diff --git a/include/wsrep/client_state.hpp b/include/wsrep/client_state.hpp
index 4ac919e..93db11d 100644
--- a/include/wsrep/client_state.hpp
+++ b/include/wsrep/client_state.hpp
@@ -4,10 +4,17 @@
 
 /** @file client_state.hpp
  *
+ *
+ * Return value conventions:
+ *
+ * The calls which may alter either client_state or associated
+ * transaction state will generally return zero on success and
+ * non-zero on failure. More detailed error information is stored
+ * into client state and persisted there until explicitly cleared.
  */
 
-#ifndef WSREP_CLIENT_CONTEXT_HPP
-#define WSREP_CLIENT_CONTEXT_HPP
+#ifndef WSREP_CLIENT_STATE_HPP
+#define WSREP_CLIENT_STATE_HPP
 
 #include "server_state.hpp"
 #include "provider.hpp"
@@ -34,7 +41,7 @@ namespace wsrep
         e_append_fragment_error
     };
 
-    static inline const char* to_string(enum client_error error)
+    static inline const char* to_c_string(enum client_error error)
     {
         switch (error)
         {
@@ -48,6 +55,10 @@ namespace wsrep
         return "unknown";
     }
 
+    static inline std::string to_string(enum client_error error)
+    {
+        return to_c_string(error);
+    }
     /**
      * Client State
      */
@@ -382,45 +393,18 @@ namespace wsrep
             transaction_.streaming_context_ = transaction.streaming_context_;
         }
 
-        //
-        // Causal reads
-        //
-
-        /**
-         * Perform a causal read in the cluster. After the call returns,
-         * all the causally preceding write sets have been committed
-         * or the error is returned.
-         *
-         * This operation may require communication with other processes
-         * in the DBMS cluster, so it may be relatively heavy operation.
-         * Method wait_for_gtid() should be used whenever possible.
-         */
-        enum wsrep::provider::status causal_read() const;
-
-        /**
-         * Wait until all the write sets up to given GTID have been
-         * committed.
-         *
-         * @return Zero on success, non-zero on failure.
-         */
-        enum wsrep::provider::status wait_for_gtid(const wsrep::gtid&) const;
-
-        //
-        //
-        //
-
         /** @name Non-transactional operations */
         /** @{*/
 
         /**
          * Enter total order isolation critical section.
          */
-        enum wsrep::provider::status enter_toi();
+        int enter_toi(const wsrep::key_array&, const wsrep::const_buffer&);
 
         /**
          * Leave total order isolation critical section.
          */
-        void leave_toi();
+        int leave_toi();
 
         /**
          * Begin non-blocking operation.
@@ -430,7 +414,7 @@ namespace wsrep
         /**
          * End non-blocking operation
          */
-        void end_nbo();
+        int end_nbo();
 
         /**
          * Get reference to the client mutex.
@@ -696,4 +680,4 @@ namespace wsrep
     };
 }
 
-#endif // WSREP_CLIENT_CONTEXT_HPP
+#endif // WSREP_CLIENT_STATE_HPP
diff --git a/include/wsrep/gtid.hpp b/include/wsrep/gtid.hpp
index 73cb11f..89d08c2 100644
--- a/include/wsrep/gtid.hpp
+++ b/include/wsrep/gtid.hpp
@@ -25,6 +25,10 @@ namespace wsrep
         { }
         const wsrep::id& id() const { return id_; }
         wsrep::seqno seqno() const { return seqno_ ; }
+        bool is_undefined() const
+        {
+            return (seqno_.is_undefined() && id_.is_undefined());
+        }
     private:
         wsrep::id id_;
         wsrep::seqno seqno_;
diff --git a/include/wsrep/id.hpp b/include/wsrep/id.hpp
index ea403d7..25a35fe 100644
--- a/include/wsrep/id.hpp
+++ b/include/wsrep/id.hpp
@@ -60,11 +60,24 @@ namespace wsrep
         {
             return (std::memcmp(data_, other.data_, sizeof(data_)) == 0);
         }
-
+        bool operator!=(const id& other) const
+        {
+            return !(*this == other);
+        }
         const void* data() const { return data_; }
 
         size_t size() const { return sizeof(data_); }
 
+        bool is_undefined() const
+        {
+            return (*this == undefined());
+        }
+
+        static const wsrep::id& undefined()
+        {
+            static wsrep::id ret = wsrep::id();
+            return ret;
+        }
     private:
         unsigned char data_[16];
     };
diff --git a/include/wsrep/provider.hpp b/include/wsrep/provider.hpp
index e769780..896f2e7 100644
--- a/include/wsrep/provider.hpp
+++ b/include/wsrep/provider.hpp
@@ -232,7 +232,7 @@ namespace wsrep
         virtual int desync() = 0;
         virtual int resync() = 0;
 
