Learn to implement microservices across a range of common tech stacks.

Scalability & System Design for Developers

In this module, we’ll look at some fundamental concepts for implementing microservices. Within each concept, learn about the different technologies used to implement it. The technologies include front-end integration with Edge Side Includes (ESI), asynchronous microservices with Kafka and REST feeds, synchronous microservices with the Netflix stack, and Consul, Docker, Kubernetes, Cloud Foundry. Each technology is described and then demonstrated with real code.

Microservice Architecture: Practical Implementation

# Introduction
With synchronous communication between microservices, it is important that the failure of one microservice does not cause other microservices to fail as well. 

Otherwise, the unavailability of a single microservice can cause further microservices to fail until the entire system is no longer available.

The microservices may return errors because they cannot deliver reasonable results due to a failed microservice.

However, it
must not happen that a microservice waits for the result of another
microservice for an infinite period of time and thereby becomes
unavailable itself.



# Resilience patterns

Michael T. Nygard's book, [*Release It!*](https://www.amazon.com/Release-Design-Deploy-Production-Ready-Software/dp/1680502395/ref=sr_1_fkmr0_1?keywords=Release+It%21%3A+Design+and+Deploy+Production-Ready+Software+%28Pragmatic+Programmers%29+2nd+Edition&qid=1573034683&s=books&sr=1-1-fkmr0) describes different patterns with
which the resilience of a system can be increased.
Hystrix implements some of these patterns. Let's discuss them.


## Timeout
A **timeout** prevents a microservice from waiting too long for another microservice. 

Without a timeout, **a thread can block
for a very long time** because the  thread does not get a response from another microservice. If all threads are blocked, the microservice will fail because there are no more threads available for new tasks.

Hystrix executes a request in a separate thread pool.
Hystrix controls these threads and can terminate the request to
implement the timeout.

## Fail fast

**Fail Fast** describes a similar pattern. It is better to
generate an error as quickly as possible. 

The code can check at the beginning of an operation whether all necessary resources are available. 


If
this is not the case, the request can be terminated immediately with
an error. This reduces the time that the caller has to block a thread
or other resources.

## Bulkhead
 Hystrix can use its own thread pool for each type of request. For
example, a separate thread pool can be set up for each called
microservice. 

If the call of a particular microservice takes too long,
only the thread pool for that particular microservice is emptied,
while the others still contain threads. 

This will limit the impact of
the problem and is called a **bulkhead**. This term was coined in
analogy to a watertight bulkhead in a ship which divides the ship into
different segments. If a leak occurs, only part of the ship is flooded
with water so that the ship does not sink.



## Circuit breaker

Finally, Hystrix implements a **circuit breaker**. 

This is a fuse analogous to the ones used in the electrical system of a house. There, a circuit breaker is used to cut off the current flow if there is a short circuit, preventing a fire from breaking out.


The
Hystrix circuit breaker has a different approach. If a **system call
results in an error, the circuit breaker is opened and does not allow
any calls to pass through**. 

After some time, a call is allowed to pass through again. Only when this call is successful, is the circuit breaker closed again. This prevents a faulty microservice from being called. This **saves resources and avoids blocked threads**. 

In addition, the circuit breakers of the different clients are closing one by one so that a failed and recovered microservice gradually handles the full load. This **reduces the probability that it will
fail again immediately** after starting up.

In this lesson, we'll look at resilience with Hystrix in detail.

Resilience: Hystrix

Introduction