Command & Control systems that are either always online or occasionally available online. Similar to Servers or IOT platforms where the link for reaching them can be stable wire/fibre LAN connections, or not-always-connected satellite links or GSM connections, or devices available over WAN connections.
- [Send to socket with netcat](#send-to-socket-with-netcat)
- [Sending a command from one Node to Another Node](#sending-a-command-from-one-node-to-another-node)
- [Example JSON for appending a message of type command into the `socket` file](#example-json-for-appending-a-message-of-type-command-into-the-socket-file)
- [Specify more messages at once do](#specify-more-messages-at-once-do)
- [Send the same message to several hosts by using the toHosts field](#send-the-same-message-to-several-hosts-by-using-the-tohosts-field)
- [Send an Op Command message for process listing with custom timeout and amount of retries](#send-an-op-command-message-for-process-listing-with-custom-timeout-and-amount-of-retries)
- [Send and Op Command to stop a subscriber on a node](#send-and-op-command-to-stop-a-subscriber-on-a-node)
- [Send and Op Command to start a subscriber on a node](#send-and-op-command-to-start-a-subscriber-on-a-node)
- [Tail a log file on a node, and save the result of the tail centrally at the directory specified](#tail-a-log-file-on-a-node-and-save-the-result-of-the-tail-centrally-at-the-directory-specified)
- [Services at startup of Steward. Could be implemented by having a local folder of messages to go through at startup. What is needed](#services-at-startup-of-steward-could-be-implemented-by-having-a-local-folder-of-messages-to-go-through-at-startup-what-is-needed)
- [Add Op option the remove messages from the queue on nodes](#add-op-option-the-remove-messages-from-the-queue-on-nodes)
Command And Control anything like Servers, Containers, VM's or others by creating and sending messages with methods who will describe what to do. Steward will then take the responsibility for making sure that the message are delivered to the receiver, and that the method specified are executed with the given parameters defined. An example of a message.
If the receiver `toNode` is down when the message was sent, it will be **retried** until delivered within the criterias set for `timeouts` and `retries`. The state of each message processed is handled by the owning steward instance where the message originated, and no state about the messages are handled in the message broker.
Since the initial connection from a Steward node is outbound towards the central message broker no inbound firewall openings are needed.
Send Request Methods to control your servers by passing a messages that will have guaranteed delivery based on the criteries set, and when/if the receiving node is available. The result of the method executed will be delivered back to you from the node you sent it from.
Examples of Request Methods:
- **Any** shell command or script of your own choice.
Steward uses **NATS** as message passing architecture for the commands back and forth from nodes. Delivery is guaranteed within the criterias set. All of the processes in the system are running concurrently, so if something breaks or some process is slow it will not affect the handling and delivery of the other messages in the system.
A node can be a server running any host operating system, a container living in the cloud somewhere, a Rapsberry Pi, or something else that needs to be controlled that have an operating system installed.
Steward can be compiled to run on all major architectures like **x86**, **amd64**,**arm64**, **ppc64** and more, with for example operating systems like **Linux**, **OSX**, **Windows** and **Android**.
The idea for how to handle processes, messages and errors are based on Joe Armstrongs idea behind Erlang described in his Thesis <https://erlang.org/download/armstrong_thesis_2003.pdf>.
Joe's document describes how to build a system where everything is based on sending messages back and forth between processes in Erlang, and where everything is done concurrently.
I used those ideas as inspiration for building a fully concurrent system to control servers or container based systems by passing messages between processes asynchronously to execute methods, handle errors, or handle the retrying if something fails.
In a push setup the commands to be executed is pushed to the receiver, but if a command fails because for example a broken network link it is up to you as an administrator to detect those failures and retry them at a later time until it is executed successfully.
In a pull setup an agent is installed at the Edge unit, and the configuration or commands to execute locally are pulled from a central repository. With this kind of setup you can be pretty certain that sometime in the future the node will reach it's desired state, but you don't know when. And if you want to know the current state you will need to have some second service which gives you that information.
In it's simplest form the idea about using an event driven system as the core for management of Edge units is that the sender/publisher are fully decoupled from the receiver/subscriber. We can get an acknowledge if a message is received or not, and with this functionality we will at all times know the current state of the receiving end.
