In case you missed it, Logic Apps moved to General Availability last week.
At the same time, pricing for Logic Apps gained a new option: Consumption charging i.e. pay by use.
I've been talking to a number of clients about this new pricing model, and every single one has expressed concerns: it's these concerns that I wanted to touch upon in this post. Most of these concerns are based around the fact that these customers feel they can no longer accurately calculate the monthly cost for their Logic Apps in advance.
I also feel that the whole discussion might be a bit overblown, as once you remove the cost of having to pay for servers and server licenses for a traditional on-premises application (not to mention the run and maintain costs), Logic Apps can be significantly cheaper, regardless of which pricing model you use.
Old Pricing Model vs New
Prior to GA, Logic Apps were charged as part of the App Service Plan (ASP) to which they belong: An App Service Plan has a monthly charge (based on the number of compute units used in the plan), but also throttles a Logic App once a certain number of executions are exceeded in a month (the limit changes depending on the type of ASP the Logic App uses).
Effectively the old way was Pay Monthly, the new way is Pay As You Go.
This table outlines the changes:
App Service Plan Model
Static Monthly Charge
Limit on Number of Logic App Executions
Pay per Logic App Execution
I can understand why Microsoft are making the change: consumption pricing favours those that have either a small number of Logic App executions per month (as they only pay for what they use); or who have large numbers of executions per month (and were therefore being throttled as they exceeded the ASP limits).
I'm not sure yet if the ASP-style pricing model will stay: there's no mention of it any more in the Logic Apps pricing page, but you can still (optionally) associate a Logic App with an ASP when you create it.
How to select either pricing model when creating or updating a Logic App
When you create a new Logic App, you used to be able to select an App Service Plan: now this option is no longer available, and all new Logic Apps use the Consumption pricing plan by default.
However, if you have an existing Logic App and you wish to switch the billing model, you can do so via PowerShell here. You can also follow the instructions at the bottom of this blog post here (I suspect this will get surfaced in the Portal if both billing models are kept).
Why the new model can be confusing
Consumption pricing makes sense: one of the benefits of Azure is that you pay for what you use. Instead of paying upfront for an expensive license fee (e.g. SQL Server and BizTalk Server licenses) you can instead pay a smaller amount every month. A lot of businesses prefer this as it helps with cash flow, and reduces capital expenditure.
The main issue with consumption pricing for Logic Apps is that instead of paying for each execution of the Logic App, you're paying for the execution of the actions within that Logic App. And this is the problem, as a Logic App is opaque: when you're designing a solution, you may know how many Logic Apps you'll have, but you may not know exactly how many actions each will contain (or how many pf those actions will be executed) and this makes it difficult to estimate the runtime cost.
Up to now, it's been easy to work out what a Logic App will cost to run. And that's usually one of the first questions from a client: how much will this cost me per month.
But now, it's harder: instead of knowing exactly you have to estimate, and this estimate has to be based not only on how many times a Logic App will execute, but also *what* the Logic App will be doing i.e. if it will be looping, or if actions in an IF branch will execute.
Effect of the consumption model on development and testing
The main concern I (and others) have with the Consumption billing model is the effect it will have on development and testing: developers (and testers) are used to executing their applications with little or no cost (other than maybe the cost of an dev/test server and dev/test licenses).
Take a BizTalk Developer: chances are the BizTalk and SQL Server Licenses they are using came from an MSDN subscription, or they bought the Dev edition. In either case, they will execute their code during the development process without paying any attention to cost.
The same applies to testers.
An argument can be made that the cost per action of a Logic App is so low that this wouldn't be an issue (e.g. a reasonably complex Logic App with 50 actions per execution would cost 2.5p (4c) per execution. But the pennies do add up: imagine a corporate customer with 100 developers: each time those developers execute a Logic App like this, it costs the company £2.50 (US $4.00) – and that's just executing it once.
Microsoft will likely point out that an MSDN Subscription comes with free Azure credit, and that therefore there is no extra cost to execute these Logic Apps as they're covered in this free credit. But this doesn't apply to most of my clients: although the developers have MSDN, the free credit applies only to their MSDN subscription not the corporate subscription where they perform dev and testing. The MSDN subscriptions are usually used for prototyping, as they're not shared amongst multiple developers, unlike the corporate subscriptions.
So to summarise:
Consumption pricing could lead to:
Note: There is questionable value in performing Load/Perf Testing on Logic Apps, as there is little you can do to tune the environment, which is the usual rationale behind Load Testing (especially in a BizTalk environment). However, some level of testing may be required if your Logic App is designed to process multiple messages and there is either a time limit or the messages are connected in some way (e.g. debatching a request and then collating responses).
The solution (in my view) is to keep both billing models:
In addition, it would help if Microsoft provided more examples on monthly cost; provided some way to help calculate the monthly cost under the consumption model; and highlighted the overall reduction in cost through using Logic Apps for most customers. For example, if your Logic App is under the ASP pricing model, and you execute it, then the portal could tell you what that execution would have cost you under the consumption model (using retail pricing). Just an idea!
Let me know if you have other opinions, or agree/disagree with this.
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