What is a microservices application?
The Microservices Example application is an example of an application that uses client-side service discovery. It is written in Scala and uses Spring Boot and Spring Cloud as the Microservice chassis . They provide various capabilities including a Netflix Eureka service registry.
What is service registry?
Implement a service registry, which is a database of services, their instances and their locations. Service instances are registered with the service registry on startup and deregistered on shutdown. Client of the service and/or routers query the service registry to find the available instances of a service. A service registry might invoke a service instance’s health check API to verify that it is able to handle requests
What is self registration?
Self registration and 3rd party registration are two different ways that service instances can be registered with the service registry. Health Check API - the service registry invokes a service instance’s health check API to verify that it is able to handle requests.
How does Chris help clients?
Chris helps clients around the world adopt the microservice architecture through consulting engagements, and training classes and workshops.
What is Chris's latest startup?
Eventuate is Chris's latest startup. It makes it easy to use the Saga pattern to manage transactions and the CQRS pattern to implement queries.
What happens if the service registry fails?
Although clients should cache data provided by the service registry, if the service registry fails that data will eventually become out of date. Consequently, the service registry must be highly available. You need to decide how service instances are registered with the service registry. There are two options:
What is Eureka server?
The Eureka Server is a small Spring Boot application:
What is service registry?
The service registry is a database populated with information on how to dispatch requests to microservice instances. Interactions between the registry and other components can be divided into two groups, each with two subgroups:
What happens when a third party registration service goes down?
For instance, what happens when a service goes down? A third-party registration service might be configured to provide safe fallbacks for services that fail. Other policies might be implemented for other cases. For instance, the service registry process might be notified of a high-load condition and automatically add a new endpoint by requesting the instantiation of a new microservice-process or VM. As you can imagine, these possibilities are critical for big architectures.
What is self registration?
Self-registration forces microservices to interact with the registry by themselves. When a service goes up, it notifies the registry. The same thing happens when the service goes down. Whatever additional data is required by the registry must be provided by the service itself. If you have been following this series, you know that microservices are all about dealing with a single concern, so self-registration might seem like an anti-pattern. However, for simple architectures, self-registration might be the right choice.
What is third party registration?
Third-party registration is normally used in the industry. In this case, there is a process or service that manages all other services. This process polls or checks in some way which microservice instances are running and it automatically updates the service registry.
What is client side discovery?
Client-side discovery forces clients to query a discovery service before performing the actual requests. As happens with self-registration, this requires clients to deal with additional concerns other than their main objective. The discovery service may or may not be located behind the API gateway. If it is not located behind the gateway, balancing, authentication and other cross-cutting concerns may need to be re-implemented for the discovery service. Additionally, each client needs to know the fixed endpoint (or endpoints) to contact the discovery service. These are all disadvantages. The one big advantage is not having to code the necessary logic in the gateway system. Study this carefully when picking your discovery method.
What is discovery in business?
As you can imagine, discovery is the counterpart to registration from the point of view of clients. When a client wants to access a service, it must find out where the service is located (and other relevant information to perform the request).
Why is systemd used?
Why systemd? It has become the de-facto service manager in most Linux installations. There are other alternatives for managing your services but all require installation and configuration. For simplicity, we picked the one that comes preinstalled in most distros, and that is systemd.
What is service in hibernate?
Service : Services are classes that provide Hibernate with pluggable implementations of various types of functionality. Specifically they are implementations of certain service contract interfaces
What is a sessionfactoryserviceregistry?
SessionFactoryServiceRegistry org.hibernate.service.spi.SessionFactoryServiceRegistry is the 3rd standard Hibernate ServiceRegistry. Typically, its parent registry is the StandardServiceRegistry. SessionFactoryServiceRegistry is designed to hold Services which need access to the SessionFactory. Currently that is just 3 Services.
What is spi.stat.statisticsimplementor?
org.hibernate.stat.spi.StatisticsImplementor is the SPI portion of the org.hibernate.stat.Statistics API. The collector portion, if you will.
