Comparison of Tradition and Live Queries

When you execute a SELECT statement, whether synchronous or asynchronous, you expect the system to return results that are currently present in the database and that match your selection criteria. You also expect a finite result-set and that your query executes within a given time frame.

Live Queries operate in a slightly different way:

• They do not return data as it exists when the query executes.
• They return changes that occur on the database from that moment on that match your selection criteria.
• They continue until you terminate the query or an error occurs.
• They are asynchronous and push-based, (that is, the server sends new results to your callback as soon as they become available).

To make the differences more explicit, you can find a pair of simple examples below.

Standard Queries

The client executes a SELECT statement on the Person class, which returns the current list of persons in the database.

orientdb> SELECT FROM Person

+-------+------------+---------+
| @rid  | name       | surname |
+-------+------------+---------+
| #12:0 | John       | Milton  |
| #12:1 | Hieronymus | Bosch   |
+-------+------------+---------+

From a different client, the user inserts a new entry into the database.

orientdb> INSERT INTO Person SET surname = "Augustus"

By issuing the INSERT statement, the second client adds a new entry to the Person class. However, OrientDB has closed the initial selection query made by the first client.

The first client remains unaware of the insertion until it reissues the SELECT statement.

Live Queries

In this example, the client again queries the Person class, but this time uses a LIVE SELECT statement.

orientdb> LIVE SELECT FROM Person

token: 1234567 // Unique identifier of this live query, needed for unsubscribe.

The immediate result of this query is to issue a unique identifier of the query itself. The statement returns no other data, even if the data is available on the database.

Insertion

From the second client, issue the above INSERT statement.

orientdb> INSERT INTO Person SET surname = "Augustus"

When OrientDB executes this statement, the first client responds with the following content (schematic):

content: {@rid: 12:2, surname: 'Augustus'}
operation: insert

Update

From the second client, issue an UPDATE statement.

orientdb> UPDATE Person SET surname = "Caesar"
          WHERE surname = "Augustus"

When OrientDB executes this statement, the first client responds with the following content (schematic):

content: {@rid: #12:2, surname = "Caesar"}
operation: update

Unsubscribe

At any time you can unsubscribe the first client from the Live Query by issuing the LIVE UNSUBSCRIBE statement with the given token.

orientdb> LIVE UNSUBSCRIBE 1234567

From now on, the Live Query does not return any other results to the client.