Speculative Execution¶

Query Idempotence¶

Speculative execution will result in executing the same query several times. Therefore, it is important that queries are idempotent i.e. a query can be applied multiple times without changing the result beyond the initial application. Queries that are not explicitly marked as idempotent will not be scheduled for speculative executions.

The following types of queries are not idempotent:

• Mutation of counter columns

• Prepending or appending to a list column

• Use of non-idempotent CQL function e.g. now() or uuid()

The driver is unable to determine if a query is idempotent therefore it is up to an application to explicitly mark a statement as being idempotent.

CassStatement* statement = cass_statement_new( "SELECT * FROM table1", 0);

/* Make the statement idempotent */
cass_statement_set_is_idempotent(statement, cass_true);

cass_statement_free(statement);

Enabling speculative execution¶

Speculative execution is enabled by connecting a CassSession with a CassCluster that has a speculative execution policy enabled. The driver currently only supports a constant policy, but may support more in the future.

CassCluster* cluster = cass_cluster_new();

cass_int64_t constant_delay_ms = 500; /* Delay before a new execution is created */
int max_speculative_executions = 2;   /* Number of executions */

cass_cluster_set_constant_speculative_execution_policy(cluster,
                                                       constant_delay_ms,
                                                       max_speculative_executions);

/* ... */

cass_cluster_free(cluster);