SERVER-49569 Create ReshardingCoordinatorService

11 files changed, 764 insertions, 231 deletions

diff --git a/src/mongo/db/namespace_string.cpp b/src/mongo/db/namespace_string.cpp
index 2ddcc220902..29bbda32cdd 100644
--- a/src/mongo/db/namespace_string.cpp
+++ b/src/mongo/db/namespace_string.cpp
@@ -85,6 +85,8 @@ const NamespaceString NamespaceString::kIndexBuildEntryNamespace(NamespaceString
                                                                  "system.indexBuilds");
 const NamespaceString NamespaceString::kRangeDeletionNamespace(NamespaceString::kConfigDb,
                                                                "rangeDeletions");
+const NamespaceString NamespaceString::kConfigReshardingOperationsNamespace(
+    NamespaceString::kConfigDb, "reshardingOperations");
 const NamespaceString NamespaceString::kConfigSettingsNamespace(NamespaceString::kConfigDb,
                                                                 "settings");
 const NamespaceString NamespaceString::kVectorClockNamespace(NamespaceString::kConfigDb,
diff --git a/src/mongo/db/namespace_string.h b/src/mongo/db/namespace_string.h
index 8418fc1993b..33c6ea13668 100644
--- a/src/mongo/db/namespace_string.h
+++ b/src/mongo/db/namespace_string.h
@@ -118,6 +118,9 @@ public:
     // Namespace for pending range deletions.
     static const NamespaceString kRangeDeletionNamespace;
 
+    // Namespace for resharding operation state.
+    static const NamespaceString kConfigReshardingOperationsNamespace;
+
     // Namespace for balancer settings and default read and write concerns.
     static const NamespaceString kConfigSettingsNamespace;
 
diff --git a/src/mongo/db/s/SConscript b/src/mongo/db/s/SConscript
index 9e4e1426a83..fe4bc629199 100644
--- a/src/mongo/db/s/SConscript
+++ b/src/mongo/db/s/SConscript
@@ -68,7 +68,8 @@ env.Library(
         'range_deletion_util.cpp',
         'read_only_catalog_cache_loader.cpp',
         'resharding_util.cpp',
-        'resharding/resharding_coordinator.cpp',
+        'resharding/resharding_coordinator_observer.cpp',
+        'resharding/resharding_coordinator_service.cpp',
         'scoped_operation_completion_sharding_actions.cpp',
         'session_catalog_migration_destination.cpp',
         'session_catalog_migration_source.cpp',
diff --git a/src/mongo/db/s/resharding/coordinator_document.idl b/src/mongo/db/s/resharding/coordinator_document.idl
index 761e55fb6b2..127d69c779b 100644
--- a/src/mongo/db/s/resharding/coordinator_document.idl
+++ b/src/mongo/db/s/resharding/coordinator_document.idl
@@ -73,6 +73,10 @@ structs:
         generate_comparison_operators: false
         strict: true
         fields:
+            tempReshardingNss:
+                type: namespacestring
+                description: "The namespace of the temporary resharding collection that exists on
+                            recipient shards."
             state: CoordinatorState
             donorShards:
                 type: array<DonorShardEntry>
diff --git a/src/mongo/db/s/resharding/resharding_coordinator.cpp b/src/mongo/db/s/resharding/resharding_coordinator.cpp
deleted file mode 100644
index 61b586a5294..00000000000
--- a/src/mongo/db/s/resharding/resharding_coordinator.cpp
+++ /dev/null
@@ -1,92 +0,0 @@
-/**
- *    Copyright (C) 2020-present MongoDB, Inc.
- *
- *    This program is free software: you can redistribute it and/or modify
- *    it under the terms of the Server Side Public License, version 1,
- *    as published by MongoDB, Inc.
- *
- *    This program is distributed in the hope that it will be useful,
- *    but WITHOUT ANY WARRANTY; without even the implied warranty of
- *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- *    Server Side Public License for more details.
- *
- *    You should have received a copy of the Server Side Public License
- *    along with this program. If not, see
- *    <http://www.mongodb.com/licensing/server-side-public-license>.
- *
- *    As a special exception, the copyright holders give permission to link the
- *    code of portions of this program with the OpenSSL library under certain
- *    conditions as described in each individual source file and distribute
- *    linked combinations including the program with the OpenSSL library. You
- *    must comply with the Server Side Public License in all respects for
- *    all of the code used other than as permitted herein. If you modify file(s)
- *    with this exception, you may extend this exception to your version of the
- *    file(s), but you are not obligated to do so. If you do not wish to do so,
- *    delete this exception statement from your version. If you delete this
- *    exception statement from all source files in the program, then also delete
- *    it in the license file.