-        virtual int pause() = 0;
+        virtual wsrep::seqno pause() = 0;
         virtual int resume() = 0;
 
         // Applier interface
@@ -277,6 +277,14 @@ namespace wsrep
         virtual enum status replay(
             const wsrep::ws_handle& ws_handle, void* applier_ctx) = 0;
 
+        /**
+         * Perform a causal read on cluster.
+         *
+         * @param timeout Timeout in seconds
+         *
+         * @return Provider status indicating the result of the call.
+         */
+        virtual enum wsrep::provider::status causal_read(int timeout) const = 0;
         virtual int sst_sent(const wsrep::gtid&, int) = 0;
         virtual int sst_received(const wsrep::gtid&, int) = 0;
 
diff --git a/include/wsrep/seqno.hpp b/include/wsrep/seqno.hpp
index 24bdd28..7e30cda 100644
--- a/include/wsrep/seqno.hpp
+++ b/include/wsrep/seqno.hpp
@@ -23,15 +23,15 @@ namespace wsrep
     {
     public:
         seqno()
-            : seqno_()
+            : seqno_(-1)
         { }
 
         explicit seqno(long long seqno)
             : seqno_(seqno)
         {
-            if (seqno_ < 0)
+            if (seqno_ < -1)
             {
-                throw wsrep::runtime_error("Negative seqno given");
+                throw wsrep::runtime_error("Too negative seqno given");
             }
         }
 
@@ -40,9 +40,9 @@ namespace wsrep
             return seqno_;
         }
 
-        bool nil() const
+        bool is_undefined() const
         {
-            return (seqno_ == 0);
+            return (seqno_ == -1);
         }
 
         bool operator<(seqno other) const
@@ -54,13 +54,19 @@ namespace wsrep
         {
             return (seqno_ > other.seqno_);
         }
-
+        bool operator==(seqno other) const
+        {
+            return (seqno_ == other.seqno_);
+        }
         seqno operator+(seqno other) const
         {
             return (seqno(seqno_ + other.seqno_));
         }
-
-        static seqno undefined() { return seqno(0); }
+        seqno operator+(long long other) const
+        {
+            return (*this + seqno(other));
+        }
+        static seqno undefined() { return seqno(-1); }
     private:
         long long seqno_;
     };
diff --git a/include/wsrep/server_state.hpp b/include/wsrep/server_state.hpp
index 33d2f1a..ab41334 100644
--- a/include/wsrep/server_state.hpp
+++ b/include/wsrep/server_state.hpp
@@ -4,11 +4,11 @@
 
 /** @file server_state.hpp
  *
- * Server Context Abstraction
+ * Server State Abstraction
  * ==========================
  *
- * This file defines an interface for WSREP Server Context.
- * The Server Context will encapsulate server identification,
+ * This file defines an interface for WSREP Server State.
+ * The Server State t will encapsulate server identification,
  * server state and server capabilities. The class also
  * defines an interface for manipulating server state, applying
  * of remote transaction write sets, processing SST requests,
@@ -56,16 +56,24 @@
  * to be aborted or in case of fully optimistic concurrency control,
  * the conflict is detected at commit.
  *
+ *
+ * # Return value conventions
+ *
+ * The calls which are proxies to corresponding provider functionality
+ * will return wsrep::provider::status enum as a result. Otherwise
+ * the return value is generally zero on success, non zero on failure.
  */
 
-#ifndef WSREP_SERVER_CONTEXT_HPP
-#define WSREP_SERVER_CONTEXT_HPP
+#ifndef WSREP_SERVER_STATE_HPP
+#define WSREP_SERVER_STATE_HPP
 
 #include "mutex.hpp"
 #include "condition_variable.hpp"
 #include "server_service.hpp"
 #include "id.hpp"
 #include "transaction_id.hpp"
+#include "provider.hpp"
+// #include "gtid.hpp"
 
 #include <vector>
 #include <string>
@@ -76,10 +84,8 @@ namespace wsrep
     // Forward declarations
     class ws_handle;
     class ws_meta;
-    class provider;
     class client_state;
     class transaction;
-    class gtid;
     class view;
     class const_buffer;
 
@@ -275,6 +281,37 @@ namespace wsrep
          */
         void wait_until_state(wsrep::server_state::state) const;
 
