Full Review on Mainstream Timing Task Solutions

By Jingxiao

As a common mode of executing expected logic according to the agreed time, timing tasks can be applied to various business scenarios in enterprise-level development, such as background timing data synchronization to generate reports, regular cleaning of disk log files, and scheduled scanning of overtime orders for compensation and callback.

There are many frameworks and solutions available to program developers in the field of timing tasks to implement business functions and product launches quickly. This article introduces and analyzes the current mainstream timing task solutions, which are expected to be used as a reference in enterprise technology selection and project architecture reconstruction.

Crontab

Target Positioning

Crontab is a built-in executable command in Linux. It can execute specified system instructions or shell scripts based on the time when cron expressions are generated.

Usage

Crontab command syntax:

crontab [-u username] [-l | -e | -r ]
Parameters：
-u : Only a root user can perform this task. It means to edit the crontab of a user
-e : It means to edit the work content of crontab
-l : It means to check the work content of crontab
-r : It means to remove all content of crontab

Example of the configuration file:

* * * * * touch ~/crontab_test
* 3 * * * ~/backup
0 */2 * * * /sbin/service httpd restart

Principle

The crond daemon is started by the init process when Linux is started. The crond daemon checks every minute whether the /etc/crontab configuration file contains tasks to be executed and outputs the execution status of timing tasks through the /var/log/cron file. Users can use the Crontab command to manage the /etc/crontab configuration file.

Solution Analysis

With the help of Crontab, users can easily and quickly implement the simple timing task function, but it has the following disadvantages:

The timing task is bound to the specified Linux machine. When the machine is scaled out or replaced, crontab needs to be reconfigured. At the same time, there is a risk of a single point of failure.
As the scale of timing tasks increases, there is no unified perspective to track and control the task progress. Therefore, it is difficult to maintain.
The function is too simple, and there are no advanced task features (such as timeout, retry, and blocking).
Observability is poor, and it is difficult to troubleshoot and locate problems.
The task is persistent, causing unnecessary waste of resource costs when no task is to be executed.

Spring Task

Target Positioning

The Spring framework provides an out-of-the-box timing scheduling. Users can identify the execution cycle of a specified method by annotations of xml or @Scheduled.

Spring Task supports a variety of task execution modes, including the corn configured with time zones, fixed latency, and fixed rate.

Usage

The code instance is listed below:

@EnableScheduling
@SpringBootApplication
public class App {

    public static void main(String[] args) {
        SpringApplication.run(App.class, args);
    }
}

@Component
public class MyTask {  

    @Scheduled(cron = "0 0 1 * * *")
    public void test() {
        System.out.println("test");  
    }  

}

Principle

The principle of Spring Task is to use ScheduledAnnotationBeanPostProcessor to intercept the methods annotated by @Scheduled when initializing Bean and build corresponding Task instances to register in the ScheduledTaskRegistrar according to each method and its annotation configuration.

After the singleton bean is initialized, configure the TaskScheduler in the ScheduledTaskRegistrar by calling back the afterSingletonsInstantiated. The implementation of the underlying layer of the TaskScheduler depends on the ScheduledThreadPoolExecutor in the JDK concurrent package. All tasks are added to the TaskScheduler to be scheduled for execution while executing the afterPropertiesSet method.

Solution Analysis

With the help of Spring Task, users can quickly implement periodic execution of specified methods through annotations, and Spring Task supports multiple periodic policies. However, similar to Crontab, it has the following disadvantages:

The task execution of Spring Task is single-threaded. If the execution time of the previous task is too long, subsequent tasks will be blocked, and users need to configure the applicable thread pool themselves.
Each node runs independently, with a risk of a single point of failure and no distributed coordination mechanism. Concurrent execution must be prohibited.
As the scale of timing tasks increases, there is no unified perspective to track and control the task progress. Therefore, it is difficult to maintain.
The function is too simple, and there are no advanced task features (such as timeout, retry, and blocking).
Observability is poor, and it is difficult to troubleshoot and locate problems.
The task is persistent, causing unnecessary waste of resource costs when no task is to be executed.

ElasticJob

Target Positioning

As an open-source distributed task framework from Dangdang, ElasticJob provides flexible scheduling, resource management, job governance, and many other features. It has become a subproject of Apache Shardingsphere.

ElasticJob currently consists of two independent subprojects: ElasticJob-Lite and ElasticJob-Cloud. ElasticJob-Lite is a lightweight, decentralized solution that provides coordination services of distributed tasks in the form of jar. ElasticJob-Cloud uses Mesos to provide additional services (such as resource management, application distribution, and process isolation). Generally, ElasticJob-Lite is enough to meet the demand.

