open-consul/docs/config/checklist-adding-config-fields.md

Adding a Consul Config Field

This is a checklist of all the places you need to update when adding a new field to config. There may be a few other special cases not included but this covers the majority of configs.

We suggest you copy the raw markdown into a gist or local file and check them off as you go (you can mark them as done by replace [ ] with [x] so github renders them as checked). Then please include the completed lists you worked through in your PR description.

Examples of special cases this doesn't cover are:

• If the config needs special treatment like a different default in -dev mode or differences between OSS and Enterprise.
• If custom logic is needed to support backwards compatibility when changing syntax or semantics of anything

There are four specific cases covered with increasing complexity:

1. adding a simple config field only used by client agents
2. adding a CLI flag to mirror that config field
3. adding a config field that needs to be used in Consul servers
4. adding a field to the Service Definition

Adding a Simple Config Field for Client Agents

• Add the field to the Config struct (or an appropriate sub-struct) in agent/config/config.go.
• Add the field to the actual RuntimeConfig struct in agent/config/runtime.go.
• Add an appropriate parser/setter in agent/config/builder.go to translate.
• Add the new field with a random value to both the JSON and HCL files in agent/config/testdata/full-config.*, which should cause the test to fail. Then update the expected value in TestLoad_FullConfig in agent/config/runtime_test.go to make the test pass again.
• Run go test -run TestRuntimeConfig_Sanitize ./agent/config -update to update the expected value for TestRuntimeConfig_Sanitize. Look at git diff to make sure the value changed as you expect.
• If your new config field needed some validation as it's only valid in some cases or with some values (often true).
  • Add validation to Validate in agent/config/builder.go.
  • Add a test case to the table test TestLoad_IntegrationWithFlags in agent/config/runtime_test.go.
• If your new config field needs a non-zero-value default.
  • Add that to DefaultSource in agent/config/defaults.go.
  • Add a test case to the table test TestLoad_IntegrationWithFlags in agent/config/runtime_test.go.
  • If the config needs to be defaulted for the test server used in unit tests, also add it to DefaultConfig() in agent/consul/config.go.
• If your config should take effect on a reload/HUP.
  • Add necessary code to to trigger a safe (locked or atomic) update to any state the feature needs changing. This needs to be added to one or more of the following places:
    • ReloadConfig in agent/agent.go if it needs to affect the local client state or another client agent component.
    • ReloadConfig in agent/consul/client.go if it needs to affect state for client agent's RPC client.
  • Add a test to agent/agent_test.go similar to others with prefix TestAgent_reloadConfig*.
• Add documentation to website/content/docs/agent/config/config-files.mdx.

Done! You can now use your new field in a client agent by accessing s.agent.Config.<FieldName>.

If you need a CLI flag, access to the variable in a Server context, or touched the Service Definition, make sure you continue on to follow the appropriate checklists below.

Adding a CLI Flag Corresponding to the new Field

If the config field also needs a CLI flag, then follow these steps.

• Do all of the steps in Adding a Simple Config Field For Client Agents.
• Add the new flag to agent/config/flags.go.
• Add a test case to TestParseFlags in agent/config/flag_test.go.
• Add a test case (or extend one if appropriate) to the table test TestLoad_IntegrationWithFlags in agent/config/runtime_test.go to ensure setting the flag works.
• Add flag (as well as config file) documentation to website/source/docs/agent/config/config-files.mdx and website/source/docs/agent/config/cli-flags.mdx.

Adding a Simple Config Field for Servers

Consul servers have a separate Config struct for reasons. Note that Consul server agents are actually also client agents, so in some cases config that is only destined for servers doesn't need to follow this checklist provided it's only needed during the bootstrapping of the server (which is done in code shared by both server and client components in agent.go). For example WAN Gossip configs are only valid on server agents but since WAN Gossip is setup in agent.go they don't need to follow this checklist. The simplest (and mostly accurate) rule is:

If you need to access the config field from code in agent/consul (e.g. RPC endpoints), then you need to follow this. If it's only in agent (e.g. HTTP endpoints or agent startup) you don't.

A final word of warning - you should never need to pass config into the FSM (agent/consul/fsm) or state store (agent/consul/state). Doing so is very dangerous and can violate consistency guarantees and corrupt databases. If you think you need this then please discuss the design with the Consul team before writing code!

