An annotated example backend (GNU Flymake 1.3.4)

;;; ruby-flymake.el --- A ruby Flymake backend -*- lexical-binding: t; -*-
(require 'cl-lib)
(defvar-local ruby--flymake-proc nil)

(defun ruby-flymake (report-fn &rest _args)
 ;; Not having a ruby interpreter is a serious problem which should cause
 ;; the backend to disable itself, so an error is signaled.
 ;;
 (unless (executable-find
 "ruby") (error "Cannot find a suitable ruby"))
 ;; If a live process launched in an earlier check was found, that
 ;; process is killed. When that process's sentinel eventually runs,
 ;; it will notice its obsoletion, since it have since reset
 ;; `ruby-flymake-proc' to a different value
 ;;
 (when (process-live-p ruby--flymake-proc)
 (kill-process ruby--flymake-proc))

 ;; Save the current buffer, the narrowing restriction, remove any
 ;; narrowing restriction.
 ;;
 (let ((source (current-buffer)))
 (save-restriction
 (widen)
 ;; Reset the `ruby--flymake-proc' process to a new process
 ;; calling the ruby tool.
 ;;
 (setq
 ruby--flymake-proc
 (make-process
 :name "ruby-flymake" :noquery t :connection-type 'pipe
 ;; Make output go to a temporary buffer.
 ;;
 :buffer (generate-new-buffer " *ruby-flymake*")
 :command '("ruby" "-w" "-c")
 :sentinel
 (lambda (proc _event)
 ;; Check that the process has indeed exited, as it might
 ;; be simply suspended.
 ;;
 (when (memq (process-status proc) '(exit signal))
 (unwind-protect
 ;; Only proceed if `proc' is the same as
 ;; `ruby--flymake-proc', which indicates that
 ;; `proc' is not an obsolete process.
 ;;
 (if (with-current-buffer source (eq proc ruby--flymake-proc))
 (with-current-buffer (process-buffer proc)
 (goto-char (point-min))
 ;; Parse the output buffer for diagnostic's
 ;; messages and locations, collect them in a list
 ;; of objects, and call `report-fn'.
 ;;
 (cl-loop
 while (search-forward-regexp
 "^\\(?:.*.rb\\|-\\):\\([0-9]+\\): \\(.*\\)$"
 nil t)
 for msg = (match-string 2)
 for (beg . end) = (flymake-diag-region
 source
 (string-to-number (match-string 1)))
 for type = (if (string-match "^warning" msg)
 :warning
 :error)
 when (and beg end)
 collect (flymake-make-diagnostic source
 beg
 end
 type
 msg)
 into diags
 finally (funcall report-fn diags)))
 (flymake-log :warning "Canceling obsolete check %s"
 proc))
 ;; Cleanup the temporary buffer used to hold the
 ;; check's output.
 ;;
 (kill-buffer (process-buffer proc)))))))
 ;; Send the buffer contents to the process's stdin, followed by
 ;; an EOF.
 ;;
 (process-send-region ruby--flymake-proc (point-min) (point-max))
 (process-send-eof ruby--flymake-proc))))

(defun ruby-setup-flymake-backend ()
 (add-hook 'flymake-diagnostic-functions 'ruby-flymake nil t))
(add-hook 'ruby-mode-hook 'ruby-setup-flymake-backend)