Collections

Collections is a library meant to simplify the experience with respect to module state handling.

Cosmos SDK modules handle their state using the KVStore interface. The problem with working with KVStore is that it forces you to think of state as a bytes KV pairings when in reality the majority of state comes from complex concrete golang objects (strings, ints, structs, etc.).

Collections allows you to work with state as if they were normal golang objects and removes the need for you to think of your state as raw bytes in your code.

It also allows you to migrate your existing state without causing any state breakage that forces you into tedious and complex chain state migrations.

Installation

To install collections in your cosmos-sdk chain project, run the following command:

go get cosmossdk.io/collections

Core types

Collections offers 5 different APIs to work with state, which will be explored in the next sections, these APIs are:

• Map: to work with typed arbitrary KV pairings.
• KeySet: to work with just typed keys
• Item: to work with just one typed value
• Sequence: which is a monotonically increasing number.
• IndexedMap: which combines Map and KeySet to provide a Map with indexing capabilities.

Preliminary components

Before exploring the different collections types and their capability it is necessary to introduce the three components that every collection shares. In fact when instantiating a collection type by doing, for example, collections.NewMap/collections.NewItem/... you will find yourself having to pass them some common arguments.

For example, in code:

package collections

import (
    "cosmossdk.io/collections"
    storetypes "cosmossdk.io/store/types"
    sdk "github.com/cosmos/cosmos-sdk/types"
)

var AllowListPrefix = collections.NewPrefix(0)

type Keeper struct {
	Schema    collections.Schema
	AllowList collections.KeySet[string]
}

func NewKeeper(storeKey *storetypes.KVStoreKey) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))

	return Keeper{
		AllowList: collections.NewKeySet(sb, AllowListPrefix, "allow_list", collections.StringKey),
	}
}

Let's analyse the shared arguments, what they do, and why we need them.

SchemaBuilder

The first argument passed is the SchemaBuilder

SchemaBuilder is a structure that keeps track of all the state of a module, it is not required by the collections to deal with state but it offers a dynamic and reflective way for clients to explore a module's state.

We instantiate a SchemaBuilder by passing it a function that given the modules store key returns the module's specific store.

We then need to pass the schema builder to every collection type we instantiate in our keeper, in our case the AllowList.

Prefix

The second argument passed to our KeySet is a collections.Prefix, a prefix represents a partition of the module's KVStore where all the state of a specific collection will be saved.

Since a module can have multiple collections, the following is expected:

• module params will become a collections.Item
• the AllowList is a collections.KeySet

We don't want a collection to write over the state of the other collection so we pass it a prefix, which defines a storage partition owned by the collection.

If you already built modules, the prefix translates to the items you were creating in your types/keys.go file, example: https://github.com/cosmos/cosmos-sdk/blob/main/x/feegrant/key.go#L27

your old:

var (
	// FeeAllowanceKeyPrefix is the set of the kvstore for fee allowance data
	// - 0x00<allowance_key_bytes>: allowance
	FeeAllowanceKeyPrefix = []byte{0x00}

	// FeeAllowanceQueueKeyPrefix is the set of the kvstore for fee allowance keys data
	// - 0x01<allowance_prefix_queue_key_bytes>: <empty value>
	FeeAllowanceQueueKeyPrefix = []byte{0x01}
)

becomes:

var (
	// FeeAllowanceKeyPrefix is the set of the kvstore for fee allowance data
	// - 0x00<allowance_key_bytes>: allowance
	FeeAllowanceKeyPrefix = collections.NewPrefix(0)

	// FeeAllowanceQueueKeyPrefix is the set of the kvstore for fee allowance keys data
	// - 0x01<allowance_prefix_queue_key_bytes>: <empty value>
	FeeAllowanceQueueKeyPrefix = collections.NewPrefix(1)
)

Rules

collections.NewPrefix accepts either uint8, string or []bytes it's good practice to use an always increasing uint8for disk space efficiency.

A collection MUST NOT share the same prefix as another collection in the same module, and a collection prefix MUST NEVER start with the same prefix as another, examples:

prefix1 := collections.NewPrefix("prefix")
prefix2 := collections.NewPrefix("prefix") // THIS IS BAD!

prefix1 := collections.NewPrefix("a")
prefix2 := collections.NewPrefix("aa") // prefix2 starts with the same as prefix1: BAD!!!

Human-Readable Name

The third parameter we pass to a collection is a string, which is a human-readable name. It is needed to make the role of a collection understandable by clients who have no clue about what a module is storing in state.

Rules

Each collection in a module MUST have a unique humanised name.

Key and Value Codecs

A collection is generic over the type you can use as keys or values. This makes collections dumb, but also means that hypothetically we can store everything that can be a go type into a collection. We are not bounded to any type of encoding (be it proto, json or whatever)

So a collection needs to be given a way to understand how to convert your keys and values to bytes. This is achieved through KeyCodec and ValueCodec, which are arguments that you pass to your collections when you're instantiating them using the collections.NewMap/collections.NewItem/... instantiation functions.

