Microservices Deployment

Question

I have:

Microservice-A Microservice-B Microservice-C

Microservice-A calls Microservice-B and Microservice-C

When I deploy Microservice-A I want make

Answer 1

The ideal solution is to never be in a position where you are deploying into an environment where you don't already know the versions of your dependencies. That way madness lies.

Avoiding this is a governance concern and so should be central to any service oriented approach to building software.

For instance, let's say you are developing version 2.0 of your service A. In your target environment, you have service B version 1.0 and service C version 1.0.

So the first step on the path to stress-free releases, as part of your development build, you should be running a set of nearest neighbour automated tests, which stub out B v1.0 and C v1.0 based on the service contracts (more on this later). This can be facilitated using test double tools such as mountebank.

Then, just as you have created your v2.0 release branch, you learn that another team is about to release v1.1 of service C. It should always be possible to work out whether v1.0 to v1.1 constitutes a breaking change to the v1.0 contract for service C (more on this later).

If v1.1 is a breaking change, no problem, you update your tests with v1.1 of the service C contract and fix any failures. You are then good to create a new v2.0.1 patch branch and release. If for whatever reason you are forced to release before service C you can still release from the v2.0 branch.

If v1.1 is not a breaking change, no problem, just release off your existing branch.

There are various strategies for coping with the overhead produced by a centralised release management protocol such as described above.

As stated earlier, contracts for all dependent services should be used when testing your service. (Note: it's very important for the nearest neighbour tests to be driven from contracts, rather than using existing code models, such as DTOs defined in the service's unit tests). Contracts for all the services should be based on a standard (such as swagger) which supplies a complete service description, and be very easy to find - the use of a service repository can simplify this.

Also stated earlier, it should always be possible to know if new versions of dependent services have the potential to break your service. One strategy is to agree on a versioning convention which bestows some kind of meaning when incrementing the version. For instance, you could use a major.minor.patch (eg v1.0.0) where a change the major version number constitutes a change to the service contract and therefore has the potential to break things. In our previous example, service C went from v1.0 to v1.1. With a convention such as the one described above, we could be sure that the change would not break us, as the major version number was unchanged.

While it can be cumbersome to set up and maintain a centralised release management protocol, the benefit is that you always have full confidence that by deploying your service, nothing will break. What's more, this avoids having any complex (and to my mind, contrived) runtime dependency resolution, such as you are proposing in your original question.

Answer 2

Each microservice could provide a versionnumber either via API or by writing to a public file / shared DB. Each microservice would then contain all the expected versionnumbers of its dependencies with and check if the version numbers in the file / database match before startup.