64 lines
2.7 KiB
Markdown
64 lines
2.7 KiB
Markdown
---
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parent: Remote Storage
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grand_parent: Code Howtos
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---
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# Remote SQL Storage
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For user documentation, see <https://docs.jabref.org/collaborative-work/sqldatabase>.
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## Handling large shared databases
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Synchronization times may get long when working with a large database containing several thousand entries. Therefore, synchronization only happens if several conditions are fulfilled:
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* Edit to another field.
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* Major changes have been made (pasting or deleting more than one character).
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Class `org.jabref.logic.util.CoarseChangeFilter.java` checks both conditions.
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Remaining changes that have not been synchronized yet are saved at closing the database rendering additional closing time. Saving is realized in `org.jabref.logic.shared.DBMSSynchronizer.java`. Following methods account for synchronization modes:
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* `pullChanges` synchronizes the database unconditionally.
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* `pullLastEntryChanges` synchronizes only if there are remaining entry changes. It is invoked when closing the shared database (`closeSharedDatabase`).
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## Database structure
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The following examples base on PostgreSQL.
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Other databases work similar.
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The database structure is created at [org.jabref.logic.shared.PostgreSQLProcessor#setUp](https://github.com/JabRef/jabref/blob/main/src/main/java/org/jabref/logic/shared/PostgreSQLProcessor.java#L37-L37).
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```mermaid
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erDiagram
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ENTRY ||--o{ FIELD : contains
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ENTRY {
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serial shared_id
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varchar type
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int version
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}
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FIELD {
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int entry_shared_id
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varchar name
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text value
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}
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METADATA {
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varchar key
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text value
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}
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```
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The "secret sauce" is the `version` of an entry.
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This version is used as version in the sense of an [Optimistic Offline Lock](https://martinfowler.com/eaaCatalog/optimisticOfflineLock.html), which in turn is a well-established technique to prevent conflicts in concurrent business transactions.
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It assumes that the chance of conflict is low.
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Implementation details are found at <https://www.baeldung.com/cs/offline-concurrency-control>.
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The `shared_id` and `version` are handled in [`org.jabref.model.entry.SharedBibEntryData`](https://github.com/JabRef/jabref/blob/main/src/main/java/org/jabref/model/entry/SharedBibEntryData.java).
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## Synchronization
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PostgreSQL supports to register listeners on the database on changes.
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(MySQL does not).
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The listening is implemented at [`org.jabref.logic.shared.listener.PostgresSQLNotificationListener`](https://github.com/JabRef/jabref/blob/main/src/main/java/org/jabref/logic/shared/listener/PostgresSQLNotificationListener.java#L16).
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It "just" fetches updates from the server when a change occurred there.
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Thus, the changes are not actively pushed from the server, but still need to be fetched by the client.
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