- */
-
-#include "mongo/db/s/resharding/resharding_coordinator.h"
-
-#include "mongo/db/service_context.h"
-#include "mongo/s/catalog/type_chunk.h"
-#include "mongo/s/shard_id.h"
-
-namespace mongo {
-
-namespace {
-const auto getReshardingCoordinator = ServiceContext::declareDecoration<ReshardingCoordinator>();
-}  // namespace
-
-ReshardingCoordinator::ReshardingCoordinator() = default;
-
-ReshardingCoordinator::~ReshardingCoordinator() {
-    // TODO SERVER-49569 uncomment line below
-    // invariant(_reshardingOperationsInProgress.empty());
-}
-
-ReshardingCoordinator& ReshardingCoordinator::get(ServiceContext* serviceContext) {
-    return getReshardingCoordinator(serviceContext);
-}
-
-ReshardingCoordinator& ReshardingCoordinator::get(OperationContext* opCtx) {
-    return get(opCtx->getServiceContext());
-}
-
-// TODO SERVER-49570
-void ReshardingCoordinator::onNewReshardCollection(const NamespaceString& nss,
-                                                   const std::vector<ShardId>& donors,
-                                                   const std::vector<ShardId>& recipients,
-                                                   const std::vector<ChunkType>& initialChunks) {}
-
-// TODO	SERVER-49572
-void ReshardingCoordinator::onRecipientReportsCreatedCollection(const NamespaceString& nss,
-                                                                const ShardId& recipient) {}
-
-// TODO SERVER-49573
-void ReshardingCoordinator::onDonorReportsMinFetchTimestamp(const NamespaceString& nss,
-                                                            const ShardId& donor,
-                                                            Timestamp timestamp) {}
-
-// TODO SERVER-49574
-void ReshardingCoordinator::onRecipientFinishesCloning(const NamespaceString& nss,
-                                                       const ShardId& recipient) {}
-
-// TODO SERVER-49575
-void ReshardingCoordinator::onRecipientReportsStrictlyConsistent(const NamespaceString& nss,
-                                                                 const ShardId& recipient) {}
-
-// TODO SERVER-49576
-void ReshardingCoordinator::onRecipientRenamesCollection(const NamespaceString& nss,
-                                                         const ShardId& recipient) {}
-
-// TODO SERVER-49577
-void ReshardingCoordinator::onDonorDropsOriginalCollection(const NamespaceString& nss,
-                                                           const ShardId& donor) {}
-
-// TODO SERVER-49578
-void ReshardingCoordinator::onRecipientReportsUnrecoverableError(const NamespaceString& nss,
-                                                                 const ShardId& recipient,
-                                                                 Status error) {}
-}  // namespace mongo
diff --git a/src/mongo/db/s/resharding/resharding_coordinator.h b/src/mongo/db/s/resharding/resharding_coordinator.h
deleted file mode 100644
index 9de335abf40..00000000000
--- a/src/mongo/db/s/resharding/resharding_coordinator.h
+++ /dev/null
@@ -1,137 +0,0 @@
-/**
- *    Copyright (C) 2020-present MongoDB, Inc.
- *
- *    This program is free software: you can redistribute it and/or modify
- *    it under the terms of the Server Side Public License, version 1,
- *    as published by MongoDB, Inc.
- *
- *    This program is distributed in the hope that it will be useful,
- *    but WITHOUT ANY WARRANTY; without even the implied warranty of
- *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- *    Server Side Public License for more details.
- *
- *    You should have received a copy of the Server Side Public License
- *    along with this program. If not, see
- *    <http://www.mongodb.com/licensing/server-side-public-license>.
- *
- *    As a special exception, the copyright holders give permission to link the
- *    code of portions of this program with the OpenSSL library under certain
- *    conditions as described in each individual source file and distribute
- *    linked combinations including the program with the OpenSSL library. You
- *    must comply with the Server Side Public License in all respects for
- *    all of the code used other than as permitted herein. If you modify file(s)
- *    with this exception, you may extend this exception to your version of the
- *    file(s), but you are not obligated to do so. If you do not wish to do so,
- *    delete this exception statement from your version. If you delete this
- *    exception statement from all source files in the program, then also delete
- *    it in the license file.
- */
-
-#pragma once
-
-#include <vector>
-
-#include "mongo/base/status.h"
-#include "mongo/bson/timestamp.h"
-#include "mongo/platform/mutex.h"
-
-namespace mongo {
-
-class ChunkType;
-class NamespaceString;
-class OperationContext;
-class ServiceContext;
-class ShardId;
-
-/**
- * Manages active resharding operations and holds a catalog of ReshardingCoordinatorStateMachine
- * objects indexed by collection namespace.
- *
- * There is one instance of this object per service context.
- */
-
-class ReshardingCoordinator {
-    ReshardingCoordinator(const ReshardingCoordinator&) = delete;
-    ReshardingCoordinator& operator=(const ReshardingCoordinator&) = delete;
-
-public:
-    ReshardingCoordinator();
-    ~ReshardingCoordinator();
-
-    static ReshardingCoordinator& get(ServiceContext* serviceContext);
-    static ReshardingCoordinator& get(OperationContext* opCtx);
-
-    /**
-     * Creates a new ReshardingCoordinatorStateMachine for the resharding operation on the
-     * collection 'nss' and adds it to _reshardingOperationsInProgress. Updates on-disk metadata to
-     * indicate that the collection is being resharded.
-     */
-    void onNewReshardCollection(const NamespaceString& nss,
-                                const std::vector<ShardId>& donors,
-                                const std::vector<ShardId>& recipients,
-                                const std::vector<ChunkType>& initialChunks);
-
-    /**
-     * Called when each recipient reports it has finished creating the collection. Triggers a state
-     * transition and updates on-disk metadata if this recipient is the last to report it has
-     * finished creating the collection.
-     */
-    void onRecipientReportsCreatedCollection(const NamespaceString& nss, const ShardId& recipient);
-
-    /**
-     * Called when each donor reports its minFetchTimestamp. If this donor is the last to report its
-     * minFetchTimestamp, selects the highest timestamp among all donors to be the fetchTimestamp,
-     * triggers a state change, and updates on-disk metadata.
