Creating a Vault instance with a TLS Consul Cluster

06 Oct 2019

So we want to set up a Vault instance, and have it’s storage be a TLS based Consul cluster. The problem is that the Consul cluster needs Vault to create the certificates for TLS, which is quite the catch-22. Luckily for us, quite easy to solve:

  1. Start a temporary Vault instance as an intermediate ca
  2. Launch Consul cluster, using Vault to generate certificates
  3. Destroy temporary Vault instance
  4. Start a permanent Vault instance, with Consul as the store
  5. Reprovision the Consul cluster with certificates from the new Vault instance

Sequence diagram of the previous numbered list

There is a repository on Github with all the scripts used, and a few more details on some options.


The Host machine needs the following software available in your PATH:

You have a TLS Certificate you can use to create an intermediate CA with. See this blog post for How to create a local CA


The script will do all of this for you, but an explanation of the steps is below:

  1. Start a Temporary Vault instance

     echo '
     storage "inmem" {}
     listener "tcp" {
       address = ""
       tls_disable = 1
     }' > "vault/temp_vault.hcl"
     vault server -config="vault/temp_vault.hcl" &
     echo "$!" >
     export VAULT_TOKEN=$(./ | tail -n 1)
  2. Generate a Vault token for the Consul machines to use to authenticate with Vault

     export CONSUL_VAULT_TOKEN=$(vault write -field=token -force auth/token/create)
  3. Launch 3 Consul nodes (uses the CONSUL_VAULT_TOKEN variable)

     vagrant up

    The vagrantfile just declares 3 identical machines:

     Vagrant.configure(2) do |config| = "pondidum/hashibox"
       config.vm.provision "consul",
         type: "shell",
         path: "./",
         env: {
       config.vm.define "c1" do |c1|
         c1.vm.hostname = "consul1"
       config.vm.define "c2" do |c2|
         c2.vm.hostname = "consul2"
       config.vm.define "c3" do |c3|
         c3.vm.hostname = "consul3"

    The provisioning script just reads a certificate from Vault, and writes out pretty much the same configuration as in the last post on creating a TLS enabled Consul Cluster, but you can view it in the repository for this demo too.

  4. Create a local Consul server to communicate with the cluster:


    This is done so that the Vault instance can always communicate with the Consul cluster, no matter which Consul node we are reprovisioning later. In a production environment, you would have this Consul server running on each machine that Vault is running on.

  5. Stop the temporary Vault instance now that all nodes have a certificate

    kill $(cat
  6. Start the persistent Vault instance, using the local Consul agent

     echo '
     storage "consul" {
       address = "localhost:8501"
       scheme = "https"
     listener "tcp" {
       address = ""
       tls_disable = 1
     }' > "$config_dir/persistent_vault.hcl"
     vault server -config="$config_dir/persistent_vault.hcl" > /dev/null &
     echo "$!" >
     export VAULT_TOKEN=$(./ | tail -n 1)
  7. Generate a new Vault token for the Consul machines to use to authenticate with Vault (same as step 2)

     export CONSUL_VAULT_TOKEN=$(vault write -field=token -force auth/token/create)
  8. Reprovision the Consul nodes with new certificates

     vagrant provision c1 --provision-with consul
     vagrant provision c2 --provision-with consul
     vagrant provision c3 --provision-with consul
  9. Profit

    To clean up the host’s copy of Vault and Consul, you can run this:

     kill $(cat
     kill $(cat

Summary & Further Actions

Luckily, this is the kind of thing that should only need doing once (or once per isolated environment). When running in a real environment, you will also want to set up:

  • ACL in Consul which locks down the KV storage Vault uses to only be visible/writeable by Vault
  • Provisioning the VAULT_TOKEN to the machines in a secure fashion
  • Periodic refresh of the Certificates uses in the Consul cluster

Consul DNS Fowarding in Ubuntu, revisited

24 Sep 2019

I was recently using my Hashibox for a test, and I noticed the DNS resolution didn’t seem to work. This was a bit worrying, as I have written about how to do DNS resolution with Consul forwarding in Ubuntu, and apparently something is wrong with how I do it. Interestingly, the Alpine version works fine, so it appears there is something not quite working with how I am configuring Systemd-resolved.

