Persisting Data

The persister module lets you save and load Store data to and from different locations, or underlying storage types.

This is useful for preserving Store data between browser sessions or reloads, saving or loading browser state to or from a server, or saving Store data to disk in a environment with filesystem access.

Types Of Persisters

Creating a Persister for a Store depends on the choice of underlying storage where the data is to be stored. Options include:

The createSessionPersister function, which returns a Persister that uses the browser's session storage.
The createLocalPersister function, which returns a Persister that uses the browser's local storage.
The createRemotePersister function, which returns a Persister that uses a remote server.
The createFilePersister function, which returns a Persister that uses a local file (in an appropriate environment).

There is also a way to developer custom Persisters of your own, which we describe in the Custom Persistence guide.

`Persister` Operations

A Persister lets you explicit save or load data, with the save method and the load method respectively. These methods are both asynchronous (since the underlying data storage may also be) and return promises. As a result you should use the await keyword to call them in a way that guarantees subsequent execution order.

In this example, a Persister saves data to, and loads it from, the browser's session storage:

const store = createStore()
  .setValues({employees: 3})
  .setTables({pets: {fido: {species: 'dog'}}});
const persister = createSessionPersister(store, 'petStore');

await persister.save();
console.log(sessionStorage.getItem('petStore'));
// -> '[{"pets":{"fido":{"species":"dog"}}},{"employees":3}]'

sessionStorage.setItem(
  'petStore',
  '[{"pets":{"toto":{"species":"dog"}}},{"employees":4}]',
);
await persister.load();
console.log(store.getTables());
// -> {pets: {toto: {species: 'dog'}}}
console.log(store.getValues());
// -> {employees: 4}

sessionStorage.clear();

Automatic Loading and Saving

When you don't want to deal with explicit persistence operations, a Persister object also provides automatic saving and loading. Automatic saving listens for changes to the Store and persists the data immediately. Automatic loading listens (or polls) for changes to the persisted data and reflects those changes in the Store.

You can start automatic saving or loading with the startAutoSave method and startAutoLoad method. Both are asynchronous since they will do an immediate save and load before starting to listen for subsequent changes. You can stop the behavior with the stopAutoSave method and stopAutoLoad method (which are synchronous).

In this example, both automatic loading and saving are configured:

await persister.startAutoLoad({pets: {fido: {species: 'dog'}}});
await persister.startAutoSave();

store.delValues().setTables({pets: {felix: {species: 'cat'}}});
// ...
console.log(sessionStorage.getItem('petStore'));
// -> '[{"pets":{"felix":{"species":"cat"}}},{}]'

sessionStorage.setItem('petStore', '[{"pets":{"toto":{"species":"dog"}}},{}]');
// -> StorageEvent('storage', {storageArea: sessionStorage, key: 'petStore'})
// ...
console.log(store.getTables());
// -> {pets: {toto: {species: "dog"}}}

persister.destroy();
sessionStorage.clear();

Note that the startAutoLoad method also takes a default set of Tables so that the Store can be instantiated with good data if the persistence layer is empty (such as when this is the first time the app has been executed).

A Caveat

You may often want to have both automatic saving and loading of a Store so that changes are constantly synchronized (allowing basic state preservation between browser tabs, for example). The framework has some basic provisions to prevent race conditions - for example it will not attempt to save data if it is currently loading it and vice-versa.

Be aware, however, that the default implementations do not provide complex synchronization heuristics and you should comprehensively test your persistence strategy to understand the opportunity for data loss (in the case of trying to save data to a server under poor network conditions, for example).

Summary