-     */
-    void onDonorReportsMinFetchTimestamp(const NamespaceString& nss,
-                                         const ShardId& donor,
-                                         Timestamp timestamp);
-
-    /**
-     * Called when each recipient finishes cloning and enters steady-state. Triggers a state
-     * transition and updates on-disk metadata if this recipient is the last to report it has
-     * finished cloning the collection.
-     */
-    void onRecipientFinishesCloning(const NamespaceString& nss, const ShardId& recipient);
-
-    /**
-     * Called when a recipient has met the following criteria:
-     * 1. Has been notified by all donors all writes must be run in transactions and
-     * 2. Has applied all oplog entries and
-     * 3. Has entered strict-consistency state
-     *
-     * If the recipient is the last to report it is in strict-consistency, commits the resharding
-     * operation, triggers a state transition, and updates on-disk metadata.
-     */
-    void onRecipientReportsStrictlyConsistent(const NamespaceString& nss, const ShardId& recipient);
-
-    /**
-     * Called when each recipient finishes renaming the temporary collection. Triggers a state
-     * transition and updates on-disk metadata if this recipient is the last to report it has
-     * finished renaming.
-     */
-    void onRecipientRenamesCollection(const NamespaceString& nss, const ShardId& recipient);
-
-    /**
-     * Called when each donor finishes dropping the original collection. Triggers a state transition
-     * and updates on-disk metadata if this donor is the last to report it has finished dropping.
-     */
-    void onDonorDropsOriginalCollection(const NamespaceString& nss, const ShardId& donor);
-
-    /**
-     * Called if a recipient reports an unrecoverable error.
-     */
-    void onRecipientReportsUnrecoverableError(const NamespaceString& nss,
-                                              const ShardId& recipient,
-                                              Status error);
-
-private:
-    // Protects the state below
-    Mutex _mutex = MONGO_MAKE_LATCH("ReshardingCoordinator::_mutex");
-
-    // Contains ReshardingCoordinatorStateMachine objects by collection namespace.
-    // TODO SERVER-49569 uncomment line below
-    // StringMap<ReshardingCoordinatorStateMachine> _reshardingOperationsInProgress;
-};
-
-}  // namespace mongo
diff --git a/src/mongo/db/s/resharding/resharding_coordinator_observer.cpp b/src/mongo/db/s/resharding/resharding_coordinator_observer.cpp
new file mode 100644
index 00000000000..dfa92b132e5
--- /dev/null
+++ b/src/mongo/db/s/resharding/resharding_coordinator_observer.cpp
@@ -0,0 +1,96 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#include "mongo/db/s/resharding/resharding_coordinator_observer.h"
+
+#include "mongo/db/service_context.h"
+#include "mongo/s/catalog/type_chunk.h"
+#include "mongo/s/shard_id.h"
+
+namespace mongo {
+
+ReshardingCoordinatorObserver::ReshardingCoordinatorObserver() = default;
+
+ReshardingCoordinatorObserver::~ReshardingCoordinatorObserver() = default;
+
+// TODO SERVER-49572
+void ReshardingCoordinatorObserver::onRecipientReportsCreatedCollection(const ShardId& recipient) {}
+
+// TODO SERVER-49573
+void ReshardingCoordinatorObserver::onDonorReportsMinFetchTimestamp(const ShardId& donor,
+                                                                    Timestamp timestamp) {}
+
+// TODO SERVER-49574
+void ReshardingCoordinatorObserver::onRecipientFinishesCloning(const ShardId& recipient) {}
+
+// TODO SERVER-49575
+void ReshardingCoordinatorObserver::onRecipientReportsStrictlyConsistent(const ShardId& recipient) {
+}
+
+// TODO SERVER-49576
+void ReshardingCoordinatorObserver::onRecipientRenamesCollection(const ShardId& recipient) {}
+
+// TODO SERVER-49577
+void ReshardingCoordinatorObserver::onDonorDropsOriginalCollection(const ShardId& donor) {}
+
+// TODO SERVER-49578
+void ReshardingCoordinatorObserver::onRecipientReportsUnrecoverableError(const ShardId& recipient,
+                                                                         Status error) {}
+
+// TODO	SERVER-49572
+SharedSemiFuture<void> ReshardingCoordinatorObserver::awaitAllRecipientsCreatedCollection() {
+    return _allRecipientsCreatedCollection.getFuture();
+}
+
+// TODO SERVER-49573
+SharedSemiFuture<Timestamp> ReshardingCoordinatorObserver::awaitAllDonorsReadyToDonate() {
+    return _allDonorsReportedMinFetchTimestamp.getFuture();
+}
+
+// TODO SERVER-49574
+SharedSemiFuture<void> ReshardingCoordinatorObserver::awaitAllRecipientsFinishedCloning() {
+    return _allRecipientsFinishedCloning.getFuture();
+}
+
+// TODO SERVER-49575
+SharedSemiFuture<void> ReshardingCoordinatorObserver::awaitAllRecipientsInStrictConsistency() {
+    return _allRecipientsReportedStrictConsistencyTimestamp.getFuture();
+}
+
+// TODO SERVER-49577
+SharedSemiFuture<void> ReshardingCoordinatorObserver::awaitAllDonorsDroppedOriginalCollection() {
+    return _allDonorsDroppedOriginalCollection.getFuture();
+}
+
+// TODO SERVER-49576
+SharedSemiFuture<void> ReshardingCoordinatorObserver::awaitAllRecipientsRenamedCollection() {
+    return _allRecipientsRenamedCollection.getFuture();
+}
+
+}  // namespace mongo
diff --git a/src/mongo/db/s/resharding/resharding_coordinator_observer.h b/src/mongo/db/s/resharding/resharding_coordinator_observer.h
new file mode 100644
index 00000000000..df637cb96c4
--- /dev/null
+++ b/src/mongo/db/s/resharding/resharding_coordinator_observer.h
@@ -0,0 +1,163 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include <vector>
+
+#include "mongo/base/status.h"
+#include "mongo/bson/timestamp.h"
+#include "mongo/db/s/resharding/coordinator_document_gen.h"
+#include "mongo/platform/mutex.h"
+#include "mongo/util/future.h"
+#include "mongo/util/string_map.h"
+
+namespace mongo {
+
+class ChunkType;
+class NamespaceString;
+class OperationContext;
+class ServiceContext;
+class ShardId;
+
+/**
+ * Observes writes that indicate state changes for a resharding operation. Holds promises that
+ * the ReshardingCoordinator waits on in order to transition to a new state.