Consul's server components for historical reasons don't use the RuntimeConfig struct they have their own struct called Config in agent/consul/config.go.

• Do all of the steps in Adding a Simple Config Field For Client Agents.
• Add the new field to Config struct in agent/consul/config.go
• Add code to set the values from the RuntimeConfig in the confusingly named consulConfig method in agent/agent.go
• If needed, add a test to agent_test.go if there is some non-trivial behavior in the code you added in the previous step. We tend not to test simple assignments from one to the other since these are typically caught by higher-level tests of the actual functionality that matters but some examples can be found prefixed with TestAgent_consulConfig*
• If your config should take effect on a reload/HUP
  • Add necessary code to ReloadConfig in agent/consul/server.go this needs to be adequately synchronized with any readers of the state being updated. - Add a new test or a new assertion to TestServer_ReloadConfig

You can now access that field from s.srv.config.<FieldName> inside an RPC handler.

Adding a New Field to Service Definition

The Service Definition syntax appears both in Consul config files but also in the /v1/agent/service/register API.

For wonderful historical reasons, our config files have always used snake_case attribute names in both JSON and HCL (even before we supported HCL!!) while our API uses CamelCase.

Because we want documentation examples to work in both config files and API bodies to avoid needless confusion, we have to accept both snake case and camel case field names for the service definition.

Finally, adding a field to the service definition implies adding the field to several internal structs and to all API outputs that display services from the catalog. That explains the multiple layers needed below.

This list assumes a new field in the base service definition struct. Adding new fields to health checks is similar but mostly needs HealthCheck structs and methods updating instead. Adding fields to embedded structs like ProxyConfig is largely the same pattern but may need different test methods etc. updating.

• Do all of the steps in Adding a Simple Config Field For Client Agents.
• agent/structs package
  • Add the field to ServiceDefinition (service_definition.go)
  • Add the field to NodeService (structs.go)
  • Add the field to ServiceNode (structs.go)
  • Update ServiceDefinition.ToNodeService to translate the field
  • Update NodeService.ToServiceNode to translate the field
  • Update ServiceNode.ToNodeService to translate the field
  • Update TestStructs_ServiceNode_Conversions
  • Update ServiceNode.PartialClone
  • Update TestStructs_ServiceNode_PartialClone (structs_test.go)
  • If needed, update NodeService.Validate to ensure the field value is reasonable
  • Add test like TestStructs_NodeService_Validate* in structs_test.go
  • Add comparison in NodeService.IsSame
  • Update TestStructs_NodeService_IsSame
  • Add comparison in ServiceNode.IsSameService
  • Update TestStructs_ServiceNode_IsSameService
  • If your field name has MultipleWords,
    • Add it to the aux inline struct in ServiceDefinition.UnmarshalJSON (service_defintion.go).
      • Note: if the field is embedded higher up in a nested struct, follow the chain and update the necessary struct's UnmarshalJSON method - you may need to add one if there are no other case transformations being done, copy and existing example.
      • Note: the tests that exercise this are in agent endpoint for historical reasons (this is where the translation used to happen).
• agent package
  • Update testAgent_RegisterService and/or add a new test to ensure your fields register correctly via API (agent_endpoint_test.go)
  • If your field name has MultipleWords,
    • Update testAgent_RegisterService_TranslateKeys to include examples with it set in snake_case and ensure it is parsed correctly. Run this via TestAgent_RegisterService_TranslateKeys (agent_endpoint_test.go).
• api package
  • Add the field to AgentService (agent.go)
  • Add/update an appropriate test in agent_test.go
    • (Note you need to use make test or ensure the consul binary on your $PATH is a build with your new field - usually make dev ensures this unless you're path is funky or you have a consul binary even further up the shell's $PATH).
• Docs
  • Update docs in website/source/docs/agent/services.html.md
  • Consider if it's worth adding examples to feature docs or API docs that show the new field's usage.

Note that although the new field will show up in the API output of /agent/services , /catalog/services and /health/services, those tests right now don't exercise anything that's super useful unless custom logic is required since they don't even encode the response object as JSON and just assert on the structs you already modified. If custom presentation logic is needed, tests for these endpoints might be warranted too. It's usual to use omit-empty for new fields that will typically not be used by existing registrations although we don't currently test for that systematically.