Building High-Availability Java Applications

To build highly available Java applications, we need to consider comprehensively from architecture design, service governance to deployment, operation and maintenance. 1. Use microservice architecture to combine load balancing (such as Nginx, Spring Cloud Gateway) to achieve service isolation and automatic failover, and combine fuse mechanisms (Hystrix or Resilience4j) and service registration discovery (Eureka, Consul or Nacos) to improve system resilience; 2. Use master-slave replication and automatic switching tools (such as MHA, Patroni) to reasonably configure connection pools (such as HikariCP) and deal with data consistency issues; 3. Implement fault tolerance and downgrade mechanisms, use Resilience4j or Sentinel to achieve fuse current limit, and combine cache to deal with service exceptions; 4. Use Docker Kubernetes combination to configure health check probes and automated operation and maintenance strategies, and use Prometheus Grafana and ELK for monitoring Set to ensure that the system status is observable and problems are traceable.

Highly available Java applications are not achieved overnight. They need to be considered in multiple aspects, including architecture design, service governance, deployment, operation and maintenance. The core goal is to enable the system to operate normally when facing failures and minimize downtime.

1. Use microservice load balancing

Once there is a problem with a single application, the entire system may be paralyzed. After splitting into microservices, the lapse of one service will not directly affect other functional modules. For example, after the order service and user service are separated, even if the user service is temporarily unavailable, the order process can continue.

In conjunction with load balancing (such as Nginx, Spring Cloud Gateway, or Zuul), requests are automatically forwarded to healthy instances. This way, even if a node goes down, it can ensure that the service is continuously available.

• Recommended calls between microservices with circuit breaker mechanism (Hystrix or Resilience4j)
• Service registration and discovery can be done using Eureka, Consul or Nacos
• Health checks must be configured and abnormal nodes should be removed in time

2. High database availability cannot be ignored

No matter how stable the Java application is, if the database is hung, it is equivalent to being hung all. Therefore, the high availability of the database must be planned in advance. Master-slave replication is the most basic solution, and read-write separation can relieve stress. In addition, tools like MySQL's MHA and PostgreSQL's Patroni can automatically switch when there is a problem with the main library.

In addition, the connection pool should also be configured reasonably, such as HikariCP sets the appropriate maximum number of connections, timeout time and retry strategy to avoid the connection being unable to be added after the database is restored or being crushed by instant requests.

• Automatic switching of master-slave structure is a basic requirement
• The connection pool configuration should be reasonable to avoid avalanche effect
• Data consistency can be handled through distributed transactions (Seata) or final consistent schemes

3. Fault tolerance and downgrade mechanisms must be in place

The system cannot never fail. The key is how to deal with it "gracefully" when it is broken. For example, if an external service call fails, there should be a retry mechanism; if it fails all the time, it should trigger a downgrade and return cached data or default value.

For example: the user center interface is hanged, and the order service can first obtain user information from the local cache instead of directly reporting an error. Although this design cannot be maintained for a long time, it can last for at least a while.

• Resilience4j or Sentinel is recommended to use Resilience4j or Sentinel to achieve fuse, current limit, and downgrade.
• Exception handling requires clear strategies and do not allow local problems to spread.
• Caching is also one of the important means of fault tolerance, but be careful about cache failure storms

4. Deployment and monitoring must not be missing.

In terms of Java application deployment, Docker Kubernetes is currently the mainstream solution. K8s provides functions such as automatic restart, rolling update, and replica management, which greatly improves the stability of the system. In conjunction with the life-time/readiness probe, some problems can be automatically fixed.

In terms of monitoring, Prometheus Grafana is a good choice. Log collection recommends ELK (Elasticsearch Logstash Kibana) for easy troubleshooting. The alarm system must also be configured, such as sending notifications through Alertmanager to know what was wrong with the system as soon as possible.

• Docker and Kubernetes are standard combinations of modern deployments
• The health check probe should be properly configured to prevent manslaughter or missed tests.
• The monitoring system must cover key points such as JVM indicators, interface response, error rate, etc.

Basically that's it. The high availability of Java applications is a system project that involves multiple links of development, testing, operation and maintenance. However, as long as the above points are implemented step by step, a relatively stable system can be built.

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