What is the purpose of a service registry?
The main purpose of a service registry is to hold, manage and provide access to services . Service registries are hierarchical. Services in one registry can depend on and utilize services in that same registry as well as any parent registries.
What is the meaning of "back up"?
Making statements based on opinion; back them up with references or personal experience.
What is EventListenerRegistry?
EventListenerRegistry org.hibernate.event.service.spi.EventListenerRegistry is the big Service managed in the SessionFactoryServiceRegistry. This is the Service that manages and exposes all of Hibernate’s event listeners. A major use-case for Integrators is to alter the listener registry.
What is Netflix Eureka service registry?
You will set up a Netflix Eureka service registry and then build a client that both registers itself with the registry and uses it to resolve its own host. A service registry is useful because it enables client-side load-balancing and decouples service providers from consumers without the need for DNS.
How long does it take for a eureka client to register?
The eureka-client will take about a minute to register itself in the registry and to refresh its own list of registered instances from the registry. Visit the eureka-client in the browser, at http://localhost:8080/service-instances/a-bootiful-client. There, you should see the ServiceInstance for the eureka-client reflected in the response. If you see an empty <List> element, wait a bit and refresh the page.
How many applications does Eureka need?
This guide needs two applications. The first application (the service application) needs only the Eureka Server dependency.
What license is used for the guides?
All guides are released with an ASLv2 license for the code, and an Attribution, NoDerivatives creative commons license for the writing.
What is a eureka client?
The eureka-client defines a Spring MVC REST endpoint ( ServiceInstanceRestController) that returns an enumeration of all the registered ServiceInstance instances at http://localhost:8080/service-instances/a-bootiful-client. See the Building a RESTful Web Service guide to learn more about building REST services with Spring MVC and Spring Boot.
How to test end to end results in Eureka?
Test the end-to-end result by starting the eureka-service first and then, once that has loaded, starting the eureka-client.
Can you use Spring to stand up Netflix Eureka?
Congratulations! You have just used Spring to stand up a Netflix Eureka service registry and to use that registry in a client application.
How to edit registry?
Manually edit the registry (Use this only if the Fix It utility does not work) 1 Go to Start and then Run 2 Type in regedit in the edit field 3 Go to HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Uninstall 4 Under this Uninstall folder you will find many registry entries within curly brackets. 5 Click on Edit tab and then Find#N#Note: Highlight Uninstall folder before performing search for particular registry. 6 Enter version string as value to find corresponding registry entry to be deleted.#N#Steps for finding registry keys for any version of Java 7#N#Steps for finding registry keys for any version of Java 6#N#Steps for finding registry keys for any version of Java 1.5#N#Steps for finding registry keys for any version of Java 1.4 7 Once you find registry key, highlight that key and then Right click on it and choose Delete 8 Confirm the delete and click Yes
Can you damage your computer by editing the registry?
Incorrectly editing your registry may severely damage your system. You should back up any valued data from your computer before making changes to the registry.
What is OSGi service platform core specification?
The OSGi Service Platform Core Specification (most current version is 4.1 at time of writing) does not specify what types and resources are visible through the context class loader when an operation is invoked on a service obtained via the service registry. Since some services may use libraries that make certain assumptions about the context class loader, Spring Dynamic Modules enables you to explicitly control the context class loader during service execution. This is achieved using the option context-class-loader attribute of the service element.
What is service element?
The service element is used to define a bean representing an exported OSGi service. At a minimum you must specify the bean to be exported, and the service interface that the service advertises.
How to traverse an OSGi collection?
The recommend way of traversing a collection is by using an Iterator . However, since OSGi services can come and go, the content of the managed service collection will be adjusted accordingly. Spring DM will transparently update all Iterator s held by the user so it is possible to safely traverse the collection while it is being modified. Moreover, the Iterator s will reflect all the changes made to the collection, even if they occurred after the Iterator s were created (that is during the iteration). Consider a case where a collection shrinks significantly (for example a big number of OSGi services are shutdown) right after an iteration started. To avoid dealing with the resulting 'dead' service references, Spring DM iterators do not take collection snapshots (that can be inaccurate) but rather are updated on each service event so they reflect the latest collection state, no matter how fast or slow the iteration is.
