportやstatus codeをうまく扱いたい
    - `creasty/apperrors`
    - これをｍiddlewareで処理する
    - これがベストとは思っていない
    - みなさんどうしてますか
    31
    

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33. ## References
    - articles
    - Gocon Spring 2016 Keynote
    - Golangのエラー処理とpkg/errors | SOTA
    - packages
    - github.com/pkg/errors
    - github.com/creasty/apperrors
    - github.com/izumii5210/grpc-errors
    - Sample application
    - izumin5210-sandbox/grpc-and-gateway-sample-app-go
    33
    

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