+        /**
+         * Set last committed GTID.
+         */
+        void last_committed_gtid(const wsrep::gtid&);
+        /**
+         * Return the last committed GTID known to be committed
+         * on server.
+         */
+        wsrep::gtid last_committed_gtid() const;
+
+        /**
+         * Wait until all the write sets up to given GTID have been
+         * committed.
+         *
+         * @return Zero on success, non-zero on failure.
+         */
+        int wait_for_gtid(const wsrep::gtid&) const;
+
+        /**
+         * Perform a causal read in the cluster. After the call returns,
+         * all the causally preceding write sets have been committed
+         * or the error is returned.
+         *
+         * This operation may require communication with other processes
+         * in the DBMS cluster, so it may be relatively heavy operation.
+         * Method wait_for_gtid() should be used whenever possible.
+         *
+         * @param timeout Timeout in seconds
+         */
+        enum wsrep::provider::status causal_read(int timeout) const;
+
         /**
          *
          */
@@ -369,6 +406,7 @@ namespace wsrep
             , address_(address)
             , working_dir_(working_dir)
             , rollback_mode_(rollback_mode)
+            , last_committed_gtid_()
             , debug_log_level_(0)
         { }
 
@@ -392,6 +430,7 @@ namespace wsrep
         std::string address_;
         std::string working_dir_;
         enum rollback_mode rollback_mode_;
+        wsrep::gtid last_committed_gtid_;
         int debug_log_level_;
     };
 
@@ -428,4 +467,4 @@ namespace wsrep
 
 }
 
-#endif // WSREP_SERVER_CONTEXT_HPP
+#endif // WSREP_SERVER_STATE_HPP
diff --git a/include/wsrep/transaction.hpp b/include/wsrep/transaction.hpp
index 5d15b14..e3dc040 100644
--- a/include/wsrep/transaction.hpp
+++ b/include/wsrep/transaction.hpp
@@ -68,7 +68,7 @@ namespace wsrep
         // Return true if the last fragment was ordered by the
         // provider
         bool ordered() const
-        { return (ws_meta_.seqno().nil() == false); }
+        { return (ws_meta_.seqno().is_undefined() == false); }
 
         /*!
          * Return true if any fragments have been succesfully certified
diff --git a/src/server_state.cpp b/src/server_state.cpp
index cc50d44..d1c2726 100644
--- a/src/server_state.cpp
+++ b/src/server_state.cpp
@@ -77,6 +77,41 @@ void wsrep::server_state::wait_until_state(
     cond_.notify_all();
 }
 
+void wsrep::server_state::last_committed_gtid(const wsrep::gtid& gtid)
+{
+    wsrep::unique_lock<wsrep::mutex> lock(mutex_);
+    assert(last_committed_gtid_.is_undefined() ||
+           last_committed_gtid_.seqno() + 1 == gtid.seqno());
+    last_committed_gtid_ = gtid;
+    cond_.notify_all();
+}
+
+wsrep::gtid wsrep::server_state::last_committed_gtid() const
+{
+    wsrep::unique_lock<wsrep::mutex> lock(mutex_);
+    return last_committed_gtid_;
+}
+
+int wsrep::server_state::wait_for_gtid(const wsrep::gtid& gtid) const
+{
+    wsrep::unique_lock<wsrep::mutex> lock(mutex_);
+    if (gtid.id() != last_committed_gtid_.id())
+    {
+        return 1;
+    }
+    while (last_committed_gtid_.seqno() < gtid.seqno())
+    {
+        cond_.wait(lock);
+    }
+    return 0;
+}
+
+enum wsrep::provider::status
+wsrep::server_state::causal_read(int timeout) const
+{
+    return provider_->causal_read(timeout);
+}
+
 void wsrep::server_state::on_connect()
 {
     wsrep::log() << "Server " << name_ << " connected to cluster";
diff --git a/src/transaction.cpp b/src/transaction.cpp
index 01d0cab..3923dab 100644
--- a/src/transaction.cpp
+++ b/src/transaction.cpp
@@ -89,7 +89,7 @@ int wsrep::transaction::start_replaying(const wsrep::ws_meta& ws_meta)
     assert(active());
     assert(client_state_.mode() == wsrep::client_state::m_high_priority);
     assert(state() == s_replaying);
-    assert(ws_meta_.seqno().nil() == false);
+    assert(ws_meta_.seqno().is_undefined() == false);
     certified_ = true;
     return 0;
 }
diff --git a/src/wsrep_provider_v26.cpp b/src/wsrep_provider_v26.cpp
index 794e7c8..21e628b 100644
--- a/src/wsrep_provider_v26.cpp
+++ b/src/wsrep_provider_v26.cpp
@@ -69,12 +69,12 @@ namespace
 
