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diff --git a/src/ceph/src/common/map_cacher.hpp b/src/ceph/src/common/map_cacher.hpp
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+++ b/src/ceph/src/common/map_cacher.hpp
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+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+/*
+ * Ceph - scalable distributed file system
+ *
+ * Copyright (C) 2013 Inktank Storage, Inc.
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software
+ * Foundation.  See file COPYING.
+ *
+ */
+
+#ifndef MAPCACHER_H
+#define MAPCACHER_H
+
+#include "common/sharedptr_registry.hpp"
+
+namespace MapCacher {
+/**
+ * Abstraction for ordering key updates
+ */
+template<typename K, typename V>
+class Transaction {
+public:
+  /// Set keys according to map
+  virtual void set_keys(
+    const std::map<K, V> &keys ///< [in] keys/values to set
+    ) = 0;
+
+  /// Remove keys
+  virtual void remove_keys(
+    const std::set<K> &to_remove ///< [in] keys to remove
+    ) = 0;
+
+  /// Add context to fire when data is readable
+  virtual void add_callback(
+    Context *c ///< [in] Context to fire on readable
+    ) = 0;
+  virtual ~Transaction() {}
+};
+
+/**
+ * Abstraction for fetching keys
+ */
+template<typename K, typename V>
+class StoreDriver {
+public:
+  /// Returns requested key values
+  virtual int get_keys(
+    const std::set<K> &keys,   ///< [in] keys requested
+    std::map<K, V> *got  ///< [out] values for keys obtained
+    ) = 0; ///< @return error value
+
+  /// Returns next key
+  virtual int get_next(
+    const K &key,       ///< [in] key after which to get next
+    pair<K, V> *next    ///< [out] first key after key
+    ) = 0; ///< @return 0 on success, -ENOENT if there is no next
+
+  virtual ~StoreDriver() {}
+};
+
+/**
+ * Uses SharedPtrRegistry to cache objects of in progress writes
+ * allowing the user to read/write a consistent view of the map
+ * without flushing writes.
+ */
+template<typename K, typename V>
+class MapCacher {
+private:
+  StoreDriver<K, V> *driver;
+
+  SharedPtrRegistry<K, boost::optional<V> > in_progress;
+  typedef typename SharedPtrRegistry<K, boost::optional<V> >::VPtr VPtr;
+  typedef ContainerContext<set<VPtr> > TransHolder;
+
+public:
+  MapCacher(StoreDriver<K, V> *driver) : driver(driver) {}
+
+  /// Fetch first key/value pair after specified key
+  int get_next(
+    K key,               ///< [in] key after which to get next
+    pair<K, V> *next     ///< [out] next key
+    ) {
+    while (true) {
+      pair<K, boost::optional<V> > cached;
+      pair<K, V> store;
+      bool got_cached = in_progress.get_next(key, &cached);
+
+      bool got_store = false;
+      int r = driver->get_next(key, &store);
+      if (r < 0 && r != -ENOENT) {
+	return r;
+      } else if (r == 0) {
+	got_store = true;
+      }
+
+      if (!got_cached && !got_store) {
+	return -ENOENT;
+      } else if (
+	got_cached &&
+	(!got_store || store.first >= cached.first)) {
+	if (cached.second) {
+	  if (next)
+	    *next = make_pair(cached.first, cached.second.get());
+	  return 0;
+	} else {
+	  key = cached.first;
+	  continue; // value was cached as removed, recurse
+	}
+      } else {
+	if (next)
+	  *next = store;
+	return 0;
+      }
+    }
+    ceph_abort(); // not reachable
+    return -EINVAL;
+  } ///< @return error value, 0 on success, -ENOENT if no more entries
+
+  /// Adds operation setting keys to Transaction
+  void set_keys(
+    const map<K, V> &keys,  ///< [in] keys/values to set
+    Transaction<K, V> *t    ///< [out] transaction to use
+    ) {
+    std::set<VPtr> vptrs;
+    for (typename map<K, V>::const_iterator i = keys.begin();
+	 i != keys.end();
+	 ++i) {
+      VPtr ip = in_progress.lookup_or_create(i->first, i->second);
+      *ip = i->second;
+      vptrs.insert(ip);
+    }
+    t->set_keys(keys);
+    t->add_callback(new TransHolder(vptrs));
+  }
+
+  /// Adds operation removing keys to Transaction
+  void remove_keys(
+    const set<K> &keys,  ///< [in]
+    Transaction<K, V> *t ///< [out] transaction to use
+    ) {
+    std::set<VPtr> vptrs;
+    for (typename set<K>::const_iterator i = keys.begin();
+	 i != keys.end();
+	 ++i) {
+      boost::optional<V> empty;
+      VPtr ip = in_progress.lookup_or_create(*i, empty);
+      *ip = empty;
+      vptrs.insert(ip);
+    }
+    t->remove_keys(keys);
+    t->add_callback(new TransHolder(vptrs));
+  }
+
+  /// Gets keys, uses cached values for unstable keys
+  int get_keys(
+    const set<K> &keys_to_get, ///< [in] set of keys to fetch
+    map<K, V> *got             ///< [out] keys gotten
+    ) {
+    set<K> to_get;
+    map<K, V> _got;
+    for (typename set<K>::const_iterator i = keys_to_get.begin();
+	 i != keys_to_get.end();
+	 ++i) {
+      VPtr val = in_progress.lookup(*i);
+      if (val) {
+	if (*val)
+	  got->insert(make_pair(*i, val->get()));
+	//else: value cached is empty, key doesn't exist
+      } else {
+	to_get.insert(*i);
+      }
+    }
+    int r = driver->get_keys(to_get, &_got);
+    if (r < 0)
+      return r;
+    for (typename map<K, V>::iterator i = _got.begin();
+	 i != _got.end();
+	 ++i) {
+      got->insert(*i);
+    }
+    return 0;
+  } ///< @return error value, 0 on success
+};
+} // namespace
+
+#endif