So this post is how I figured out what was wrong, and how to do DNS resolution with Consul forwarding on Ubuntu properly!

The Problem

If Consul is running on the host, I can only resolve .consul domains, and if Consul is not running, I can resolve anything else. Clearly I have configured something wrong!

To summarise, I want to be able to resolve 3 kinds of address:

  • *.consul addresses should be handled by the local Consul instance
  • $ should be handled by the Hyper-V DNS server (running on the Host machine)
  • public DNS should be resolved properly


To make sure that hostname resolution even works by default, I create a blank Ubuntu box in Hyper-V, using Vagrant.

Vagrant.configure(2) do |config| = "bento/ubuntu-16.04"
  config.vm.hostname = "test"

I set the hostname so that I can test that dns resolution works from the host machine to the guest machines too. I next bring up the machine, SSH into it, and try to dig my hostmachine’s DNS name (

> vagrant up
> vagrant ssh
> dig

; <<>> DiG 9.10.3-P4-Ubuntu <<>>
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 12333
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;            IN      A

;; ANSWER SECTION:     0       IN      A

;; Query time: 0 msec
;; WHEN: Mon Sep 23 21:57:26 UTC 2019
;; MSG SIZE  rcvd: 70

> exit
> vagrant destroy -f

As you can see, the host machine’s DNS server responds with the right address. Now that I know that this should work, we can tweak the Vagrantfile to start an instance of my Hashibox:

Vagrant.configure(2) do |config| = "pondidum/hashibox"
  config.vm.hostname = "test"

When I run the same command sin this box, I get a slighty different response:

; <<>> DiG 9.10.3-P4-Ubuntu <<>>
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 57216
;; flags: qr aa rd; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 1
;; WARNING: recursion requested but not available

; EDNS: version: 0, flags:; udp: 4096
;            IN      A

consul.                 0       IN      SOA     ns.consul. hostmaster.consul. 1569276784 3600 600 86400 0

;; Query time: 1 msec
;; WHEN: Mon Sep 23 22:13:04 UTC 2019
;; MSG SIZE  rcvd: 103

As intended, the DNS server on localhost responded…but it looks like Consul answered, not the inbuilt dns server (systemd-resolved), as I intended.

The reason for this is that I am running Consul’s DNS endpoint on 8600, and Systemd-Resolved cannot send requests to anything other than port 53, so I use iptables to redirect the traffic from port 53 to 8600, which means any local use of DNS will always be sent to Consul.

The reason it works when Consul is not running is that we have both specified as a nameserver, and a fallback set to be the eth0’s Gateway, so when Consul doesn’t respond, the request hits the default DNS instead.

The Solution: Dnsmasq.

Basically, stop using systemd-resolved and use something that has a more flexible configuration. Enter Dnsmasq.

Starting from the blank Ubuntu box, I install dnsmasq, and disable systemd-resolved. Doing this might prevent any DNS resolutio working for a while…

sudo apt-get install -yq dnsmasq
sudo systemctl disable systemd-resolved.service

If you would rather not disable systemd-resolved entirely, you can use these two lines instead to just switch off the local DNS stub:

echo "DNSStubListener=no" | sudo tee --append /etc/systemd/resolved.conf
sudo systemctl restart systemd-resolved

Next I update /etc/resolv.conf to not be managed by Systemd, and point to where dnsmasq will be running:

sudo rm /etc/resolv.conf
echo "nameserver" | sudo tee /etc/resolv.conf

The reason for deleting the file is that it was symlinked to the Systemd-Resolved managed file, so that link needed to be broken first to prevent Systemd interfering.

