dynamo

dynamo

import "github.com/guregu/dynamo/v2"

dynamo is an expressive DynamoDB client for Go, with an easy but powerful API. dynamo integrates with the official AWS SDK v2.

This library is stable and versioned with Go modules.

[!TIP] dynamo v2 is finally released! See v2 Migration for tips on migrating from dynamo v1.

For dynamo v1, which uses aws-sdk-go v1, see: dynamo v1 documentation.

Example

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Expressions

dynamo will help you write expressions used to filter results in queries and scans, and add conditions to puts and deletes.

Attribute names may be written as is if it is not a reserved word, or be escaped with single quotes (''). You may also use dollar signs ($) as placeholders for attribute names and list indexes. DynamoDB has very large amount of reserved words so it may be a good idea to just escape everything.

Question marks (?) are used as placeholders for attribute values. DynamoDB doesn't have value literals, so you need to substitute everything.

Please see the DynamoDB reference on expressions for more information. The Comparison Operator and Function Reference is also handy.

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Encoding support

dynamo automatically handles the following interfaces:

• dynamo.Marshaler and dynamo.Unmarshaler
• dynamodbattribute.Marshaler and dynamodbattribute.Unmarshaler
• encoding.TextMarshaler and encoding.TextUnmarshaler

This allows you to define custom encodings and provides built-in support for types such as time.Time.

Struct tags and fields

dynamo handles struct tags similarly to the standard library encoding/json package. It uses dynamo for the struct tag's name, taking the form of: dynamo:"attributeName,option1,option2,etc". You can omit the attribute name to use the default: dynamo:",option1,etc".

Renaming

By default, dynamo will use the name of your fields as the name of the DynamoDB attribute it corresponds to. You can specify a different name with the dynamo struct tag like so: dynamo:"other_name_goes_here". If two fields have the same name, dynamo will prioritize the higher-level field.

Omission

If you set a field's name to "-" (as in dynamo:"-") that field will be ignored. It will be omitted when marshaling and ignored when unmarshaling. Also, fields that start with a lowercase letter will be ignored. However, embedding a struct whose type has a lowercase letter but contains uppercase fields is OK.

Sets

By default, slices will be marshaled as DynamoDB lists. To marshal a field to sets instead, use the dynamo:",set" option. Empty sets will be automatically omitted.

You can use maps as sets too. The following types are supported:

• []T
• map[T]struct{}
• map[T]bool

where T represents any type that marshals into a DynamoDB string, number, or binary value.

Note that the order of objects within a set is undefined.

Omitting empty values (omitempty)

Using the omitempty option (as in dynamo:",omitempty") will omit the field if it has a zero (ex. an empty string, 0, nil pointer) value. Structs are supported.

It also supports the isZeroer interface below:

type isZeroer interface {
	IsZero() bool
}

If IsZero() returns true, the field will be omitted. This gives us built-in support for time.Time.

You can also use the dynamo:",omitemptyelem" option to omit empty values inside of slices.

Automatic omission

Some values will be automatically omitted.

• Empty strings
• Empty sets
• Empty structs
• Nil pointers and interfaces
• Types that implement encoding.TextMarshaler and whose MarshalText method returns 0-length or nil slice.
• Zero-length binary (byte slices)

To override this behavior, use the dynamo:",allowempty" flag. Not all empty types can be stored by DynamoDB. For example, empty sets will still be omitted.

To override auto-omit behavior for children of a map, for example map[string]string, use the dynamo:",allowemptyelem" option.

Using the NULL type

DynamoDB has a special NULL type to represent null values. In general, this library avoids marshaling things as NULL and prefers to omit those values instead. If you want empty/nil values to marshal to NULL, use the dynamo:",null" option.

Unix time

By default, time.Time will marshal to a string because it implements encoding.TextMarshaler.

If you want time.Time to marshal as a Unix time value (number of seconds since the Unix epoch), you can use the dynamo:",unixtime" option. This is useful for TTL fields, which must be Unix time.

