The Old Joel on Software Forum: Part 3 (of 5)

2.5-tier

I have seen once somebody mention here 2.5-tier architectures (but I couldn't find the post). What does it mean exactly ?

Pakter
Monday, November 3, 2003

A classic three tier architecture is where the layers are:
Database
Business Logic
User Interface
Each layer only talks to the layer below/above. In practice to get good UI performance sometimes the Business layer is skipped and the UI talks direct to the database. Often this skipping is only done for readonly access. i.e. the transactions are all done three tier but for reports direct access to the database is used.
It's nicknamed 2.5 tier 'cos it ain't three tier and it isn't two tier either.
It's developers admitting to kludging it for speed.

Peter Ibbotson
Monday, November 3, 2003

Peter, do you have a cite for that?

In my experience, "2.5 tier" refers to the idea that you don't have a discrete data tier, but rather that the Business Layer talks to the database via stored procedures and views. In other words, it's not 2-tier (BL pulls data directly) but it's not 3-tier, because there's no "real" data layer.

So it's 2.5 tier.

More importantly, it's why the term "n-tier" came into existence - to sideline all the stupid pedantic arguments designers would get into about whether a design was 2-tier, 2.5 tier, or 3 tier. :-)

Philo

Philo
Monday, November 3, 2003

Nope, I don't have any cites. I've seen it used in the context I was talking about by I think one of the navision/axapta developers in an interview a couple of years ago. I've used it internally here as part of a discussion i.e. we'll try for three tier but if times get tough we can go to a two and half tier model for performance.
I used half because all writes to the database would three layer but some of the reads would be two.
Google isn't very helpful on this one, mind you I can't find any cites for your definition either.
Anyone else got alternative definitions?

Peter Ibbotson
Monday, November 3, 2003

Thanks, the two definitions make sense.
I suppose there is no established standard of what a 2.5-tier architecture is. (As Philo said, even for "integer" tiers it is not always very clear).

Pakter
Tuesday, November 4, 2003