Lastly a minimal configuration for dnsmasq:

echo '
' | sudo tee /etc/dnsmasq.d/default

sudo systemctl restart dnsmasq

This config does a few things, the two most important lines are:

  • resolv-file=/var/run/dnsmasq/resolv.conf which is pointing to the default resolv.conf written by dnsmasq. This file contains the default nameserver supplied by the default network connection, and I want to use this as a fallback for anything dnsmasq cannot resolve directly (which will be everything, except .consul). In my case, the content of this file is just nameserver

  • server=/consul/ specifies that any address ending in .consul should be forwarded to Consul, running at on port 8600. No more iptables rules!


Now that I have a (probably) working DNS system, let’s look at testing it properly this time. There are 3 kinds of address I want to test:

  • Consul resolution, e.g. consul.service.consul should return the current Consul instance address.
  • Hostname resolution, e.g. should resolve to the machine hosting the VM.
  • Public resolution, e.g. should resolve to…reddit.

I also want to test that the latter two cases work when Consul is not running too.

So let’s write a simple script to make sure these all work. This way I can reuse the same script on other machines, and also with other VM providers to check DNS works as it should. The entire script is here:


consul agent -dev -client -bind '{{ GetInterfaceIP "eth0" }}' > /dev/null &
sleep 1

consul_ip=$(dig consul.service.consul +short)
self_ip=$(dig $HOSTNAME.$local_domain +short | tail -n 1)
host_ip=$(dig $host_machine.$local_domain +short | tail -n 1)
reddit_ip=$(dig +short | tail -n 1)

kill %1

[ "$consul_ip" == "" ] && echo "Didn't get consul ip" >&2 && exit 1
[ "$self_ip" == "" ] && echo "Didn't get self ip" >&2 && exit 1
[ "$host_ip" == "" ] && echo "Didn't get host ip" >&2 && exit 1
[ "$reddit_ip" == "" ] && echo "Didn't get reddit ip" >&2 && exit 1

echo "==> Consul Running: Success!"

consul_ip=$(dig consul.service.consul +short | tail -n 1)
self_ip=$(dig $HOSTNAME.$local_domain +short | tail -n 1)
host_ip=$(dig $host_machine.$local_domain +short | tail -n 1)
reddit_ip=$(dig +short | tail -n 1)

[[ "$consul_ip" != *";; connection timed out;"* ]] && echo "Got a consul ip ($consul_ip)" >&2 && exit 1
[ "$self_ip" == "" ] && echo "Didn't get self ip" >&2 && exit 1
[ "$host_ip" == "" ] && echo "Didn't get host ip" >&2 && exit 1
[ "$reddit_ip" == "" ] && echo "Didn't get reddit ip" >&2 && exit 1

echo "==> Consul Stopped: Success!"

exit 0

What this does is:

  1. Read two command line arguments, or use defaults if not specified
  2. Start Consul as a background job
  3. Query 4 domains, storing the results
  4. Stop Consul (kill %1)
  5. Check an IP address came back for each domain
  6. Query the same 4 domains, storing the results
  7. Check that a timeout was received for consul.service.consul
  8. Check an IP address came back for the other domains

To further prove that dnsmasq is forwarding requests correctly, I can include two more lines to /etc/dnsmasq.d/default to enable logging, and restart dnsmasq

echo "log-queries" | sudo tee /etc/dnsmasq.d/default
echo "log-facility=/var/log/dnsmasq.log" | sudo tee /etc/dnsmasq.d/default
sudo systemctl restart dnsmasq
dig consul.service.consul

Now I can view the log file and check that it received the DNS query and did the right thing. In this case, it recieved the consul.service.consul query, and forwarded it to the local Consul instance:

Sep 24 06:30:50 dnsmasq[13635]: query[A] consul.service.consul from
Sep 24 06:30:50 dnsmasq[13635]: forwarded consul.service.consul to
Sep 24 06:30:50 dnsmasq[13635]: reply consul.service.consul is

I don’t tend to keep DNS logging on in my Hashibox as the log files can grow very quickly.

Wrapping Up

Now that I have proven my DNS resolution works (I think), I have rolled it back into my Hashibox, and can now use machine names for setting up clusters, rather than having to specify IP addresses initially.

