14.14 The BLACKHOLE Storage Engine

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 configure with the --with-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.

You can check whether the BLACKHOLE storage engine is available with the 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 slave servers. This can be useful as a repeater or filter mechanism.

Note

When using the row based format for the binary log, updates and deletes are skipped, and neither logged nor applied. For this reason, you should use STATEMENT for the binary logging format, and not ROW or MIXED.

Suppose that your application requires slave-side filtering rules, but transferring all binary log data to the slave first results in too much traffic. In such a case, it is possible to set up on the master host a dummy slave process whose default storage engine is BLACKHOLE, depicted as follows:

Replication using BLACKHOLE for filtering

The master writes to its binary log. The dummy mysqld process acts as a slave, applying the desired combination of replicate-do-* and replicate-ignore-* rules, and writes a new, filtered binary log of its own. (See Section 16.1.3, “Replication and Binary Logging Options and Variables”.) This filtered log is provided to the slave.

The dummy process does not actually store any data, so there is little processing overhead incurred by running the additional mysqld process on the replication master host. This type of setup can be repeated with additional replication slaves.

INSERT triggers for BLACKHOLE tables work as expected. However, because the BLACKHOLE table does not actually store any data, UPDATE and 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:

The BLACKHOLE storage engine does not support INSERT DELAYED, LOCK TABLES, or UNLOCK TABLES statements prior to MySQL 5.1.19.

As of MySQL 5.1.4, 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 will have no effect. This should be born in mind when considering the behavior of primary key columns that auto increment. The engine will not automatically increment field values, and does not retain auto increment field state. This has important implications in replication.

Consider the following replication scenario where all three of the following conditions apply:

  1. On a master server there is a blackhole table with an auto increment field that is a primary key.

  2. On a slave the same table exists but using the MyISAM engine.

  3. Inserts are performed into the master'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 will fail with a duplicate entry error on the primary key column.

In statement based replication, the value of INSERT_ID in the context event will always be the same. Replication will therefore fail 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 will result in the slave attempting to replay two insert log entries using the same value for the primary key column, and so replication will fail.

Column Filtering

When using row-based replication, (binlog_format=ROW), a slave where the last columns are missing from a table is supported, as described in the section Section 16.4.1.9, “Replication with Differing Table Definitions on Master and Slave”.

This filtering works on the slave side, that is, the columns are copied to the slave before they are filtered out. There are at least two cases where it is not desirable to copy the columns to the slave:

  1. If the data is confidential, so the slave server should not have access to it.

  2. If the master has many slaves, filtering before sending to the slaves may reduce network traffic.

Master column filtering can be achieved using the BLACKHOLE engine. This is carried out in a way similar to how master table filtering is achieved - by using the BLACKHOLE engine and the --replicate-do-table or --replicate-ignore-table option.

The setup for the master is:

CREATE TABLE t1 (public_col_1, ..., public_col_N,
                 secret_col_1, ..., secret_col_M) ENGINE=MyISAM;

The setup for the trusted slave is:

CREATE TABLE t1 (public_col_1, ..., public_col_N) ENGINE=BLACKHOLE;

The setup for the untrusted slave is:

CREATE TABLE t1 (public_col_1, ..., public_col_N) ENGINE=MyISAM;