conductor.go

Conductor

Light orchestrator for Containers and CGI scripts for systemd servers running Podman and a Caddy reverse-proxy.

Sometimes Kubernetes is overkill, even lightweight versions. When configuration is static, you just need to manage and run services on a different set of machines. However, you might need a light orchestrator to handle zero downtime deployments, load balancer integration or CGI (now known as lambdas or serverless functions)

Conductor will handle all of this for you with a few core concepts:

• Application: this is what it means, an application.

• Service: this is a particular instance of an application deployed. Example: production, staging, ...

• Deployment: within an instance, the application can be deployed a single time (as normal) or multiple times (to provide parallelism or handle version updates in deployments)

The idea is that the Conductor configuration is static on all machines, and conditionals (to be implemented) control which services run where. CGI scripts can be then used to control communication between the machines if needed and a global replicated static configuration allows every service to know where each other service resides (no need of etcd when the data store never changes).

Installation

Download the executable, place it in your PATH and run conductor system install. You should have systemd and podman on your server. It also expects to be able to connect to the Caddy via the standard API endpoint on port 2019. The Caddy server should be described in caddy.service (others units depends on it).

Service anatomy

To declare a service, just put a JSON file in /etc/conductor/services/my-service/conductor-service.json (replace my-service by your service name):

{
  "inherit" [
    "/path/to/the/base/conductor-service.json"
  ],
  "app_name": "beautiful-app",
  "instance_name": "staging",
  "config": {
    "BEAUTIFUL_APP_ENV": "staging",
    "DOCKER_IMAGE": "example.org/beautiful/app",
    "DOCKER_TAG": "latest"
  },
  "pods": [
    {
      "name": "",
      "pod_template": "./pod.template",
      "config_map_template": "./config-map.template",
      "service_directives": []
    }
  ],
  "proxy_config_template": "./proxy-config.template",
  "hooks": [
    {
      "id": "my-start-hook",
      "when": "post-start",
      "exec": ["./post-start.hook"]
    },
    {
      "id": "my-stop-hook",
      "when": "pre-stop",
      "exec": ["./pre-stop.hook"]
    }
  ],
  "commands" {
    "migrate": {
      "service": true,
      "service_any_deployment": false,
      "deployment": false,
      "description": "Run data migrations",
      "exec": ["./migrate.cmd"]
    }
  },
  "display_service_config": ["BEAUTIFUL_APP_ENV", "DOCKER_TAG"],
  "display_deployment_config": ["DOCKER_VERSION"]
}

Hooks

Hooks are scripts executed with the config as environment variables.

Additional variables for services and deployments:

• CONDUCTOR_APP
• CONDUCTOR_INSTANCE
• CONDUCTOR_SERVICE_NAME
• CONDUCTOR_SERVICE_DIR
• CONDUCTOR_SERVICE_UNIT
• CONDUCTOR_SERVICE_CONFIG_UNIT

Additional variables for deployments only:

• CONDUCTOR_SERVICE_PART name of the pod in the service
• CONDUCTOR_DEPLOYMENT contains the deployment name
• CONDUCTOR_DEPLOYMENT_UNIT
• CONDUCTOR_DEPLOYMENT_CONFIG_UNIT
• POD_NAME contains the pod name
• POD_IP_ADDRESS contains the pod IP address

Templates

Templates are any executable script, variables are passed to them via the command-line arguments or the environment variables, use what you prefer. It should return on the standard output the templated document.

Templates executed in the context of the service are executed in the service directory. If executed in the context of a deployment, the template is executed in the deployment directory.

The important templates are:

• the pod template: it should return a Kubernetes pod in YAML compatible with podman kube play. This is what will run in the deployment.

• the config map template: this is an optional template that can generate a ConfigMap file suitable for consumption within podman

• the proxy configuration template: this should generate JSON Caddy configuration snippets suitable for injection via the Caddy API. It should be a JSON array of objects, each with:

  - `id`: where the snipped should be POSTed via the config API
  - `config`: the snipped, containing a unique `@id` key suitable for
    removal when undeploying the service
  - `register_only`: can be true to only register but never deregister the
    snippet, then the `@id` is optional. Can be used to add domain names for
    certificate automation.
  

  The template is called both with the service and with the deployment, at startup and shutdown. The template can detect it is run with the deployment because the variable POD_IP_ADDRESS is then present.

Variables accessible within templates are visible with the commands conductor service env or conductor deployment env. It is possible to simulate a template execution with conductor _ service template or conductor _ deployment template.

Example template:

#!/bin/bash

cat <<YAML
---
apiVersion: v1
kind: Pod
metadata:
  labels:
    app: $CONDUCTOR_APP
  name: $POD_NAME
spec:

  hostAliases:
    - ip: "127.0.0.1"
      hostnames:
        - mongodb

  containers:

    - name: main
      image: ${DOCKER_IMAGE}:${DOCKER_TAG}
      env:
        - name: CONDUCTOR_DEPLOYMENT
          value: "${CONDUCTOR_DEPLOYMENT}"
      resources:
        limits:
          memory: "4Gi"

    - name: mongodb
      image: mongo:7.0
      command: ["--replSet", "rs0"]
      # args: []
YAML

Display config

It is possible to add columns to the conductor service ls and conductor deployment ls commands via the display_service_config and display_deployment_config configuration options. It will show the variables for those services and deployments

Inheritance

It is possible to inherit from a base configuration, in which case the path locations inherited from the base configuration will be adjusted to be relative to the file they were declared in.

Commands

It is possible to declare commands that can be execute with conductor run. Each command must have an exec array to execute a script relative to the current file.