Creating a TLS enabled Consul cluster

14 Sep 2019

This post is going to go through how to set up a Consul cluster to communicate over TLS. I will be using Vagrant to create three machines locally, which will form my cluster, and in the provisioning step will use Vault to generate the certificates needed.

How to securely communicate with Vault to get the TLS certificates is out of scope for this post.

Host Configuration

Unless you already have Vault running somewhere on your network, or have another mechanism to generate TLS certificates for each machine, you’ll need to start and configure Vault on the Host machine. I am using my Vault Dev Intermediate CA script from my previous post.

To set this up, all I need to do is run this on the host machine, which starts Vault in a docker container, and configures it as an intermediate certificate authority:


I also have DNS on my network setup for the domain so will be using that to test with.

Consul Machine Configuration

The Vagrantfile is very minimal - I am using my Hashibox (be aware the libvirt provider for this might not work, for some reason vagrant package with libvirt produces a non-bootable box).

Vagrant.configure(2) do |config| = "pondidum/hashibox"
  config.vm.provision "consul", type: "shell", path: "./"

  config.vm.define "c1" do |c1|
    c1.vm.hostname = "consul1"

  config.vm.define "c2" do |c2|
    c2.vm.hostname = "consul2"

  config.vm.define "c3" do |c3|
    c3.vm.hostname = "consul3"

The hashibox script already has all the tools we’ll need installed already: Consul, Vault, and jq.

First up, we request a certificate from Vault to use for Consul - How you get this certificate in a secure manner in a production environment is up to you. There is a catch-22 here for me, in that in a production environment I use Vault with Consul as it’s backing store…but Consul needs Vault to start! I’ll go over how I get around this in a future post.

export VAULT_ADDR=""
export VAULT_TOKEN="vault"

response=$(vault write pki/issue/cert -format=json common_name=$ alt_names="server.dc1.consul")

The first thing to note is that we have specified an alt_names for the certificate - you must have a SAN of server.$DC.$DOMAIN so either server.dc1.consul or, and the server prefix is required!.

Next, we need to take all the certificates from the response and write them to the filesystem.

mkdir -p "$config_dir/ca"

for (( i=0; i<$(echo "$response" | jq '.data.ca_chain | length'); i++ )); do
  cert=$(echo "$response" | jq -r ".data.ca_chain[$i]")
  name=$(echo "$cert" | openssl x509 -noout -subject -nameopt multiline | sed -n 's/ *commonName *= //p' | sed 's/\s//g')

  echo "$cert" > "$config_dir/ca/$name.pem"

echo "$response" | jq -r .data.private_key > $config_dir/consul.key
echo "$response" | jq -r .data.certificate > $config_dir/consul.crt
echo "$response" | jq -r .data.issuing_ca >> $config_dir/consul.crt

The for loop iterates through all of the certificates returned in the ca_chain and writes them into a ca directory. We use openssl to get the name of the certificate, so the files are named nicely!

Finally, it writes the private_key for the node’s certificate to consul.key, and both the certificate and issuing_ca to the consul.crt file.

Now for the consul.json. To setup a secure cluster, first of all we need to add the certificate configuration, pointing to the files we wrote earlier:

"ca_path": "$config_dir/ca/",
"cert_file": "$config_dir/consul.crt",
"key_file": "$config_dir/consul.key",

We will also disable the HTTP port, and enable the HTTPS port:

"ports": {
    "http": -1,
    "https": 8501

Finally, we need to add some security settings. First is encrypt, which sets that the key that all Consul nodes will use to encrypt their communications. It must match on all nodes. The easiest way to generate this is just run consul keygen and use the value that produces.

  • "encrypt": "oNMJiPZRlaP8RnQiQo9p8MMK5RSJ+dXA2u+GjFm1qx8=":

    The key the cluster will use to encrypt all it’s traffic. It must be the same on all nodes, and the easiest way to generate the value is to use the output of consul keygen.

  • "verify_outgoing": true:

    All the traffic leaving this node will be encrypted with the TLS certificates. However, the node will still accept non-TLS traffic.