NOTE: Generally speaking you will never be required to implement your own Key/ValueCodec as the SDK and collections libraries already come with default, safe and fast implementation of those. You might need to implement them only if you're migrating to collections and there are state layout incompatibilities.

Let's explore an example:

package collections

import (
	"cosmossdk.io/collections"
	storetypes "cosmossdk.io/store/types"
	sdk "github.com/cosmos/cosmos-sdk/types"
)

var IDsPrefix = collections.NewPrefix(0)

type Keeper struct {
	Schema    collections.Schema
	IDs   collections.Map[string, uint64]
}

func NewKeeper(storeKey *storetypes.KVStoreKey) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))

	return Keeper{
		IDs: collections.NewMap(sb, IDsPrefix, "ids", collections.StringKey, collections.Uint64Value),
	}
}

We're now instantiating a map where the key is string and the value is uint64. We already know the first three arguments of the NewMap function.

The fourth parameter is our KeyCodec, we know that the Map has string as key so we pass it a KeyCodec that handles strings as keys.

The fifth parameter is our ValueCodec, we know that the Map has a uint64 as value so we pass it a ValueCodec that handles uint64.

Collections already comes with all the required implementations for golang primitive types.

Let's make another example, this falls closer to what we build using cosmos SDK, let's say we want to create a collections.Map that maps account addresses to their base account. So we want to map an sdk.AccAddress to an auth.BaseAccount (which is a proto):

package collections

import (
	"cosmossdk.io/collections"
	storetypes "cosmossdk.io/store/types"
	"github.com/cosmos/cosmos-sdk/codec"
	sdk "github.com/cosmos/cosmos-sdk/types"
	authtypes "github.com/cosmos/cosmos-sdk/x/auth/types"
)

var AccountsPrefix = collections.NewPrefix(0)

type Keeper struct {
	Schema    collections.Schema
	Accounts   collections.Map[sdk.AccAddress, authtypes.BaseAccount]
}

func NewKeeper(storeKey *storetypes.KVStoreKey, cdc codec.BinaryCodec) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
	return Keeper{
		Accounts: collections.NewMap(sb, AccountsPrefix, "accounts",
			sdk.AccAddressKey, codec.CollValue[authtypes.BaseAccount](cdc)),
	}
}

As we can see here since our collections.Map maps sdk.AccAddress to authtypes.BaseAccount, we use the sdk.AccAddressKey which is the KeyCodec implementation for AccAddress and we use codec.CollValue to encode our proto type BaseAccount.

Generally speaking you will always find the respective key and value codecs for types in the go.mod path you're using to import that type. If you want to encode proto values refer to the codec codec.CollValue function, which allows you to encode any type implement the proto.Message interface.

Map

We analyse the first and most important collection type, the collections.Map. This is the type that everything else builds on top of.

Use case

A collections.Map is used to map arbitrary keys with arbitrary values.

Example

It's easier to explain a collections.Map capabilities through an example:

package collections

import (
	"cosmossdk.io/collections"
	storetypes "cosmossdk.io/store/types"
	"fmt"
	"github.com/cosmos/cosmos-sdk/codec"
	sdk "github.com/cosmos/cosmos-sdk/types"
	authtypes "github.com/cosmos/cosmos-sdk/x/auth/types"
)

var AccountsPrefix = collections.NewPrefix(0)

type Keeper struct {
	Schema    collections.Schema
	Accounts   collections.Map[sdk.AccAddress, authtypes.BaseAccount]
}

func NewKeeper(storeKey *storetypes.KVStoreKey, cdc codec.BinaryCodec) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
	return Keeper{
		Accounts: collections.NewMap(sb, AccountsPrefix, "accounts",
			sdk.AccAddressKey, codec.CollValue[authtypes.BaseAccount](cdc)),
	}
}

func (k Keeper) CreateAccount(ctx sdk.Context, addr sdk.AccAddress, account authtypes.BaseAccount) error {
	has, err := k.Accounts.Has(ctx, addr)
	if err != nil {
		return err
	}
	if has {
		return fmt.Errorf("account already exists: %s", addr)
	}
	
	err = k.Accounts.Set(ctx, addr, account)
	if err != nil {
		return err
	}
	return nil
}

func (k Keeper) GetAccount(ctx sdk.Context, addr sdk.AccAddress) (authtypes.BaseAccount, error) {
	acc, err := k.Accounts.Get(ctx, addr)
	if err != nil {
		return authtypes.BaseAccount{}, err
	}
	
	return acc,	nil
}

func (k Keeper) RemoveAccount(ctx sdk.Context, addr sdk.AccAddress) error {
	err := k.Accounts.Remove(ctx, addr)
	if err != nil {
		return err
	}
	return nil
}

Set method

Set maps with the provided AccAddress (the key) to the auth.BaseAccount (the value).

