Project: Open Policy Agent (OPA) (Provisioning / Security & Compliance)
File: management-discovery.md

title: "Discovery" kind: management weight: 5 OPA can be configured to download bundles of policy and data, report status, and upload decision logs to remote endpoints. The discovery feature helps you centrally manage the OPA configuration for these features. You should use the discovery feature if you want to avoid managing OPA configuration updates in a number of different locations. When the discovery feature is enabled, OPA will periodically download a discovery bundle. Like regular bundles, the discovery bundle may contain JSON and Rego files. OPA will evaluate the data and policies contained in the discovery bundle to generate the rest of the configuration. There are two main ways to structure the discovery bundle: Include static JSON configuration files that define the OPA configuration. Include Rego files that can be evaluated to produce the OPA configuration. If you need OPA to select which policy to download dynamically (e.g., based on environment variables like the region where OPA is running), use the second option. If discovery is enabled, other features like bundle downloading and status reporting can be configured manually. In case of conflicts, the bootstrap configuration for plugins would override the discovered configuration. In general, the bootstrap configuration overrides the discovered configuration. See the for configuration details. OPA expects the service to expose an API endpoint that serves bundles. ```http GET /<service_url>/<discovery.resource> HTTP/1.1 ``` If the bundle exists, the server should respond with an HTTP 200 OK status followed by a gzipped tarball in the message body. ```http HTTP/1.1 200 OK Content-Type: application/gzip ``` You can enable discovery with an OPA configuration file similar to the example below. In some places in the documentation, the initial configuration provided to OPA is referred to as the "boot configuration". ```yaml services: acmecorp: url: https://example.com/control-plane-api/v1 credentials: bearer: token: "bGFza2RqZmxha3NkamZsa2Fqc2Rsa2ZqYWtsc2RqZmtramRmYWxkc2tm" discovery: service: acmecorp resource: /configuration/example/discovery.tar.gz ``` Using the boot configuration above, OPA will fetch discovery bundles from: ``` https://example.com/control-plane-api/v1/configuration/example/discovery.tar.gz ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ services[discovery.service].url discovery.resource ``` The `discovery.resource` field defaults to `bundles/<discovery.name>`. The default is convenient if you want to serve discovery bundles and normal bundles from the same API endpoint. If only one service is defined, there is no need to set `discovery.service`. The optional `discovery.signing` field can be used to specify the `keyid` and `scope` that should be used for verifying the signature of the discovery bundle. See section for details. OPA generates it's subsequent configuration by querying the Rego and JSON files contained inside the discovery bundle. The default query is `data` however this can be overridden by specifying the `discovery.decision`. ```yaml services: name: acmecorp url: https://example.com/control-plane-api/v1 credentials: bearer: token: "bGFza2RqZmxha3NkamZsa2Fqc2Rsa2ZqYWtsc2RqZmtramRmYWxkc2tm" discovery: name: example resource: /configuration/example/discovery.tar.gz decision: example/discovery ``` OPA executes the following query: ```

``` If the discovery bundle contained the following Rego file: ```ruby package example discovery := { "bundles": { "main": { "service": "acmecorp", "resource": bundle_name }, }, "default_decision": "acmecorp/httpauthz/allow" } bundle_name := "acmecorp/httpauthz" ``` The subsequent configuration would be: ```json { "bundles": { "main": { "service": "acmecorp", "resource": "acmecorp/httpauthz" }, }, "default_decision": "acmecorp/httpauthz/allow" } ``` The discovery bundle contents above are essentially static. The same result could be achieved by constructing the discovery bundle with a static JSON file: ```json { "example": { "discovery": { "bundles": { "main": { "service": "acmecorp", "resource": "acmecorp/httpauthz" }, }, "default_decision": "acmecorp/httpauthz/allow" } } } ``` For an example of how to configure OPA dynamically see the section below. The subsequent configuration does not have to specify `services` or include a reference to a service in the `bundle`, `status,` or `decision_log` sections. If the either the `services` or references to services are missing, OPA will default them to the value from the boot configuration. Let's see an example of how the discovery feature can be used to dynamically configure an OPA to download one of two bundles based on a label in the boot configuration. Let's say the label `region` indicates the region in which the OPA is running and it's value will decide the bundle to download. Below is a policy file which generates an OPA