  • "verify_incoming_rpc": true:

    All the gossip traffic arriving at this node must be signed with an authority in the ca_path.

  • "verify_incoming_https": false:

    We are going to use the Consul Web UI, so we want to allow traffic to hit the API without a client certificate. If you are using the UI from a non-server node, you can set this to true.

  • "verify_server_hostname": true:

    Set Consul to verify outgoing connections have a hostname in the format of server.<datacenter>.<domain>. From the docs: “This setting is critical to prevent a compromised client from being restarted as a server and having all cluster state including all ACL tokens and Connect CA root keys replicated to it”

The complete config we will use is listed here:

cat <<-EOF
    "bootstrap_expect": 3,
    "client_addr": "",
    "data_dir": "/var/consul",
    "leave_on_terminate": true,
    "rejoin_after_leave": true,
    "retry_join": ["consul1", "consul2", "consul3"],
    "server": true,
    "ui": true,
    "encrypt": "oNMJiPZRlaP8RnQiQo9p8MMK5RSJ+dXA2u+GjFm1qx8=",
    "verify_incoming_rpc": true,
    "verify_incoming_https": false,
    "verify_outgoing": true,
    "verify_server_hostname": true,
    "ca_path": "$config_dir/ca/",
    "cert_file": "$config_dir/consul.crt",
    "key_file": "$config_dir/consul.key",
    "ports": {
        "http": -1,
        "https": 8501
) | sudo tee $config_dir/consul.json

Lastly, we’ll make a systemd service unit to start consul:

cat <<-EOF
Description=consul agent

ExecStart=/usr/bin/consul agent -config-file=$config_dir/consul.json -bind '{{ GetInterfaceIP "eth0" }}'
ExecReload=/bin/kill -HUP $MAINPID

) | sudo tee /etc/systemd/system/consul.service

sudo systemctl daemon-reload
sudo systemctl enable consul.service
sudo systemctl start consul

As the machines we are starting also have docker networks (and potentially others), our startup line specifies to bind to the eth0 network, using a Consul Template.


First, we need to run our intermediate CA, then provision our three machines:

vagrant up

After a few moments, you should be able to curl the consul ui (curl or open in your browser.

Note, however, the if your root CA is self-signed, like mine is, some browsers (such as FireFox) won’t trust it, as they won’t use your machine’s Trusted Certificate Store, but their own in built store. You can either accept the warning or add your root certificate to the browser’s store.


Now that we have our cluster seemingly running with TLS, what happens if we try to connect a Consul client without TLS to it? On the host machine, I just run a single node, and tell it to connect to one of the cluster nodes:

consul agent \
  -join \
  -bind '{{ GetInterfaceIP "eth0" }}' \
  -data-dir /tmp/consul

The result of this is a refusal to connect, as the cluster has TLS configured, but this instance does not:

==> Starting Consul agent...
==> Log data will now stream in as it occurs:
==> Joining cluster...
==> 1 error occurred:
  * Failed to join Remote state is encrypted and encryption is not configured


In the next post, I’ll go through how we can set up a Vault cluster which stores its data in Consul, but also provision that same Consul cluster with certificates from the Vault instance!

Using Vault as a Development CA

25 Aug 2019

Often when developing or testing some code, I need (or want) to use SSL, and one of the easiest ways to do that is to use Vault. However, it gets pretty annoying having to generate a new CA for each project, and import the CA cert into windows (less painful in Linux, but still annoying), especially as I forget which cert is in use, and accidentally clean up the wrong ones.

My solution has been to generate a single CA certificate and PrivateKey, import this into my Trusted Root Certificate Store, and then whenever I need a Vault instance, I just setup Vault to use the existing certificate and private key. The documentation for how to do this seems somewhat lacking, so here’s how I do it.

