Merge pull request #3632 from matrix-org/erikj/refactor_repl_servlet

 Add helper base class for generating new replication endpoints

1 files changed, 215 insertions, 0 deletions

diff --git a/synapse/replication/http/_base.py b/synapse/replication/http/_base.py
new file mode 100644
index 0000000000..5e5376cf58
--- /dev/null
+++ b/synapse/replication/http/_base.py
@@ -0,0 +1,215 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import abc
+import logging
+import re
+
+from six.moves import urllib
+
+from twisted.internet import defer
+
+from synapse.api.errors import CodeMessageException, HttpResponseException
+from synapse.util.caches.response_cache import ResponseCache
+from synapse.util.stringutils import random_string
+
+logger = logging.getLogger(__name__)
+
+
+class ReplicationEndpoint(object):
+    """Helper base class for defining new replication HTTP endpoints.
+
+    This creates an endpoint under `/_synapse/replication/:NAME/:PATH_ARGS..`
+    (with an `/:txn_id` prefix for cached requests.), where NAME is a name,
+    PATH_ARGS are a tuple of parameters to be encoded in the URL.
+
+    For example, if `NAME` is "send_event" and `PATH_ARGS` is `("event_id",)`,
+    with `CACHE` set to true then this generates an endpoint:
+
+        /_synapse/replication/send_event/:event_id/:txn_id
+
+    For POST/PUT requests the payload is serialized to json and sent as the
+    body, while for GET requests the payload is added as query parameters. See
+    `_serialize_payload` for details.
+
+    Incoming requests are handled by overriding `_handle_request`. Servers
+    must call `register` to register the path with the HTTP server.
+
+    Requests can be sent by calling the client returned by `make_client`.
+
+    Attributes:
+        NAME (str): A name for the endpoint, added to the path as well as used
+            in logging and metrics.
+        PATH_ARGS (tuple[str]): A list of parameters to be added to the path.
+            Adding parameters to the path (rather than payload) can make it
+            easier to follow along in the log files.
+        METHOD (str): The method of the HTTP request, defaults to POST. Can be
+            one of POST, PUT or GET. If GET then the payload is sent as query
+            parameters rather than a JSON body.
+        CACHE (bool): Whether server should cache the result of the request/
+            If true then transparently adds a txn_id to all requests, and
+            `_handle_request` must return a Deferred.
+        RETRY_ON_TIMEOUT(bool): Whether or not to retry the request when a 504
+            is received.
+    """
+
+    __metaclass__ = abc.ABCMeta
+
+    NAME = abc.abstractproperty()
+    PATH_ARGS = abc.abstractproperty()
+
+    METHOD = "POST"
+    CACHE = True
+    RETRY_ON_TIMEOUT = True
+
+    def __init__(self, hs):
+        if self.CACHE:
+            self.response_cache = ResponseCache(
+                hs, "repl." + self.NAME,
+                timeout_ms=30 * 60 * 1000,
+            )
+
+        assert self.METHOD in ("PUT", "POST", "GET")
+
+    @abc.abstractmethod
+    def _serialize_payload(**kwargs):
+        """Static method that is called when creating a request.
+
+        Concrete implementations should have explicit parameters (rather than
+        kwargs) so that an appropriate exception is raised if the client is
+        called with unexpected parameters. All PATH_ARGS must appear in
+        argument list.
+
+        Returns:
+            Deferred[dict]|dict: If POST/PUT request then dictionary must be
+            JSON serialisable, otherwise must be appropriate for adding as
+            query args.
+        """
+        return {}
+
+    @abc.abstractmethod
+    def _handle_request(self, request, **kwargs):
+        """Handle incoming request.
+
+        This is called with the request object and PATH_ARGS.
+
+        Returns:
+            Deferred[dict]: A JSON serialisable dict to be used as response
+            body of request.
+        """
+        pass
+
+    @classmethod
+    def make_client(cls, hs):
+        """Create a client that makes requests.
+
+        Returns a callable that accepts the same parameters as `_serialize_payload`.
+        """
+        clock = hs.get_clock()
+        host = hs.config.worker_replication_host
+        port = hs.config.worker_replication_http_port
+
+        client = hs.get_simple_http_client()
+
+        @defer.inlineCallbacks
+        def send_request(**kwargs):
+            data = yield cls._serialize_payload(**kwargs)
+
+            url_args = [urllib.parse.quote(kwargs[name]) for name in cls.PATH_ARGS]
+
+            if cls.CACHE:
+                txn_id = random_string(10)
+                url_args.append(txn_id)
+
+            if cls.METHOD == "POST":
+                request_func = client.post_json_get_json
+            elif cls.METHOD == "PUT":
+                request_func = client.put_json
+            elif cls.METHOD == "GET":
+                request_func = client.get_json
+            else:
+                # We have already asserted in the constructor that a
+                # compatible was picked, but lets be paranoid.
+                raise Exception(
+                    "Unknown METHOD on %s replication endpoint" % (cls.NAME,)
+                )
+
+            uri = "http://%s:%s/_synapse/replication/%s/%s" % (
+                host, port, cls.NAME, "/".join(url_args)
+            )
+
+            try:
+                # We keep retrying the same request for timeouts. This is so that we
+                # have a good idea that the request has either succeeded or failed on
+                # the master, and so whether we should clean up or not.
+                while True:
+                    try:
+                        result = yield request_func(uri, data)
+                        break
+                    except CodeMessageException as e:
+                        if e.code != 504 or not cls.RETRY_ON_TIMEOUT:
+                            raise
+
+                    logger.warn("%s request timed out", cls.NAME)
+
+                    # If we timed out we probably don't need to worry about backing
+                    # off too much, but lets just wait a little anyway.
+                    yield clock.sleep(1)
+            except HttpResponseException as e:
+                # We convert to SynapseError as we know that it was a SynapseError
+                # on the master process that we should send to the client. (And
+                # importantly, not stack traces everywhere)
+                raise e.to_synapse_error()
+
+            defer.returnValue(result)
+
+        return send_request
+
+    def register(self, http_server):
+        """Called by the server to register this as a handler to the
+        appropriate path.
+        """
+
+        url_args = list(self.PATH_ARGS)
+        handler = self._handle_request
+        method = self.METHOD
+
+        if self.CACHE:
+            handler = self._cached_handler
+            url_args.append("txn_id")
+
+        args = "/".join("(?P<%s>[^/]+)" % (arg,) for arg in url_args)
+        pattern = re.compile("^/_synapse/replication/%s/%s$" % (
+            self.NAME,
+            args
+        ))
+
+        http_server.register_paths(method, [pattern], handler)
+
+    def _cached_handler(self, request, txn_id, **kwargs):
+        """Called on new incoming requests when caching is enabled. Checks
+        if there is a cached response for the request and returns that,
+        otherwise calls `_handle_request` and caches its response.
+        """
+        # We just use the txn_id here, but we probably also want to use the
+        # other PATH_ARGS as well.
+
+        assert self.CACHE
+
+        return self.response_cache.wrap(
+            txn_id,
+            self._handle_request,
+            request, **kwargs
+        )