This plugin(org.simantics.history) contains a library for history service.

== Details ==

HistoryManager manages persistent and stateful objects called "Subscription".
Subscription has globally unique id (GUID).

When a subscription is recorded, it is opened and a handle is received. 
To record the user supplies time code and values for each variable. 
  
A subscription consists of items that describe how a variable is to be recorded
and stored in a file. For each item: interval, deadband, variableId, enabled, 
and the sample format are provided.  

Sample format describes the format how the data is stored in the stream file, the
primitive types and fields to record. There is a well-known set of 
fields that the recording supports. Unknown fields are left unchanged. 
Fields "time", "endTime", and "value" are mandatory.

Sample = {
   // Time
   time         : Double,      // Time of the first sample (mandatory field) 
   endTime      : Double,      // Time of the last sample (mandatory field)

   // Values
   value        : Double,      // First value (mandatory field)  
   lastValue    : Double,      // Last value 
   avg          : Double,      // Avg value of source valus within the band's time range
   median       : Double,      // Median value
   min          : Double, 
   max          : Double,
   
   quality      : Byte,        // 0-Good, -1=Novalue
   count        : Integer      // The number of source values acquired 
}

One variable is typically subscribed with multiple items. For instance, simantics
chart subscribes the chart_raw, chart_1s, chart_10s, chart_60s, and chart_min-max 
items. 1s, 10s, and 60s have corresponding interval value (eg. max. 1 entry per 10s of data). 
"min-max" is an subscription item that tracks down the minimum and maximum
value of the variable. It has infinitely long interval value and thus records
only one sample (unless there is discontinuation). 
 
The sample entry is basically a band of values. The entry is packed even if 
deadband and interval are disabled. The system packs the unchanged set of values 
into a single entry. 

Simple = {
   time         : Double, 
   endTime      : Double,
   value        : Double
}

The system can write down any primitive fields. Even arrays, enums, records
and strings. Time and endTime fields must be numeric, values not.

Example1 = {
   time         : Long, 
   endTime      : Long,
   value        : Double,
   avg          : Float
}

VectorSample = {
   time         : Long, 
   endTime      : Long,
   value        : Double[ 3 ]
}


Discontinuation
--------------- 

The assumption is that two consecutive stream entries describe a data that is sampled 
continuously from the source. If there is non-continuation in the source data or
if the recording was disabled temporarily, a discontinuation marker is added. 

To support discontinuation in the data stream, there must be quality-field.

Example = {
   time         : Long, 
   endTime      : Long,
   value        : Double,
   quality      : Byte    // 0 Good, -1 No value
}


File History
=================

File based implementation is the only existing implementation. Files are 
managed in workarea (folder). Subscription state and recording metadata is in one 
file, and all subscription items each in a separate file. 

 Subscription        -    [SubscriptionId].dbb
 SubscriptionItem    -    [SubscriptionId]-VariableId-[sampling hash hex]-[sampling name].stm

When a subscription is created, it prepares all related files. If a subscription
is modified, it reflects to files by deleting and creating new. 

An open subscription recording can be closed and opened, the state data is persisted 
when the handle is closed. 