+ *
+ * An instance of this object is specific to one resharding operation.
+ */
+
+class ReshardingCoordinatorObserver {
+public:
+    ReshardingCoordinatorObserver();
+    ~ReshardingCoordinatorObserver();
+
+    /**
+     * Called when each recipient updates its 'state' field to 'initializing' in the
+     * 'recipientShards' field in a config.reshardingOperations entry.
+     */
+    void onRecipientReportsCreatedCollection(const ShardId& recipient);
+
+    /**
+     * Called when each donor updates its 'state' field to 'donating' and writes its
+     * 'minFetchTimestamp' in the 'donorShards' field in a config.reshardingOperations entry.
+     */
+    void onDonorReportsMinFetchTimestamp(const ShardId& donor, Timestamp timestamp);
+
+    /**
+     * Called when each recipient updates its 'state' field to 'steady-state' in the
+     * 'recipientShards' field in a config.reshardingOperations entry.
+     */
+    void onRecipientFinishesCloning(const ShardId& recipient);
+
+    /**
+     * Called when each recipient updates its 'state' field to 'strictly-consistent' and writes its
+     * 'strictConsistencyTimestamp' in the 'recipientShards' field in a config.reshardingOperations
+     * entry.
+     */
+    void onRecipientReportsStrictlyConsistent(const ShardId& recipient);
+
+    /**
+     * Called when each recipient updates its 'state' field to 'done' in the 'recipientShards'
+     * field in a config.reshardingOperations entry.
+     */
+    void onRecipientRenamesCollection(const ShardId& recipient);
+
+    /**
+     * Called when each donor updates its 'state' field to 'done' in the 'donorShards' field in
+     * a config.reshardingOperations entry.
+     */
+    void onDonorDropsOriginalCollection(const ShardId& donor);
+
+    /**
+     * Called if a recipient reports an unrecoverable error.
+     */
+    void onRecipientReportsUnrecoverableError(const ShardId& recipient, Status error);
+
+    /**
+     * Fulfills the '_allRecipientsCreatedCollection' promise when the last recipient writes that it
+     * is in 'initializing' state.
+     */
+    SharedSemiFuture<void> awaitAllRecipientsCreatedCollection();
+
+    /**
+     * When the last donor reports its 'minFetchTimestamp', selects the highest 'minFetchTimestamp'
+     * of all donors to be the 'fetchTimestamp'. Fulfills the '_allDonorsReportedMinFetchTimestamp'
+     * promise with this 'fetchTimestamp'.
+     */
+    SharedSemiFuture<Timestamp> awaitAllDonorsReadyToDonate();
+
+    /**
+     * Fulfills the '_allRecipientsFinishedCloning' promise when the last recipient writes that it
+     * is in 'steady-state'.
+     */
+    SharedSemiFuture<void> awaitAllRecipientsFinishedCloning();
+
+    /**
+     * Fulfills the '_allRecipientsReportedStrictConsistencyTimestamp' promise when the last
+     * recipient writes that it is in 'strict-consistency' state as well as its
+     * 'strictConsistencyTimestamp'.
+     */
+    SharedSemiFuture<void> awaitAllRecipientsInStrictConsistency();
+
+    /**
+     * Fulfills the '_allDonorsDroppedOriginalCollection' promise when the last donor writes that it
+     * is in 'done' state.
+     */
+    SharedSemiFuture<void> awaitAllDonorsDroppedOriginalCollection();
+
+    /**
+     * Fulfills the '_allRecipientsRenamedCollection' promise when the last recipient writes
+     * that it is in 'done' state.
+     */
+    SharedSemiFuture<void> awaitAllRecipientsRenamedCollection();
+
+
+private:
+    // Protects the state below
+    Mutex _mutex = MONGO_MAKE_LATCH("ReshardingCoordinatorObserver::_mutex");
+
+    /**
+     * Promises indicating that either all donors or all recipients have entered a specific state.
+     * The ReshardingCoordinator waits on these in order to transition states.