Usage

Users need to configure the address of zk registry and the configuration information of the task in yaml:

elasticjob:
  regCenter:
    serverLists: localhost:6181
    namespace: elasticjob-lite-springboot
  jobs:
    simpleJob:
      elasticJobClass: org.apache.shardingsphere.elasticjob.lite.example.job.SpringBootSimpleJob
      cron: 0/5 * * * * ?
      timeZone: GMT+08:00
      shardingTotalCount: 3
      shardingItemParameters: 0=Beijing,1=Shanghai,2=Guangzhou

Implement the corresponding API to identify the corresponding task and configure a monitor to implement callbacks before and after a task execution:

public class MyElasticJob implements SimpleJob {

    @Override
    public void execute(ShardingContext context) {
        switch (context.getShardingItem()) {
            case 0: 
                // do something by sharding item 0
                break;
            case 1: 
                // do something by sharding item 1
                break;
            case 2: 
                // do something by sharding item 2
                break;
            // case n: ...
        }
    }
}


public class MyJobListener implements ElasticJobListener {

    @Override
    public void beforeJobExecuted(ShardingContexts shardingContexts) {
        // do something ...
    }

    @Override
    public void afterJobExecuted(ShardingContexts shardingContexts) {
        // do something ...
    }

    @Override
    public String getType() {
        return "simpleJobListener";
    }
}

Principle

The underlying time scheduling of ElasticJob is based on Quartz. Quartz needs to persist business Bean to an underlying data table. Thus, the system is intruded severely. At the same time, Quartz preempts tasks by db locks, and parallel scheduling and scalability are not supported.

ElasticJob implements horizontal scaling through the feature of data sharding and custom sharding parameters, which means one task can be split into N independent task items, and then distributed servers can execute the shard items assigned to them in parallel. For example, a database contains 0.1 billion pieces of data. If we need to read and calculate the data, we can divide the 0.1 billion pieces of data into ten shards. Each shard reads 10 million pieces of data and then writes the data to databases after calculation.

If there are three machines to execute tasks, machine A is allocated with shard (0,1,2,9), machine B is allocated with shard (3,4,5), and machine C is allocated with shard (6,7,8). This is also the most significant advantage over Quartz.

In the implementation, ElasticJob uses Zookeeper as the registry to carry out distributed scheduling coordination and leader election and to sense the increase or decrease of servers by the change of temporary nodes in the registry. Every time the leader node is elected, the server goes online and offline, and the total number of shards changes; subsequent re-shards will be triggered. The leader node will perform a specific shard division when the next scheduled task is triggered. Then, each node will obtain shard information from the registry to execute as the running parameter of the task.

Solution Analysis

As a distributed task framework, ElasticJob solves the problem that the single-node task execution mentioned above cannot guarantee high availability and good performance during highly concurrent task execution and supports advanced mechanisms (such as failover and misfire). However, it has the following disadvantages:

The framework is heavyweight and relies on the external zk registry, which increases the cost and maintenance complexity of at least three servers.
As the number of tasks continues increasing, zk as a stateful middleware will become a performance bottleneck.
The observability is weak. We need to introduce db and configure the event tracking feature.
The task is persistent, causing unnecessary waste of resource costs when no task is to be executed.

XXLJob

Target Positioning

XXLJob is an open-source distributed task framework from Dianping employees. Its core design goals are rapid development, simple learning, lightweight, and ease of expansion.

XXLJob provides various features, such as task sharding broadcasts, timeout control, failure retries, and blocking policies. It also manages and maintains tasks through the experience-friendly automated O&M console.

Usage

XXLJob consists of two parts: central scheduler and distributed executor, which need to be started separately when used. When the scheduling center starts, the dependent db configuration needs to be configured.

The executor needs to configure the address of the scheduling center:

xxl.job.admin.addresses=http://127.0.0.1:8080/xxl-job-admin
xxl.job.accessToken=
xxl.job.executor.appname=xxl-job-executor-sample
xxl.job.executor.address=
xxl.job.executor.ip=
xxl.job.executor.port=9999
xxl.job.executor.logpath=/data/applogs/xxl-job/jobhandler
xxl.job.executor.logretentiondays=30

We can use the following methods to create tasks and callback before and after tasks in bean mode:

@XxlJob(value = "demoJobHandler2", init = "init", destroy = "destroy")
public void demoJobHandler() throws Exception {
   int shardIndex = XxlJobHelper.getShardIndex();
   int shardTotal = XxlJobHelper.getShardTotal();
   XxlJobHelper.log("Shard parameter: sequence number of current shard = {}, total number of shards= {}", shardIndex, shardTotal);
}

public void init(){
        logger.info("init");
}

public void destroy(){
        logger.info("destory");
}

After the task is created, we need to configure the task execution policy on the console:

Principle

The XXLJob implementation decouples the scheduling system from tasks. The self-developed scheduler of XXLJob is responsible for managing scheduling information and issuing scheduling requests based on scheduling configurations. It supports visualized, simple, and dynamic management of scheduling information, automatic discovery and routing, scheduling expiration policy, retry policy, and executor Failover.

The executor is responsible for receiving scheduling requests, executing task logic, and receiving task termination requests and log requests. It is also responsible for task timeout and blocking policies. The scheduler and executor communicate through RESTful APIs. The scheduler supports stateless cluster deployment, improving the disaster recovery and availability of the scheduling system and maintaining lock information and persistence through MySQL. Executor supports stateless cluster deployment, improving scheduling system availability and task processing capabilities.

