Cyclic retrieval

Cyclic retrieval is the retrieval of stored data for the given time period based on a specified cyclic retrieval resolution, regardless of whether or not the value of the tag(s) has changed. It works with all types of tags. Cyclic retrieval produces a virtual rowset, which may or may not correspond to the actual data rows stored on the AVEVA Historian.

In cyclic retrieval, one row is returned for each "cycle boundary." You specify the number of cycles either directly or by means of a time resolution, that is, the spacing of cycle boundaries in time. If you specify a number of cycles, the AVEVA Historian returns that number of rows, evenly spaced in time over the requested period. The cyclic resolution is calculated by dividing the requested time period by the number of cycle boundaries. If you specify a resolution, the number of cycles is calculated by dividing the time period by the resolution.

If no data value is actually stored at a cycle boundary, the last value before the boundary is returned.

Beginning with AVEVA System Platform 2014 R2 SP1, Historian cyclic storage rules improve the handling of "slow rate change" data tags. Instead of delaying posts to the database if tag values do not arrive in a timely manner, new rules define a cyclic timeout when the database will be updated anyway. That timeout is typically one-half the period for the tag’s cycle storage rate or the database server’s maximum cyclic storage timeout, whichever is shorter. The time out is controlled by a the system parameter MaxCyclicStorageTimeout. For more information, see System Parameters in the AVEVA Historian Administration Guide.

The default retrieval mode is cyclic for retrieval from analog tables, including analog and state summary tables.

Cyclic retrieval is fast and therefore consumes little server resources. However, it may not correctly reflect the stored data because important process values (gaps, spikes, etc.) might fall between cycle boundaries. For an alternative, see Best fit retrieval.