The command must declare in what context it can be executed:

• "service": true if the command is executed for a service globally
• "service_any_deployment": true if the command is executed for a service in which case it will be executed in the context of any active deployment, or will fail if there is no compatible active deployment
• "deployment": true if the command is executed in the context of a deployment

The command can be executed for a service with conductor run -s SERVICE_NAME or for a deployment with conductor run -d DEPLOYMENT_NAME.

The configuration is available as environment variables just as hooks, with the addition of:

• CONDUCTOR_COMMAND contains the command name.
• CONDUCTOR_COMMAND_DIR working directory of the invocation, the command itself run relative to the service or deployment directory.

Basic How-To

• declare a service
• execute conductor reload to start the systemd units for the services you declared

CGI / Serverless

There is an untested implementation. You can declare functions in your service:

{
  "functions": [
    {
      "name": "",
      "format": "cgi",
      "exec": ["./cgi-script.sh", "--cgi"],
      "stderr_as_stdout": false,
      "response_headers": [
        "Content-Type: text/plain; charset=utf-8"
      ],
      "no_response_headers": false,
      "path_info_strip": 2,
      "service_directives": []
    }
  ]
}

When the deployment corresponding to the function is started, a systemd socket with Accept=yes is started and socket activation is used to start the script that will handle the request.

The formats supported are:

• cgi: a should be compatible CGI interface
• http-stdio: the stdin contains a HTTP request and stdout should be replied with the http response. This is just passthrough of the accepted socket.

In the proxy config template, you can add this shell snippet to configure your functions:

if [[ -n "$CONDUCTOR_FUNCTION_ID" ]]; then
  exec conductor function caddy-config
fi

Future developments

If systemd socket activation is not enough for CGI, perhaps Conductor should take the socket activation in its own hands. This would be necessary in order to pre-provision functions to get faster response times (the CGI format would not work in this case).

It should also be possible to declare such functions as daemons that could be pre-provisioned or scaled down to zero depending on the configuration.

Basically, there are three modes of operations possible:

• single shot (CGI): each request calls a single executable that is started for the request and ends with the request. This is standard systemd socket activation with Accept=yes. Pre-provisioning could be difficult if sticking with the CGI interface and systemd cannot handle it.

• multi-shot: each executable can handle multiple requests but not in parallel. After a configurable number of requests, the executable can be recycled. This could be an HTTP/1.1 connection with keep-alive. Systemd to my knowledge is not able to pre-provision such services.

• parallel: a single executable can handle all requests, like a daemon, but it can scale down to zero in the absence of requests and be reloaded when a connection arrives. Systemd can handle this with standard socket activation (Accept=no)

Roadmap:

• A service pod should be optional in which case it does not generate a deployment
• A service can declare CGI functions, each function is then started as a systemd socket and service that executes the CGI script via cgi-adapter (to be included in conductor)
• The proxy config template should be called for the CGI functions too, and be given the service unique ID as variable
• Add socket activation to pod (allow multiple unix sockets for a single pod).
• Allow multiple pods in a single service
• Allow raw HTTP CGI scripts with Accept=yes that can handle multiple requests on a single keep alive connection
• Let Conductor handle socket activation for the multiple requests use case, and let Conductor handle preloading of the CGI executable.
  • Add conductor-fast-function@.service which will depend on
  • conductor-fast-function-manager.service which will receive start and stop signal from individual functions
  • it will create the socket and manage the function execution either via systemd-run (so it can pass around the socket) or by other means
  • it can handle the equivalent of Accept=yes sockets and pre-provision the function processes. This would improve over systemd socket activation by: - pre-provisioning the process, no cold start - the same process could be used for two separate clients that would otherwise use separate connections and separate processes (wrong because there is a single connection with Caddy, except that perhaps Caddy does not maintain an open connection) instead of using systemd-run, the conductor fast function manager could listen on a socket A (bound to the reverse proxy) and manage a reverse proxy to a socket B (managed by conductor-cgi-function@.socket). By reverse-proxying, the manager could handle pre-provisioning by opening a connection and not writing to it
  • it can handle the equivalent of Accept=no sockets in which case the main socket would have to be passed as a file descriptor. This would improve on systemd socket activation by: - pre-provisioning the process, no cold start - because otherwise Conductor does not support socket activation yet
• It should be possible to have commands accessible as functions with a sane protocol and security
• Handle security policies (see below)

Security (not yet implemented)

Security can be handled using Caddy reverse_proxy rewrite handlers. See:

• https://caddyserver.com/docs/caddyfile/directives/forward_auth
• https://caddyserver.com/docs/json/apps/http/servers/routes/handle/reverse_proxy/#rewrite
• https://caddyserver.com/docs/json/apps/http/servers/routes/handle/reverse_proxy/handle_response/routes/handle/headers/

For CGI/lanbdas, the reverse proxy should be configured to forward a copy of the request to the authentication service which can then decide if the request is authorized (is the API token authorized or not).

The key here is to have two handlers: the first a reverse proxy to the auth service with the rewrite configuration. In this first handler the handle_response block is configured to use upon auth success the headers handler to copy the headers to the request, and upon failure a handler can respond with an unauthorized message. Then the second handler follows the first with the rewrite and forwards to the legitimate upstream if the auth succeeds.

Conductor should setup such an auth service which can detect a list of bearer tokens to authorize CGI functions. CGI functions can be associated with policies, and policies can specify a number of rules.

The policy could contain :

• a list of static bearer tokens accepted
• a list of JWT public keys accepted (RS*, ES*)

Services making use of these tokens will have to have the tokens or JWT private keys configured

Actions

It should be possible to declare actions in the services. For example an action could perform a data migration, another could enter a console program to debug the application.

It should be as simple as running conductor action run SERVICE db:migrate ....

It should be easy to declare an action to be accessible remotely via a CGI script, security should prevent unauthorized access.