Add doc for api node

book/src/SUMMARY.md

@@ -24,6 +24,7 @@
   - [The Runtime](runtime.md)
 
 - [API Reference](api-reference.md)
+  - [API Node](api-node.md)
   - [JSON RPC API](jsonrpc-api.md)
   - [JavaScript API](javascript-api.md)
   - [solana-wallet CLI](wallet.md)

book/src/api-node.md

@@ -0,0 +1,37 @@
+# API Node
+
+Solana supports a fullnode type called an *api node*. This node type is intended
+for applications that need to observe the data plane, without participating in
+transaction validation or ledger replication.
+
+An api node runs without a vote signer, and can optionally stream ledger entries
+out to a Unix domain socket as they are processed. The JSON-RPC service still
+functions as on any other node.
+
+To run an api node, include the argument `no-signer` and (optional)
+`entry-stream` socket location:
+
+```bash
+$ ./multinode-demo/fullnode-x.sh --no-signer --entry-stream <SOCKET>
+```
+
+The stream will output a series of JSON objects:
+- An Entry event JSON object is sent when each ledger entry is processed, with
+the following fields:
+
+   * `dt`, the system datetime, as RFC3339-formatted string
+   * `t`, the event type, always "entry"
+   * `s`, the slot height, as unsigned 64-bit integer
+   * `h`, the tick height, as unsigned 64-bit integer
+   * `entry`, the entry, as JSON object
+
+
+- A Block event JSON object is sent when a block is complete, with the
+following fields:
+
+   * `dt`, the system datetime, as RFC3339-formatted string
+   * `t`, the event type, always "block"
+   * `s`, the slot height, as unsigned 64-bit integer
+   * `h`, the tick height, as unsigned 64-bit integer
+   * `l`, the slot leader id, as base-58 encoded string
+   * `id`, the block id, as base-58 encoded string