complete docs

Signed-off-by: Kavindu Dodanduwa <kavindudodanduwa@gmail.com>

docs/architecture.md

@@ -35,7 +35,9 @@ erDiagram
 
 In-process deployments embed the flagd evaluation engine directly into the client application through the use of an [in-process provider](./installation.md#in-process).
 The in-process provider is connected via the sync protocol to an implementing [gRPC service](./concepts/syncs.md#grpc-sync) that provides the flag definitions.
-This pattern requires an in-process implementation of the flagd evaluation engine, but has the benefit of no I/O overhead, since no inter-process communication is required.
+You can use flagd itself as a [gRPC service](./reference/grpc-sync-service.md).
+In this mode, the flag sync stream will expose aggregated flag configurations currently configured through [syncs](./concepts/syncs.md).
+This pattern requires an in-process implementation of the flagd evaluation engine but has the benefit of no I/O overhead, since no inter-process communication is required.
 
 ```mermaid
 ---

docs/index.md

@@ -13,6 +13,7 @@ With flagd you can:
 * perform pseudorandom assignments for experimentation
 * perform progressive roll-outs of new features
 * aggregate flag definitions from multiple sources
+* expose aggregated flags as a gRPC stream to be used by in-process providers
 
 It doesn't include a UI, management console or a persistence layer.
 It's configurable entirely via a [POSIX-style CLI](./reference/flagd-cli/flagd.md).

docs/reference/grpc-sync-service.md

@@ -0,0 +1,41 @@
+---
+description: flagd as a gRPC sync service
+---
+
+# Overview
+
+flagd can expose a gRPC sync service, allowing in-process providers to obtain their flag configurations.
+This mode is **disabled** by default, and you can enable it by using startup flag `--sync-enabled` (or `-e` shorthand flag).
+The gRPC sync stream contains flag configurations currently configured at flagd as [sync-configurations](./sync-configuration.md).
+
+```mermaid
+---
+title: gRPC sync
+---
+erDiagram
+    flagd ||--o{ "sync (file)" : watches
+    flagd ||--o{ "sync (http)" : polls
+    flagd ||--o{ "sync (grpc)" : "sync.proto (gRPC/stream)"
+    flagd ||--o{ "sync (kubernetes)" : watches
+    "In-Process provider" ||--|| flagd : "gRPC sync stream (default port 8015)"
+```
+
+You may change the default port of the service using startup flag `--sync-port` (or `-g` shothand flag).
+
+By default, the gRPC stream expose all the flag configurations merged following merge strategy of flagd.
+You can read more about the merge strategy in our dedicated [concepts guide on syncs](../concepts/syncs.md).
+
+If you specify a `selector` in the gRPC sync request, the gRPC service will attempt match the provided selector valur to a source and stream specific flags.
+For example, if `selector` is set to `myFlags.json`, service will stream flags observed from `myFlags.json` file.
+And the request will fail if there is no flag source matching the requested `selector`.
+flagd provider implementations expose the ability to define the `selector` value.
+
+```java
+final FlagdProvider flagdProvider =
+        new FlagdProvider(FlagdOptions.builder()
+                .resolverType(Config.Evaluator.IN_PROCESS)
+                .host("localhost")
+                .port(8015)
+                .selector("myFlags.json")
+                .build());
+```