Ergo

Ergo checks that your deployments, pull requests or expenses follow rules written as plain sentences, like approved_by is a non-empty string, and gives back a report that says what it checked, what it found and why each check passed or failed. Whoever owns a rule can write it, and whoever has to trust the result can read the report without knowing how it was computed.

Ergo is at 0.1. Any 0.x release may still change what a policy says or what a report shows, and CHANGELOG.md says when it does.

What Ergo does

Policies read like sentences

A check is one line: approved_by is a non-empty string, every commits.verified is true, some licenses is in $params.allowed_licenses. Sentences can also count, compare times, match patterns and allow a margin, as in count of approvers is at least 2, reviewed_at is before deployed_at, branch matches '^release/' or cost is 2.5 within 0.01. When a check can pass in more than one way, any_of names the ways.

The policy says what it's about

from: deployments whose environment is "prod" picks the subjects, which is what Ergo calls the things a policy checks. When a rule only applies sometimes, whose says when, and a subject can be built on another, like the pull requests of each artifact. Values that change between runs, like $params.allowed_licenses, come from outside the policy. There are no defaults, so a policy run without its params fails instead of checking something nobody configured.

Every run gives a report

The report has one row per subject and check, passed or failed alike, with the value Ergo read. A failed row also has a cause: absent when the field isn't there, null when it holds null, value when it was read fine and just doesn't pass, and unusable when it's the wrong kind of thing.

The report has the same shape whatever the policy, so you can keep reports and compare them, and a reporting tool can read all of them the same way. compliant gives the yes or no, and ergo.violations gives the failed rows that are problems, each with its description and its sentence, for whoever has to fix them.

Ergo adds checks of its own

A subject the policy isn't about is recorded as out of scope and stays in the report. A policy that finds nothing to check fails, unless it says finding nothing is fine. Two subjects with the same id fail, and so does a policy that's written wrong, with a row that says what's wrong.

A missing field, a null, a wrong type or an empty list where a check expects items fails the check, unless the policy says it's fine, like , if any, or does not exist.

The same input gives the same report

Ergo reads no clock and nothing random, and sorts everything it writes. The same policy, input and params give the same report, row for row, whatever order the policy was written in.

One file, with a spec behind it

Ergo is one file for Open Policy Agent, with no dependencies. Run it with opa eval, or compile it to Wasm and run it with OPA's JavaScript runtime, as REFERENCE.md shows.

Every rule Ergo follows is written down in the spec, with cases that pin it, and both the policy and the report have a JSON Schema. A policy says which spec version it's written for, and a report says which engine made it and which version it follows, so other tools can read the reports.

Getting started

Ergo is a single file. Download it into your project and write a policy next to it.

You'll need OPA 1.20.1 or later to run it, and we test Ergo with OPA 1.20.2. Older versions give wrong results or don't load it: OPA 1.19 says 1 >= 100.0 is true, so a check can pass when it should fail, OPA 1.20.0 stops with panic: illegal value on 0.0 == 1.5, and OPA 1.2 stops with rego_parse_error: unexpected as keyword. Run opa version to see which one you have.

1. Download Ergo

Download ergo.rego from the latest release into the directory that holds your policies:

curl -L --create-dirs -o policy/ergo.rego https://github.com/kosli-dev/ergo/releases/latest/download/ergo.rego

policy/
  ergo.rego

Each release has a version, like 0.1.0, and the file says which one it is. To stay on one, download it from that release instead of the latest, like https://github.com/kosli-dev/ergo/releases/download/rego-v0.1.0/ergo.rego.

2. Write a policy

Here is a policy that says "every production deployment must have an approver". It goes in a YAML file next to ergo.rego.

policy/policy.yaml:

ergo_spec: v0.1
subjects:
  deployment:
    from: deployments whose environment is "prod"
    identified_by: id
requirements:
  prod_deploy:
    description: Every production deployment is approved
    subject: deployment
    checks:
      approved:
        description: Someone approved the deployment
        assert: approved_by is a non-empty string

ergo_spec says which version of Ergo's spec the policy is written for. Ergo reads only the version it follows, so a policy without it, or written for another version, fails.

A policy says what to check. First it says what it's about, the things we call subjects. Each kind of subject gets a name, here deployment, that the report uses for each one, and says where to find them:

• from says where the subjects are in the input. deployments means input.deployments, and release.deployments would mean input.release.deployments. whose keeps only the ones this policy is about, here the deployments whose environment is prod.
• identified_by names the field that tells the subjects apart, here id. No two subjects can share one.

Then the requirement says what must be true of them. Its description says what it requires, in words anyone can read, and subject names the subject it checks:

• checks are the rules each subject must pass, each one written as a sentence. Here, approved_by must be a non-empty string. A check with nothing to add can be the sentence alone: approved: approved_by is a non-empty string.

Several requirements can check the same subject. Each one gets its own rows in the report.

