MySQL Reference Manual :: 14.6.2 Description of the FEDERATED Storage Engine

When you create a FEDERATED table, the server creates a table definition 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 database. 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 records are stored in a format particular to the handler. To read records, the handler must parse data into columns. To write records, 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. This necessitates the use of the MySQL client API to read, delete, update, and insert data. 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 converted from the columns in the SELECT result set to the format that the FEDERATED handler expects.

The basic flow is as follows: