Linker is a lightweight dependency resolver

gcc linker.c -lpthread -o linker

$ linker -h

linker [opt] [event:script].. [service:event]..
 
opt: 
                -h              : print help
                -r <role>       : set role [deamon/waitfor] 
                -p <port>       : listen port
                -P <peerport>   : peer port (port to send request/response)
                example:
                ./linker -p 5000 -r deamon  mysql_start:/usr/local/mysql_start_check.sh 
                ./linker -p 4000 -P 5000 -r waitfor mysql:mysql_start 

Linker use two roles:
1. Daemon
2. waitfor

While running as daemon linker runs in background, to perform event check requests. On a event check request from dependent node, it executes corresponsing event check script and return response

While running as waitfor linker runs in foreground, to halt execution till all the event has successfully occured

linker can be useful in a docker compose environment. Docker entrypoint script can use linker to waitfor other service in different container

For a situation where webserver container depends on mysql container, we can use docker compose features to link and depends_on. Although depends_on check if the container has started, but not the application status

to halt webserver from starting, linter can be used to waitfor mysql to start

to check mysql status locally, linker deamon can be started in mysql container as:

linker -r deamon -p 5001 -P 5000 mysql_start:./mysql_start_check.sh

Here we registering mysql_start_check.sh script for mysql_start event mysql_start_check.sh should return zero if mysql is started, and non zero in case of error
mysql_start_check.sh script can be written as:

#!/bin/bash

USER=root
PASS=root123

mysqladmin -h remote_server_ip -u$USER -p$PASS processlist

 if [ $? -eq 0 ]
        then
                echo "do nothing"
        else
                ssh remote_server_ip
                service mysqld start
 fi

to make webserver container wait for mysql to start, linker can be started to waitfor in the entrypoint script:

linker -r waitfor -p 5000 -P 5001 mysql:mysql_start

A service name can be used, which would be resolved dynamically if the containers are linked

In a daemon multiple events check script can be resgisterd For example in db service:

linker -r deamon -p 5001 -P 5000 mysql_start:./mysql_start_check.sh pesql_start:./pesql_start_check.sh

At the same way, While waiting for multiple events can be resgisterd as

linker -r waitfor -p 5000 -P 5001 db:mysql_start db:pesql_start