Under the hood the collections.Map will convert the key and value to bytes using the key and value codec. It will prepend to our bytes key the prefix and store it in the KVStore of the module.

Has method

The has method reports if the provided key exists in the store.

Get method

The get method accepts the AccAddress and returns the associated auth.BaseAccount if it exists, otherwise it errors.

Remove method

The remove method accepts the AccAddress and removes it from the store. It won't report errors if it does not exist, to check for existence before removal use the Has method.

Iteration

Iteration has a separate section.

KeySet

The second type of collection is collections.KeySet, as the word suggests it maintains only a set of keys without values.

Implementation curiosity

A collections.KeySet is just a collections.Map with a key but no value. The value internally is always the same and is represented as an empty byte slice []byte{}.

Example

As always we explore the collection type through an example:

package collections

import (
	"cosmossdk.io/collections"
	storetypes "cosmossdk.io/store/types"
	"fmt"
	sdk "github.com/cosmos/cosmos-sdk/types"
)

var ValidatorsSetPrefix = collections.NewPrefix(0)

type Keeper struct {
	Schema        collections.Schema
	ValidatorsSet collections.KeySet[sdk.ValAddress]
}

func NewKeeper(storeKey *storetypes.KVStoreKey) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
	return Keeper{
		ValidatorsSet: collections.NewKeySet(sb, ValidatorsSetPrefix, "validators_set", sdk.ValAddressKey),
	}
}

func (k Keeper) AddValidator(ctx sdk.Context, validator sdk.ValAddress) error {
	has, err := k.ValidatorsSet.Has(ctx, validator)
	if err != nil {
		return err
	}
	if has {
		return fmt.Errorf("validator already in set: %s", validator)
	}
	
	err = k.ValidatorsSet.Set(ctx, validator)
	if err != nil {
		return err
	}
	
	return nil
}

func (k Keeper) RemoveValidator(ctx sdk.Context, validator sdk.ValAddress) error {
	err := k.ValidatorsSet.Remove(ctx, validator)
	if err != nil {
		return err
	}
	return nil
}

The first difference we notice is that KeySet needs use to specify only one type parameter: the key (sdk.ValAddress in this case). The second difference we notice is that KeySet in its NewKeySet function does not require us to specify a ValueCodec but only a KeyCodec. This is because a KeySet only saves keys and not values.

Let's explore the methods.

Has method

Has allows us to understand if a key is present in the collections.KeySet or not, functions in the same way as collections.Map.Has

Set method

Set inserts the provided key in the KeySet.

Remove method

Remove removes the provided key from the KeySet, it does not error if the key does not exist, if existence check before removal is required it needs to be coupled with the Has method.

Item

The third type of collection is the collections.Item. It stores only one single item, it's useful for example for parameters, there's only one instance of parameters in state always.

implementation curiosity

A collections.Item is just a collections.Map with no key but just a value. The key is the prefix of the collection!

Example

package collections

import (
	"cosmossdk.io/collections"
	storetypes "cosmossdk.io/store/types"
	"github.com/cosmos/cosmos-sdk/codec"
	sdk "github.com/cosmos/cosmos-sdk/types"
	stakingtypes "cosmossdk.io/x/staking/types"
)

var ParamsPrefix = collections.NewPrefix(0)

type Keeper struct {
	Schema        collections.Schema
	Params collections.Item[stakingtypes.Params]
}

func NewKeeper(storeKey *storetypes.KVStoreKey, cdc codec.BinaryCodec) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
	return Keeper{
		Params: collections.NewItem(sb, ParamsPrefix, "params", codec.CollValue[stakingtypes.Params](cdc)),
	}
}

func (k Keeper) UpdateParams(ctx sdk.Context, params stakingtypes.Params) error {
	err := k.Params.Set(ctx, params)
	if err != nil {
		return err
	}
	return nil
}

func (k Keeper) GetParams(ctx sdk.Context) (stakingtypes.Params, error) {
	return k.Params.Get(ctx)
}

The first key difference we notice is that we specify only one type parameter, which is the value we're storing. The second key difference is that we don't specify the KeyCodec, since we store only one item we already know the key and the fact that it is constant.

Iteration

One of the key features of the KVStore is iterating over keys.

Collections which deal with keys (so Map, KeySet and IndexedMap) allow you to iterate over keys in a safe and typed way. They all share the same API, the only difference being that KeySet returns a different type of Iterator because KeySet only deals with keys.

:::note

Every collection shares the same Iterator semantics.

:::

Let's have a look at the Map.Iterate method:

func (m Map[K, V]) Iterate(ctx context.Context, ranger Ranger[K]) (Iterator[K, V], error) 

It accepts a collections.Ranger[K], which is an API that instructs map on how to iterate over keys. As always we don't need to implement anything here as collections already provides some generic Ranger implementers that expose all you need to work with ranges.

