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5 |
A store is a data structure that holds the state of the application. |
- Anatomy of an SDK application {prereq}
The Cosmos SDK comes with a large set of stores to persist the state of applications. By default, the main store of SDK applications is a multistore, i.e. a store of stores. Developers can add any number of key-value stores to the multistore, depending on their application needs. The multistore exists to support the modularity of the Cosmos SDK, as it lets each module declare and manage their own subset of the state. Key-value stores in the multistore can only be accessed with a specific capability key, which is typically held in the keeper of the module that declared the store.
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| | KVStore 1 - Manage by keeper of Module 1 |
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| | KVStore 2 - Manage by keeper of Module 2 | |
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| | KVStore 3 - Manage by keeper of Module 2 | |
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| | KVStore 4 - Manage by keeper of Module 3 | |
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| | KVStore 5 - Manage by keeper of Module 4 | |
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| Main Multistore |
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Application's State
At its very core, a Cosmos SDK store is an object that holds a CacheWrapper and implements a GetStoreType() method:
The GetStoreType is a simple method that returns the type of store, whereas a CacheWrapper is a simple interface that specifies cache-wrapping and Write methods:
Cache-wrapping is used ubiquitously in the Cosmos SDK and required to be implemented on every store type. A cache-wrapper creates a light snapshot of a store that can be passed around and updated without affecting the main underlying store. This is used to trigger temporary state-transitions that may be reverted later should an error occur. If a state-transition sequence is performed without issue, the cached store can be comitted to the underlying store at the end of the sequence.
A commit store is a store that has the ability to commit changes made to the underlying tree or db. The Cosmos SDK differentiates simple stores from commit stores by extending the basic store interfaces with a Committer:
The Committer is an interface that defines methods to persist changes to disk:
The CommitID is a deterministic commit of the state tree. Its hash is returned to the underlying consensus engine and stored in the block header. Note that commit store interfaces exist for various purposes, one of which is to make sure not every object can commit the store. As part of the object-capabilities model of the Cosmos SDK, only baseapp should have the ability to commit stores. For example, this is the reason why the ctx.KVStore() method by which modules typically access stores returns a KVStore and not a CommitKVStore.
The Cosmos SDK comes with many types of stores, the most used being CommitMultiStore, KVStore and GasKv store. Other types of stores include Transient and TraceKV stores.
Each Cosmos SDK application holds a multistore at its root to persist its state. The multistore is a store of KVStores that follows the Multistore interface:
If tracing is enabled, then cache-wrapping the multistore will wrap all the underlying KVStore in TraceKv.Store before caching them.
The main type of Multistore used in the Cosmos SDK is CommitMultiStore, which is an extension of the Multistore interface:
As for concrete implementation, the [rootMulti.Store] is the go-to implementation of the CommitMultiStore interface.
The rootMulti.Store is a base-layer multistore built around a db on top of which multiple KVStores can be mounted, and is the default multistore store used in baseapp.
Whenever the rootMulti.Store needs to be cached-wrapped, a cachemulti.Store is used.
cachemulti.Store cache wraps all substores in its constructor and hold them in Store.stores. Store.GetKVStore() returns the store from Store.stores, and Store.Write() recursively calls CacheWrap.Write() on all the substores.
A KVStore is a simple key-value store used to store and retrieve data. A CommitKVStore is a KVStore that also implements a Committer. By default, stores mounted in baseapp's main CommitMultiStore are CommitKVStores. The KVStore interface is primarily used to restrict modules from accessing the committer.
Individual KVStores are used by modules to manage a subset of the global state. KVStores can be accessed by objects that hold a specific key. This key should only be exposed to the keeper of the module that defines the store.
CommitKVStores are declared by proxy of their respective key and mounted on the application's multistore in the main application file. In the same file, the key is also passed to the module's keeper that is responsible for managing the store.
