# What is service discovery?

* A key problem in deployment is getting services to be able to find each-other
* A database might be at 10.1.1.1:6543 while the webserver is at 10.1.1.2:8080

### Level 1 Service Descovery: Baiscs

* Everything is manually configured

<figure><img src="/files/GJ0kdGAm8kMjvEve3C7A" alt=""><figcaption></figcaption></figure>

<figure><img src="/files/Llv9iJzyDPN0FHBGVQjh" alt=""><figcaption></figcaption></figure>

{% hint style="warning" %}
You'll know you should start caring about service discovery when you start seeing one of the following:

1. You want "zero downtime deployments" or to use other, more complex deploy strategies
2. You have more than a couple of microservices
3. You are deploying to several environments (dev, staging, prod) and it's getting unwieldy
   {% endhint %}

### Lever 2: Service Discovery via hash table

* A straightforward approach: Store the service IPs in a hash table&#x20;

<figure><img src="/files/lPPwhlskjkYqDSgo5lxW" alt=""><figcaption></figcaption></figure>

* The hash table in question in the config is here:&#x20;

<figure><img src="/files/APSCkGo6o3KZAAR83r79" alt=""><figcaption></figcaption></figure>

{% hint style="info" %}
In simple term:

* All you need to do is update your frontend to set the keys in the hash table to be the forntend's IP
* Then make the backend do the same for the backend
  {% endhint %}

#### Problems with this approach

* It's complex
* It's error prone
* It requires to write custom config files

### Level 3 Service Discovery via DNS

* The idea for DNS is just to map hostnames to IPs. This is the industry standard
* It would be ideal if we could connect to "<http://frontend>" from our reverse proxy and having DNS respond with the IP for the correct versions of the frontend

<figure><img src="/files/XsVd3pqfUSu6Hr7vuLq9" alt=""><figcaption></figcaption></figure>

* So instead of the first example where you use the IP
* You would use `mongo` where `mongo` is a key in the key:value pairs

<figure><img src="/files/V0iMsqthstIQbNwRoj89" alt=""><figcaption></figcaption></figure>


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