Example

We have a collections.Map that maps accounts using uint64 IDs.

package collections

import (
	"cosmossdk.io/collections"
	storetypes "cosmossdk.io/store/types"
	"github.com/cosmos/cosmos-sdk/codec"
	sdk "github.com/cosmos/cosmos-sdk/types"
	authtypes "github.com/cosmos/cosmos-sdk/x/auth/types"
)

var AccountsPrefix = collections.NewPrefix(0)

type Keeper struct {
	Schema   collections.Schema
	Accounts collections.Map[uint64, authtypes.BaseAccount]
}

func NewKeeper(storeKey *storetypes.KVStoreKey, cdc codec.BinaryCodec) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
	return Keeper{
		Accounts: collections.NewMap(sb, AccountsPrefix, "accounts", collections.Uint64Key, codec.CollValue[authtypes.BaseAccount](cdc)),
	}
}

func (k Keeper) GetAllAccounts(ctx sdk.Context) ([]authtypes.BaseAccount, error) {
	// passing a nil Ranger equals to: iterate over every possible key
	iter, err := k.Accounts.Iterate(ctx, nil)
	if err != nil {
		return nil, err
	}
	accounts, err := iter.Values()
	if err != nil {
		return nil, err
	}

	return accounts, err
}

func (k Keeper) IterateAccountsBetween(ctx sdk.Context, start, end uint64) ([]authtypes.BaseAccount, error) {
	// The collections.Range API offers a lot of capabilities
	// like defining where the iteration starts or ends.
	rng := new(collections.Range[uint64]).
		StartInclusive(start).
		EndExclusive(end).
		Descending()

	iter, err := k.Accounts.Iterate(ctx, rng)
	if err != nil {
		return nil, err
	}
	accounts, err := iter.Values()
	if err != nil {
		return nil, err
	}

	return accounts, nil
}

func (k Keeper) IterateAccounts(ctx sdk.Context, do func(id uint64, acc authtypes.BaseAccount) (stop bool)) error {
	iter, err := k.Accounts.Iterate(ctx, nil)
	if err != nil {
		return err
	}
	defer iter.Close()

	for ; iter.Valid(); iter.Next() {
		kv, err := iter.KeyValue()
		if err != nil {
			return err
		}

		if do(kv.Key, kv.Value) {
			break
		}
	}
	return nil
}

Let's analyse each method in the example and how it makes use of the Iterate and the returned Iterator API.

GetAllAccounts

In GetAllAccounts we pass to our Iterate a nil Ranger. This means that the returned Iterator will include all the existing keys within the collection.

Then we use the Values method from the returned Iterator API to collect all the values into a slice.

Iterator offers other methods such as Keys() to collect only the keys and not the values and KeyValues to collect all the keys and values.

IterateAccountsBetween

Here we make use of the collections.Range helper to specialise our range. We make it start in a point through StartInclusive and end in the other with EndExclusive, then we instruct it to report us results in reverse order through Descending

Then we pass the range instruction to Iterate and get an Iterator, which will contain only the results we specified in the range.

Then we use again the Values method of the Iterator to collect all the results.

collections.Range also offers a Prefix API which is not applicable to all keys types, for example uint64 cannot be prefix because it is of constant size, but a string key can be prefixed.

IterateAccounts

Here we showcase how to lazily collect values from an Iterator.

:::note

Keys/Values/KeyValues fully consume and close the Iterator, here we need to explicitly do a defer iterator.Close() call.

:::

Iterator also exposes a Value and Key method to collect only the current value or key, if collecting both is not needed.

:::note

For this callback pattern, collections expose a Walk API.

:::

Composite keys

So far we've worked only with simple keys, like uint64, the account address, etc. There are some more complex cases in, which we need to deal with composite keys.

A key is composite when it is composed of multiple keys, for example bank balances as stored as the composite key (AccAddress, string) where the first part is the address holding the coins and the second part is the denom.

Example, let's say address BOB holds 10atom,15osmo, this is how it is stored in state:

(bob, atom) => 10
(bob, osmos) => 15

Now this allows to efficiently get a specific denom balance of an address, by simply getting (address, denom), or getting all the balances of an address by prefixing over (address).

Let's see now how we can work with composite keys using collections.

Example

In our example we will show-case how we can use collections when we are dealing with balances, similar to bank, a balance is a mapping between (address, denom) => math.Int the composite key in our case is (address, denom).