+     */
+    SharedPromise<void> _allRecipientsCreatedCollection;
+
+    SharedPromise<Timestamp> _allDonorsReportedMinFetchTimestamp;
+
+    SharedPromise<void> _allRecipientsFinishedCloning;
+
+    SharedPromise<void> _allRecipientsReportedStrictConsistencyTimestamp;
+
+    SharedPromise<void> _allDonorsDroppedOriginalCollection;
+
+    SharedPromise<void> _allRecipientsRenamedCollection;
+};
+
+}  // namespace mongo
diff --git a/src/mongo/db/s/resharding/resharding_coordinator_service.cpp b/src/mongo/db/s/resharding/resharding_coordinator_service.cpp
new file mode 100644
index 00000000000..f21e90ccb9f
--- /dev/null
+++ b/src/mongo/db/s/resharding/resharding_coordinator_service.cpp
@@ -0,0 +1,286 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#define MONGO_LOGV2_DEFAULT_COMPONENT ::mongo::logv2::LogComponent::kCommand
+
+#include "mongo/db/s/resharding/resharding_coordinator_service.h"
+
+#include "mongo/db/dbdirectclient.h"
+#include "mongo/db/ops/write_ops.h"
+#include "mongo/logv2/log.h"
+#include "mongo/rpc/get_status_from_command_result.h"
+#include "mongo/s/catalog/type_chunk.h"
+#include "mongo/s/shard_id.h"
+#include "mongo/util/string_map.h"
+#include "mongo/util/uuid.h"
+
+namespace mongo {
+
+ReshardingCoordinatorService::ReshardingCoordinator::ReshardingCoordinator(const BSONObj& state)
+    : PrimaryOnlyService::TypedInstance<ReshardingCoordinator>(),
+      _id(state["_id"].wrap().getOwned()),
+      _stateDoc(ReshardingCoordinatorDocument::parse(
+          IDLParserErrorContext("ReshardingCoordinatorStateDoc"), state)) {
+    _reshardingCoordinatorObserver = std::make_shared<ReshardingCoordinatorObserver>();
+}
+
+void ReshardingCoordinatorService::ReshardingCoordinator::run(
+    std::shared_ptr<executor::ScopedTaskExecutor> executor) noexcept {
+    ExecutorFuture<void>(**executor)
+        .then([this, executor] { return _init(executor); })
+        .then([this] { _tellAllRecipientsToRefresh(); })
+        .then([this, executor] { return _awaitAllRecipientsCreatedCollection(executor); })
+        .then([this] { _tellAllDonorsToRefresh(); })
+        .then([this, executor] { return _awaitAllDonorsReadyToDonate(executor); })
+        .then([this] { _tellAllRecipientsToRefresh(); })
+        .then([this, executor] { return _awaitAllRecipientsFinishedCloning(executor); })
+        .then([this] { _tellAllDonorsToRefresh(); })
+        .then([this, executor] { return _awaitAllRecipientsInStrictConsistency(executor); })
+        .then([this] { return _commit(); })
+        .then([this] {
+            if (_stateDoc.getState() > CoordinatorStateEnum::kRenaming) {
+                return;
+            }
+
+            this->_runUpdates(CoordinatorStateEnum::kRenaming);
+            return;
+        })
+        .then([this, executor] { return _awaitAllRecipientsRenamedCollection(executor); })
+        .then([this] { _tellAllDonorsToRefresh(); })
+        .then([this, executor] { return _awaitAllDonorsDroppedOriginalCollection(executor); })
+        .then([this] { _tellAllRecipientsToRefresh(); })
+        .then([this] { _tellAllDonorsToRefresh(); })
+        .onError([this](Status status) {
+            _runUpdates(CoordinatorStateEnum::kError);
+
+            LOGV2(4956902,
+                  "Resharding failed",
+                  "namespace"_attr = _stateDoc.getNss().ns(),
+                  "newShardKeyPattern"_attr = _stateDoc.getReshardingKey(),
+                  "error"_attr = status);
+
+            // TODO wait for donors and recipients to abort the operation and clean up state
+            _tellAllRecipientsToRefresh();
+            _tellAllDonorsToRefresh();
+
+            return status;
+        })
+        .getAsync([this](Status status) {
+            if (!status.isOK()) {
+                invariant(_stateDoc.getState() == CoordinatorStateEnum::kError);
+                _completionPromise.setError(status);
+                return;
+            }
+
+            invariant(_stateDoc.getState() == CoordinatorStateEnum::kDone);
+            _completionPromise.emplaceValue();
+        });
+}
+
+void ReshardingCoordinatorService::ReshardingCoordinator::setInitialChunksAndZones(
+    std::vector<ChunkType> initialChunks, std::vector<TagsType> newZones) {
+    if (_stateDoc.getState() > CoordinatorStateEnum::kInitializing ||
+        _initialChunksAndZonesPromise.getFuture().isReady()) {
+        return;
+    }
+
+    _initialChunksAndZonesPromise.emplaceValue(
+        ChunksAndZones{std::move(initialChunks), std::move(newZones)});
+}
+
+ExecutorFuture<void> ReshardingCoordinatorService::ReshardingCoordinator::_init(
+    const std::shared_ptr<executor::ScopedTaskExecutor>& executor) {
+    if (_stateDoc.getState() > CoordinatorStateEnum::kInitializing) {
+        return ExecutorFuture<void>(**executor, Status::OK());
+    }
+
+    return _initialChunksAndZonesPromise.getFuture()
+        .thenRunOn(**executor)
+        .then([this](const ChunksAndZones& initialChunksAndZones) {
+            // TODO SERVER-50304 Run this insert in a transaction with writes to config.collections,
+            // config.chunks, and config.tags
+            auto opCtx = cc().makeOperationContext();
+            DBDirectClient client(opCtx.get());
+
+            const auto commandResponse = client.runCommand([&] {
+                write_ops::Insert insertOp(NamespaceString::kConfigReshardingOperationsNamespace);
+                insertOp.setDocuments({_stateDoc.toBSON()});
+                return insertOp.serialize({});
+            }());
+            uassertStatusOK(getStatusFromWriteCommandReply(commandResponse->getCommandReply()));
+
+            invariant(_stateDoc.getState() == CoordinatorStateEnum::kInitializing);
+            _stateDoc.setState(CoordinatorStateEnum::kInitialized);
+        });
+}
+
+ExecutorFuture<void>
+ReshardingCoordinatorService::ReshardingCoordinator::_awaitAllRecipientsCreatedCollection(
+    const std::shared_ptr<executor::ScopedTaskExecutor>& executor) {
+    if (_stateDoc.getState() > CoordinatorStateEnum::kInitialized) {
+        return ExecutorFuture<void>(**executor, Status::OK());
+    }
+
+    return _reshardingCoordinatorObserver->awaitAllRecipientsCreatedCollection()
+        .thenRunOn(**executor)
+        .then([this]() { this->_runUpdates(CoordinatorStateEnum::kPreparingToDonate); });
+}
+
+ExecutorFuture<void>
+ReshardingCoordinatorService::ReshardingCoordinator::_awaitAllDonorsReadyToDonate(
+    const std::shared_ptr<executor::ScopedTaskExecutor>& executor) {
+    if (_stateDoc.getState() > CoordinatorStateEnum::kPreparingToDonate) {
+        return ExecutorFuture<void>(**executor, Status::OK());
+    }
+
+    return _reshardingCoordinatorObserver->awaitAllDonorsReadyToDonate()
+        .thenRunOn(**executor)
+        .then([this](Timestamp fetchTimestamp) {
+            this->_runUpdates(CoordinatorStateEnum::kCloning, fetchTimestamp);
+        });
+}
+
+ExecutorFuture<void>
+ReshardingCoordinatorService::ReshardingCoordinator::_awaitAllRecipientsFinishedCloning(
+    const std::shared_ptr<executor::ScopedTaskExecutor>& executor) {
+    if (_stateDoc.getState() > CoordinatorStateEnum::kCloning) {
+        return ExecutorFuture<void>(**executor, Status::OK());
+    }
+
+    return _reshardingCoordinatorObserver->awaitAllRecipientsFinishedCloning()
+        .thenRunOn(**executor)
+        .then([this]() { this->_runUpdates(CoordinatorStateEnum::kMirroring); });
+}
+
+SharedSemiFuture<void>
+ReshardingCoordinatorService::ReshardingCoordinator::_awaitAllRecipientsInStrictConsistency(
+    const std::shared_ptr<executor::ScopedTaskExecutor>& executor) {
+    if (_stateDoc.getState() > CoordinatorStateEnum::kMirroring) {
+        return Status::OK();
+    }
+
+    return _reshardingCoordinatorObserver->awaitAllRecipientsInStrictConsistency();
+}
+
+Future<void> ReshardingCoordinatorService::ReshardingCoordinator::_commit() {
+    if (_stateDoc.getState() > CoordinatorStateEnum::kMirroring) {
+        return Status::OK();
+    }
+
+    // TODO SERVER-50304 Run this update in a transaction with writes to config.collections,
+    // config.chunks, and config.tags
+    this->_runUpdates(CoordinatorStateEnum::kCommitted);
+
+    return Status::OK();
+};
+
+ExecutorFuture<void>
+ReshardingCoordinatorService::ReshardingCoordinator::_awaitAllRecipientsRenamedCollection(
+    const std::shared_ptr<executor::ScopedTaskExecutor>& executor) {
+    if (_stateDoc.getState() > CoordinatorStateEnum::kRenaming) {
+        return ExecutorFuture<void>(**executor, Status::OK());
+    }
+
+    return _reshardingCoordinatorObserver->awaitAllRecipientsRenamedCollection()
+        .thenRunOn(**executor)
+        .then([this]() { this->_runUpdates(CoordinatorStateEnum::kDropping); });
+}
+
+ExecutorFuture<void>
+ReshardingCoordinatorService::ReshardingCoordinator::_awaitAllDonorsDroppedOriginalCollection(
+    const std::shared_ptr<executor::ScopedTaskExecutor>& executor) {
+    if (_stateDoc.getState() > CoordinatorStateEnum::kDropping) {
+        return ExecutorFuture<void>(**executor, Status::OK());
+    }
+
+    return _reshardingCoordinatorObserver->awaitAllDonorsDroppedOriginalCollection()
+        .thenRunOn(**executor)
+        .then([this]() { this->_runUpdates(CoordinatorStateEnum::kDone); });
+}
+
+// TODO SERVER-50304 Run this write in a transaction with updates to config.collections (and
+// the initial chunks to config.chunks and config.tags if nextState is kInitialized)
+void ReshardingCoordinatorService::ReshardingCoordinator::_runUpdates(
+    CoordinatorStateEnum nextState, boost::optional<Timestamp> fetchTimestamp) {
+    // Build new state doc for update
+    ReshardingCoordinatorDocument updatedStateDoc = _stateDoc;
+    updatedStateDoc.setState(nextState);
+    if (fetchTimestamp) {
+        auto fetchTimestampStruct = updatedStateDoc.getFetchTimestampStruct();
+        if (fetchTimestampStruct.getFetchTimestamp())