What is Spring Dynamic Modules?
Spring Dynamic Modules provides an osgi namespace for Spring (see Appendix H, Spring Dynamic Modules Schema) that can be used to export Spring beans as OSGi services, and to define references to services obtained via the service registry. The namespace elements may be used nested inside another top-level namespace (typically the Spring beans namespace), or within the top-level osgi element.
What is service interface?
The OSGi Service Platform Core Specification defines the term service interface to represent the specification of a service's public methods. Typically this will be a Java interface, but the specification also supports registering service objects under a class name, so the phrase service interface can be interpreted as referring to either an interface or a class.
What is OSGI service registry?
The OSGi service registry enables a bundle to publish objects to a shared registry, advertised via a given set of Java interfaces. Published services also have service properties associated with them in the registry.
Why use proxy in OSGI?
The reason behind using a proxy is to prevent the listener from holding strong reference to the service (which can disappear at any point). Listeners interested in tracking certain services should not rely on instance equality ( == ). Object equality ( equals / hashcode) can be used but only if the backing service has exposed the aforementioned methods as part of its contract (normally by declaring them on a certain published interface/class). If these methods are not published, the proxy will invoke its own method, not the targets. This is on purpose since, while the proxy tries to be as transparent as possible, it is up to the developer to define the desired semantics.
Context
- Clients of a service use either Client-side discovery or Server-side discoveryto determine the location of a service instance to which to send requests.
Problem
- How do clients of a service (in the case of Client-side discovery) and/or routers (in the case of Server-side discovery) know about the available instances of a service?
Forces
- Each instance of a service exposes a remote API such as HTTP/REST, or Thrift etc. at a particular location (host and port)
- The number of services instances and their locations changes dynamically. Virtual machines and containers are usually assigned a dynamic IP address. An EC2 Autoscaling Group, for example, adjusts t...
Solution
- Implement a service registry, which is a database of services, their instances and their locations.Service instances are registered with the service registry on startup and deregistered on shutdown.Client of the service and/or routers query the service registry to find the available instances of a service.A service registry might invoke a service instance’s health check APIto ve…
Examples
- The Microservices Example application is an example of an application that uses client-side service discovery.It is written in Scala and uses Spring Boot and Spring Cloud as the Microservice chassis.They provide various capabilities including a Netflix Eurekaservice registry. The Eureka Server is a small Spring Boot application: It is deployed using Docker: Other examples of servic…
Resulting Context
- The benefits of the Service Registry pattern include: 1. Client of the service and/or routers can discover the location of service instances. There are also some drawbacks: 1. Unless the service registry is built in to the infrastructure, it is yet another infrastructure component that must be setup, configured and managed.Moreover, the service registry is a critical system component.Alt…
Related Patterns
- Client-side discovery and Server-side discoverycreate the need for a service registry
- Self registration and 3rd party registrationare two different ways that service instances can be registered with the service registry
- Health Check API- the service registry invokes a service instance’s health check API to verify that it is able to handle requests
The Service Registry
Example: A Registry Service
- In part 2 we worked on a simple API gateway implementation. In that example we implemented dynamic dispatching of requests through queries to a service database. In other words, we implemented server-side discovery. For this example, we will extend our microservice architecture by working on the registrationaspect. We will do so in two ways: 1. By providing a simple registra…
Aside: Use Auth0 For Your Microservices
- Auth0 and microservices go hand-in-hand thanks to the magic of JWT. Check it out: Your client id and client secret are available through the Auth0 dashboard. Create a new account and start hacking!
Conclusion
- The service registry is an essential part of a microservice-based architecture. There are different ways of handling registration and discovery that fit different architectural complexities. Consider the pros and cons described above for each alternative before committing to one. In Part 4 we will study service dependencies in detail and how to manage them efficiently.