Instantiation of a composite key collection

package collections

import (
	"cosmossdk.io/collections"
	"cosmossdk.io/math"
	storetypes "cosmossdk.io/store/types"
	sdk "github.com/cosmos/cosmos-sdk/types"
)


var BalancesPrefix = collections.NewPrefix(1)

type Keeper struct {
	Schema   collections.Schema
	Balances collections.Map[collections.Pair[sdk.AccAddress, string], math.Int]
}

func NewKeeper(storeKey *storetypes.KVStoreKey) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
	return Keeper{
		Balances: collections.NewMap(
			sb, BalancesPrefix, "balances",
			collections.PairKeyCodec(sdk.AccAddressKey, collections.StringKey),
			sdk.IntValue,
		),
	}
}

The Map Key definition

First of all we can see that in order to define a composite key of two elements we use the collections.Pair type:

collections.Map[collections.Pair[sdk.AccAddress, string], math.Int]

collections.Pair defines a key composed of two other keys, in our case the first part is sdk.AccAddress, the second part is string.

The Key Codec instantiation

The arguments to instantiate are always the same, the only thing that changes is how we instantiate the KeyCodec, since this key is composed of two keys we use collections.PairKeyCodec, which generates a KeyCodec composed of two key codecs. The first one will encode the first part of the key, the second one will encode the second part of the key.

Working with composite key collections

Let's expand on the example we used before:

var BalancesPrefix = collections.NewPrefix(1)

type Keeper struct {
	Schema   collections.Schema
	Balances collections.Map[collections.Pair[sdk.AccAddress, string], math.Int]
}

func NewKeeper(storeKey *storetypes.KVStoreKey) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
	return Keeper{
		Balances: collections.NewMap(
			sb, BalancesPrefix, "balances",
			collections.PairKeyCodec(sdk.AccAddressKey, collections.StringKey),
			sdk.IntValue,
		),
	}
}

func (k Keeper) SetBalance(ctx sdk.Context, address sdk.AccAddress, denom string, amount math.Int) error {
	key := collections.Join(address, denom)
	return k.Balances.Set(ctx, key, amount)
}

func (k Keeper) GetBalance(ctx sdk.Context, address sdk.AccAddress, denom string) (math.Int, error) {
	return k.Balances.Get(ctx, collections.Join(address, denom))
}

func (k Keeper) GetAllAddressBalances(ctx sdk.Context, address sdk.AccAddress) (sdk.Coins, error) {
	balances := sdk.NewCoins()

	rng := collections.NewPrefixedPairRange[sdk.AccAddress, string](address)

	iter, err := k.Balances.Iterate(ctx, rng)
	if err != nil {
		return nil, err
	}

	kvs, err := iter.KeyValues()
	if err != nil {
		return nil, err
	}

	for _, kv := range kvs {
		balances = balances.Add(sdk.NewCoin(kv.Key.K2(), kv.Value))
	}
	return balances, nil
}

func (k Keeper) GetAllAddressBalancesBetween(ctx sdk.Context, address sdk.AccAddress, startDenom, endDenom string) (sdk.Coins, error) {
    rng := collections.NewPrefixedPairRange[sdk.AccAddress, string](address).
        StartInclusive(startDenom).
        EndInclusive(endDenom)

    iter, err := k.Balances.Iterate(ctx, rng)
    if err != nil {
        return nil, err
	}
    ...
}

SetBalance

As we can see here we're setting the balance of an address for a specific denom. We use the collections.Join function to generate the composite key. collections.Join returns a collections.Pair (which is the key of our collections.Map)

collections.Pair contains the two keys we have joined, it also exposes two methods: K1 to fetch the 1st part of the key and K2 to fetch the second part.

As always, we use the collections.Map.Set method to map the composite key to our value (math.Int in this case)

GetBalance

To get a value in composite key collection, we simply use collections.Join to compose the key.

GetAllAddressBalances

We use collections.PrefixedPairRange to iterate over all the keys starting with the provided address. Concretely the iteration will report all the balances belonging to the provided address.

The first part is that we instantiate a PrefixedPairRange, which is a Ranger implementer aimed to help in Pair keys iterations.

	rng := collections.NewPrefixedPairRange[sdk.AccAddress, string](address)

As we can see here we're passing the type parameters of the collections.Pair because golang type inference with respect to generics is not as permissive as other languages, so we need to explicitly say what are the types of the pair key.

GetAllAddressesBalancesBetween

This showcases how we can further specialise our range to limit the results further, by specifying the range between the second part of the key (in our case the denoms, which are strings).

IndexedMap

collections.IndexedMap is a collection that uses under the hood a collections.Map, and has a struct, which contains the indexes that we need to define.

Example

Let's say we have an auth.BaseAccount struct which looks like the following:

type BaseAccount struct {
	AccountNumber uint64     `protobuf:"varint,3,opt,name=account_number,json=accountNumber,proto3" json:"account_number,omitempty"`
	Sequence      uint64     `protobuf:"varint,4,opt,name=sequence,proto3" json:"sequence,omitempty"`
}

First of all, when we save our accounts in state we map them using a primary key sdk.AccAddress. If it were to be a collections.Map it would be collections.Map[sdk.AccAddres, authtypes.BaseAccount].

