Split out the local backend (#2052)

This splits the backend into two parts: the local backend for desktop cellxgene and the AWS backend for hosted cellxgene. The local backend is in local_server while the hosted remains in server. The general idea is to copy everything from server to local_server, pull unneeded stuff out of local_server, and keep server as-is for this PR. Not touching server means all the infra and deployment code will continue working just as it did before so we can make those changes incrementally.

local_server/common/health.py

@@ -0,0 +1,33 @@
+from http import HTTPStatus
+from flask import make_response, jsonify
+
+from local_server import __version__ as cellxgene_version
+from local_server.common.data_locator import DataLocator
+
+
+def _is_accessible(path, config):
+    if path is None:
+        return True
+
+    try:
+        dl = DataLocator(path, region_name=config.data_locator__s3__region_name)
+        return dl.exists()
+    except RuntimeError:
+        return False
+
+
+def health_check(config):
+    """
+    simple health check - return HTTP response.
+    See https://tools.ietf.org/id/draft-inadarei-api-health-check-01.html
+    """
+    health = {"status": None, "version": "1", "releaseID": cellxgene_version}
+
+    server_config = config.server_config
+    check = _is_accessible(server_config.single_dataset__datapath, server_config)
+
+    health["status"] = "pass" if check else "fail"
+    code = HTTPStatus.OK if health["status"] == "pass" else HTTPStatus.BAD_REQUEST
+    response = make_response(jsonify(health), code)
+    response.headers["Content-Type"] = "application/health+json"
+    return response