configuration. example.rego ```ruby package discovery config := { "bundles": { "main": { "service": "acmecorp", "resource": bundle_name # line 7 } } } rt := opa.runtime() region := rt.config.labels.region bundlename := regionbundle[region] region_bundle := { "US": "example/test1/p", "UK": "example/test2/p" } ``` The `bundle_name` variable in `line 7` of the above policy will be dynamically selected based on the value of the label `region`. So if an OPA was started with `region: "US"`, then the `bundle_name` will be `example/test1/p`. Start an OPA with a boot configuration as shown below: config.yaml ```yaml services: name: acmecorp url: https://example.com/control-plane-api/v1 credentials: bearer: token: "bGFza2RqZmxha3NkamZsa2Fqc2Rsa2ZqYWtsc2RqZmtramRmYWxkc2tm" discovery: resource: bundles/discovery.tar.gz decision: discovery/config labels: region: "US" ``` Run OPA: ```bash opa run -s -c config.yaml ``` You should see a log like below, which shows the bundle being downloaded. In this case, the bundle name is `example/test1/p` as `region` is `US`. ```raw INFO Bundle downloaded and activated successfully. name=example/test1/p plugin=bundle ``` Now start another OPA with a boot configuration as shown below. Notice the `region` is `UK`: config.yaml ```yaml services: name: acmecorp url: https://example.com/control-plane-api/v1 credentials: bearer: token: "bGFza2RqZmxha3NkamZsa2Fqc2Rsa2ZqYWtsc2RqZmtramRmYWxkc2tm" discovery: resource: bundles/discovery.tar.gz decision: discovery/config labels: region: "UK" ``` Run OPA: ```bash opa run -s -c config.yaml ``` In this case, the bundle being downloaded is `example/test2/p` as `region` is `UK`. ```raw INFO Bundle downloaded and activated

name=example/test2/p plugin=bundle ``` This shows how the discovery feature can help in centrally managing the bundle to be downloaded by an OPA based on a configuration label. You can use the same strategy to dynamically configure other plugins based on the running OPA's configuration labels or environment variables. In practice, discovery services do not change frequently. These configuration sections are treated as immutable to avoid accidental configuration errors rendering OPA unable to discover a new configuration. If the discovered configuration changes the `discovery` section, those changes are ignored. If the discovered configuration changes the discovery service, an error will be logged. If the discovered configuration changes the `labels` section, only labels that are additional compared to the bootstrap configuration are used, all other changes are ignored. If the discovery document changes its `labels` section over time, the effective set of labels is always the bootstrap configuration plus added labels from the latest discovery document. Like regular bundles, if the discovery bundle contains a `.signatures.json` file, OPA will verify the discovery bundle before activating it. The format of the `.signatures.json` file and the verification steps are same as that for regular bundles. Since the discovered configuration ignores changes to the `discovery` section, any key used for signature verification of a discovery bundle CANNOT be modified via discovery. We recommend that if you are using discovery you should be signing the discovery bundles because those bundles include the keys used to verify the non-discovery bundles. However, OPA does not enforce that recommendation. You may use unsigned discovery bundles that themselves require non-discovery bundles to be signed. OPA can optionally persist the activated discovery bundle to disk for recovery purposes. To enable persistence, set the `discovery.persist` field to `true`. When bundle persistence is enabled, OPA will attempt to read the discovery bundle from disk on startup. This allows OPA to start with the most recently activated bundle in case OPA cannot communicate with the bundle server. OPA will try to load and activate the persisted discovery bundle on a best-effort basis. Any errors encountered during the process will be surfaced in the bundle's status update. When communication between OPA and the bundle server is restored, the latest bundle is downloaded, activated, and persisted. Like regular bundles, only the discovery bundle itself is persisted. The discovered configuration that is generated by evaluating the data and policies contained in the discovery bundle will NOT be persisted. {{< info >}} The discovery bundle is persisted at `<persistence_directory>/bundles/<discovery.name>/bundle.tar.gz`. By default `persistence_directory` is `.opa` in the working directory of the OPA process. If `persistence_directory` is changed through discovery this will not affect where the discovery plugin will store the discovery bundles, the boot configuration will always be used. {{< /info >}} Configuring OPA using Discovery Bundles is a powerful production feature. {{< ecosystemfeatureembed key="opa-bundles-discovery" topic="or support distributing discovery bundles" >}}