Then we also want to be able to get an account not only by its sdk.AccAddress, but also by its AccountNumber.

So we can say we want to create an Index that maps our BaseAccount to its AccountNumber.

We also know that this Index is unique. Unique means that there can only be one BaseAccount that maps to a specific AccountNumber.

First of all, we start by defining the object that contains our index:

var AccountsNumberIndexPrefix = collections.NewPrefix(1)

type AccountsIndexes struct {
	Number *indexes.Unique[uint64, sdk.AccAddress, authtypes.BaseAccount]
}

func NewAccountIndexes(sb *collections.SchemaBuilder) AccountsIndexes {
	return AccountsIndexes{
		Number: indexes.NewUnique(
			sb, AccountsNumberIndexPrefix, "accounts_by_number",
			collections.Uint64Key, sdk.AccAddressKey,
			func(_ sdk.AccAddress, v authtypes.BaseAccount) (uint64, error) {
				return v.AccountNumber, nil
			},
		),
	}
}

We create an AccountIndexes struct which contains a field: Number. This field represents our AccountNumber index. AccountNumber is a field of authtypes.BaseAccount and it's a uint64.

Then we can see in our AccountIndexes struct the Number field is defined as:

*indexes.Unique[uint64, sdk.AccAddress, authtypes.BaseAccount]

Where the first type parameter is uint64, which is the field type of our index. The second type parameter is the primary key sdk.AccAddress. And the third type parameter is the actual object we're storing authtypes.BaseAccount.

Then we create a NewAccountIndexes function that instantiates and returns the AccountsIndexes struct.

The function takes a SchemaBuilder. Then we instantiate our indexes.Unique, let's analyse the arguments we pass to indexes.NewUnique.

NOTE: indexes list

The AccountsIndexes struct contains the indexes, the NewIndexedMap function will infer the indexes form that struct using reflection, this happens only at init and is not computationally expensive. In case you want to explicitly declare indexes: implement the Indexes interface in the AccountsIndexes struct:

func (a AccountsIndexes) IndexesList() []collections.Index[sdk.AccAddress, authtypes.BaseAccount] {
    return []collections.Index[sdk.AccAddress, authtypes.BaseAccount]{a.Number}
}

Instantiating a indexes.Unique

The first three arguments, we already know them, they are: SchemaBuilder, Prefix which is our index prefix (the partition where index keys relationship for the Number index will be maintained), and the human name for the Number index.

The second argument is a collections.Uint64Key which is a key codec to deal with uint64 keys, we pass that because the key we're trying to index is a uint64 key (the account number), and then we pass as fifth argument the primary key codec, which in our case is sdk.AccAddress (remember: we're mapping sdk.AccAddress => BaseAccount).

Then as last parameter we pass a function that: given the BaseAccount returns its AccountNumber.

After this we can proceed instantiating our IndexedMap.

var AccountsPrefix = collections.NewPrefix(0)

type Keeper struct {
	Schema   collections.Schema
	Accounts *collections.IndexedMap[sdk.AccAddress, authtypes.BaseAccount, AccountsIndexes]
}

func NewKeeper(storeKey *storetypes.KVStoreKey, cdc codec.BinaryCodec) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
	return Keeper{
		Accounts: collections.NewIndexedMap(
			sb, AccountsPrefix, "accounts",
			sdk.AccAddressKey, codec.CollValue[authtypes.BaseAccount](cdc),
			NewAccountIndexes(sb),
		),
	}
}

As we can see here what we do, for now, is the same thing as we did for collections.Map. We pass it the SchemaBuilder, the Prefix where we plan to store the mapping between sdk.AccAddress and authtypes.BaseAccount, the human name and the respective sdk.AccAddress key codec and authtypes.BaseAccount value codec.

Then we pass the instantiation of our AccountIndexes through NewAccountIndexes.

Full example:

package docs

import (
	"cosmossdk.io/collections"
	"cosmossdk.io/collections/indexes"
	storetypes "cosmossdk.io/store/types"
	"github.com/cosmos/cosmos-sdk/codec"
	sdk "github.com/cosmos/cosmos-sdk/types"
	authtypes "github.com/cosmos/cosmos-sdk/x/auth/types"
)

var AccountsNumberIndexPrefix = collections.NewPrefix(1)

type AccountsIndexes struct {
	Number *indexes.Unique[uint64, sdk.AccAddress, authtypes.BaseAccount]
}

func (a AccountsIndexes) IndexesList() []collections.Index[sdk.AccAddress, authtypes.BaseAccount] {
	return []collections.Index[sdk.AccAddress, authtypes.BaseAccount]{a.Number}
}

