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Handling Errors

go-storage returns errors that satisfy the Go error interface type. The most basic form of error handling looks similar to the following example:

if err != nil {    log.Printf("error: %s", err.Error())    // Handle error    return}

Of course, we can do more. Our goal is:

  • Let users know where and why an error happens: rich contextual information
  • Let users handle error gracefully: identifiable and unified error codes

Error Codes and Fine-grained Error Types#

Simply put, we defined a finite set of error codes and all errors returned by the public APIs of go-storage will have an error code. You can use errors.Is to check the error code and handle the error correspondingly.

The error codes can be either globally shared by all services (defined in go-storage/services/error.go), or service-specific (defined in go-service-*/error.go).

An error code can have a more fine-grained internal classification: it can have 0, 1 or more related error struct types, which have fields carrying contextual information. And you can use errors.As to handle them specially.


In errors.As(originErr, &targetErr), the type of targetErr should be the struct value type instead of pointer type.

Example of Using Error Codes and Fine-grained Error Types#

import (    s3 ""    ""    "")
// ...
store, err := services.NewStorager("s3",     pairs.WithCredential(os.Getenv("STORAGE_S3_CREDENTIAL")),    pairs.WithEndpoint(os.Getenv("STORAGE_S3_ENDPOINT")),    // ...)if err != nil {    if errors.Is(initErr, services.ErrServiceNotRegistered) {        // handle ErrServiceNotRegistered        return    }    // handle error    return}
n, err := store.Read(path, &buf)if err != nil {    if errors.Is(err, services.ErrServiceInternal) {        // handle ErrServiceInternal        // (global error)        return    }    if errors.Is(err, services.ErrRequestThrottled) {        // handle ErrRequestThrottled        // (global error)        return    }    if errors.Is(err, services.ErrObjectNotExist) {        // handle ErrObjectNotExist         // (global error)        return    }    if errors.Is(err, s3.ErrServerSideEncryptionCustomerKeyInvalid) {        // handle ErrServerSideEncryptionCustomerKeyInvalid        // (s3-specific error)        return    }    if errors.Is(err, services.ErrRestrictionDissatisfied) {        // You can also using errors.As outside, without checking the error code        var e services.MetadataUnrecognizedError        if errors.As(err, &e) {            // handle MetadataUnrecognizedError            return        }        // handle ErrRestrictionDissatisfied        return    }    // handle error    return}

List of Global Error Codes#

There's a ErrUnexpected for uncovered cases.

Client-side Errors#

Error CodeFine-grained Error Type

Server-side Errors#

Error CodeFine-grained Error Type

Top-level Errors#


Note: Usually using error codes and fine-grained error structs is enough and you don't have to know the content in this section.

Actually there's some top-level errors wrapping the error codes and fine-grained error types, and they are the actual error types returned in go-storage. The figure below shows the structure:

  • InitError: returned by NewServicer and NewStorager
  • ServiceError: returned by the methods of Servicer
  • StorageError: returned by the methods of Storager

You can use errors.As to convert errors into top-level error types and retrieve the fields you are interested in, just like handling fine-grained error structs.