Building Patterns

In this guide, we’ll go over building and signing Patterns in Etcha.

Build Process

Etcha builds Patterns by executing the build Command list on the local instance, and then generating a JWT containing the Pattern and other metadata.

The JWT is then cryptographically signed to prevent modification and prove to other Etcha instances that it was created by a trusted authority.

The output of the the build process is this JWT, a cryptographically secure file containing base64 encoded text.

Signing Patterns

We need a way to sign Patterns before building them. Out of the box, Etcha cannot build or sign Patterns without cryptographic keys. Specifically, we need to provide values in our Etcha configuration for signingKey or signingCommands.

signingKey

A signingKey will use a static, private key to sign the Pattern JWT. This value can be hardcoded in your “builder”, or you can retrieve it from environment variables, a remote URL, a DNS record, etc. This text is like a password though and should be protected.

We can generate keys appropriate for signing using etcha gen-keys. We’ll use the privateKey value during build for the signingKey, and then we’ll save the publicKey and use that for the verifyKeys when we push/pull.

Here is an example Etcha configuration showing a static signingKey:

{
  "build": {
    "signingKey": "ed25519private:MC4CAQAwBQYDK2VwBCIEIDZUP0nKuVhDJu5h0QKAQVrZBLrDa9CA09SPJKj/21eG:qsX98cmrLW".
  }
}

We can also leverage some of the dynamic Jsonnet functions to pull the key from a separate tool, like HashiCorp Vault:

local getEnv(key) = std.native('getEnv')(key);
local getPath(path, fallback=null) = std.native('getPath')(path, fallback);

{
  build: {
    signingKey: std.parseJson(getPath('https://vault.mycompany.com/v1/kv/secret/etcha#x-vault-token:%s' % getEnv('VAULT_TOKEN'))).data.private_key,
  },
}

This example looks a little intense, lets walk through it:

• We declared two functions at the top to leverage Jsonnet native functions, getEnv which will retrieve an environment variable, and getPath which will retrieve the contents of a local file or URL.
• We defined our build object and signingKey using those function, working inside out:
  • We looked up the VAULT_TOKEN value using getEnv
  • We created a path for getPath using string formatting. The path is the Vault API path to our signing key. At the end, we added a syntax that getPath will use to set an HTTP header, X-Vault-Token, with the value from getEnv, which is used to authenticate to Vault with.
  • We parse the string returned by getPath to turn it into a Jsonnet object.
  • We retrieve the data.private_key value from the Jsonnet object.

signingCommands

Some organizations may need to perform signing in a more secure, restrictive manner, like delegating signing to a HSM or another key provider. We can use signingCommands for this. signingCommands are a list of Commands that will use environment variables to sign a Token and return the signed Token to Etcha.

Etcha will set the Environment Variable ETCHA_PAYLOAD containing the base64 raw URL encoded string that needs to be signed.

Our signing Commands need to construct the rest of the JWT, and then print the JWT during a Command change that triggers the event jwt, like this:

[
  {
    "always": true,
    "change": "<commands to build and print JWT>",
    "id": "build JWT",
    "onChange": [
      "etcha:jwt"
    ]
  }
]

Building

Now that we’ve setup our signing keys, lets build this Pattern:

// patterns/myapp.jsonnet

local app_name = 'myapp';
local restart = function(name)
  {
    change: 'systemctl restart %s' % name,
    id: 'restart %s' % name,
  };

{
  build: [
    {
      always: true,
      change: 'make %s' % app_name,
      id: 'build %s' % app_name,
      onChange: [
        'etcha:buildManifest',
        'etcha:runEnv_myapp',
      ]
    },
  ],
  subject: app_name,
  run: [
    {
      change: 'curl -L https://s3.example.com/myapp_v2 -o /myapp',
      check: "myapp --version | grep v2",
      id: "copy %s v2' app_name,
      onChange: [
        'restart myapp',
      ],
    },
    restart(app_name),
  ]
}

We’ll run etcha -c config.jsonnet build patterns/myapp.jsonnet myapp.jwt myapp. This does a few things:

1. Import the Pattern Jsonnet file, myapp.jsonnet, and any other files it imports
2. Render the Pattern Jsonnet file
3. Run all of the Commands in build in Change Mode. Optionally using the source configuration exec, myapp, specified by the last parameter.
4. Collect metadata from these Commands to populate the JWT, like buildManifest and runEnv_. In our Pattern, the build command build %s will fire these events, and these values will contain the stdout of the change execution.
5. Create a JWT containing the raw Pattern Jsonnet files collected in step 1, the metadata collected in step 4, and any JWT values set in our Pattern.
6. Sign the JWT using signingKey or signingCommands
7. Save the JWT file to myapp.jwt, the parameter after the path to the Pattern.

local native = 'etcha/lib/etcha/native.libsonnet';
{
  run: [
    id: 'set a secret',
    always: true,
    change: 'echo %s > /tmp/mysecret' % native.get(field='secret', default=native.getEnv('SECRET')),
  ]
}