Merge branch 'master' into split_provision_and_start.sh

Author: wangkuiyi

dev.md

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+## Develop, Release, and Use SQLFlow-in-a-VM
+
+### For Developers
+
+1. Install VirtualBox and Vagrant on a computer with a relatively large memory size.
+1. `git clone https://github.com/sql-machine-learning/desktop && cd desktop`
+1. To build and start the VM: `vagrant up`
+1. To SSH into the VM: `vagrant ssh`
+1. The `vagrant up` command starts the VM and runs `provision.sh` as user `root`.  The `provision.sh` installs Docker and minikube and pulls related Docker images.  It also generates a `start.sh`.  Users should run `vagrant ssh` and execute `start.sh` inside the VM as user `vagrant`.  The `start.sh` starts minikube, deploys Argo/Tekton, and run all the services in the VM.
+1. Every time we edit the `provison.sh`, we can run `vagrant provision` to rerun `provision.sh` as root.  This rerun would see the previously installed software.  So `provision.sh` contains some `if..else` structures to skip reinstalling software.
+1. To suspend the VM, run `vagrant halt`.  You can run `vagrant up` later to resume it.
+1. To completely destroy the VM and re-provision it, run `vagrant reload`.
+
+### For Releaser
+
+The releaser, which, in most cases, is a developer, can export a running VirtualBox VM into a VM image file with extension `.ova`.  An `ova` file is a tarball of a directory, whose content follows the OVF specification.  For the concepts, please refer to this [explanation](https://damiankarlson.com/2010/11/01/ovas-and-ovfs-what-are-they-and-whats-the-difference/).
+
+According to this [tutorial](https://www.techrepublic.com/article/how-to-import-and-export-virtualbox-appliances-from-the-command-line/), releasers can run the following command to list running VMs.
+
+```bash
+vboxmanage list vms
+```
+
+Then, they can run the following command to export the `.ova` file.
+
+```bash
+vboxmanage export UBUNTUSERVER164 -o ubuntu_server_new.ova
+```
+
+### For End-users
+
+To run SQLFlow on a desktop computer running Windows, Linux, or macOS, an end-user needs to download
+
+1. the `sqlflow` command-line tool released by SQLFlow CI, and
+1. the released `.ova` file.
+
+If the end-user has VirtualBox installed -- no Vagrant required -- s/he could import the `.ova` file and start an VM.
+
+Or, if s/he has an AWS or Google Cloud account, s/he could upload the `.ova` file to start the VM on the cloud.  AWS users can follow [these steps](https://aws.amazon.com/ec2/vm-import/).
+
+Anyway, given a running VM, the end-user can run the following command to connect to it:
+
+```bash
+sqlflow -server=my-vm.aws.com:3306
+```