Spring batch - special aspects of batch processing

Spring batch what for?

The spring batch is a framework specially designed for batch processing.

Intended for processing e.g. files with large amount of data, providing a clear DSL defined based on XML.

The framework comes with abstractions and defaults that have "extension points" where business or processing logic can be placed.

Why should I use spring batch?

It is regarded as the standard framework for batch processing. A lot of developers know how to use it.

It provides useful abstractions and can be configured in many regards to support more advanced requirements:

• transaction support
• retry support
• skip functionality
• perfectly integrated in the spring world (DI etc.)
• strong layered architecture
• very scalable due to support of step partitioning, multi-threaded steps, …

Basic concept

In spring batch the processing starts with spring batch job. It consists of steps. 

The steps can be chunk-oriented or so called TaskletStep but this is for supporting legacy code.

The main components of spring batch are: 

• ItemReader - reads one item
• ItemProcessor - processes one item, but is optional
• ItemWriter - writes a list of items

Besides that there are different type of listeners for placing business

logic:

• job/step execution listener
• chunk listener
• ItemRead/Process/Write- Listener
• SkipListener

The transaction boundary can never be around steps or a complete job.

Metadata like execution starting/end point, amount of commits/rollbacks, step status etc. are saved at several points:

• step execution context - a map that is used for serializing data
• chunk execution context - used inside a chunk transaction for knowing the current item in process

The default behavior for rollbacks is that if a non-caught exceptions happened during the processing of a chunk, the step is rolled back.

All committed chunks until that stay committed, but the complete job fails.

The meta data of a step are initialized in the beginning of the step and updated at the end of the step. This is done in separate transactions, in order to update the state of the step status this has to be done in an own transaction as the processing of the step itself can fail and has to be rollbacked then.

A spring batch job consists of steps as we know now and these steps consists of chunks. Each chunk is executed in its own transaction.

When does spring batch know how much data has to be read into the chunk?

This is specified by a policy: CompletionPolicy.

Specifying the commitInterval on the chunk tag leads to a SimpleCompletionPolicy.

As soon as the amount of items are read satisfying the completionPolicy, the items read and processed are passed to the ItemWriter.

Restart of a job

How is a restart of a job done?

Well, for batch processing the data retrieval if not from a file but retrieved from a database table(s) or messaging system are done over the declaration of a non-transactional datasource. As on a processing error a rollback would close the data retrieval channel as well.

On this non-transactional datasource no other job, component or module should operate. The data is normally read by use a database cursor in order to avoid memory issues. Spring batch provides here the JdbcCursorItemReader.

The restart of a job itself is analyzed by spring batch. If a job is called with the same parameters (same job parameters) spring batch sees this as a restart of the job if the job before ended with a failure.

In order to accomplish that the state is saved in the execution context of the chunk.

Here every reader persists the counter of read items inside the transaction of the chunk. Every chunk commits his work inside his transaction towards the end of work. Meaning if the completionPolicy is set to 10. After the 10 item the chunk tries to commit its work. On job restart the execution point of the last successful chunk is taken and the not committed items are processed.

The execution context is seen by all the readers - that means the state of the counter can be modified by different counters. This setup is not thread-safe!

Ordering

For the restart-ability of a job the ordering of the read data must be well-defined.

If the data comes from database or from a file the ordering in the data retrieval must be explicit set in order that the restart of the job get the items that have to be reprocessed in the same order as on the first run.

Benefits and usage of spring data JPA

What are the benefits of using spring data JPA?

Spring data JPA addresses the following situations:

• unclear how the persistence layer will develop
• the first prototype starts because of time and focus with a map, later it probably it will be replaced with a longterm persistence
• persistence layer might change from relational to NoSQL or vice versa 
• for the sake of a set of fast running unit-tests the persistence layer might be configured to use light-weight persistence like a simple map
• …

To be really open regarding the persistence layer, the domain layer should be separated from the data access layer. For this an approach like the repository pattern from Martin Fowler is common praxis.

The repository enforces to treat objects of a type as a "conceptual set" like a collection. 

With a simple DAO approach you see the DAO as a gateway for accessing the database. 

This DAO tend to grow extensively as new querying or update functionality is needed.

This leads to poor responsibility. With the repository you treat all the objects as a conceptual set. 

For querying and update extensions the repository will make usage of DAO(s). 

So the DAO are well-focussed and have a single responsibility for gathering / changing data.

The set objects of a type are handled in the repository.

In the beginning of your development you can just have a simple in-memory storage as a map 

in order to focus on the domain logic etc. Later you can delegate the storage and access to sophisticated DAO(s).

So the domain objects used by the business logic in the domain layer are developed against the interfaces that are used by the repository interfaces.

On the top of the jpa entities the repository layer is placed.

Next to the domain objects the repository interfaces are placed. They always present to the outside the interfaces that are used by the domain layer which are exposed by the repository interfaces. These repository interfaces provide basic CRUD functionalities.

Example:

domain layer: Customer implements ICustomer

repository layer: CustomerRepository delivers ICustomer

persistence layer: CustomerRepositoryImpl implements the CustomerRepository

The CustomerRepositoryImpl also could make further usage of DAOs to access the objects.

The CustomerRepositoryImpl will make usage of the EntityManager of JPA and will define the transactional context:

public class CustomerRepositoryImpl implements CustomerRepository {

    private EntityManager entityManager;

    @Transactional

    public ICustomer save(ICustomer customer) {

      Customer c = new Customer(customer);

       entityManager.persist©;

       return c;

    } 

}

Usage of spring data jpa

The spring data jpa has the objective to simply the development of the repository layer, mentioned above as this code is boilerplate-code. With spring data jpa you only have to define the interface, an implementation for delegation and provide the corresponding spring configuration.

The rest will be instantiated and delivered by Spring.

So first of all we have to define the repository interface:

public interface CustomerJpaRepository extends JpaRepository<Customer, Long> {

    Customer save(Customer customer);

}

Second we need the spring configuration:

<jpa:repositories base-package="…">

   <jpa:repository id="customerJpaRepository" />

</jpa:repositories>

Unfortunately spring data jpa can't operate directly on the interface defined as the CustomerJpaRepository. It always needs the specific jpa entity. 

Therefore we need to implement the interface of CustomerJpaRepository.

But we will inject an instance of the CustomerJpaRepository and all operations will delegate to this instance:

public class CustomerJpaRepositoryImpl implements CustomerJpaRepository {

       private CustomerJpaRepository repo;

      public Customer save(Customer customer) {

           return repos.save(customer);

      }  

}

In the background spring data jpa will dynamically provide an instance of the interface and make it available under the id customerJpaRepository.

Advantages of spring data jpa

Spring data jpa provides finder methods out of the box. 

So based on naming conventions findByX will be provided by spring data jpa dynamically and will result to an entity result where all the entities will have for their field X the corresponding parameter value.

Besides there are other useful features like paging including sorting and others.