Things you’ll need:

  • Docker
  • Vault cli
  • JQ

Generating the Root Certificate

First we need to create a Certificate, which we will do using the Vault docker container, and our local Vault CLI. We start the docker container in the background, and mark it for deletion when it stops (--rm):

container=$(docker run -d --rm  --cap-add=IPC_LOCK -p 8200:8200 -e "VAULT_DEV_ROOT_TOKEN_ID=vault" vault:latest)

export VAULT_ADDR="http://localhost:8200"
export VAULT_TOKEN="vault"

max_ttl="87600h" # 10 years why not

mkdir -p $certs_dir
rm -rf $certs_dir/*.*

vault secrets enable pki
vault secrets tune -max-lease-ttl=$max_ttl pki

Finally, we generate a certificate by writing to the pki/root/generate/exported path. If the path ends with exported the Private Key is returned too. If you specify /internal then the Private Key is stored internally to Vault, and never accessible.

result=$(vault write -format "json" \
  pki/root/generate/exported \
  common_name="Local Dev CA" \
  alt_names="localhost," \

echo "$result" > $certs_dir/response.json
echo "$result" | jq -r .data.certificate > $certs_dir/ca.crt
echo "$result" | jq -r .data.private_key > $certs_dir/private.key

docker stop $container

We put the entire response into a json file just incase there is something interesting we want out of it later, and store the certificate and private key into the same directory too. Note for the certificate’s alt_names I have specified both localhost and, which is the domain that Hyper-V machines use.

Lastly, we can now import the root CA into our machine/user’s Trusted Root Certification Authorities store, meaning our later uses of this certificate will be trusted by our local machine.

Creating a Vault CA

As before, we use a Docker container to run the Vault instance, except this time we import the existing CA certificate into the PKI backend. The first half of the script ( is pretty much the same as before, except we don’t delete the contents of the ./ca directory, and our certificate max_ttl is much lower:

docker run -d --rm  --cap-add=IPC_LOCK -p 8200:8200 -e "VAULT_DEV_ROOT_TOKEN_ID=vault" vault:latest

export VAULT_ADDR="http://localhost:8200"
export VAULT_TOKEN="vault"


vault secrets enable pki
vault secrets tune -max-lease-ttl=$max_ttl pki

The last part is to read in the certificate and private key, bundle them together, and configure the pki backend to use them, and add a single role to use for issuing certificates:

pem=$(cat $certs_dir/ca.crt $certs_dir/private.key)

vault write pki/config/ca pem_bundle="$pem"

vault write pki/roles/cert \
  allowed_domains=localhost, \
  allow_subdomains=true \

Also note how we don’t stop the docker container either. Wouldn’t be much of a CA if it stopped the second it was configured…

Creating a Vault Intermediate CA

Sometimes, I want to test that a piece of software works when I have issued certificates from an Intermediate CA, rather than directly from the root. We can configure Vault to do this too, with a modified script which this time we start two PKI secret backends, one to act as the root, and onc as the intermediate:


set -e

docker run -d --rm  --cap-add=IPC_LOCK -p 8200:8200 -e "VAULT_DEV_ROOT_TOKEN_ID=vault" vault:latest

export VAULT_ADDR="http://localhost:8200"
export VAULT_TOKEN="vault"

# create root ca
pem=$(cat $certs_dir/ca.crt $certs_dir/private.key)

vault secrets enable -path=pki_root pki
vault secrets tune -max-lease-ttl=87600h pki_root
vault write pki_root/config/ca pem_bundle="$pem"

# create the intermediate
vault secrets enable pki
vault secrets tune -max-lease-ttl=43800h pki

csr=$(vault write pki/intermediate/generate/internal \
  -format=json common_name="Spectre Dev Intermdiate CA" \
  | jq -r .data.csr)

intermediate=$(vault write pki_root/root/sign-intermediate \
  -format=json csr="$csr" format=pem_bundle ttl=43800h \
  | jq -r .data.certificate)

chained=$(echo -e "$intermediate\n$(cat $certs_dir/ca.crt)")

vault write pki/intermediate/set-signed certificate="$chained"

echo "$intermediate" > intermediate.crt

vault write pki/roles/cert \
  allowed_domains=localhost, \
  allow_subdomains=true \

# destroy the temp root
vault secrets disable pki_root

We use the pki_root backend to sign a CSR from the pki (intermediate) backend, and once the signed response is stored in pki, we delete the pki_root backend, as it is no longer needed for our Development Intermediate CA.

