AVEVA Historian - SQL Server Standard CAL license count?

Note that some older releases of Wonderware Historian did include SQL Server Standard licenses and some of the corresponding "Wonderware CALs" did include SQL Server Standard CALs--if you have those products, you might already be covered, so check your licenses. 

This is fundamentally a Microsoft SQL Server licensing question, so you should ultimately refer to Microsoft policies. SQL Server clients aren't licensed for "concurrent" usage, only for "device" or "user". This does create a mismatch in cases where you have concurrent Historian Client users: for example, a 10-concurrent user Historian Client Web license might be used by 25 different people from any of 15 different workstations, so you'd need either "25 SQL Server 'user' CALs" or "15 SQL Server 'device' CALs". Alternatively, you can ignore the clients and license SQL Server based on the "processor cores", which may make sense for higher numbers of users/devices. Also keep in mind that a SQL Server CAL gives access to any number of SQL Servers (not just those related to AVEVA products). 

As for which software requires a SQL Server CAL (when licensing based on CALs), we'd recommend a "device CAL" for the Historian server itself (which covers all the local connections and services), plus licenses for: Remote IDAS nodes, Historian Client (Desktop and/or/Web), and nodes/users with any embedded controls (Trend, Alarm History Grid, Trend Pen, History Summary Custom Property, etc.) and any SQL-based reports accessing the "Runtime" database. 

In addition to standard SQL Server tools (like Profiler), there are some unsupported example SQL queries that might be useful to capture more specific Historian usage data. See the "ClientUsageTracking.sql" and "SummarizeSQLClientLoad.sql" specifically. 

Thank you, Elliott, for the detailed answer.

The customer currently holds the historian license, which does not include the SQL standard license. They are looking to ascertain the SQL CAL license count in order to make a corresponding purchase.

I should have noted this in my original reply, but there is not a strict requirement for SQL Server Standard based on the tag count, though at one time AVEVA did include that in larger Historian licenses. Historian is actually a very lightweight application as far as traditional SQL Server database applications go, and that will easily scale to the 2M tag limit for Historian using SQL Express without any material impact on data collection. 

Where SQL Express becomes more of a limiting factor is SQL query load, which is normally used for Historian Client Trend, AVEVA Reports, and other applications. Internally, a "tier 1" Historian also uses SQL for managing the "sync queue". The biggest benefit for SQL Server Standard will be in applications with a lot of concurrent SQL queries. System tags can give an indication of when an application is bound by SQL Express limitations (see: SysPerfSQLServerCPU, SysPerfSQLServerVirtualMBytes, SysPerfSQLServerPageFaults). The exact limits may vary by SQL Express release,

Since the time-series data isn't actually stored in database tables, applications using the REST interface or the .NET SDK will see minimal differences between Express and Standard, but they will still require a SQL CAL when using SQL Server Standard. 

Thanks Elliott.