func NewAccountIndexes(sb *collections.SchemaBuilder) AccountsIndexes {
	return AccountsIndexes{
		Number: indexes.NewUnique(
			sb, AccountsNumberIndexPrefix, "accounts_by_number",
			collections.Uint64Key, sdk.AccAddressKey,
			func(_ sdk.AccAddress, v authtypes.BaseAccount) (uint64, error) {
				return v.AccountNumber, nil
			},
		),
	}
}

var AccountsPrefix = collections.NewPrefix(0)

type Keeper struct {
	Schema   collections.Schema
	Accounts *collections.IndexedMap[sdk.AccAddress, authtypes.BaseAccount, AccountsIndexes]
}

func NewKeeper(storeKey *storetypes.KVStoreKey, cdc codec.BinaryCodec) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
	return Keeper{
		Accounts: collections.NewIndexedMap(
			sb, AccountsPrefix, "accounts",
			sdk.AccAddressKey, codec.CollValue[authtypes.BaseAccount](cdc),
			NewAccountIndexes(sb),
		),
	}
}

Working with IndexedMaps

Whilst instantiating collections.IndexedMap is tedious, working with them is extremely smooth.

Let's take the full example, and expand it with some use-cases.

package docs

import (
	"cosmossdk.io/collections"
	"cosmossdk.io/collections/indexes"
	storetypes "cosmossdk.io/store/types"
	"github.com/cosmos/cosmos-sdk/codec"
	sdk "github.com/cosmos/cosmos-sdk/types"
	authtypes "github.com/cosmos/cosmos-sdk/x/auth/types"
)

var AccountsNumberIndexPrefix = collections.NewPrefix(1)

type AccountsIndexes struct {
	Number *indexes.Unique[uint64, sdk.AccAddress, authtypes.BaseAccount]
}

func (a AccountsIndexes) IndexesList() []collections.Index[sdk.AccAddress, authtypes.BaseAccount] {
	return []collections.Index[sdk.AccAddress, authtypes.BaseAccount]{a.Number}
}

func NewAccountIndexes(sb *collections.SchemaBuilder) AccountsIndexes {
	return AccountsIndexes{
		Number: indexes.NewUnique(
			sb, AccountsNumberIndexPrefix, "accounts_by_number",
			collections.Uint64Key, sdk.AccAddressKey,
			func(_ sdk.AccAddress, v authtypes.BaseAccount) (uint64, error) {
				return v.AccountNumber, nil
			},
		),
	}
}

var AccountsPrefix = collections.NewPrefix(0)

type Keeper struct {
	Schema   collections.Schema
	Accounts *collections.IndexedMap[sdk.AccAddress, authtypes.BaseAccount, AccountsIndexes]
}

func NewKeeper(storeKey *storetypes.KVStoreKey, cdc codec.BinaryCodec) Keeper {
	sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
	return Keeper{
		Accounts: collections.NewIndexedMap(
			sb, AccountsPrefix, "accounts",
			sdk.AccAddressKey, codec.CollValue[authtypes.BaseAccount](cdc),
			NewAccountIndexes(sb),
		),
	}
}

func (k Keeper) CreateAccount(ctx sdk.Context, addr sdk.AccAddress) error {
	nextAccountNumber := k.getNextAccountNumber()
	
	newAcc := authtypes.BaseAccount{
		AccountNumber: nextAccountNumber,
		Sequence:      0,
	}
	
	return k.Accounts.Set(ctx, addr, newAcc)
}

func (k Keeper) RemoveAccount(ctx sdk.Context, addr sdk.AccAddress) error {
	return k.Accounts.Remove(ctx, addr)
} 

func (k Keeper) GetAccountByNumber(ctx sdk.Context, accNumber uint64) (sdk.AccAddress, authtypes.BaseAccount, error) {
	accAddress, err := k.Accounts.Indexes.Number.MatchExact(ctx, accNumber)
	if err != nil {
		return nil, authtypes.BaseAccount{}, err
	}
	
	acc, err := k.Accounts.Get(ctx, accAddress)
	return accAddress, acc, nil
}

func (k Keeper) GetAccountsByNumber(ctx sdk.Context, startAccNum, endAccNum uint64) ([]authtypes.BaseAccount, error) {
	rng := new(collections.Range[uint64]).
		StartInclusive(startAccNum).
		EndInclusive(endAccNum)
	
	iter, err := k.Accounts.Indexes.Number.Iterate(ctx, rng)
	if err != nil {
		return nil, err
	}
	
	return indexes.CollectValues(ctx, k.Accounts, iter)
}


func (k Keeper) getNextAccountNumber() uint64 {
	return 0
}

Collections with interfaces as values

Although cosmos-sdk is shifting away from the usage of interface registry, there are still some places where it is used. In order to support old code, we have to support collections with interface values.

The generic codec.CollValue is not able to handle interface values, so we need to use a special type codec.CollValueInterface. codec.CollValueInterface takes a codec.BinaryCodec as an argument, and uses it to marshal and unmarshal values as interfaces. The codec.CollValueInterface lives in the codec package, whose import path is github.com/cosmos/cosmos-sdk/codec.

