PAK Core – IPersistenceService
2. Basics
The IPersistenceService-interface is the main component involved in storing and persisting your data. While an Engine is running, the Persistence Service functions as the datastore, making it the direct entry point for all data created or consumed by any Command. The Persistence Service should essentially function as a key-value database.
3. Signature
Before giving a short example on how the interface should be used, this section describes the signature of the IPersistenceService interface, as well as an overview on which methods have to be implemented.
3.1. Transaction Model
The interface offers the ability to implement the commonly known ACID-principle by declaring different methods based on transactions.
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Begins a new transaction in the current Persistence; Should always be thread dependent |
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Finishes the current transaction by committing all changes of the current thread |
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Undoes all changes made in the current transaction and finishes it. |
| While you can choose not to follow the transaction model, be advised that your implementation should ensure thread-safety |
3.2. Data Processing
The interface offers two operations relating to data processing. find for looking up values stored in the datastore and store in order to populate it.
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Tries retrieving the object for the specified key from the Persistence, returns null if no value was found. |
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Tries retrieving all objects for the specified keys from the Persistence, if no value was found for a key, there will be no entry in the resulting map. |
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Stores the given object for the specified key in the Persistence. |
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Stores all given entries in the Persistence. |
You must implement the find and store method for single values. By default, the respective operations which take a map as an input only call your single element operations for every element of the map.
If the need for data deletion arises, implement this functionality for the store operation using IPersistenceService.InitialValue.class
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4. Usage
This section briefly outlines a few examples on how an implementing Persistence should be used.
4.1. Accessing Data
private final IPersistenceService persistence = ...;
public void storeExample() {
this.persistence.begin(); (1)
this.persistence.store("testKey", "Hello World"); (2)
this.persistence.commit(); (3)
}
public Object retrieveExample() {
this.persistence.begin(); (1)
try {
return this.persistence().find("testKey"); (2)
} finally {
this.persistence.commit(); (3)
}
}| 1 | Before we perform actions on our Persistence we should start a new transaction, so possible mistakes may be rolled back |
| 2 | After we initialized our transaction we can either store or retrieve a value to/from the Persistence |
| 3 | Finally, when we are finished editing, we commit the transaction, thus closing it and saving the state |
4.2. Data Deletion
The deletion of data is not a built-in specification as the deleting of data from the datastore should hardly be necessary. However, if you need the functionality the IPersistenceService offers a marker class for that reason:
/** marker class */
final class InitialValue {
private InitialValue() {
// private constructor this is only used via InitialValue.class
}
}Let’s suppose we want to delete the object behind the key superSecretVar. We simply call store using our marker class like
persistence.store("superSecretVar", IPersistenceService.InitialValue.class);
4.3. Data Transformation
All data that is written to the Persistence Service will be in the JSON format by default. However, should your implementation be dependent on another data format, do not solve this in your Persistence, but rather use the designated interface IDataTransformer.
5. Default Implementations
There are two default implementations of the IPersistenceService offered by PAK.
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The
SimplePersistenceService: Functions as an in-memory Persistence which can be used analogous to a map. It is mostly used for prototyping and testing (Dependency: de.asap.pak.core:simple) -
The
RocksDBPersistence: Functions as a bridge between PAK and the RocksDB implementation. (Dependency: de.asap.pak.extra:pak-rocksdb)