is prohibited.
Clients submit read-only and update transactions to the middleware. The middleware
schedules and routes updates and queries to cluster nodes. The scheduler generates a
correct interleaved execution order. The master nodes serve as primary nodes where
all updates will first be executed. In the following discussion, we consider only a
single master node. However, the master node could actually be clustered itself. Its
internal organisation is not of interest to the coordinator. It only needs to know the
serialisation order and maintains a high-level log.
Queries arrive at an input queue. The input queue is not processed in a ???first-in-firstout???
manner. Instead, the scheduler decides in which order to process the incoming
requests (a waiting time limit avoids starvation). In general, there can be several
secondary nodes where a query of a read transaction may execute. The router chooses
one of these nodes for each query. To do so, the coordination middleware maintains
some global system state information, for example, the version of each node.
Transaction.Model
With regard to transactions submitted by clients, that is, client transactions, we
distinguish between read-only (OLAP) transactions and update transactions. A
read-only transaction only consists of queries. An update transaction comprises at
least one insert, delete, or update statement??”shortly referred to as updates??”next
to arbitrarily many further SQL statements.
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