feat: no schema in live mode (#238)

This PR addresses
https://github.com/electric-sql/electric-next/issues/236.

.changeset/flat-tigers-check.md

@@ -0,0 +1,6 @@
+---
+"@electric-sql/next": patch
+"@core/sync-service": patch
+---
+
+Only include schema in header of responses to non-live requests.

packages/sync-service/lib/electric/plug/serve_shape_plug.ex

@@ -116,6 +116,7 @@ defmodule Electric.Plug.ServeShapePlug do
   plug :put_resp_content_type, "application/json"
   plug :validate_query_params
   plug :load_shape_info
+  plug :put_schema_header
 
   # We're starting listening as soon as possible to not miss stuff that was added since we've asked for last offset
   plug :listen_for_new_changes
@@ -154,7 +155,6 @@ defmodule Electric.Plug.ServeShapePlug do
     |> assign(:last_offset, last_offset)
     |> put_resp_header("x-electric-shape-id", shape_id)
     |> put_resp_header("x-electric-chunk-last-offset", "#{last_offset}")
-    |> put_resp_header("x-electric-schema", schema(shape))
   end
 
   defp schema(shape) do
@@ -165,6 +165,14 @@ defmodule Electric.Plug.ServeShapePlug do
     |> Jason.encode!()
   end
 
+  # Only adds schema header when not in live mode
+  defp put_schema_header(conn, _) when not conn.assigns.live do
+    shape = conn.assigns.shape_definition
+    put_resp_header(conn, "x-electric-schema", schema(shape))
+  end
+
+  defp put_schema_header(conn, _), do: conn
+
   # If the offset requested is -1, noop as we can always serve it
   def validate_shape_offset(%Conn{assigns: %{offset: @before_all_offset}} = conn, _) do
     # noop

packages/sync-service/test/electric/plug/serve_shape_plug_test.exs

@@ -322,6 +322,7 @@ defmodule Electric.Plug.ServeShapePlugTest do
       assert Plug.Conn.get_resp_header(conn, "pragma") == ["no-cache"]
       assert Plug.Conn.get_resp_header(conn, "expires") == ["0"]
       assert Plug.Conn.get_resp_header(conn, "x-electric-chunk-last-offset") == [next_offset_str]
+      assert Plug.Conn.get_resp_header(conn, "x-electric-schema") == []
     end
 
     test "handles shape rotation" do

packages/typescript-client/src/client.ts

@@ -1,5 +1,5 @@
 import { ArgumentsType } from 'vitest'
-import { Message, Value, Offset } from './types'
+import { Message, Value, Offset, Schema } from './types'
 import { MessageParser, Parser } from './parser'
 
 export type ShapeData = Map<string, { [key: string]: Value }>
@@ -169,6 +169,7 @@ export class ShapeStream {
   private options: ShapeStreamOptions
   private backoffOptions: BackoffOptions
   private fetchClient: typeof fetch
+  private schema?: Schema
 
   private subscribers = new Map<
     number,
@@ -255,12 +256,15 @@ export class ShapeStream {
         this.lastOffset = lastOffset as Offset
       }
 
-      const schemaHeader = headers.get(`X-Electric-Schema`)!
-      const schema = schemaHeader ? JSON.parse(schemaHeader) : {}
+      const getSchema = (): Schema => {
+        const schemaHeader = headers.get(`X-Electric-Schema`)
+        return schemaHeader ? JSON.parse(schemaHeader) : {}
+      }
+      this.schema = this.schema ?? getSchema()
 
       const messages = status === 204 ? `[]` : await response.text()
 
-      const batch = this.messageParser.parse(messages, schema)
+      const batch = this.messageParser.parse(messages, this.schema)
 
       // Update isUpToDate
       if (batch.length > 0) {
@@ -346,6 +350,7 @@ export class ShapeStream {
     this.lastOffset = `-1`
     this.shapeId = shapeId
     this.isUpToDate = false
+    this.schema = undefined
   }
 
   private validateOptions(options: ShapeStreamOptions): void {