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14.7.1. Description of the FEDERATED Storage Engine

When you create a FEDERATED table, the server creates a table format file in the database directory. The file begins with the table name and has an .frm extension. No other files are created, because the actual data is in a remote table. This differs from the way that storage engines for local tables work.

For local database tables, data files are local. For example, if you create a MyISAM table named users, the MyISAM handler creates a data file named users.MYD. A handler for local tables reads, inserts, deletes, and updates data in local data files, and rows are stored in a format particular to the handler. To read rows, the handler must parse data into columns. To write rows, column values must be converted to the row format used by the handler and written to the local data file.

With the MySQL FEDERATED storage engine, there are no local data files for a table (for example, there is no .MYD file). Instead, a remote database stores the data that normally would be in the table. The local server connects to a remote server, and uses the MySQL client API to read, delete, update, and insert data in the remote table. Data retrieval is initiated via a SELECT * FROM tbl_name SQL statement. To read the result, rows are fetched one at a time by using the mysql_fetch_row() C API function, and then converting the columns in the SELECT result set to the format that the FEDERATED handler expects.

The flow of information is as follows:

  1. SQL calls issued locally

  2. MySQL handler API (data in handler format)

  3. MySQL client API (data converted to SQL calls)

  4. Remote database -> MySQL client API

  5. Convert result sets (if any) to handler format

  6. Handler API -> Result rows or rows-affected count to local


 
 
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