Interactive Debugging of Transaction Conflicts with TokuDB

I am developing a concurrent application that uses TokuDB to store its database. Sometimes, one of my SQL statements returns with a ‘lock wait timeout exceeded’ error. How do I identify the cause of this error? First, I need to understand a little bit about how TokuDB transactions use locks. Then, I need to understand how to use the MySQL information schema to look at the current state of the locks.

Transactions and Locks

TokuDB uses key range locks to implement serializable transactions. These locks are acquired as the transaction progresses. The locks are released when the transaction commits or aborts.

TokuDB stores these locks in a data structure called the lock tree. The lock tree stores the set of range locks granted to each transaction. In addition, the lock tree stores the set of locks that are not granted due to a conflict with locks granted to some other transaction.

TokuDB 7.1 provides access to the set of locks taken and the set of locks that are blocked via tables in the MySQL information schema. In addition, TokuDB stores the last lock conflict information in a session variable that can be retrieved by the application.

Example

Here is an example debug session.

I create a table with a single column that is the primary key.

I have 2 MySQL clients with ID’s 1 and 2 respectively. MySQL client 1 inserts some values into the table ‘t’. TokuDB transaction 51 is created for the insert statement. Since autocommit is off, transaction 51 is still live after the insert statement completes, and I can query the ‘tokudb_locks’ information schema table to see the locks that are held by the transaction.

The keys are currently hex dumped. Someday we will pretty print the keys.
Now we switch to the other MySQL client with ID 2.

The insert gets blocked since there is a conflict on the primary key with value 100.
The granted TokuDB locks are:

The locks that are pending due to a conflict are:

Eventually, the lock for client 2 times out, and we can retrieve a JSON coded document that describes the conflict.

The JSON document encodes the database, table and index where the conflict occurred in the ‘dname’ key. The ‘key’ encodes the conflicting key. The ‘blocking_txnid’ encodes the transaction identifier that is blocking my transaction. I can find the MySQL client that created the blocking transaction via joins of information schema tables.

Now I have information about the two client operations that are conflicting and can hopefully change my application to avoid this conflict.