Apart from the traditional Get and Set methods, a KVStore is expected to implement an Iterator() method which returns an Iterator object. The Iterator() method is used to iterate over a domain of keys, typically keys that share a common prefix. Here is a common pattern of using an Iterator that might be found in a module's keeper:
store := ctx.KVStore(keeper.storeKey)
iterator := sdk.KVStorePrefixIterator(store, prefix) // proxy for store.Iterator
defer iterator.Close()
for ; iterator.Valid(); iterator.Next() {
var object types.Object
keeper.cdc.MustUnmarshalBinaryLengthPrefixed(iterator.Value(), &object)
if cb(object) {
break
}
}The default implementation of KVStore and CommitKVStore used in baseapp is the iavl.Store.
iavl stores are based around an IAVL Tree, a self-balancing binary tree which guarantees that:
GetandSetoperations are O(log n), where n is the number of elements in the tree.- Iteration efficiently returns the sorted elements within the range.
- Each tree version is immutable and can be retrieved even after a commit (depending on the pruning settings).
dbadapter.Store is a adapter for dbm.DB making it fulfilling the KVStore interface.
dbadapter.Store embeds dbm.DB, meaning most of the KVStore interface functions are implemented. The other functions (mostly miscellaneous) are manually implemented. This store is primarily used within Transient Stores
Transient.Store is a base-layer KVStore which is automatically discarded at the end of the block.
Transient.Store is a dbadapter.Store with a dbm.NewMemDB(). All KVStore methods are reused. When Store.Commit() is called, a new dbadapter.Store is assigned, discarding previous reference and making it garbage collected.
This type of store is useful to persist information that is only relevant per-block. One example would be to store parameter changes (i.e. a bool set to true if a parameter changed in a block).
Transient stores are typically accessed via the context via the TransientStore() method:
cachekv.Store is a wrapper KVStore which provides buffered writing / cached reading functionalities over the underlying KVStore.
This is the type used whenever an IAVL Store needs to be cache-wrapped (typically when setting value that might be reverted later).
Store.Get() checks Store.cache first in order to find if there is any cached value associated with the key. If the value exists, the function returns it. If not, the function calls Store.parent.Get(), sets the key-value pair to the Store.cache, and returns it.
Store.Set() sets the key-value pair to the Store.cache. cValue has the field dirty bool which indicates whether the cached value is different from the underlying value. When Store.Set() cache new pair, the cValue.dirty is set true so when Store.Write() is called it can be written to the underlying store.
Store.Iterator() have to traverse on both caches items and the original items. In Store.iterator(), two iterators are generated for each of them, and merged. memIterator is essentially a slice of the KVPairs, used for cached items. mergeIterator is a combination of two iterators, where traverse happens ordered on both iterators.
Cosmos SDK applications use gas to track resources usage and prevent spam. GasKv.Store is a KVStore wrapper that enables automatic gas consumption each time a read or write to the store is made. It is the solution of choice to track storage usage in Cosmos SDK applications.
When methods of the parent KVStore are called, GasKv.Store automatically consumes appropriate amount of gas depending on the Store.gasConfig:
By default, all KVStores are wrapped in GasKv.Stores when retrieved. This is done in the KVStore() method of the context:
In this case, the default gas configuration is used:
tracekv.Store is a wrapper KVStore which provides operation tracing functionalities over the underlying KVStore. It is applied automatically by the Cosmos SDK on all KVStore if tracing is enabled on the parent MultiStore.
When each KVStore methods are called, tracekv.Store automatically logs traceOperation to the Store.writer. traceOperation.Metadata is filled with Store.context when it is not nil. TraceContext is a map[string]interface{}.
prefix.Store is a wrapper KVStore which provides automatic key-prefixing functionalities over the underlying KVStore.
When Store.{Get, Set}() is called, the store forwards the call to its parent, with the key prefixed with the Store.prefix.
When Store.Iterator() is called, it does not simply prefix the Store.prefix, since it does not work as intended. In that case, some of the elements are traversed even they are not starting with the prefix.
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