I have a microservice to wrap the access to a MongoDB (DaaS). It is implemented with Spring Data Reactive Repository (ReactiveMongoRepository).

I have deployed within a docker image, running on Kubernetes (in Google Cloud). I have configured the orchestrator to keep a minimum of 2 pods of my microservice (and a maximum of 4).

In other microservice, I have implemented a batch process with multithreading, where I call my daas, with the following sequence:

1. findById
2. Modify some fields (including increments of counters)
3. save

Here is the relevant code:

public Mono<Element> updateElement(String id) {
    return this.daasClient.findById(id)
        .map(elem -> modify(elem))
        .flatMap(elem -> this.daasClient.save(elem));
}

When there are lots of operations, each pod runs some of then (including the previuos code), so I have seen that the access to the resource (Mongo) is not thread safe, so the resut is not as expected.

I guess, 2 pods run the findById simultaneously, so the update is not to the "real" document, so, the last in invoking the save method overrides the changes of the other.

Anybody know how I could do to avoid this, i.e., to implement this operation in thread-safe (pod-safe) way?

Thanks