1. A message in valid format is appended to the in socket.
1. The message is picked up by the system and put on a FIFO ringbuffer.
1. The method type of the message is checked, a subject is created based on the content of the message, and a publisher process to handle the message type for that specific receiving node is started if it does not exist.
1. The message is then serialized to binary format, and sent to the subscriber on the receiving node.
1. If the message is expected to be ACK'ed by the subcriber then the publisher will wait for an ACK if the message was delivered. If an ACK was not received within the defined timeout the message will be resent. The amount of retries are defined within the message.
1. The receiving end will need to have a subscriber process started on a specific subject and be allowed handle messages from the sending nodes to execute the method defined in the message.
1. When a message have been deserialized, it will lookup the correct handler for the method type specified within the message, and execute that handler.
1. If the output of the method called is supposed to be returned to the publiser it will do so by using the replyMethod specified, and pick up the next message in the queue.
Tue Sep 21 09:17:55 2021, info: toNode: ship2, fromNode: central, method: REQOpProcessList: max retries reached, check if node is up and running and if it got a subscriber for the given REQ type
- The handling of all messages is done by spawning up a process for handling the message in it's own thread. This allows us to down on the **individual message level** keep the state for each message both in regards to ACK's, error handling, send retries, and rerun of a method for a message if the first run was not successful.
- Processes for handling messages on a host can be **restarted** upon **failure**, or asked to just terminate and send a message back to the operator that something have gone seriously wrong. This is right now just partially implemented to test that the concept works, where the **action=no-action**.
- Messages not fully processed or not started yet will be automatically rehandled if the service is restarted since the current state of all the messages being processed are stored on the local node in a **key value store** until they are finished.
- All messages processed by a publisher will be written to a log file as they are processed, with all the information needed to recreate the same message if needed, or it can be used for auditing.
- All handling down to the process and message level are handled concurrently. So if there are problems handling one message sent to a node on a subject it will not affect the messages being sent to other nodes, or other messages sent on other subjects to the same host.
- Message types of both **ACK** and **NACK**, so we can decide if we want or don't want an Acknowledge if a message was delivered succesfully.
Example: We probably want an **ACK** when sending some **REQCLICommand** to be executed, but we don't care for an acknowledge **NACK** when we send an **REQHello** event.
- Default timeouts to wait for ACK messages and max attempts to retry sending a message are specified upon startup. This can be overridden on the message level.
- Timeouts can be specified on both the **message**, and the **method**.
- A message can have a timeout.
- If the method triggers a shell command, the command can have its own timeout, allowing process timeout for long/stuck commands, or for telling how long the command is supposed to run.
Will run the command given, and return the stdout output of the command continously while the command runs. Uses the methodTimeout to define for how long the command will run.
Tail log files on some node, and get the result for each new line read sent back in a reply message. Uses the methodTimeout to define for how long the command will run.
Scrape web url, and get the html sent back in a reply message. Uses the methodTimeout for how long it will wait for the http get method to return result.
Write the output of the reply message to a file specified with the `directory` and `fileName` fields, where the writing will write over any existing content of that file.
**ReqCliCommand** is a bit special in that it can be used as both **method** and **replyMethod**
The final result, if any, of the replyMethod will be sent to the central server.
By using the `{{STEWARD_DATA}}` you can grab the output of your initial request method, and then use it as input in your reply method.
**NB:** The echo command in the example below will remove new lines from the data. To also keep any new lines we need to put escaped **quotes** around the template variable. Like this:
The broker for messaging is Nats-server from <https://nats.io>. Download, run it, and use the `-brokerAddress` flag on **Steward** to point to the ip and port:
There is a lot of different variants of how you can setup and confiure Nats. Full mesh, leaf node, TLS, Authentication, and more. You can read more about how to configure the Nats broker called nats-server at <https://nats.io/>.
### Add Op option the remove messages from the queue on nodes
If messages have been sent, and not picked up by a node it might make sense to have some method to clear messages on a node. This could either be done by message ID, and/or time duration.
All code in this repository are to be concidered not-production-ready, and the use is at your own responsibility. The code are the attempt to concretize the idea of a purely async management system where the controlling unit is decoupled from the receiving unit, and that that we know the state of all the receiving units at all times.