Instantiating Collections with interface values

In order to instantiate a collection with interface values, we need to use codec.CollValueInterface instead of codec.CollValue.

package example

import (
    "cosmossdk.io/collections"
    storetypes "cosmossdk.io/store/types"
    "github.com/cosmos/cosmos-sdk/codec"
    sdk "github.com/cosmos/cosmos-sdk/types"
	authtypes "github.com/cosmos/cosmos-sdk/x/auth/types"
)

var AccountsPrefix = collections.NewPrefix(0)

type Keeper struct {
    Schema   collections.Schema
    Accounts *collections.Map[sdk.AccAddress, sdk.AccountI]
}

func NewKeeper(cdc codec.BinaryCodec, storeKey *storetypes.KVStoreKey) Keeper {
    sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
    return Keeper{
        Accounts: collections.NewMap(
            sb, AccountsPrefix, "accounts",
            sdk.AccAddressKey, codec.CollInterfaceValue[sdk.AccountI](cdc),
        ),
    }
}

func (k Keeper) SaveBaseAccount(ctx sdk.Context, account authtypes.BaseAccount) error {
    return k.Accounts.Set(ctx, account.GetAddress(), account)
}

func (k Keeper) SaveModuleAccount(ctx sdk.Context, account authtypes.ModuleAccount) error {
    return k.Accounts.Set(ctx, account.GetAddress(), account)
}

func (k Keeper) GetAccount(ctx sdk.context, addr sdk.AccAddress) (sdk.AccountI, error) {
    return k.Accounts.Get(ctx, addr)
}

Triple key

The collections.Triple is a special type of key composed of three keys, it's identical to collections.Pair.

Let's see an example.

package example

import (
 "context"

 "cosmossdk.io/collections"
 storetypes "cosmossdk.io/store/types"
 "github.com/cosmos/cosmos-sdk/codec"
)

type AccAddress = string
type ValAddress = string

type Keeper struct {
 // let's simulate we have redelegations which are stored as a triple key composed of
 // the delegator, the source validator and the destination validator.
 Redelegations collections.KeySet[collections.Triple[AccAddress, ValAddress, ValAddress]]
}

func NewKeeper(storeKey *storetypes.KVStoreKey) Keeper {
 sb := collections.NewSchemaBuilder(sdk.OpenKVStore(storeKey))
 return Keeper{
  Redelegations: collections.NewKeySet(sb, collections.NewPrefix(0), "redelegations", collections.TripleKeyCodec(collections.StringKey, collections.StringKey, collections.StringKey)
 }
}

// RedelegationsByDelegator iterates over all the redelegations of a given delegator and calls onResult providing
// each redelegation from source validator towards the destination validator.
func (k Keeper) RedelegationsByDelegator(ctx context.Context, delegator AccAddress, onResult func(src, dst ValAddress) (stop bool, err error)) error {
 rng := collections.NewPrefixedTripleRange[AccAddress, ValAddress, ValAddress](delegator)
 return k.Redelegations.Walk(ctx, rng, func(key collections.Triple[AccAddress, ValAddress, ValAddress]) (stop bool, err error) {
  return onResult(key.K2(), key.K3())
 })
}

// RedelegationsByDelegatorAndValidator iterates over all the redelegations of a given delegator and its source validator and calls onResult for each
// destination validator.
func (k Keeper) RedelegationsByDelegatorAndValidator(ctx context.Context, delegator AccAddress, validator ValAddress, onResult func(dst ValAddress) (stop bool, err error)) error {
 rng := collections.NewSuperPrefixedTripleRange[AccAddress, ValAddress, ValAddress](delegator, validator)
 return k.Redelegations.Walk(ctx, rng, func(key collections.Triple[AccAddress, ValAddress, ValAddress]) (stop bool, err error) {
  return onResult(key.K3())
 })
}

Advanced Usages

Alternative Value Codec

The codec.AltValueCodec allows a collection to decode values using a different codec than the one used to encode them. Basically it enables to decode two different byte representations of the same concrete value. It can be used to lazily migrate values from one bytes representation to another, as long as the new representation is not able to decode the old one.

A concrete example can be found in x/bank where the balance was initially stored as Coin and then migrated to Int.

var BankBalanceValueCodec = codec.NewAltValueCodec(sdk.IntValue, func(b []byte) (sdk.Int, error) {
    coin := sdk.Coin{}
    err := coin.Unmarshal(b)
    if err != nil {
        return sdk.Int{}, err
    }
    return coin.Amount, nil
})

The above example shows how to create an AltValueCodec that can decode both sdk.Int and sdk.Coin values. The provided decoder function will be used as a fallback in case the default decoder fails. When the value will be encoded back into state it will use the default encoder. This allows to lazily migrate values to a new bytes representation.