Update docs with the information about GCS sync source

Signed-off-by: Alan Kutniewski <kutniewski@google.com>

docs/concepts/syncs.md

@@ -68,6 +68,23 @@ In this example, `default/my_example` expected to be a valid FeatureFlag resourc
 namespace and `my_example` being the resource name.
 See [sync source](../reference/sync-configuration.md#source-configuration) configuration for details.
 
+---
+
+### GCS sync
+
+The GCS sync provider fetches flags from a GCS blob and periodically poll the GCS for the flag definition updates.
+It uses [application default credentials](https://cloud.google.com/docs/authentication/application-default-credentials) if they
+are [configured](https://cloud.google.com/docs/authentication/provide-credentials-adc) to authorize the calls to GCS.
+
+```shell
+flagd start --uri gs://my-bucket/my-flags.json
+```
+
+In this example, `gs://my-bucket/my-flags.json` is expected to be a valid GCS URI accessible by the flagd
+(either by being public or together with application default credentials).
+The polling interval can be configured.
+See [sync source](../reference/sync-configuration.md#source-configuration) configuration for details.
+
 ## Merging
 
 Flagd can be configured to read from multiple sources at once, when this is the case flagd will merge all flag definition into a single

docs/reference/sync-configuration.md

@@ -17,6 +17,7 @@ it is passed to the correct implementation:
 | `file`                | `file:`                | `file:etc/flagd/my-flags.json`        |
 | `http`                | `http(s)://`           | `https://my-flags.com/flags`          |
 | `grpc`                | `grpc(s)://`           | `grpc://my-flags-server`              |
+| `gcs`                 | `gs://`                | `gs://my-bucket/my-flags.json`        |
 
 ## Source Configuration
 
@@ -30,10 +31,10 @@ Alternatively, these configurations can be passed to flagd via config file, spec
 | Field       | Type               | Note                                                                                                                                                                                                             |
 | ----------- | ------------------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | uri         | required `string`  | Flag configuration source of the sync                                                                                                                                                                            |
-| provider    | required `string`  | Provider type - `file`, `kubernetes`, `http`, or `grpc`                                                                                                                                                          |
+| provider    | required `string`  | Provider type - `file`, `kubernetes`, `http`, `grpc` or `gcs`                                                                                                                                                         |
 | authHeader  | optional `string`  | Used for http sync; set this to include the complete `Authorization` header value for any authentication scheme (e.g., "Bearer token_here", "Basic base64_credentials", etc.). Cannot be used with `bearerToken` |
 | bearerToken | optional `string`  | (Deprecated) Used for http sync; token gets appended to `Authorization` header with [bearer schema](https://www.rfc-editor.org/rfc/rfc6750#section-2.1). Cannot be used with `authHeader`                        |
-| interval    | optional `uint32`  | Used for http sync; requests will be made at this interval. Defaults to 5 seconds.                                                                                                                               |
+| interval    | optional `uint32`  | Used for http and gcs syncs; requests will be made at this interval. Defaults to 5 seconds.                                                                                                                               |
 | tls         | optional `boolean` | Enable/Disable secure TLS connectivity. Currently used only by gRPC sync. Default (ex: if unset) is false, which will use an insecure connection                                                                 |
 | providerID  | optional `string`  | Value binds to grpc connection's providerID field. gRPC server implementations may use this to identify connecting flagd instance                                                                                |
 | selector    | optional `string`  | Value binds to grpc connection's selector field. gRPC server implementations may use this to filter flag configurations                                                                                          |
@@ -54,6 +55,7 @@ Sync providers:
 - `kubernetes` - default/my-flag-config
 - `grpc`(insecure) - grpc-source:8080
 - `grpcs`(secure) - my-flag-source:8080
+- `gcs` - gs://my-bucket/my-flags.json
 
 Startup command:
 
@@ -66,7 +68,8 @@ Startup command:
             {"uri":"default/my-flag-config","provider":"kubernetes"},
             {"uri":"grpc-source:8080","provider":"grpc"},
             {"uri":"my-flag-source:8080","provider":"grpc", "maxMsgSize": 5242880},
-            {"uri":"my-flag-source:8080","provider":"grpc", "certPath": "/certs/ca.cert", "tls": true, "providerID": "flagd-weatherapp-sidecar", "selector": "source=database,app=weatherapp"}]'
+            {"uri":"my-flag-source:8080","provider":"grpc", "certPath": "/certs/ca.cert", "tls": true, "providerID": "flagd-weatherapp-sidecar", "selector": "source=database,app=weatherapp"},
+            {"uri":"gs://my-bucket/my-flag.json","provider":"gcs"}]'
 ```
 
 Configuration file,
@@ -91,4 +94,6 @@ sources:
     tls: true
     providerID: flagd-weatherapp-sidecar
     selector: "source=database,app=weatherapp"
+  - uri: gs://my-bucket/my-flag.json
+    provider: gcs
 ```