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Scaling Unit - Why we should not have a master unit

In the last post we talk about the base concept of a Scale Units. In today post we will go further and talk about why we can end up with different types of scale units based on application needs.
Until now we defined a scale unit as a group of resources that are grouped together to server a specific number of clients.
This will work with success for application that can have scale units that are 100% independent and are not managed by an authority.
Let’s imagine the following scenario:
We need to create an application that pushes the same binary content to clients when an administrator decide.

It is pretty simple to define a scale unit that manage a X number of clients. This scale unit will contain the content that needs to be pushed to clients replicated as many as time needs to satisfy the SLA required by each client connected to that scale unit.

But there are some steps that need to be done before a release of a binary content can be done. For example we need to be sure that the binary content was downloaded with success by all scaling units. Also, we need to be able to control from only one point the release of a specific binary content.
If all this are not enough, when a client needs to be registered into the system, he will know an initial registration endpoint that will need to help him to find the scaling unit that will manage him.
Based on this requirement, we can already identify another type of scaling unit that will need to handle this functionality. Let’s call this scaling unit “Global Unit” – GU. The scaling units that handle our clients can be called “Client Unit” – CU.
The Global Unit will be aware all the time about the rest of the Client Units and will need to manage and communicate with each scaling unit.

This approach can be risky, because Global Unit can become a bottle neck. In case something goes wrong on it we will not be able anymore to trigger a global push of a binary content. Scaling a Global Unit is not an easy job, because it is the master controlling unit over the rest of our scaling units. We can have an Active-Active or Active-Passive failover mechanism at Global Unit, but it will be a hard thing to do because we’ll need to replicate content and storage between Active and the Passive (Active) unit (We will talk more about Active-Active and Active-Passive topic in another posts).
This is a risk that we need to accept if we will go on an approach with a Global Unit that manage and control Client Units.
It is important to know that not all the systems needs a Global Unit that control different commands over all Client Units - like a binary content release or redirect clients at the first handshake to their Client Unit. 
In general I would try to make everything that is possible to avoid having a Global Unit.

Our main scope should be to define scaling units that don’t have any kind of contact or shared resources between them or a 'master' unit. In this way scaling, management and control can be made clean and easily.

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