> ## Documentation Index > Fetch the complete documentation index at: https://developer.upsun.com/llms.txt > Use this file to discover all available pages before exploring further. # Relationships > A dictionary of relationships that defines the connections to other services and apps. Optional in [single-runtime](https://developer.upsun.com/docs/configure-apps/app-reference/single-runtime-image.md#primary-application-properties) and [composable](https://developer.upsun.com/docs/configure-apps/app-reference/composable-image.md#primary-application-properties) images. To allow containers in your project to communicate with one another, you need to define relationships between them. You can define a relationship between an app and a service, or [between two apps](https://developer.upsun.com/docs/configure-apps/multi-app/relationships.md). The quickest way to define a relationship between your app and a service is to use the service's default endpoint.\ However, some services allow you to define multiple databases, cores, and/or permissions. In these cases, you can't rely on default endpoints. Instead, you can explicitly define multiple endpoints when setting up your relationships. > [!NOTE] > > App containers don't have a default endpoint like services. > To connect your app to another app in your project, > you need to explicitly define the `http` endpoint as the endpoint to connect both apps.\\ > > For more information, see how to [define relationships between your apps](https://developer.upsun.com/docs/configure-apps/multi-app/relationships.md). > [!NOTE] > > Availability > New syntax (default and explicit endpoints) described below is supported by most, but not all, image types > (`Relationship 'SERVICE_NAME' of application 'app' ... targets a service without a valid default endpoint configuration.`). > This syntax is currently being rolled out for all images. > If you encounter this error, use the "legacy" Upsun Cloud configuration noted at the bottom of this section. To define a relationship between your app and a service: > [tab] Using default endpoints > > Use the following configuration: > > applications: > \: > \# ... > relationships: > \: > > The `SERVICE_NAME` is the name of the service as defined in its [configuration](https://developer.upsun.com/docs/add-services). > It is used as the relationship name, and associated with a `null` value. > This instructs Upsun Cloud to use the service's default endpoint to connect your app to the service. > > For example, if you define the following configuration: > > applications: > \: > \# ... > relationships: > mariadb: > > Upsun Cloud looks for a service named `mariadb` in your `.upsun/config.yaml` file, > and connects your app to it through the service's default endpoint. > > For reference, the equivalent configuration using explicit endpoints would be the following: > > applications: > \: > \# ... > relationships: > mariadb: > service: mariadb > endpoint: mysql > > You can define any number of relationships in this way: > > applications: > \: > \# ... > relationships: > mariadb: > redis: > elasticsearch: > > > [!NOTE] > > > > Tip > > An even quicker way to define many relationships is to use the following single-line configuration: > > > > applications: > > \: > > \# ... > > relationships: > > \ > > \ > > \ > > > > services: > > \: > > type: mariadb:\{\{version:mariadb:latest}} > > \: > > type: redis:\{\{version:redis:latest}} > > \: > > type: elasticsearch:\{\{version:elasticsearch:latest}} > [tab] Using explicit endpoints > > Use the following configuration: > > applications: > \: > \# ... > relationships: > \: > service: \ > endpoint: \ > > * `RELATIONSHIP_NAME` is the name you want to give to the relationship. > * `SERVICE_NAME` is the name of the service as defined in its [configuration](https://developer.upsun.com/docs/add-services). > * `ENDPOINT_NAME` is the endpoint your app will use to connect to the service (refer to the service reference to know which value to use). > > For example, to define a relationship named `database` that connects your app to a service called `mariadb` through the `db1` endpoint, > use the following configuration: > > applications: > \: > \# ... > relationships: > database: # The name of the relationship. > service: mariadb > endpoint: db1 > > For more information on how to handle multiple databases, multiple cores, > and/or different permissions with services that support such features, > see each service's dedicated page: > > * [MariaDB/MySQL](https://developer.upsun.com/docs/add-services/mysql#multiple-databases) (multiple databases and permissions) > * [PostgreSQL](https://developer.upsun.com/docs/add-services/postgresql#multiple-databases) (multiple databases and permissions) > * [Redis](https://developer.upsun.com/docs/add-services/redis.md#multiple-databases) (multiple databases) > * [Solr](https://developer.upsun.com/docs/add-services/solr.md#solr-6-and-later) (multiple cores) > * [Vault KMS](https://developer.upsun.com/docs/add-services/vault.md#multiple-endpoints-example) (multiple permissions) > > You can add as many relationships as you want to your app configuration, > using both default and explicit endpoints according to your needs: > > applications: > \: > \# ... > relationships: > database1: > service: mariadb > endpoint: admin > database2: > service: mariadb > endpoint: legacy > cache: > service: redis > search: > service: elasticsearch > [!NOTE] > > Legacy > The following legacy syntax for specifying relationships is still supported by Upsun Cloud: > > applications: > \: > \# ... > relationships: > \: "\:\" > > services: > SERVICE\_NAME\_A: > type: mariadb:\{\{version:mariadb:latest}} > > For example: > > applications: > \: > \# ... > relationships: > database: "db:mysql" > > services: > db: > type: mariadb:\{\{version:mariadb:latest}} > > Feel free to use this until the default and explicit endpoint syntax is supported on all images. 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