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Design Applications for High Availability
17.8.3 Oracle CQL Query Restrictions
In a high availability application, Oracle CQL queries have the following restrictions.
For more information about Oracle CQL and the topics covered in this section, see
Oracle CQL Language Reference for Oracle Stream Explorer.
17.8.3.1 Range-Based Windows
In a Type 1 application where the application must generate exactly the same sequence
of output events as existing secondaries, all range-based Oracle CQL windows must
be shorter than the warm-up-window-length time. See Choose an Adequate warmup-window-length Time.
Channels must use application time when Oracle CQL queries contain range-based
Windows. See Prefer Application Time .
17.8.3.2 Tuple-Based Windows
In a Type 1 application where the application must generate exactly the same sequence
of output events as existing secondaries, all tuple-based windows must be qualified by
time. See Choose an Adequate warm-up-window-length Time.
17.8.3.3 Partitioned Windows
Avoid partitioned windows because there are situations in which a partition cannot be
rebuilt. If you do use partitioned windows, configure a warm-up-window-length
time long enough to give the Oracle Stream Explorer server time to rebuild the
partition. See Choose an Adequate warm-up-window-length Time.
17.8.3.4 Sliding Windows
Oracle CQL queries should not use sliding windows when new stages that join the
multiserver domain are expected to generate exactly the same output events as
existing stages. See Rejoining the High Availability MultiServer Domain.
17.8.3.5 DURATION Clause and Non-Event Detection
You must use application time when Oracle CQL queries contain a DURATION clause
for non-event detection. See Prefer Application Time .
17.8.3.6 Prefer Application Time
In Oracle Stream Explorer each event is associated with a point in time at which the
event occurred. Oracle CQL recognizes two types of time:
• Application time: A time value assigned to each event outside of Oracle CQL by
the application before the event enters the Oracle CQL processor.
• System time: A time value associated with an event when it arrives at the Oracle
CQL processor, essentially by calling System.nanoTime().
Application time is generally the best approach for applications that need to be highly
available. The application time is associated with an event before the event is sent
downstream, so it is consistent across active primary and secondary servers.
System time can cause application instances to generate different results because the
time value associated with an event can be different on each server due to system
clocks not being synchronized. You can use system time for applications when Oracle
CQL queries do not use time-based windows. Applications that use only event-based
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