There are few things I enjoy more than seeing the light go on in someone’s mind, and few better places to see it than at an OPC Training Workshop. A student had asked a very good question, and a few days later, when visiting a customer site, the same question came up again. So – here I am, blogging about it.
The question asked both times was this: how do I access the data files/tables on your HDA Historian?
Some of you are probably chuckling under your breath. But these are very smart people. Experts with a wide variety of technology. So why the disconnect? OPC-HDA wasn’t something they were familiar with. So if you don’t think that’s a funny question, here are two answers for you: short and long.
Short answer: You don’t. You don’t _need_ to. That’s the point. Any HDA tool will get you all the access to the archived data you need.
Long answer: You don’t. You don’t _need_ to. The whole point of the OPC HDA standard is to remove the need to deeply understand how the data is archived, accessed or organized. I can store my data in a single file, or fifty-thousand. I can store them as csv, Excel, or DAT. I can store them with an advanced configuration database with advanced lookups, or in one giant long, insanely over-indexed table. I can store the data how ever I want. HDA makes no restriction about which mechanism I should choose. The OPC HDA Server’s job is to make that invisible to you, whichever archive you choose.
That’s the theory anyway: in reality, not all HDA Servers or archives are created equal.
Archive structures are defined by good engineering, design constraints, and project requirements. Choosing poorly can have implications related to security, reliability, auditability and availability of the data. Ultimately, whichever archive you or the vendor choose, the HDA Server needs to understand the database schema, or the file structure. The HDA Server is responsible for figuring out which files to open, how to structure the query, and for packaging them up so any OPC HDA Client can understand the answer. The HDA Server is almost always separate from the archive – just as the OPC DA server is almost always separate from the device.
Like DA, HDA frees you to pick and choose whatever tool to access the data from your historian. That’s how OPC Excel Reporter and OPC Easy Trender work with any historian on the market, and how OPC Desktop Historian works with any HDA tool.
So instead of data-base schemas and file structures, gain knowledge about your processes and equipment performance, from a wider variety of data sources, with a greater selection of tools. That’s the power of OPC.