Persister

A Persister object lets you save and load Store data to and from different locations, or underlying storage types.

This is useful for preserving Store or MergeableStore 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.

Creating a Persister depends on the choice of underlying storage where the data is to be stored. Options include the createSessionPersister function, the createLocalPersister function, the createRemotePersister function, and the createFilePersister function, as just simple examples. The createCustomPersister function can also be used to easily create a fully customized way to save and load Store data.

Using the values of the Persists enum, the generic parameter to the Persister indicates whether it can handle a regular Store, a MergeableStore, or either. Consult the table in the overall persisters module documentation to see current support for each. The different levels of support are also described for each of the types of Persister themselves.

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.

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).

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 - and will sequentially schedule methods that could cause race conditions.

That said, be aware that you should always 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).

To help debug such issues, since v4.0.4, the create methods for all Persister objects take an optional onIgnoredError argument. This is a handler for the errors that the Persister would otherwise ignore when trying to save or load data (such as when handling corrupted stored data). It's recommended you use this for debugging persistence issues, but only in a development environment. Database-based Persister objects also take an optional onSqlCommand argument for logging commands and queries made to the underlying database.

import {createSessionPersister} from 'tinybase/persisters/persister-browser';
import {createStore} from 'tinybase';

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

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

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

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

import {createSessionPersister} from 'tinybase/persisters/persister-browser';
import {createStore} from 'tinybase';

const store = createStore();
const persister = createSessionPersister(store, 'pets');

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

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

// In another browser tab:
sessionStorage.setItem('pets', '[{"pets":{"toto":{"species":"dog"}}},{}]');
// -> StorageEvent('storage', {storageArea: sessionStorage, key: 'pets'})

// ...
console.log(store.getTables());
// -> {pets: {toto: {species: 'dog'}}}

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