Issuing Certificates

We can now use the cert role to issue certificates for our applications, which I have in a script called


export VAULT_ADDR="http://localhost:8200"
export VAULT_TOKEN="vault"

vault write \
  -format=json \
  pki/issue/cert \

This script I usually use with jq to do something useful with:

response=$(./ consul)

cert=$(echo "$response" | jq -r .data.certificate)
key=$(echo "$response" | jq -r .data.private_key)

Cleaning Up

When I have finished with an application or demo, I can just stop the Vault container, and run the script again if I need Vault for another project.

Architecture Decision Records

29 Jun 2019

This is a text version of a short talk (affectionately known as a “Coffee Bag”) I gave at work this week, on Architecture Design Records. You can see the slides here, but there isn’t a recording available, unfortunately.

It should be noted; these are not to replace full architecture diagrams; you should definitely still write C4 Models to cover the overall architecture. ADRs are for the details, such as serializer formats, convention-over-configuration details, number precisions for timings, or which metrics library is used and why.


Architecture Design Records are there to solve the main question people repeatedly ask when they view a new codebase or look at an older part of their current codebase:

Why on earth was it done like this?!

Generally speaking, architectural decisions have been made in good faith at the time, but as time marches on, things change, and the reasoning gets lost. The reasoning might be discoverable through the commit history, or some comments in a type somewhere, and every once in a while, people remember the Wiki exists, and hope that someone else remembered and put some docs there. They didn’t by the way.

Architecture Design Records are aiming to solve all of this, with three straightforward attributes: Easy to Write, Easy to Read, and Easy to Find. Let’s look at these on their own, and then have a look at an example.

Easy to Find

As I alluded to earlier, “easy to find” doesn’t mean “hidden in confluence” (or any other wiki, for that matter.) The best place to put records of architecture decisions is in the repository. If you want them elsewhere, that’s fine, but the copy in the repository should be the source of truth.

As long as the location is consistent (and somewhat reasonable), it doesn’t matter too much where they go. I like to put them in the docs/arch path, but a common option is docs/adr too:

$ tree ~/dev/projects/awesome-api
|-- docs
|   `-- arch
|       |--
|       |--
|       `--
|-- src
|-- test

The file names for each architecture decision are imperative - e.g. “serialization format”, rather than “figure out what format to use”, much like your commit messages are (right?) You might also note that the files are Markdown. Because what else would they be really?

Easy to Write

As just mentioned, I usually use Markdown for writing all documents, but as long as you are consistent (notice a pattern here?) and that it is plain-text viewable (i.e. in a terminal), it doesn’t matter too much. Try and pick a format that doesn’t add much mental overhead to writing the documents, and if it can be processed by tools easily, that’s a bonus, as we will look into later.

Easy to Read

There are two components to this: Rendering and Format.

Rendering is covering how we actually read it - plain text in a terminal, syntax highlighting in an editor, or rendered into a web page. Good ADRs can handle all three, and Markdown is a good fit for all of them! By using Markdown, not only can we render to HTML, we can even use Confluences’s questionable “Insert Markdown Markup” support to write them into a wiki location if desired.

Format is covering what the content of the document is. There are many different templates you can use, which have different levels of detail, and are aimed at different levels of decisions. I like to use a template based off Michael Nygard’s, which I modified a little bit to have the following sections:

  • Title
  • Status
  • Context
  • Considered Options
  • Chosen Decision
  • Consequences

Let’s have a look at these in an example.


We have a new API we are developing, and we need to figure out which serialization format we should use for all the requests and responses it will handle.

