> ## Documentation Index > Fetch the complete documentation index at: https://docs.open-metadata.org/llms.txt > Use this file to discover all available pages before exploring further. # Understand Code Layout # Understand Code Layout Use this document as a quick start guide to begin developing in OpenMetadata. Below, we address the following topics: 1. Schema (Metadata Models) 2. APIs 3. System and Components ## Schema (Metadata Models) OpenMetadata takes a schema-first approach to model metadata. We define entities, types, API requests, and relationships between entities. We define the OpenMetadata schema using the [JSON Schema](https://json-schema.org/) vocabulary. We convert models defined using JSON Schema to [Plain Old Java Objects (POJOs)](https://www.jsonschema2pojo.org/) using the `jsonschema2pojo-maven-plugin` plugin as defined in [`pom.xml`](https://github.com/open-metadata/OpenMetadata/blob/main/openmetadata-service/pom.xml#L517). You can find the generated POJOs under `OpenMetadata/openmetadata-service/target/generated-sources/jsonschema2pojo`. ### Entities You can locate defined entities in the directory [`OpenMetadata/openmetadata-spec/src/main/resources/json/schema/entity`](https://github.com/open-metadata/OpenMetadata/tree/main/openmetadata-spec/src/main/resources/json/schema/entity). Currently, OpenMetadata supports the following entities: * data * feed * policies * services * tags * teams ### Types All OpenMetadata supported types are defined under [`OpenMetadata/openmetadata-spec/src/main/resources/json/schema/type`](https://github.com/open-metadata/OpenMetadata/tree/main/openmetadata-spec/src/main/resources/json/schema/type). ### API request objects The API request objects are defined under [`OpenMetadata/openmetadata-spec/src/main/resources/json/schema/api`](https://github.com/open-metadata/OpenMetadata/tree/main/openmetadata-spec/src/main/resources/json/schema/api). ## API OpenMetadata uses the [Dropwizard](https://www.dropwizard.io/) Java framework to build REST APIs. You can locate defined APIs in the directory [`OpenMetadata/openmetadata-service/src/main/java/org/openmetadata/service/resources`](https://github.com/open-metadata/OpenMetadata/tree/main/openmetadata-service/src/main/java/org/openmetadata/service/resources). OpenMetadata uses [Swagger](https://swagger.io/) to generate API documentation following OpenAPI standards. ## System and Components System and Components ### Events OpenMetadata captures changes to entities as `events` and stores them in the OpenMetadata server database. OpenMetadata also indexes change events in Elasticsearch to make them searchable. The event handlers are defined under [`OpenMetadata/openmetadata-service/src/main/java/org/openmetadata/service/events`](https://github.com/open-metadata/OpenMetadata/tree/main/openmetadata-service/src/main/java/org/openmetadata/service/events) and are applied globally to any outgoing response using the `ContainerResponseFilter`. ### Database OpenMetadata uses MySQL or Postgres for the metadata catalog. The catalog code is located in the directory [`OpenMetadata/openmetadata-service/src/main/java/org/openmetadata/service/jdbi3`](https://github.com/open-metadata/OpenMetadata/tree/main/openmetadata-service/src/main/java/org/openmetadata/service/jdbi3). The database entity tables are created with the script [`OpenMetadata/bootstrap/openmetadata-ops.sh`](https://github.com/open-metadata/OpenMetadata/blob/main/bootstrap/openmetadata-ops.sh). [Flyway](https://flywaydb.org/) is used for managing the database table versions. ### Elasticsearch OpenMetadata uses Elasticsearch to store the Entity change events and makes it searchable by search index. The [`OpenMetadata/openmetadata-service/src/main/java/org/openmetadata/service/elasticsearch/ElasticSearchEventPublisher.java`](https://github.com/open-metadata/OpenMetadata/blob/main/openmetadata-service/src/main/java/org/openmetadata/service/elasticsearch/ElasticSearchEventPublisher.java) is responsible for capturing the change events and updating Elasticsearch. Elasticsearch indices are created when the [`OpenMetadata/ingestion/pipelines/metadata_to_es.json`](https://github.com/open-metadata/OpenMetadata/blob/main/ingestion/pipelines/metadata_to_es.json) ingestion connector is run. ### Authentication/Authorization OpenMetadata uses Google OAuth for authentication. All incoming requests are filtered by validating the JWT token using the Google OAuth provider. Access control is provided by [`Authorizer`](https://github.com/open-metadata/OpenMetadata/blob/main/openmetadata-service/src/main/java/org/openmetadata/service/security/Authorizer.java). See the configuration file `OpenMetadata` [`/conf/openmetadata.yaml`](https://github.com/open-metadata/OpenMetadata/blob/main/conf/openmetadata.yaml) for the authentication and authorization configurations. ### Ingestion Ingestion is a simple Python framework to ingest metadata from external sources into OpenMetadata. **Connectors** OpenMetadata defines and uses a set of components called `Connectors` for metadata ingestion. Each data service requires its own connector. See the documentation on how to [build a connector](/v1.12.x/developers/contribute/developing-a-new-connector) for details on developing connectors for new services. Workflow is a simple orchestration job that runs `Source`, `Processor`, `Sink`, `Stage` and `BulkSink` based on the configurations present under [`OpenMetadata/ingestion/examples/workflows`](https://github.com/open-metadata/OpenMetadata/tree/main/ingestion/src/metadata/examples/workflows). There are some popular connectors already developed and can be found under: 1. Source → [`OpenMetadata/ingestion/src/metadata/ingestion/source`](https://github.com/open-metadata/OpenMetadata/tree/main/ingestion/src/metadata/ingestion/source) 2. Processor → [`OpenMetadata/ingestion/src/metadata/ingestion/processor`](https://github.com/open-metadata/OpenMetadata/tree/main/ingestion/src/metadata/ingestion/processor) 3. Sink → [`OpenMetadata/ingestion/src/metadata/ingestion/sink`](https://github.com/open-metadata/OpenMetadata/tree/main/ingestion/src/metadata/ingestion/sink) 4. Stage → [`OpenMetadata/ingestion/src/metadata/ingestion/stage`](https://github.com/open-metadata/OpenMetadata/tree/main/ingestion/src/metadata/ingestion/stage) 5. BulkSink → [`OpenMetadata/ingestion/src/metadata/ingestion/bulksink`](https://github.com/open-metadata/OpenMetadata/tree/main/ingestion/src/metadata/ingestion/bulksink) **Airflow** For simplicity, OpenMetadata ingests metadata from external sources using a pull-based model. OpenMetadata uses Apache Airflow to orchestrate ingestion workflows. See the directory [`OpenMetadata/ingestion/examples/airflow/dags`](https://github.com/open-metadata/OpenMetadata/tree/main/ingestion/examples/airflow/dags) for reference DAG definitions. **JsonSchema python typings** You can generate Python types for OpenMetadata models defined using Json Schema using the make generate command of the [`Makefile`](https://github.com/open-metadata/OpenMetadata/blob/main/Makefile). Generated files are located in the directory `OpenMetadata/ingestion/src/metadata/generated`