Then a Rego file hands the policy to Ergo. OPA reads every YAML and JSON file in the directory you pass with -d, so the YAML file becomes data.ergo_spec, data.subjects and data.requirements.

policy/deploy.rego:

package deploy

import data.ergo

default ergo_spec := null

ergo_spec := data.ergo_spec

default subjects := {}

subjects := data.subjects

default requirements := {}

requirements := data.requirements

report := ergo.report(input, {"ergo_spec": ergo_spec, "subjects": subjects, "requirements": requirements}, {})

violations := ergo.violations(report)

ergo.report takes the input, the policy and the params. The policy holds its version, the subjects and the requirements, and this one has no params, so they're {}. The version comes from the YAML file and isn't written in the Rego, so that it always says what the file was written for.

The defaults keep the report defined when the file leaves out ergo_spec:, subjects: or requirements:, or spells one wrong. That report still fails: without ergo_spec every requirement is written wrong, with no requirements it isn't compliant, and with no subjects the requirement names one that isn't there.

The old way of writing checks

Before 0.1, a check was an object with an op and a path, like {op: non_empty_string, path: [approved_by]}, and a subject used applies_to filters written the same way. Spec 0.1 doesn't read that syntax any more. A policy written that way is written wrong, and the report says so. ergo.as_sentences rewrites what it can as sentences, and Moving a policy to sentences in the reference shows what it leaves for you to rewrite.

3. Get the report

With the following deployments.json input:

{
  "deployments": [
    { "id": "d-1", "environment": "prod", "approved_by": "alice" },
    { "id": "d-2", "environment": "prod" },
    { "id": "d-3", "environment": "staging" }
  ]
}

Ask for the report:

opa eval -d policy -i deployments.json --format=pretty 'data.deploy.report'

The report records every check Ergo ran, including the ones that passed. It comes in four parts, after the policy's own description and meta, which this one doesn't have, and engine and ergo, which say what made the report and which version of the spec it follows:

{
  "compliant": false,
  "description": "",
  "engine": {
    "name": "ergo.rego",
    "version": "0.2.0-dev"
  },
  "ergo": "v0.1",
  "meta": {},
  "requirements": { ... },
  "results": [ ... ],
  "subjects": { ... }
}

• compliant is the overall answer. When you need a plain yes or no to allow or block something, this is the one to use: allow := report.compliant.

• requirements has an entry for each requirement. It repeats the requirement's description, and its status is met, not_met, or not_applicable when no subject was left to check. When it's not_met, causes says why, worst first, with how many rows each: here [{"cause": "absent", "rows": 1}]. It also says how many subjects were found and kept, and lists every check with a readable expression, which is the sentence as you wrote it.

• results has one row per subject and check, with the value Ergo read, whether the check passed, and a cause.

• subjects says where each subject the requirements use comes from. Here it holds one entry, and the requirement's own entry has "subject": "deployment" to point at it:

  "subjects": {
    "deployment": {
      "description": "",
      "from": "deployments whose environment is \"prod\"",
      "identified_by": "id",
      "parts": {}
    }
  }

Here's what those rows look like for our example:

subject	check	value read	passed	cause
	$well_formed	count(checks) = 1, require = "every"	true	satisfied
	$min_subjects	in-scope deployment count = 2	true	satisfied
	$unique_ids	repeated deployment ids = []	true	satisfied
d-1	$applies	environment = "prod"	true	satisfied
d-2	$applies	environment = "prod"	true	satisfied
d-3	$applies	environment = "staging"	false	value
d-1	approved	approved_by = "alice"	true	satisfied
d-2	approved	approved_by = null	false	absent

You didn't write the checks starting with $. Ergo adds them for you:

• $well_formed makes sure the requirement itself makes sense, for example that it has at least one check.
• $min_subjects makes sure at least one subject was found. If from points to nothing, this check fails, so the policy can't pass without checking anything.
• $unique_ids makes sure no two deployments share an id. Otherwise their rows could look the same, and you couldn't tell which one failed.
• $applies records whether each subject passed the whose conditions. d-3 didn't, and the report says so rather than leaving it out. A deployment with no environment at all would fail $applies with the cause absent, and the requirement would fail with it, because Ergo can't tell whether it's a production deployment.

When a check fails, cause tells you why. d-2 failed with absent because it has no approved_by field at all: no approval was ever recorded. Had approved_by been "", the cause would be value instead, since an approval was recorded but it's empty. Both fail, but they're different problems and you'd fix them differently.

4. Get the violations

Because the report has everything, it gets long. When all you want is to tell someone what went wrong, ask for the violations instead:

opa eval -d policy -i deployments.json --format=pretty 'data.deploy.violations'

[
  {
    "cause": "absent",
    "check": "approved",
    "description": "Someone approved the deployment",
    "expression": "approved_by is a non-empty string",
    "inputs": [{ "name": "approved_by", "value": null }],
    "requirement": "prod_deploy",
    "subject": { "id": "d-2", "type": "deployment" }
  }
]

violations is built from the report. It keeps only the failed rows that are real problems and adds each check's description and expression, so you have what you need to write a message. d-3 isn't listed: it failed $applies, but being out of scope isn't a problem.

Going further

The reference covers every field, operator and cause, including the ones this example doesn't use.

Updating

Download the new ergo.rego over the old one and run your policy's tests. CHANGELOG.md says what each release changed, and which spec version it follows.

Contributing

Bug reports, ideas and pull requests are welcome. CONTRIBUTING.md explains how, and everyone taking part follows the code of conduct.

License

Ergo is licensed under the Apache License 2.0.