Fix: Clean up command line directory inconsistencies

[pniedzielski]

We noticed two inconsistencies while reviewing the bmqbrkr main function:

1. --config was parsed as an optional argument, but if it was not given on the command line, we would detect and fail after parsing. We can just make it required.

2. All environment variables that the BlazingMQ broker knows about can be overridden by command line arguments, except BMQ_PREFIX. We can add a new command line argument to do that.

[chrisbeard]

I guess the only real question here is should the env var win even if --prefixDir is provided on the cli? Unsure.

[hallfox]

Usually the order of precedence I expect is config file, CLI arg, environment variable in order of lowest to highest precedence (i.e. BMQ_PREFIX should win)

[pniedzielski]

I have no strong opinions either way.

With bmqtool, it looks like the CLI param --broker overrides BMQ_PORT:

blazingmq/src/applications/bmqtool/bmqtool.m.cpp

Line 173 in 112f750

char* bmqPort = bsl::getenv("BMQ_PORT");

But BMQ_BROKER_URI overrides the user's session options:

blazingmq/src/groups/bmq/bmqa/bmqa_session.cpp

Line 298 in 112f750

// Override session options from the environment.

So it's a mess even with the only two examples we have.

[pniedzielski]

Did a little research, it seems that most other CLI tools have CLI arg as the highest precedence.

• Docker: https://docs.docker.com/reference/cli/docker/#configuration-files "Command line options override environment variables and environment variables override properties you specify in a config.json file."
• Git: Does not document this very clearly from what I can tell (https://git-scm.com/docs/git#_environment_variables). But, running the following test confirms that the CLI argument takes higher precedence:

  cd ~
  GIT_DIR=~/blazingmq/.git git --git-dir=~/blazingmq-sdk-java/.git fetch
  # remote: Enumerating objects: 23, done.
  # remote: Counting objects: 100% (23/23), done.
  # remote: Compressing objects: 100% (9/9), done.
  # remote: Total 23 (delta 8), reused 13 (delta 8), pack-reused 0 (from 0)
  # Unpacking objects: 100% (23/23), 5.93 KiB | 276.00 KiB/s, done.
  # From https://github.com/bloomberg/blazingmq-sdk-java
  #    9203baf..a342729  main       -> upstream/main

• HashiCorp Vault: I think this is a bit more like BlazingMQ as a network server. https://developer.hashicorp.com/vault/docs/commands#configure-environment-variables "You can use environment variables to configure the CLI globally. Some configuration settings have a corresponding CLI flag to configure a specific command. For example, export VAULT_ADDR='http://localhost:8200' sets the address of your Vault server globally, while -address='http://someotherhost:8200' overrides the value for a specific command."

I have not been able to find anything that follows the precedence config file < CLI arg < environment variable.

Second thing I've found is it's hard to find this documented explicitly. So I think we should document explicitly what we choose.

[hallfox]

Coming back to this after two months but I have no idea what I was thinking when I wrote this originally. Most tools I use do the config file < environment var < CLI arg order. Maybe I was thinking about how one might control broker configurations from a docker image?

Either way I do think the most important thing is to document the precedence regardless of what takes precedent. I just don't agree with what I originally wrote anymore.

[pniedzielski]

See above comment ^