15.6 The BLACKHOLE Storage Engine
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
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.
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.
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:
The master writes to its binary log. The “dummy” mysqld process acts as a slave, applying the desired combination of
replicate-ignore-* rules, and writes a new, filtered binary log of its own. (See Section 16.1.6, “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,
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
BLACKHOLEwith and without binary logging enabled.
BLACKHOLEis essentially a “no-op” storage engine, so it could be used for finding performance bottlenecks not related to the storage engine itself.
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:
On a master server there is a blackhole table with an auto increment field that is a primary key.
On a slave the same table exists but using the MyISAM engine.
Inserts are performed into the master's table without explicitly setting the auto increment value in the
INSERTstatement itself or through using a
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.
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 220.127.116.11, “Replication with Differing Table Definitions on Source and Replica”.
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:
If the data is confidential, so the slave server should not have access to it.
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
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;