How to reset a counter

Purpose: Original value is origVal, we want to use increase/decrease to update to a specified number

How?

1. origValue = origValue + expectedValue
2. diff = expectedValue - origValue
3. origValue = origValue + diff

Ex. origValue = 54, newValue = 50

1. origValue = origValue (54) + expectedValue (50) = 104
2. diff = expectedValue (50) - origValue (104) = -54
3. origValue = origValue (104) + diff (-54) = 50

Microservice Patterns - 3.2.4 Interprocess communication in a microservice architecture - Using Service Discovery

Reference

https://www.amazon.com/Microservices-Patterns-examples-Chris-Richardson/dp/1617294543

in order for one service to invoke another service using RPI, it needs to know the network location of a service instance. => Service Discovery

Using service discovery

Instance IP may change, then Service client need a way to find instance

OVERVIEW OF SERVICE DISCOVERY

• Its key component is a service registry, which is a database of the network locations of an application’s service instances
• The service discovery mechanism updates the service registry when service instances start and stop.
• When a client invokes a service, the service discovery mechanism queries the service registry to obtain a list of available service instances and routes the request to one of them.

APPLYING THE APPLICATION-LEVEL SERVICE DISCOVERY PATTERNS

1. Service register when start (Self registration pattern, instance information may contain health check URL)
   (Pattern: Self registration A service instance registers itself with the service registry. See http://microservices.io/patterns/self-registration.html.)
2. Client query service and send request with cache mechanism and load balance strategy such as round-robin or random 
   ( Pattern: Client-side discovery A service client retrieves the list of available service instances from the service registry and load balances across them. See http://microservices.io/patterns/clientside-discovery.html.)

Good and Bad

Good: Handle the scenarios, including legacy system can support this way

Bad: When using some tool such as supported by JVM or Spring, can't be supported in non-JVM environment

it’s usually better to use a service discovery mechanism that’s provided by the deployment infrastructure because of application later implementation limitation.

APPLYING THE PLATFORM-PROVIDED SERVICE DISCOVERY PATTERNS

Platform such as Docker and Kubernetes have a built-in service registry and service discovery mechanism. Ex. DNS, VIP

Good and bad

Good: No application code needed

Bad: Need deployed to Docker or Kubernates

• 3rd party registration pattern:  a third party called the registrar, which is typically part of the deployment platform, handles the registration. ( See http://microservices.io/patterns/3rd-party-registration.html.)
• Server-side discovery pattern: Client makes a request to a DNS name, which resolves to a request router that queries the service registry and load balances requests. ( See http://microservices.io/patterns/server-side-discovery.html.)