* [**Configuration for Eureka**](#configuration-for-eureka)
* [**Upstream setting**](#upstream-setting)
## Summary
When system traffic changes, the number of servers of the upstream service also increases or decreases, or the server needs to be replaced due to its hardware failure. If the gateway maintains upstream service information through configuration, the maintenance costs in the microservices architecture pattern are unpredictable. Furthermore, due to the untimely update of these information, will also bring a certain impact for the business, and the impact of human error operation can not be ignored. So it is very necessary for the gateway to automatically get the latest list of service instances through the service registry。As shown in the figure below：
1. When the service starts, it will report some of its information, such as the service name, IP, port and other information to the registry. The services communicate with the registry using a mechanism such as a heartbeat, and if the registry and the service are unable to communicate for a long time, the instance will be cancel.When the service goes offline, the registry will delete the instance information.
2. The gateway gets service instance information from the registry in near-real time.
3. When the user requests the service through the gateway, the gateway selects one instance from the registry for proxy.
Common registries: Eureka, Etcd, Consul, Zookeeper, Nacos etc.
## How extend the discovery client?
### Basic steps
It is very easy for APISIX to extend the discovery client. , the basic steps are as follows
1. Add the implementation of registry client in the 'apisix/discovery/' directory;
2. Implement the `_M. init_worker()` function for initialization and the `_M. nodes(service_name)` function for obtaining the list of service instance nodes;
3. Convert the registry data into data in APISIX;
### the example of Eureka
#### Implementation of eureka.lua
First, add [`eureka.lua`](../apisix/discovery/eureka.lua) in the `apisix/discovery/` directory;
Then implement the `_M.init_worker()` function for initialization and the `_M.nodes(service_name)` function for obtaining the list of service instance nodes in ` eureka.lua`:
```lua
local _M = {
version = 1.0,
}
function _M.nodes(service_name)
... ...
end
function _M.init_worker()
... ...
end
return _M
```
#### How convert Eureka's instance data to APISIX's node?
Here's an example of Eureka's data：
```json
{
"applications": {
"application": [
{
"name": "USER-SERVICE", # service name
"instance": [
{
"instanceId": "192.168.1.100:8761",
"hostName": "192.168.1.100",
"app": "USER-SERVICE", # service name
"ipAddr": "192.168.1.100", # IP address
"status": "UP",
"overriddenStatus": "UNKNOWN",
"port": {
"$": 8761,
"@enabled": "true"
},
"securePort": {
"$": 443,
"@enabled": "false"
},
"metadata": {
"management.port": "8761",
"weight": 100 # Setting by 'eureka.instance.metadata-map.weight' of the spring boot application
Deal with the Eureka's instance data need the following steps :
1. select the UP instance. When the value of `overriddenStatus` is "UP" or the value of `overriddenStatus` is "UNKNOWN" and the value of `status` is "UP".
2. Host. The `ipAddr` is the IP address of instance; and must be IPv4 or IPv6.
3. Port. If the value of `port["@enabled"]` is equal to "true", using the value of `port["\$"]`, If the value of `securePort["@enabled"]` is equal to "true", using the value of `securePort["\$"]`.
4. Weight. `local weight = metadata.weight or local_conf.eureka.weight or 100`
The result of this example is as follows:
```json
[
{
"host" : "192.168.1.100",
"port" : 8761,
"weight" : 100,
"metadata" : {
"management.port": "8761",
}
}
]
```
## Configuration for discovery client
### Select discovery client
Add the following configuration to `conf/config.yaml` and select one discovery client type which you want:
```yaml
apisix:
discovery: eureka
```
This name should be consistent with the file name of the implementation registry in the `apisix/discovery/` directory.
The supported discovery client: Eureka.
### Configuration for Eureka
Add following configuration in `conf/config.yaml`：
```yaml
eureka:
host: # it's possible to define multiple eureka hosts addresses of the same eureka cluster.
Suppose both A-SERVICE and B-SERVICE provide a `/test` API. The above configuration allows access to A-SERVICE's `/test` API through `/a/test` and B-SERVICE's `/test` API through `/b/test`.
**Notice**：When configuring `upstream.service_name`, `upstream.nodes` will no longer take effect, but will be replaced by 'nodes' obtained from the registry.