+            invariant(fetchTimestampStruct.getFetchTimestamp().get() == fetchTimestamp.get());
+
+        fetchTimestampStruct.setFetchTimestamp(std::move(fetchTimestamp));
+    }
+
+    // Run update
+    auto opCtx = cc().makeOperationContext();
+    DBDirectClient client(opCtx.get());
+
+    const auto commandResponse = client.runCommand([&] {
+        write_ops::Update updateOp(NamespaceString::kConfigReshardingOperationsNamespace);
+        updateOp.setUpdates({[&] {
+            write_ops::UpdateOpEntry entry;
+            entry.setQ(_id);
+            entry.setU(updatedStateDoc.toBSON());
+            return entry;
+        }()});
+        return updateOp.serialize(
+            BSON(WriteConcernOptions::kWriteConcernField << WriteConcernOptions::Majority));
+    }());
+
+    const auto commandReply = commandResponse->getCommandReply();
+    uassertStatusOK(getStatusFromWriteCommandReply(commandReply));
+
+    // Throw if the update did not match a document. This means the state doc was removed out from
+    // under the operation.
+    uassert(495690,
+            str::stream() << "Found that the resharding coordinator state document is missing when "
+                             "attempting to update state for namespace "
+                          << _stateDoc.getNss().ns(),
+            commandReply.getIntField("n") == 1);
+
+    // Update in-memory state doc
+    _stateDoc = updatedStateDoc;
+}
+
+// TODO
+void ReshardingCoordinatorService::ReshardingCoordinator::
+    _markCoordinatorStateDocAsGarbageCollectable() {}
+
+// TODO
+void ReshardingCoordinatorService::ReshardingCoordinator::_removeReshardingFields() {}
+
+// TODO
+void ReshardingCoordinatorService::ReshardingCoordinator::_tellAllRecipientsToRefresh() {}
+
+// TODO
+void ReshardingCoordinatorService::ReshardingCoordinator::_tellAllDonorsToRefresh() {}
+
+}  // namespace mongo
diff --git a/src/mongo/db/s/resharding/resharding_coordinator_service.h b/src/mongo/db/s/resharding/resharding_coordinator_service.h
new file mode 100644
index 00000000000..ad27fa27851
--- /dev/null
+++ b/src/mongo/db/s/resharding/resharding_coordinator_service.h
@@ -0,0 +1,207 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include "mongo/db/repl/primary_only_service.h"
+#include "mongo/db/s/resharding/coordinator_document_gen.h"
+#include "mongo/db/s/resharding/resharding_coordinator_observer.h"
+#include "mongo/platform/mutex.h"
+#include "mongo/s/catalog/type_chunk.h"
+#include "mongo/s/catalog/type_tags.h"
+#include "mongo/s/shard_id.h"
+#include "mongo/util/future.h"
+
+namespace mongo {
+
+class ServiceContext;
+class OperationContext;
+
+constexpr StringData kReshardingCoordinatorServiceName = "ReshardingCoordinatorService"_sd;
+
+class ReshardingCoordinatorService final : public repl::PrimaryOnlyService {
+public:
+    explicit ReshardingCoordinatorService(ServiceContext* serviceContext)
+        : PrimaryOnlyService(serviceContext) {}
+    ~ReshardingCoordinatorService() = default;
+
+    StringData getServiceName() const override {
+        return kReshardingCoordinatorServiceName;
+    }
+    NamespaceString getStateDocumentsNS() const override {
+        return NamespaceString::kConfigReshardingOperationsNamespace;
+    }
+
+    ThreadPool::Limits getThreadPoolLimits() const override {
+        // TODO Limit the size of ReshardingCoordinatorService thread pool.
+        return ThreadPool::Limits();
+    }
+    std::shared_ptr<PrimaryOnlyService::Instance> constructInstance(
+        BSONObj initialState) const override {
+        return std::make_shared<ReshardingCoordinatorService::ReshardingCoordinator>(
+            std::move(initialState));
+    }
+
+    class ReshardingCoordinator final
+        : public PrimaryOnlyService::TypedInstance<ReshardingCoordinator> {
+    public:
+        explicit ReshardingCoordinator(const BSONObj& state);
+
+        SharedSemiFuture<void> getCompletionPromise() {
+            return _completionPromise.getFuture();
+        }
+
+        void run(std::shared_ptr<executor::ScopedTaskExecutor> executor) noexcept override;
+
+        void setInitialChunksAndZones(std::vector<ChunkType> initialChunks,
+                                      std::vector<TagsType> newZones);
+
+    private:
+        struct ChunksAndZones {
+            std::vector<ChunkType> initialChunks;
+            std::vector<TagsType> newZones;
+        };
+
+        /**
+         * Does the following writes:
+         * 1. Inserts coordinator state document into config.reshardingOperations
+         * 2. Adds reshardingFields to the config.collections entry for the original collection
+         * 3. Inserts an entry into config.collections for the temporary collection
+         * 4. Inserts entries into config.chunks for ranges based on the new shard key
+         * 5. Upserts entries into config.tags for any zones associated with the new shard key
+         *
+         * Transitions to 'kInitialized'.
