kvexpress

kvexpress

Why?

Small Go utility to:

1. Put data into Consul's KV store.
2. Pull data out of Consul's KV store and write it to a file.

Why a dedicated utility though? Can't I just do it with curl?

Yes you can - but we kept wanting to:

1. Make sure the file was long enough. 0-length configuration files are bad.
2. Load the file from some other custom templating process - not just from straight KV files.
3. Put the file into any location in the filesystem.
4. Restart/reload/stop/start daemon after writing the file.
5. Run some other custom command after writing the file.
6. Verify that the file we put into the KV was the same file that was written on the other end.
7. Stop the process on all nodes - in or out - if we want everything to stay as it is for the moment.

We did this at first with some custom Ruby scripts - but the pattern was apparent and could be applied to many other files as well.

This replaces our previous custom Ruby/shell scripts with a single Go binary we can use to get data in and out of Consul's KV store.

How does it work? - 1000 foot view

There's an introductory blog post available that shows how it can be used.

In

kvexpress in --key hosts --file /etc/consul-template/output/hosts.consul --length 100 --sorted=true

1. Check that at least --file and --key are passed along with the command. Quit if they're not present - there are no safe defaults for those flags.
2. Check for the existence of a stop key - if it's there - stop and exit.
3. Read the file into a string, and sort the string if requested.
4. Check if the file is long enough - if not - stop and exit.
5. Save the file to a .compare file - we will use this data from now on.
6. Check for the existence of a .last file - if it's not there - create it.
7. Are the .compare and .last files blank? If not - let's continue.
8. Compare the checksums of the .compare and .last files - if they're different - continue.
9. Grab the checksum from Consul and compare with the .compare file - if it's different - then let's update. This is to guard against it running on multiple server nodes that might have different .last files.
10. Save data, and checksum keys.
11. Copy .compare to .last
12. If --exec is passed - run that command.

Out

kvexpress out -k hosts -f /etc/hosts.consul -l 100 -e 'sudo pkill -HUP dnsmasq'

1. Check that at least --file and --key are passed along with the command. Quit if they're not present - there are no safe defaults for those flags.
2. Check for the existence of a stop key - if it's there - stop and exit.
3. Pull the data and checksum keys out of Consul.
4. If data is long enough and the checksum as computed on this side matches the checksum key - then continue.
5. Write the contents of data to the passed --file location.
6. If --exec is passed - run that command.

Where can I get it?

Build instructions are available below.

Debian packages for Ubuntu can be downloaded from packagecloud.

Additional binaries can be downloaded from the releases page.

A Chef cookbook is available as well.

How to use it with Consul watches.

Instructions are available here.

Ad-Hoc Usage with Consul exec

If you want to push a file to all nodes in your Consul cluster quickly - you can use Consul and kvexpress.

kvexpress in --prefix tmp --key config --length 5 --url https://git.io/vzz5P

This places the file and checksum in:

tmp/config/data
tmp/config/checksum

To get the file on every node:

consul exec "sudo kvexpress out --prefix tmp --key config --length 5 -f /etc/file.conf -e 'sudo restart something here'"

If you're not seeing it work the way you expect - you can always add --verbose to the kvexpress command - that will add lots of additional output.

Commands Available

A detailed list of commands is available here.

Consul KV Structure

How are keys organized in Consul's KV store to work with kvexpress?

Underneath a global prefix /kvexpress/ - each directory represents a specific file we are distributing through the KV store.

Each directory is named for the unique key and has the following keys underneath it:

1. data - where the configuration file is stored.
2. checksum - where the SHA256 of the data is stored.

For example - the hosts file is arranged like this:

/kvexpress/hosts/data
/kvexpress/hosts/checksum

There is an optional stop key - that if present - will cause all in and out processes to stop before writing anything. Allows us to freeze the automatic process if we need to.

Logging

All logs are sent to syslog and are tagged with kvexpress. To enable debug logs, please export KVEXPRESS_DEBUG=1

Build

Can be built with the standard go toolchain: go get -u -v github.com/DataDog/kvexpress

To build manually - clone the repo then: make deps && make

To run integration tests: make deps && make && make test - it will spin up an empty Consul and kill it after the run.

Because we use user.Current() - you can't cross compile this. If you want to build for Linux - you must build on Linux. Closed Issue

To install Consul - there are instructions here.

To launch an empty Consul instance: make consul

Contributing

We love pull requests from anyone. Details are available here.

Code of Conduct

This project adheres to the [Open Code of Conduct][code-of-conduct]. By participating, you are expected to honor this code. [code-of-conduct]: http://todogroup.org/opencodeofconduct/#kvexpress/darron@froese.org