Read views

A read view is an in-memory snapshot of the entire database that isn’t affected by future data modifications. Read views provide access to database spaces and their indexes and enable you to retrieve data using the same select and pairs operations.

Read views can be used to make complex analytical queries. This reduces the load on the main database and improves RPS for a single Tarantool instance.

To improve memory consumption and performance, Tarantool creates read views using the copy-on-write technique. In this case, duplication of the entire data set is not required: Tarantool duplicates only blocks modified after a read view is created.

Примечание

Tarantool Enterprise Edition supports read views starting from v2.11.0 and enables the ability to work with them using both Lua and C API.

Limitations

Read views have the following limitations:

Only the memtx engine is supported.
Only TREE and HASH indexes are supported.
Pagination is not supported (the after option).

Working with read views

Creating a read view

To create a read view, call the box.read_view.open() function. The snippet below shows how to create a read view with the read_view1 name.

tarantool> read_view1 = box.read_view.open({name = 'read_view1'})

After creating a read view, you can see the information about it by calling read_view_object:info().

tarantool> read_view1:info()
---
- timestamp: 66.606817935
  signature: 24
  is_system: false
  status: open
  vclock: {1: 24}
  name: read_view1
  id: 1
...

To list all the created read views, call the box.read_view.list() function.

Querying data

After creating a read view, you can access database spaces using the read_view_object.space field. This field provides access to a space object that exposes the select, get, and pairs methods with the same behavior as corresponding box.space methods.

The example below shows how to select 4 records from the bands space:

tarantool> read_view1.space.bands:select({}, {limit = 4})
---
- - [1, 'Roxette', 1986]
  - [2, 'Scorpions', 1965]
  - [3, 'Ace of Base', 1987]
  - [4, 'The Beatles', 1960]
...

Similarly, you can retrieve data by the specific index.

tarantool> read_view1.space.bands.index.year:select({}, {limit = 4})
---
- - [4, 'The Beatles', 1960]
  - [2, 'Scorpions', 1965]
  - [1, 'Roxette', 1986]
  - [3, 'Ace of Base', 1987]
...

Closing a read view

When a read view is no longer needed, close it using the read_view_object:close() method because a read view may consume a substantial amount of memory.

tarantool> read_view1:close()
---
...

Otherwise, a read view is closed implicitly when the read view object is collected by the Lua garbage collector.

After the read view is closed, its status is set to closed. On an attempt to use it, an error is raised.

Example

A Tarantool session below demonstrates how to open a read view, get data from this view, and close it. To repeat these steps, you need to bootstrap a Tarantool instance as described in Using data operations (you can skip creating secondary indexes).

Insert test data.

tarantool> bands:insert{1, 'Roxette', 1986}
           bands:insert{2, 'Scorpions', 1965}
           bands:insert{3, 'Ace of Base', 1987}
           bands:insert{4, 'The Beatles', 1960}

Create a read view by calling the open function. Then, make sure that the read view status is open.

tarantool> read_view1 = box.read_view.open({name = 'read_view1'})

tarantool> read_view1.status
---
- open
...

Change data in a database using the delete and update operations.

tarantool> bands:delete(4)
---
- [4, 'The Beatles', 1960]
...
tarantool> bands:update({2}, {{'=', 2, 'Pink Floyd'}})
---
- [2, 'Pink Floyd', 1965]
...

Query a read view to make sure it contains a snapshot of data before a database is updated.

tarantool> read_view1.space.bands:select()
---
- - [1, 'Roxette', 1986]
  - [2, 'Scorpions', 1965]
  - [3, 'Ace of Base', 1987]
  - [4, 'The Beatles', 1960]
...

Close a read view.
```
tarantool> read_view1:close()
---
...
```

Версия:

Read views

Limitations

Working with read views

Creating a read view

Querying data

Closing a read view

Example