Definitions of terms in IndexedDB – Database (HTML5)

22Dec11

database

A repository of information, typically comprising one or more object stores. Each database must have the following:
Name. It identifies the database within a specific origin and stays constant throughout its lifetime. The name can be any string value (including an empty string). Current version. When a database is first created, its version is the integer 1. Each database can have only one version at any given time.
object store
The mechanism by which data is stored in the database. The object store persistently holds records, which are key-value pairs. Records within an object store are sorted according to the keys in an ascending order.

Every object store must have a name that is unique within its database. The object store can optionally have a key generator and a key path. If the object store has a key path, it is using in-line keys; otherwise, it is using out-of-line keys.

For the reference documentation on object store, see IDBObjectStore or IDBObjectStoreSync.

version
When a database is first created, its version is the integer 1. Each database has one version at a time; a database can’t exist in multiple versions at once. The only way to change the version is by opening it with a greater version than the current one. This will start a VERSION_CHANGE transaction and fire an upgradeneeded event. The only place where the schema of the database can be updated is inside the handler of that event.
Note: This definition describes the most recent specifications, which is only implemented in some up-to-date browsers. Old browsers implemented the now deprecated and removed IDBDatabase.setVersion() method.
database connection
An operation created by opening a database. A given database can have multiple connections at the same time.

transaction
An atomic and durable set of data-access and data-modification operations on a particular database. It is how you interact with the data in a database. In fact, any reading or changing of data in the database must happen in a transaction.

A database connection can have several active transaction associated with it at a time, so long as the writing transactions do not have overlapping scopes. The scope of transactions, which is defined at creation, determines which object stores the transaction can interact with and remains constant for the lifetime of the transaction. So, for example, if a database connection already has a writing transaction with a scope that just covers the flyingMonkey object store, you can start a second transaction with a scope of the unicornCentaur and unicornPegasus object stores. As for reading transactions, you can have several of them—even overlapping ones.

Transactions are expected to be short-lived, so the browser can terminate a transaction that takes too long, in order to free up storage resources that the long-running transaction has locked. You can abort the transaction, which rolls back the changes made to the database in the transaction. And you don’t even have to wait for the transaction to start or be active to abort it.

The three modes of transactions are: read/write, read only, and version change. The only way to create and delete object stores and indexes is by using a version-change transaction. To learn more about transaction types, see the reference article for IndexedDB.

Because everything happens within a transaction, it is a very important concept in IndexedDB. To learn more about transactions, especially on how it relates to versioning, see IDBTransaction, which also has reference documentation. For the documentation on the synchronous API, see IDBTransactionSync.

request
The operation by which reading and writing on a database is done. Every request represents one read or write operation.

index
A specialized object store for looking up records in another object store, called the referenced object store. The index is a persistent key-value storage where the value part of its records is the key part of a record in the referenced object store. The records in an index are automatically populated whenever records in the referenced object store are inserted, updated, or deleted. Each record in an index can point to only one record in its referenced object store, but several indexes can reference the same object store. When the object store changes, all indexes that refers to the object store are automatically updated.

Alternatively, you can also look up records in an object store using the key.

To learn more on using indexes, see Using IndexedDB. For the reference documentation on index, see IDBKeyRange.

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