Limit maximum of replication

[kedric]

Hello,

is it possible to make an implemetation have a maximum number of replications?

I'm thinking of the use case of an element of my application that uses resources that are themselves limited

Use case:

• a database with a maximum number of connections
• a cache with a writer and several readers

[mwhittaker]

Hi @kedric! Right now, we have no official support for specifying the maximum number of replicas of a component. If you use the weaver kube deployer, however, you can edit the generated Kubernetes YAML files to specify a maximum number of replicas. This is definitely something we might add in the future, so I'll leave the issue open.

I think guaranteeing a maximum number of replicas might be tricky. For example, if you currently have $n$ replicas, where $n$ is the max, and you lose contact to one of the replicas, do you start a new replica? I don't think you can without possible having $n+1$ replicas. But we might be able to enforce a maximum replica limit on a best effort basis.

[kedric]

Hi @mwhittaker,

If I lose contact with a replica, I think it should be considered obsolete. if the replica was in demand, I'll have to start a new one. After that, it's up to me to decide whether to keep a resource margin for myself, so that I can have a period of time when I'm likely to use more resources.

But this means that for a writer/reader case, it's a tricky situation.

Is there any way of making a leader election system for the second case with what exists?

[mwhittaker]

If I lose contact with a replica, I think it should be considered obsolete. if the replica was in demand, I'll have to start a new one. After that, it's up to me to decide whether to keep a resource margin for myself, so that I can have a period of time when I'm likely to use more resources.

Makes sense! If we implement a limit on the number of replicas on a best effort basis, it should be fine to use for performance reasons.

But this means that for a writer/reader case, it's a tricky situation.

Yup, I don't think you should rely on a maximum number of replicas for correctness.

Is there any way of making a leader election system for the second case with what exists?

Because this would be very complicated, I don't think we have any short term plans to implement it in Service Weaver. If you need something like this, I recommend using an existing system like ZooKeeper in conjunction with Service Weaver.

[kedric]

Hi @mwhittaker,

Thank you for this information, I'll see how I can do.

[rgrandl]

@kedric, this is somewhat orthogonal, but you can specify the max number of replicas if you use the GKE deployer.