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#590 improve Retry explanation

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Ilkka Seppälä 2020-07-23 17:50:20 +03:00
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retry/README.md
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@ -8,16 +8,12 @@ tags:
- Performance
---
## Retry / resiliency
Enables an application to handle transient failures from external resources.
## Intent
Transparently retry certain operations that involve communication with external
resources, particularly over the network, isolating calling code from the
retry implementation details.
Transparently retry certain operations that involve communication with external resources, particularly over the
network, isolating calling code from the retry implementation details.
## Explanation
The `Retry` pattern consists retrying operations on remote resources over the
Retry pattern consists retrying operations on remote resources over the
network a set number of times. It closely depends on both business and technical
requirements: how much time will the business allow the end user to wait while
the operation finishes? What are the performance characteristics of the
@ -30,11 +26,7 @@ Another concern is the impact on the calling code by implementing the retry
mechanism. The retry mechanics should ideally be completely transparent to the
calling code (service interface remains unaltered). There are two general
approaches to this problem: from an enterprise architecture standpoint
(**strategic**), and a shared library standpoint (**tactical**).
*(As an aside, one interesting property is that, since implementations tend to
be configurable at runtime, daily monitoring and operation of this capability
is shifted over to operations support instead of the developers themselves.)*
(strategic), and a shared library standpoint (tactical).
From a strategic point of view, this would be solved by having requests
be redirected to a separate intermediary system, traditionally an
@ -42,11 +34,26 @@ be redirected to a separate intermediary system, traditionally an
a [Service Mesh](https://medium.com/microservices-in-practice/service-mesh-for-microservices-2953109a3c9a).
From a tactical point of view, this would be solved by reusing shared libraries
like [Hystrix](https://github.com/Netflix/Hystrix)[1]. This is the type of
solution showcased in the simple example that accompanies this *README*.
like [Hystrix](https://github.com/Netflix/Hystrix) (please note that *Hystrix* is a complete implementation of
the [Circuit Breaker](https://java-design-patterns.com/patterns/circuit-breaker/) pattern, of which the Retry pattern
can be considered a subset of.). This is the type of solution showcased in the simple example that accompanies this
*README*.
In our hypothetical application, we have a generic interface for all
operations on remote interfaces:
Real world example
> Our application uses a service providing customer information. Once in a while the service seems to be flaky and can return errors or sometimes it just times out. To circumvent these problems we apply the retry pattern.
In plain words
> Retry pattern transparently retries failed operations over network.
[Microsoft documentation](https://docs.microsoft.com/en-us/azure/architecture/patterns/retry) says
> Enable an application to handle transient failures when it tries to connect to a service or network resource, by transparently retrying a failed operation. This can improve the stability of the application.
**Programmatic Example**
In our hypothetical application, we have a generic interface for all operations on remote interfaces.
```java
public interface BusinessOperation<T> {
@ -54,8 +61,7 @@ public interface BusinessOperation<T> {
}
```
And we have an implementation of this interface that finds our customers
by looking up a database:
And we have an implementation of this interface that finds our customers by looking up a database.
```java
public final class FindCustomer implements BusinessOperation<String> {
@ -122,20 +128,12 @@ more importantly we did *not* instruct our `Retry` to ignore, then the operation
would have failed immediately upon receiving the error, not matter how many
attempts were left.
<br/><br/>
[1] Please note that *Hystrix* is a complete implementation of the *Circuit
Breaker* pattern, of which the *Retry* pattern can be considered a subset of.
## Class diagram
![alt text](./etc/retry.png "Retry")
## Applicability
Whenever an application needs to communicate with an external resource,
particularly in a cloud environment, and if the business requirements allow it.
## Presentations
You can view Microsoft's article [here](https://docs.microsoft.com/en-us/azure/architecture/patterns/retry).
Whenever an application needs to communicate with an external resource, particularly in a cloud environment, and if
the business requirements allow it.
## Consequences
**Pros:**
@ -150,4 +148,9 @@ You can view Microsoft's article [here](https://docs.microsoft.com/en-us/azure/a
## Related Patterns
* [Circuit Breaker](https://martinfowler.com/bliki/CircuitBreaker.html)
* [Circuit Breaker](https://java-design-patterns.com/patterns/circuit-breaker/)
## Credits
* [Retry pattern](https://docs.microsoft.com/en-us/azure/architecture/patterns/retry)
* [Cloud Design Patterns: Prescriptive Architecture Guidance for Cloud Applications](https://www.amazon.com/gp/product/1621140369/ref=as_li_tl?ie=UTF8&tag=javadesignpat-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=1621140369&linkId=3e3f686af5e60a7a453b48adb286797b)