This is a complete task scheduling communication process of XXLJob:

First, the scheduling center sends an http scheduling request to the embedded Server of the executor, and then the executor executes the corresponding task logic. After the task execution is completed or timed out, the executor sends an http callback to return the scheduling result to the scheduling center.

Solution Analysis

XXLJob is widely popular in open-source communities. With its simple operation and rich functions, it has been put into use in many companies, which can better meet the needs of the production level. However, it has the following disadvantages:

It depends on an external DB, increasing the cost and maintenance complexity of databases.
It depends on DB locks to ensure the consistency of distributed scheduling in clusters. When the number of tasks increases in a short period, it will put great pressure on DB and become a performance bottleneck.
Compared with the no-center mode, XXLJob needs to deploy additional schedulers, both the scheduler and the executor need to be persistent, and at least two devices are needed respectively to ensure high availability, causing unnecessary waste of resource costs when no tasks are to be executed.

Serverless Job

Target Positioning

Serverless is the best practice and future evolution trend of cloud computing. Its usage experience (fully managed and O&M-free) and cost advantage (pay-as-you-go) make it highly regarded in the cloud-native era.

As the first task-oriented Serverless PaaS, Serverless App Engine (SAE) Job provides a traditional user experience.

The current focus supports tasks in the standalone, broadcast, and parallel sharding models. It also supports many features (such as event-driven, concurrent policies, and timeout retries) and provides low-cost, multi-specification, and highly elastic resource instances to fulfill the execution of short-term tasks.

Usage

For existing applications that use all of the preceding solutions, SAE Job supports zero code modifications and imperceptible migration while being compatible with the feature experience.

If users use Crontab, Spring Task, ElasticJob, or XXLJob, they can directly deploy code packages or images to SAE Job and upgrade them to the Serverless architecture. This immediately provides the technical advantages of Serverless and saves resource costs and O&M costs.

Java, Shell, Python, Go, and Php can be directly deployed to SAE Job for programs stopped immediately after the operation. Then, the automated O&M control, the experience in fully managed service and free O&M, and out-of-the-box observability are available.

Principle

The underlying layer of Serverless App Engine (SAE) Job is the multi-tenant Kubernetes, which provides cluster computing resources by two means (including ECS bare metal instance and VK connecting ECI).

Tasks that users run in SAE Job are mapped to corresponding resources in Kubernetes. The multi-tenancy capability isolates tenants by system isolation, data isolation, service isolation, and network isolation.

SAE Job uses EventBridge as an event-driven source, which supports various calling methods and avoids the problem that Kubernetes built-in timers are not triggered on time and time zone precision.

At the same time, the enterprise-level features of the Job controller are continuously improved. Mechanisms are added (such as custom sharding, injection configuration, differentiated GC, active sidecar exit, real-time log persistence, and event notification). With the help of Java bytecode enhancement technology, SAE Job takes over task scheduling and realizes the Serverless mode of the universal Java target framework with zero code modifications.

With the KubeVela software delivery platform as a carrier for task release and management, the entire cloud-native delivery is completed in a declarative manner in the guidance of centering on tasks and open-source. SAE Job continuously optimizes the efficiency of etcd and scheduler in short-term high-concurrency task creation scenarios and the ultimate elasticity of instance startup. It combines the elastic resource pools to ensure low latency and high availability of task scheduling.

Solution Analysis

SAE Job solves various problems of the preceding timing task solutions. Users do not need to pay attention to task scheduling and cluster resources, deploy additional O&M dependency components, and build a complete monitoring and alerting system. More importantly, they do not need to have cloud hosting standing around 24/7, continuously consuming idle resources in a low resource utilization environment.

Compared with traditional timing task solutions, SAE Job provides three core values:

Complete and Fully Managed: It provides an overall, fully managed management interface with task lifecycle management and out-of-the-box observability. Users can learn to use SAE with low mental burden and zero cost.
Simple and O&M-Free: The underlying resources are blocked. Users only need to focus on the development of their core business logic without worrying about cluster availability, capacity, performance, etc.
Cost-Effective: The pay-as-you-go billing method is used. Users are charged only when the business logic of a task is executed. No fees are charged for the rest of the time, saving significant resource costs.

Summary

This article expounds on the target positioning, usage, and principle of several mainstream timing task solutions (Crontab, Spring Task, ElasticJob, XXLJob, and Serverless Job) and makes a horizontal evaluation of the functional experience and performance cost issues enterprises pay close attention to.

Finally, I hope you can choose Serverless Job and experience the new changes it has brought to traditional tasks.

Community

Full Review on Mainstream Timing Task Solutions

Crontab

Target Positioning

Usage

Principle

Solution Analysis

Spring Task

Target Positioning

Usage

Principle

Solution Analysis

ElasticJob

Target Positioning

Usage

Principle

Solution Analysis

XXLJob

Target Positioning

Usage

Principle

Solution Analysis

Serverless Job

Target Positioning

Usage

Principle

Solution Analysis

Summary

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