In the database, channels that use the Channel Access control-system support can be identified by having their control-system type set to “channel_access”. The Channel Access control-system support stores all samples in a single table with the name channel_access_samples. The columns of this table are described by Table C.1, “Columns of table channels_access_samples”.

The channel_data_id, decimation_level, and bucket_start_time form a composite partition key that identifies the sample bucket. These parameters are passed to the control-system support by the Cassandra PV Archiver server and are simply used “as-is”.

The sample_time is used as the clustering key. This way, it is easily possible to select only those samples from a sample bucket that have a time stamp within a specific interval.

The current_bucket_size is a static column because it obviously is the same for the whole sample bucket. This column is updated by the control-system support each time a sample is added to the sample bucket.

All other columns are used for storing the sample’s data. For each sample, exactly one of these columns has a non-null value. The disabled and disconnected columns are simple boolean columns. If one of them is true, it means that the sample is a marker of the corresponding type. Each column that stores a regular (non-marker) sample uses a user-defined type (UDT) that is only used by that column.

Note

The names of the data columns have intentionally been chosen to be very short. The reason for this is simple: Due to how regular columns are internally handled by Cassandra, the column name is serialized for each row. When there are many rows, a long column name can contribute to the total data size significantly. Most of this overhead is compensated by the compression that is applied to SSTables before storing them on disk. However, the sample bucket size that is limited to about 100 MB is measured before applying the compression. For this reason, longer column names would significantly reduce the number of samples that could be stored in each sample bucket. User-defined types (UDTs) are used for the same reason: When the various fields that are needed to store a sample would be represented as separate columns, the overhead that is caused by the meta-data for each column would increase the total data size significantly. Frozen UDTs, on the other hand, are as efficient as frozen tuples, allowing for the space-efficient storage of sample data while having human-readable names for their fields.

The UDTs that are used by the Channel Access control-system support all share a similar structure. The fields that may be present in these UDTs are listed in Table C.2, “Fields of the user-defined types”.

Not all of these fields are present in each UDT. The value, alarm_severity, and alarm_status fields are the only ones that are present in all UDTs. The std, min, max, and covered_period_fraction fields are only present in the channel_access_aggregated_* UDTs. The precision field is only present in UDTs representing samples of a floating-point type. The units, lower_warning_limit, upper_warning_limit, lower_alarm_limit, upper_alarm_limit, lower_display_limit, upper_display_limit, lower_control_limit, and upper_control_limit fields are only present in UDTs that represent samples of a numeric type. The labels field is only present in the channel_access_array_enum and channel_access_scalar_enum UDTs.

The type of the value field depends on the type of the sample that is represented by the UDT. The same applies to the lower_warning_limit, upper_warning_limit, lower_alarm_limit, upper_alarm_limit, lower_display_limit, upper_display_limit, lower_control_limit, and upper_control_limit fields. The types used for those fields are listed in Table C.3, “Type of UDT fields”.

For aggregated samples, the value field is always of type double because it stores the mean of all source samples. The array types store the value elements in a blob. The reason for this is that Cassandra’s list type comes with an overhead that is significant when representing a large number of elements as it is commonly encountered for Channel Access channels that have array values.

Storing these arrays inside a blob is very efficient because the size occupied by each element is not more than the element’s actual size (e.g. two bytes for a each element of a DBR_SHORT sample). The numbers inside the blob are stored in big endian format, so that when using Java, they can easily be converted back to numbers by interpreting the ByteBuffer representing the blob as a buffer of numbers (e.g. an IntBuffer for samples of type DBR_LONG).

For array samples of type DBR_STRING, the blob stores 40 bytes for each element. These 40 bytes represent the raw value as it has been received from the Channel Access server.

The complete list of Java element and buffer types that correspond to the data stored in the value fields of the array UDTs is given by Table C.4, “Java types corresponding to blobs storing sample values”.