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All right Nuno's asking I need my docker file command to start Cassandra server and after that I need
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to import a database schema.
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What is the best way to do that.
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So if someone hasn't already answered you in chat because sometimes that happens while I'm rambling
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on the the entry point script is sort of the universal way to do things on container startup.
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So this means that you have inside your Docker file you have the entry point and you have this BMD.
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Now in my course Docker mastery I focus on the CMT because that's really the thing that you need to
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understand to be at the beginning what I need to add is a section at least one lecture on the entry
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point and discussing the various ways that you can do entry points.
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So an entry point when we break it down the entry point in the CND are really two parts of the same
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single line or single command that Docker runs when your container starts.
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But the trick is is with the magic of bash scripting you can take the entry point and instead of it
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just being the beginning of the command that might be on in front of the CMT it can run a script and
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then at the end of that script you can tell it to pass execution to the rest of the command line.
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So it may not make a whole lot of sense but let's go check a real world went out and see what we can
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see so I'm going to pull up my sequel because really any persistent data storage system is going to
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act the same way here
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and if I look up on Docker Hub
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so I'm at the Docker Hub my sequel official image.
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And if I click on any one of these releases to top I will get the docker file which is open source and
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in this file you'll find lots of things.
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Obviously it's installing my sequel and getting sequel properly configured but down at the bottom you'll
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notice a couple of things one is it's copying a shell script into a certain location and it's running
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assembling there but that's just really for backwards compatibility.
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But down here this is the key part the entry point script
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which the entry point itself the command in the darker file doesn't have to be a script.
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Again it's really just taking if the if it exists the entry point and they see OMD and it's just putting
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them together and running them all in one line.
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So you would see this the two common way as the entry point is used is either for the beginning of a
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command that maybe if you're using Docker to run a command line utility I do this in 82 ping
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see if I can uh so I have a github.
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I have a Docker image rather and I get hub repo behind it and what it's doing is it's running a single
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command line utility in a container.
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And the way that it works here is I have the entry point being the command itself.
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And then since the CMB goes after that when I have both of them in my Docker file it just switches them
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together.
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I have it running the default CRM D so if you think about it when I run this command from the command
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I can do Docker run.
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Brett Fisher slash HP paying.
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And then I just hit enter and then it will automatically run the version command.
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But if I keep typing in that command line right.
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Tucker run.
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Brett Fisher if I just do that it's gonna pull the image.
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Whoops sorry let me uh switch screens on you
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so if I run this image it's gonna run it by default and it basically dumps out the help because that's
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what the docker file told it to do in the CRM D line.
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But if I just change something on the end because remember the format of a docker run command is that
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it will do Docker run and then do your options then do the image you're going to set you're going to
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run and then anything after that replaces the CND in line.
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So it's like a dynamic replacement of the CMT.
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So if I just run here Google dot com it will replace the dash dash version that it would run by default.
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So this is a really cool way that you can use Docker run as a command line utility and that's a good.
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That's my example essentially of how to do that.
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Now the other way to do it is with
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is not that that's the that's the one way to do it the other way is a shell script so we'll see here
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the entry point here is defining the file that it should run.
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And then if I just scroll all the way to the top here and I do this eight point no directory I sort
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of go up one directory I'll actually find it there's another file there.
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That's the docker entry point script that it's copying in on the build.
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Now in this script what's going on.
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There's tons of stuff in here.
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Probably at least a hundred lines.
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Yep.
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In fact it's hundreds of lines.
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But the magic part here is this.
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Zoom out.
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Okay.
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Go away scroll bar.
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I do not want to see you.
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Let's just go to Ras
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that very bottom line there that.
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Exactly.
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And then the dollar sign at that tells the shell hey I'm ending this script please pass the execution
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to the rest of my command line which would be whatever is in the CMT and in this case it was the my
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sequel demon.
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So essentially what's gonna happen when a container starts up in this case is it will run the script.
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It gets to the end of the script the exact says Please do whatever else I told you at the end of this
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command and that runs your demon.
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You're you're my sequel demon in the foreground so that it stays running.
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Now what the script does is all the things you probably didn't realize were happening in the background
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when you started a database container.
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It creates the default user that you want to set.
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It changes the root password to whatever you want it to set.
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It creates the database based on what you told it and it's doing all that and environment variables
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right.
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Because if we backed up to just duplicate this and if we backed up to the library
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you know on most of these official images you're gonna have a whole bunch of environment variables and
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it specifies them all down here.
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And all those environment variables are used in that script to pre configure my sequel.
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Now in the file itself if you were actually to dive into it it is setting some environment variables
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and finding files in case you're using Drucker's warm secret stuff like that and then it's basically
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getting every all the data it needs to configure it.
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Then it starts up my sequel configured it including changing the root password in all the other things.
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Then it shuts down my sequel and then ends the script and then passes that execution to my sequel to
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start up again but this time for good.
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So it's essentially starting sequel doing some things stopping sequels starting a backup.
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And that's the way that all these work all these persistent storage systems inside of the official containers
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work.
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So for you The long answer to this question is that you're going to want to use it in your own custom
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entrapment script maybe you copy the default one if there is one or maybe you make your own and then
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you would make your own image that would then create that schema initially on the first startup.
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Now once you have your fear a schema inside the database you know you're not going to really want to
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update schemas that way you would want to do further schemas.
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Through some other utility outside of that container right you would run your my cycle commands from
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another container and then they would run over the network something like that.
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Now you don't have to change this at all you could simply have it start up your Cassandra and then add
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the schema from a remote from a different container but you would have to control that order right.
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You would have to wait for Cassandra to start up.
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You would have to have another container that maybe has its own script and then that script maybe a
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smart enough to keep checking for Cassandra to see if it's ready.
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And then when it's ready then run your schema either schema update or your original schema installation.
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Essentially you could do it that way and that and a lot of cases is probably the right way to do it
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but especially when you're doing local development or you just want to speed up something small and
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easy and you just want to build a scheme of the first time and you wanted to do it automatically then
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what you would do here is you would use the official default image and you would add your own schema
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and file to that in your own custom image and essentially it would when it started up the first time
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it would do that now.
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Here's the problem and you gotta watch out for this this entry point script runs every time the container
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starts at least from that image right from that image that you had built the custom one.
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So you want to make sure that your script is smart enough that it doesn't rerun the schema over and
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over again on an ad database it already has the schema.
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So you have to build on that intelligence and that's why sometimes depending on your model if you're
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thinking of updating things regularly or if you're constantly moving databases around in a swarm cluster
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or Cuban ideas cluster then doing it inside the database container itself may not be the right strategy
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for you but this option exists.
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So check it out.
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And that is like the Cliff Notes on what to do with entry points which was not in my original course
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so I could probably say that this video in there and least have supplemental material for now that's
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better than having nothing.