The 'BLACKHOLE' storage engine acts as a 'black hole' that accepts data but throws it away and does not store it. Retrievals always return an empty result:
mysql> CREATE TABLE test(i INT, c CHAR(10)) ENGINE = BLACKHOLE;
Query OK, 0 rows affected (0.03 sec)
mysql> INSERT INTO test VALUES(1,'record one'),(2,'record two');
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> SELECT * FROM test;
Empty set (0.00 sec)To enable the 'BLACKHOLE' storage engine if you build MySQL from source, invoke 'CMake' with the '-DWITH_BLACKHOLE_STORAGE_ENGINE' option.
To examine the source for the 'BLACKHOLE' engine, look in the 'sql' directory of a MySQL source distribution.
When you create a 'BLACKHOLE' table, the server creates a table format file in the database directory. The file begins with the table name and has an '.frm' extension. There are no other files associated with the table.
The 'BLACKHOLE' storage engine supports all kinds of indexes. That is, you can include index declarations in the table definition.
The maximum key length is 1000 bytes.
You can check whether the 'BLACKHOLE' storage engine is available with the *note 'SHOW ENGINES': show-engines. statement.
Inserts into a 'BLACKHOLE' table do not store any data, but if statement based binary logging is enabled, the SQL statements are logged and replicated to replica servers. This can be useful as a repeater or filter mechanism.
Suppose that your application requires replica-side filtering rules, but transferring all binary log data to the replica first results in too much traffic. In such a case, it is possible to set up on the source host a 'dummy' replica process whose default storage engine is 'BLACKHOLE', depicted as follows:
FIGURE GOES HERE: Replication using BLACKHOLE for Filtering
The source writes to its binary log. The 'dummy' note 'mysqld': mysqld. process acts as a replica, applying the desired combination of 'replicate-do-' and 'replicate-ignore-' rules, and writes a new, filtered binary log of its own. (See note replication-options::.) This filtered log is provided to the replica.
The dummy process does not actually store any data, so there is little processing overhead incurred by running the additional *note 'mysqld': mysqld. process on the replication source host. This type of setup can be repeated with additional replication replicas.
note 'INSERT': insert. triggers for 'BLACKHOLE' tables work as expected. However, because the 'BLACKHOLE' table does not actually store any data, note 'UPDATE': update. and *note 'DELETE': delete. triggers are not activated: The 'FOR EACH ROW' clause in the trigger definition does not apply because there are no rows.
Other possible uses for the 'BLACKHOLE' storage engine include:
Verification of dump file syntax.
Measurement of the overhead from binary logging, by comparing performance using 'BLACKHOLE' with and without binary logging enabled.
'BLACKHOLE' is essentially a 'no-op' storage engine, so it could be used for finding performance bottlenecks not related to the storage engine itself.
The 'BLACKHOLE' engine is transaction-aware, in the sense that committed transactions are written to the binary log and rolled-back transactions are not.
Blackhole Engine and Auto Increment Columns
The Blackhole engine is a no-op engine. Any operations performed on a table using BLACKHOLE have no effect. This should be borne in mind when considering the behavior of primary key columns that auto increment. The engine does not automatically increment field values, and does not retain auto increment column state. This has important implications in replication.
Consider the following replication scenario where all three of the following conditions apply:
On a source server there is a blackhole table with an auto increment field that is a primary key.
On a replica the same table exists but using the MyISAM engine.
Inserts are performed into the source's table without explicitly setting the auto increment value in the 'INSERT' statement itself or through using a 'SET INSERT_ID' statement.
In this scenario, replication fails with a duplicate entry error on the primary key column.
In statement based replication, the value of 'INSERT_ID' in the context event is always the same. Replication therefore fails due to trying insert a row with a duplicate value for a primary key column.
In row based replication, the value that the engine returns for the row always be the same for each insert. This results in the replica attempting to replay two insert log entries using the same value for the primary key column, and so replication fails.
Column Filtering
When using row-based replication, ('binlog_format=ROW'), a replica where the last columns are missing from a table is supported, as described in the section *note replication-features-differing-tables::.
This filtering works on the replica side, that is, the columns are copied to the replica before they are filtered out. There are at least two cases where it is not desirable to copy the columns to the replica:
If the data is confidential, so the replica server should not have access to it.
If the source has many replicas, filtering before sending to the replicas may reduce network traffic.
Source column filtering can be achieved using the 'BLACKHOLE' engine. This is carried out in a way similar to how source table filtering is achieved - by using the 'BLACKHOLE' engine and the '--replicate-do-table' or '--replicate-ignore-table' option.
The setup for the source is:
CREATE TABLE t1 (public_col_1, ..., public_col_N,
secret_col_1, ..., secret_col_M) ENGINE=MyISAM;The setup for the trusted replica is:
CREATE TABLE t1 (public_col_1, ..., public_col_N) ENGINE=BLACKHOLE;The setup for the untrusted replica is:
CREATE TABLE t1 (public_col_1, ..., public_col_N) ENGINE=MyISAM;File: manual.info.tmp, Node: merge-storage-engine, Next: federated-storage-engine, Prev: blackhole-storage-engine, Up: storage-engines