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Table Of Contents

Chapter 31

Using a RowLoader to Load Existing Data

When you use SQLFire as a cache, you can conﬁgure a SQL data loader that is triggered to load data from a backend

repository on a miss in SQLFire.

When an incoming query request for a uniquely-identiﬁed row cannot be satisﬁed by the distributed cache, the loader

is invoked to retrieve the data from an external source. SQLFire locks the associated row and prevents concurrent

readers that are trying to fetch the same row from overloading the backend database.

Note: When a loader is conﬁgured for a table, performing a select statement against the table internally fetches

and inserts data into a SQLFire table. Because of this behavior, it is possible to receive a constraint violation

when you query a table that is conﬁgured with a loader (for example, if the loader attempts to insert data from

a backend database that violates a table constraint in SQLFire).

How SQLFire Invokes a RowLoader

SQLFire only invokes a conﬁgured RowLoader when certain conditions are met.

SQLFire invokes a conﬁgured RowLoader when:

• A SELECT query is executed against a table with a WHERE clause that includes an equality (=) test for every

component that is part of the table's primary key, and

• A row for the speciﬁed primary key does not exist in memory.

Note that it does not matter which columns are requested or used from the resulting rows.

For example, consider the following example table deﬁnition:

CREATE TABLE LoaderTest(id INTEGER, name VARCHAR(10), result

VARCHAR(10), primary key (id, name));

In the "LoaderTest" table, columns "id" and "name" form the primary key. Queries that use an equality test on

the primary key invoke an attached loader implementation for the table, if the data was not already available in

SQLFire. For example, these queries would invoke a loader:

SELECT * FROM LoaderTest WHERE id = 5 AND name='Joe';

SELECT result FROM LoaderTest WHERE id = 6 AND name='Sam';

However, these queries would not invoke an attached loader:

SELECT result FROM LoaderTest WHERE id < 6 AND name='Sam';

SELECT * FROM LoaderTest;

SELECT * FROM LoaderTest WHERE id = 5;

In the above examples, none of the SELECT statements use an equality test for all components of the primary

key (both "id" and "name").

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