184.108.40.206 Limits Relating to Transaction Handling in NDB Cluster
A number of limitations exist in NDB Cluster with regard to the handling of transactions. These include the following:
Transaction isolation level. The
NDBCLUSTERstorage engine supports only the
READ COMMITTEDtransaction isolation level. (
InnoDB, for example, supports
REPEATABLE READ, and
SERIALIZABLE.) You should keep in mind that
READ COMMITTEDon a per-row basis; when a read request arrives at the data node storing the row, what is returned is the last committed version of the row at that time.
Uncommitted data is never returned, but when a transaction modifying a number of rows commits concurrently with a transaction reading the same rows, the transaction performing the read can observe “before” values, “after” values, or both, for different rows among these, due to the fact that a given row read request can be processed either before or after the commit of the other transaction.
To ensure that a given transaction reads only before or after values, you can impose row locks using
SELECT ... LOCK IN SHARE MODE. In such cases, the lock is held until the owning transaction is committed. Using row locks can also cause the following issues:
Increased frequency of lock wait timeout errors, and reduced concurrency
Increased transaction processing overhead due to reads requiring a commit phase
Possibility of exhausting the available number of concurrent locks, which is limited by
READ COMMITTEDfor all reads unless a modifier such as
LOCK IN SHARE MODEor
FOR UPDATEis used.
LOCK IN SHARE MODEcauses shared row locks to be used;
FOR UPDATEcauses exclusive row locks to be used. Unique key reads have their locks upgraded automatically by
NDBto ensure a self-consistent read;
BLOBreads also employ extra locking for consistency.
See Section 220.127.116.11, “NDB Cluster Backup Troubleshooting”, for information on how NDB Cluster's implementation of transaction isolation level can affect backup and restoration of
Transactions and BLOB or TEXT columns.
NDBCLUSTERstores only part of a column value that uses any of MySQL's
TEXTdata types in the table visible to MySQL; the remainder of the
TEXTis stored in a separate internal table that is not accessible to MySQL. This gives rise to two related issues of which you should be aware whenever executing
SELECTstatements on tables that contain columns of these types:
SELECTfrom an NDB Cluster table: If the
READ COMMITTEDtransaction isolation level is converted to a read with read lock. This is done to guarantee consistency.
SELECTwhich uses a unique key lookup to retrieve any columns that use any of the
TEXTdata types and that is executed within a transaction, a shared read lock is held on the table for the duration of the transaction—that is, until the transaction is either committed or aborted.
For example, consider the table
tdefined by the following
CREATE TABLE t ( a INT NOT NULL AUTO_INCREMENT PRIMARY KEY, b INT NOT NULL, c INT NOT NULL, d TEXT, INDEX i(b), UNIQUE KEY u(c) ) ENGINE = NDB,
The following query on
tcauses a shared read lock, because it uses a unique key lookup:
SELECT * FROM t WHERE c = 1;
However, none of the four queries shown here causes a shared read lock:
SELECT * FROM t WHERE b = 1; SELECT * FROM t WHERE d = '1'; SELECT * FROM t; SELECT b,c WHERE a = 1;
This is because, of these four queries, the first uses an index scan, the second and third use table scans, and the fourth, while using a primary key lookup, does not retrieve the value of any
You can help minimize issues with shared read locks by avoiding queries that use unique key lookups that retrieve
TEXTcolumns, or, in cases where such queries are not avoidable, by committing transactions as soon as possible afterward.
Unique key lookups and transaction isolation. Unique indexes are implemented in
NDBusing a hidden index table which is maintained internally. When a user-created
NDBtable is accessed using a unique index, the hidden index table is first read to find the primary key that is then used to read the user-created table. To avoid modification of the index during this double-read operation, the row found in the hidden index table is locked. When a row referenced by a unique index in the user-created
NDBtable is updated, the hidden index table is subject to an exclusive lock by the transaction in which the update is performed. This means that any read operation on the same (user-created)
NDBtable must wait for the update to complete. This is true even when the transaction level of the read operation is
One workaround which can be used to bypass potentially blocking reads is to force the SQL node to ignore the unique index when performing the read. This can be done by using the
IGNORE INDEXindex hint as part of the
SELECTstatement reading the table (see Section 8.9.4, “Index Hints”). Because the MySQL server creates a shadowing ordered index for every unique index created in
NDB, this lets the ordered index be read instead, and avoids unique index access locking. The resulting read is as consistent as a committed read by primary key, returning the last committed value at the time the row is read.
Reading via an ordered index makes less efficient use of resources in the cluster, and may have higher latency.
It is also possible to avoid using the unique index for access by querying for ranges rather than for unique values.
Rollbacks. There are no partial transactions, and no partial rollbacks of transactions. A duplicate key or similar error causes the entire transaction to be rolled back.
This behavior differs from that of other transactional storage engines such as
InnoDBthat may roll back individual statements.
Transactions and memory usage. As noted elsewhere in this chapter, NDB Cluster does not handle large transactions well; it is better to perform a number of small transactions with a few operations each than to attempt a single large transaction containing a great many operations. Among other considerations, large transactions require very large amounts of memory. Because of this, the transactional behavior of a number of MySQL statements is affected as described in the following list:
DELETE FROM(even with no
WHEREclause) is transactional. For tables containing a great many rows, you may find that performance is improved by using several
DELETE FROM ... LIMIT ...statements to “chunk” the delete operation. If your objective is to empty the table, then you may wish to use
Transactions and the COUNT() function. When using NDB Cluster Replication, it is not possible to guarantee the transactional consistency of the
COUNT()function on the replica. In other words, when performing on the source a series of statements (
DELETE, or both) that changes the number of rows in a table within a single transaction, executing
SELECT COUNT(*) FROMqueries on the replica may yield intermediate results. This is due to the fact that
SELECT COUNT(...)may perform dirty reads, and is not a bug in the
NDBstorage engine. (See Bug #31321 for more information.)