+         */
+        ExecutorFuture<void> _init(const std::shared_ptr<executor::ScopedTaskExecutor>& executor);
+
+        /**
+         * Waits on _reshardingCoordinatorObserver to notify that all recipients have created the
+         * temporary collection. Transitions to 'kPreparingToDonate'.
+         */
+        ExecutorFuture<void> _awaitAllRecipientsCreatedCollection(
+            const std::shared_ptr<executor::ScopedTaskExecutor>& executor);
+
+        /**
+         * Waits on _reshardingCoordinatorObserver to notify that all donors have picked a
+         * minFetchTimestamp and are ready to donate. Transitions to 'kCloning'.
+         */
+        ExecutorFuture<void> _awaitAllDonorsReadyToDonate(
+            const std::shared_ptr<executor::ScopedTaskExecutor>& executor);
+
+        /**
+         * Waits on _reshardingCoordinatorObserver to notify that all recipients have finished
+         * cloning. Transitions to 'kMirroring'.
+         */
+        ExecutorFuture<void> _awaitAllRecipientsFinishedCloning(
+            const std::shared_ptr<executor::ScopedTaskExecutor>& executor);
+
+        /**
+         * Waits on _reshardingCoordinatorObserver to notify that all recipients have entered
+         * strict-consistency.
+         */
+        SharedSemiFuture<void> _awaitAllRecipientsInStrictConsistency(
+            const std::shared_ptr<executor::ScopedTaskExecutor>& executor);
+
+        /**
+         * Does the following writes:
+         * 1. Updates the config.collections entry for the new sharded collection
+         * 2. Updates config.chunks entries for the new sharded collection
+         * 3. Updates config.tags for the new sharded collection
+         *
+         * Transitions to 'kCommitted'.
+         */
+        Future<void> _commit();
+
+        /**
+         * Waits on _reshardingCoordinatorObserver to notify that all recipients have renamed the
+         * temporary collection to the original collection namespace. Transitions to 'kDropping'.
+         */
+        ExecutorFuture<void> _awaitAllRecipientsRenamedCollection(
+            const std::shared_ptr<executor::ScopedTaskExecutor>& executor);
+
+        /**
+         * Waits on _reshardingCoordinatorObserver to notify that all donors have dropped the
+         * original collection. Transitions to 'kDone'.
+         */
+        ExecutorFuture<void> _awaitAllDonorsDroppedOriginalCollection(
+            const std::shared_ptr<executor::ScopedTaskExecutor>& executor);
+
+        /**
+         * Updates the entry for this resharding operation in config.reshardingOperations and the
+         * catalog entries for the original and temporary namespaces in config.collections.
+         */
+        void _runUpdates(CoordinatorStateEnum nextState,
+                         boost::optional<Timestamp> fetchTimestamp = boost::none);
+
+        /**
+         * Marks the state doc as garbage collectable so that it can be cleaned up by the TTL
+         * monitor.
+         */
+        void _markCoordinatorStateDocAsGarbageCollectable();
+
+        /**
+         * Removes the 'reshardingFields' from the config.collections entry.
+         */
+        void _removeReshardingFields();
+
+        /**
+         * Sends 'flushRoutingTableCacheUpdates' for the temporary namespace to all recipient
+         * shards.
+         */
+        void _tellAllRecipientsToRefresh();
+
+        /**
+         * Sends 'flushRoutingTableCacheUpdates' for the original namespace to all donor shards.
+         */
+        void _tellAllDonorsToRefresh();
+
+        // The unique key for a given resharding operation. InstanceID is an alias for BSONObj. The
+        // value of this is the UUID that will be used as the collection UUID for the new sharded
+        // collection. The object looks like: {_id: 'reshardingUUID'}
+        const InstanceID _id;
+
+        // Promise containing the initial chunks and new zones based on the new shard key. These are
+        // not a part of the state document, so must be set by configsvrReshardCollection after
+        // construction.
+        SharedPromise<ChunksAndZones> _initialChunksAndZonesPromise;
+
+        // Observes writes that indicate state changes for this resharding operation and notifies
+        // 'this' when all donors/recipients have entered some state so that 'this' can transition
+        // states.
+        std::shared_ptr<ReshardingCoordinatorObserver> _reshardingCoordinatorObserver;
+
+        // The updated coordinator state document.
+        ReshardingCoordinatorDocument _stateDoc;
+
+        // Fulfilled only after transitioned to kDone or kError
+        SharedPromise<void> _completionPromise;
+    };
+};
+
+}  // namespace mongo
diff --git a/src/mongo/s/resharding/common_types.idl b/src/mongo/s/resharding/common_types.idl
index 6eed708138e..699499c5f7b 100644
--- a/src/mongo/s/resharding/common_types.idl
+++ b/src/mongo/s/resharding/common_types.idl
@@ -47,8 +47,8 @@ enums:
             kCloning: "cloning"
             kMirroring: "mirroring"
             kCommitted: "committed"
-            kDropping: "dropping"
             kRenaming: "renaming"
+            kDropping: "dropping"
             kDone: "done"
             kError: "error"