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plugin-system.md

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The Plugin System

The cardano-db-sync functionality is intended to be reasonably minimal. However, in order to make it more flexible, it offers a plugin/extension system. This plugin system allows the existing cardano-db-sync functionality to be used as a library which can then be reasonably easily extended.

Limitations

The main limitation is that extensions should never extend, modify or write to any existing table (reading however is OK). The extension should only ever write or modify tables that are not part of the core set of database tables.

Operation

The plugin system is defined in the Plugin modules. Plugins would implement 3 functions, one for each of three operations:

  • Startup - Called once every time the program is started.
  • Insert block - Called each time a new block arrive (including Epoch Boundary Blocks).
  • Rollback - Called on a chain rollback event.

The default plugin specifies the set of functions (only insert and rollback in this case) needed for standard operation of the cardano-db-sync program.

In order to run a version of the cardano-db-sync with extension, it is necessary to duplicate the top-level cardano-db-sync program which just does command line parsing and then calls into the library that implements the database/node functionality with the required extended DbSyncNodePlugin struct.

New tables are added to the database schema by updating the schema definition, and then following the schema management documentation.

An Example Plugin

An useful example plugin has been written to create and update an Epoch table. This table was initially done as a PostgreSQL VIEW but the performance was very poor. Changing it from a VIEW to a table updated by a plugin, resulted in a huge performance improvement over the VIEW version.

The epoch table plugin is implemented in epoch which specifies all three of the plugin operations. The extended DbSyncNodePlugin is extended by calling the plugin functions after the default functions (it is also possible to call them before if needed, but extending at the head of the lists in the DbSyncNodePlugin instead of the tail) and adding a new top-level program.