Design of WebGIS back-end: architecture considerations

I have spent last two years doing a lot of Python development and designing and implementing Web GIS which included ArcGIS Server, geoprocessing services and ArcGIS API for JavaScript (further JS) web client. What I would like to do is to share an idea which I got to like.

If you need to do something, try doing it at the back-end

Imagine you have a JS web application where users will work with some feature services via a web map. They can select multiple features and calculate the sum of the values features have in a field (or fields). Let’s go through alternatives you have now.

  1. Pre-calculate the values you think your users will query and store them in the database.
    This would work fine actually when you know that your users are going to generate reports on a certain fields often and the performance is crucial. It might actually make sense to calculate certain values beforehand and store them. The disadvantage of this is additional storage and that you need to keep the values updated – the calculated field depends on other fields and their values can change. This would imply re-calculating the report field often as a part of the daily or weekly routine depending on the workflow.
  1. Get the feature’s data from the ArcGIS Server feature service and calculate the requested value on-the-fly in the client.
    Unless you are retrieving complex geometry, this operation wouldn’t cost you much. The problem is that the volume of JS code (or TypeScript) will increase and every upcoming modification in the code would imply new release which can be a painful process if you need to compress your code and move things around. Another thing is that if the amount of data you work with is rather large, there is a good chance the web browser might get slow and the performance will degrade significantly.
  1. Use the database server to calculate the values.
    This became my favorite over last years. This approach has multiple advantages.
    First, this operation runs on the database server machine with enough RAM and CPU resources. So you are not limited by the web browser capacity. The database servers are very good at calculating the values: this kind of operation is very inexpensive because in most cases it does not involve use of cursors. You have a privilege to work in transaction which provides a higher level of data integrity (it would be hard to mess up the database since you can roll back).
    Second, you can use SQL. It might not sound as an advantage first, but remember that code is written once, but is read many times. Readability counts. SQL is a clean way of communicating the workflow and the database code (such as stored procedures) is very easy to maintain. Unlike JS, you work with just one database object and don’t really have any dependencies on the system provided that you have a database server of a certain version and privileges required to create and execute stored procedures.
    Finally, allowing the database server do the work for you, you expose a certain procedure to other clients which could work with it. You don’t need to modify the client code and by updating the SQL code at one place, you automatically make it available for all the applications that work with it.

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