`filewriter`: selog blocks

从 filewriter 入口开始，跟踪 block 更新如何变成 .nxs 输出。将所需的 raw_data_1/selog/ 路径与当前 writer 进行比较，包括初始更新所需的历史查找。完成的标准是：每个 block 都写入所有列出的字段，并解决 string-log 表示方式的决定。

The filewriter must be able to write the selog data.

For each block, this includes:

• raw_data_1/selog/<block>/read_control (was a LabVIEW VI control name; not populated by current system; could be populated as EPICS PV name)
• raw_data_1/selog/<block>/set_control (was a LabVIEW VI control name; not populated by current system)
• raw_data_1/selog/<block>/setpoint
• raw_data_1/selog/<block>/setpoint_spread
• raw_data_1/selog/<block>/value
• raw_data_1/selog/<block>/value_log/alarm_severity
• raw_data_1/selog/<block>/value_log/alarm_status
• raw_data_1/selog/<block>/value_log/alarm_time
• raw_data_1/selog/<block>/value_log/name
• raw_data_1/selog/<block>/value_log/time
• raw_data_1/selog/<block>/value_log/value
• raw_data_1/selog/<block>/value_log/value_valid
• raw_data_1/selog/<block>/value_spread
• raw_data_1/selog/<block>/vi_name (was a LabVIEW VI name; not populated by current system)

[!IMPORTANT]
When implementing, we must ensure that the filewriter 'goes back' in time to find the most recent update for each block and log that as the initial update. Otherwise, values will be missing in the .nxs until the block happens to update or the forwarder happens to schedule one of it's periodic updates. For short runs and blocks that don't update much, it is possible that may never happen.

Questions

The ISISICP currently writes strings in a way that is not nexus-compliant. Do we want to continue to do that?

There is a new NXtextlog nexus class for storing string-logs, and as ESS vs00 schema for emitting string-type updates through Kafka.

Potential differences from existing files

• The update frequency of blocks logged here may be different than old system; streaming system will see and be able to log all block updates (unless we choose to implement downsampling)