Creating tables

You can use struct tags to specify hash keys, range keys, and indexes when creating a table.

For example:

type UserAction struct {
	UserID string    `dynamo:"ID,hash" index:"Seq-ID-index,range"`
	Time   time.Time `dynamo:",range"`
	Seq    int64     `localIndex:"ID-Seq-index,range" index:"Seq-ID-index,hash"`
	UUID   string    `index:"UUID-index,hash"`
}

This creates a table with the primary hash key ID and range key Time. It creates two global secondary indices called UUID-index and Seq-ID-index, and a local secondary index called ID-Seq-index.

Retrying

As of v2, dynamo relies on the AWS SDK for retrying. See: Retries and Timeouts documentation for information about how to configure its behavior.

By default, canceled transactions (i.e. errors from conflicting transactions) will not be retried. To get automatic retrying behavior like in v1, use dynamo.RetryTxConflicts.

import (
	"context"
	"log"

	"github.com/aws/aws-sdk-go-v2/aws"
	"github.com/aws/aws-sdk-go-v2/aws/retry"
	"github.com/aws/aws-sdk-go-v2/config"
	"github.com/guregu/dynamo/v2"
)

func main() {
	cfg, err := config.LoadDefaultConfig(context.Background(), config.WithRetryer(func() aws.Retryer {
		return retry.NewStandard(dynamo.RetryTxConflicts)
	}))
	if err != nil {
		log.Fatal(err)
	}
	db := dynamo.New(cfg)
	// use db
}

Compatibility with the official AWS library

dynamo has been in development before the official AWS libraries were stable. We use a different encoder and decoder than the dynamodbattribute package. dynamo uses the dynamo struct tag instead of the dynamodbav struct tag, and we also prefer to automatically omit invalid values such as empty strings, whereas the dynamodbattribute package substitutes null values for them. Items that satisfy the dynamodbattribute.(Un)marshaler interfaces are compatibile with both libraries.

In order to use dynamodbattribute's encoding facilities, you must wrap objects passed to dynamo with dynamo.AWSEncoding. Here is a quick example:

// Notice the use of the dynamodbav struct tag
type book struct {
	ID    int    `dynamodbav:"id"`
	Title string `dynamodbav:"title"`
}
// Putting an item
err := db.Table("Books").Put(dynamo.AWSEncoding(book{
	ID:    42,
	Title: "Principia Discordia",
})).Run(ctx)
// When getting an item you MUST pass a pointer to AWSEncoding!
var someBook book
err := db.Table("Books").Get("ID", 555).One(ctx, dynamo.AWSEncoding(&someBook))

Migrating from v1

The API hasn't changed much from v1 to v2. Here are some migration tips:

• All request methods now take a context as their first argument.
• Retrying relies on the AWS SDK configuration, see: Retrying.
  • Transactions won't retry TransactionCanceled responses by default anymore, make sure you configure that if you need it.
• Arguments that took int64 (such as in Query.Limit) now take int instead.
• Compatibility with the official AWS library uses v2 interfaces instead of v1.
• KMSMasterKeyArn renamed to KMSMasterKeyARN.

Integration tests

By default, tests are run in offline mode. In order to run the integration tests, some environment variables need to be set.

To run the tests against DynamoDB Local:

# Use Docker to run DynamoDB local on port 8880
docker compose -f '.github/docker-compose.yml' up -d

# Run the tests with a fresh table
# The tables will be created automatically
# The '%' in the table name will be replaced the current timestamp
DYNAMO_TEST_ENDPOINT='http://localhost:8880' \
	DYNAMO_TEST_REGION='local' \
	DYNAMO_TEST_TABLE='TestDB-%' \
	AWS_ACCESS_KEY_ID='dummy' \
	AWS_SECRET_ACCESS_KEY='dummy' \
	AWS_REGION='local' \
	go test -v -race ./... -cover -coverpkg=./...

License

BSD 2-Clause