We’ll start off with our empty document and add in the Title, and Status:

# Serialization Format

## Status

In Progress

The Title is usually the same as the file name, but not necessarily. The Status indicates where the document is in its lifespan. What statuses you choose is up to you, but I usually have:

  • In Progress
  • Accepted
  • Rejected
  • Superseded
  • Deprecated

Once an ADR is Accepted (or Rejected), the content won’t change again. Any subsequent changes will be a new ADR, and the previous one will be marked as either Deprecated or Superseded, along with a link to the ADR which replaces it, for example:

## Status

Superseded by [Api Transport Mechanisms](

Next, we need to add some context for the decision being made. In our serialization example, this will cover what area of the codebase we are covering (the API, rather than storage), and any key points, such as message volume, compatibilities etc.

## Context

We need to have a consistent serialization scheme for the API.  It needs to be backwards and forwards compatible, as we don't control all of the clients.  Messages will be fairly high volume and don't *need* to be human readable.

Now that we have some context, we need to explain what choices we have available. This will help when reading past decisions, as it will let us answer the question “was xxxx or yyyy considered?”. In our example, we consider JSON, Apache Avro, the inbuilt binary serializer, and a custom built serializer (and others, such as Thrift, ProtoBufs, etc.)

## Considered Options

1. **Json**: Very portable, and with serializers available for all languages.  We need to agree on a date format, and numeric precision, however.  The serialization should not include white space to save payload size.  Forwards and Backwards compatibility exists but is the developer's responsibility.

2. **Apache Avro**: Binary format which includes the schema with the data, meaning no need for schema distribution.  No code generator to run, and libraries are available for most languages.

3. **Inbuilt Binary**: The API is awkward to use, and its output is not portable to other programming languages, so wouldn't be easy to consume for other teams, as well as some of our internal services.

4. **Custom Built**: A lot of overhead for little to no benefit over Avro/gRPC etc.

5. **Thrift**: ...

The second to last section is our Chosen Decision, which will not only list which one we picked (Avro, in this case) but also why it was chosen over other options. All this helps reading older decisions, as it lets you know what was known at the time the decision was made - and you will always know less at the time of the decision than you do now.

## Chosen Decision

**2. Apache Avro**

Avro was chosen because it has the best combination of message size and schema definition.  No need to have a central schema repository set up is also a huge benefit.

In this example, we have selected Avro and listed that our main reasons were message size, and the fact that Avro includes the schema with each message, meaning we don’t need a central (or distributed) schema repository to be able to read messages.

The final section is for Consequences of the decision. This is not to list reasons that we could have picked other decisions, but to explain things that we need to start doing or stop doing because of this decision. Let’s see what our example has:

## Consequences

As the messages are binary format, we cannot directly view them on the wire.  However, a small CLI will be built to take a message and pretty print it to aid debugging.

As we have selected a binary message format, the messages can’t be easily viewed any more, so we will build a small CLI which when given a message (which as noted, contains the schema), renders a human-readable version of the message.


You might notice that the record doesn’t contain any dates so far. That is because it’s tracked in source control, which means we can pull all the relevant information from the commit history. For example, a full list of changes to any ADR could be fetched from Git with this command:

git log --format='%ci %s' -- docs/arch/

Likewise, when you’re running your build process, you could extract the commit history which effects a single ADR:

git log --reverse --format='%ci %s' -- docs/arch/

And then take that list and insert it into the rendered output so people can see what changed, and when:

<div style="float: right">
        <li><strong>2018-09-26</strong> start serialization format docs</li>
        <li><strong>2018-09-26</strong> consider json</li>
        <li><strong>2018-09-26</strong> consider avro, inbuilt binary and custom binary</li>
        <li><strong>2018-09-27</strong> should consider thrift too</li>
        <li><strong>2018-09-28</strong> select Avro</li>
        <li><strong>2018-09-28</strong> accepted :)</li>
        <li><strong>2019-03-12</strong> accept api transport mechanisms</li>

Note how that last log entry is the deprecation of this ADR. You can, of course, expand your log parsing only to detect Status changes etc.


Hopefully, this gives you a taste of how easily useful documentation can be written, read and found. I’m interested to hear anyone else’s thoughts on whether they find this useful, or any other alternatives.