18.6 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 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 the table definition in the global data dictionary. There are no 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 3072 bytes as of MySQL 8.0.27. Prior to 8.0.27, the maximum key length is 1000 bytes.

The BLACKHOLE storage engine does not support partitioning.

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 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 replication source server a “dummy” replica process whose default storage engine is BLACKHOLE, depicted as follows:

Figure 18.1 Replication using BLACKHOLE for Filtering

The source writes to its binary log. The “dummy” 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 Section 19.1.6, “Replication and Binary Logging Options and Variables”.) 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 mysqld process on the replication source server. This type of setup can be repeated with additional replicas.

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:

• 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 field state. This has important implications in replication.

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

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

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

3. 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 Section 19.5.1.9, “Replication with Differing Table Definitions on Source and Replica”.

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:

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

2. 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;