     static inline wsrep_seqno_t seqno_to_native(wsrep::seqno seqno)
     {
-        return (seqno.nil() ? WSREP_SEQNO_UNDEFINED : seqno.get());
+        return seqno.get();
     }
 
     static inline wsrep::seqno seqno_from_native(wsrep_seqno_t seqno)
     {
-        return wsrep::seqno(seqno == WSREP_SEQNO_UNDEFINED ? 0 : seqno);
+        return wsrep::seqno(seqno);
     }
     template <typename F, typename T>
     inline uint32_t map_one(const int flags, const F from,
@@ -520,9 +520,9 @@ int wsrep::wsrep_provider_v26::resync()
     return (wsrep_->resync(wsrep_) != WSREP_OK);
 }
 
-int wsrep::wsrep_provider_v26::pause()
+wsrep::seqno wsrep::wsrep_provider_v26::pause()
 {
-    return (wsrep_->pause(wsrep_) != WSREP_OK);
+    return wsrep::seqno(wsrep_->pause(wsrep_));
 }
 
 int wsrep::wsrep_provider_v26::resume()
@@ -633,6 +633,12 @@ wsrep::wsrep_provider_v26::replay(const wsrep::ws_handle& ws_handle,
         wsrep_->replay_trx(wsrep_, mwsh.native(), applier_ctx));
 }
 
+enum wsrep::provider::status
+wsrep::wsrep_provider_v26::causal_read(int timeout) const
+{
+    return map_return_value(wsrep_->sync_wait(wsrep_, 0, timeout, 0));
+}
+
 int wsrep::wsrep_provider_v26::sst_sent(const wsrep::gtid& gtid, int err)
 {
     wsrep_gtid_t wsrep_gtid;
diff --git a/src/wsrep_provider_v26.hpp b/src/wsrep_provider_v26.hpp
index 3e67935..c8d9e57 100644
--- a/src/wsrep_provider_v26.hpp
+++ b/src/wsrep_provider_v26.hpp
@@ -25,7 +25,7 @@ namespace wsrep
 
         int desync();
         int resync();
-        int pause();
+        wsrep::seqno pause();
         int resume();
 
         enum wsrep::provider::status run_applier(void*);
@@ -49,6 +49,9 @@ namespace wsrep
                                const wsrep::ws_meta&);
         int release(wsrep::ws_handle&);
         enum wsrep::provider::status replay(const wsrep::ws_handle&, void*);
+
+        enum wsrep::provider::status causal_read(int) const;
+
         int sst_sent(const wsrep::gtid&,int);
         int sst_received(const wsrep::gtid& gtid, int);
 
diff --git a/test/mock_provider.hpp b/test/mock_provider.hpp
index a8388c6..5980b64 100644
--- a/test/mock_provider.hpp
+++ b/test/mock_provider.hpp
@@ -46,7 +46,7 @@ namespace wsrep
         int capabilities() const { return 0; }
         int desync() { return 0; }
         int resync() { return 0; }
-        int pause() { return 0; }
+        wsrep::seqno pause() { return wsrep::seqno(0); }
         int resume() { return 0; }
         enum wsrep::provider::status run_applier(void*)
         {
@@ -104,7 +104,7 @@ namespace wsrep
             else
             {
                 enum wsrep::provider::status ret;
-                if (it->second.nil())
+                if (it->second.is_undefined())
                 {
                     ws_meta = wsrep::ws_meta(wsrep::gtid(), wsrep::stid(),
                                              wsrep::seqno::undefined(), 0);
@@ -140,7 +140,7 @@ namespace wsrep
                            const wsrep::ws_meta& ws_meta)
         {
             BOOST_REQUIRE(ws_handle.opaque());
-            BOOST_REQUIRE(ws_meta.seqno().nil() == false);
+            BOOST_REQUIRE(ws_meta.seqno().is_undefined() == false);
             return commit_order_enter_result_;
         }
 
@@ -148,7 +148,7 @@ namespace wsrep
                                const wsrep::ws_meta& ws_meta)
         {
             BOOST_REQUIRE(ws_handle.opaque());
-            BOOST_REQUIRE(ws_meta.seqno().nil() == false);
+            BOOST_REQUIRE(ws_meta.seqno().is_undefined() == false);
             return commit_order_leave_result_;
         }
 
@@ -170,7 +170,7 @@ namespace wsrep
             {
                 // If the ws_meta was not assigned yet, the certify
                 // returned early due to BF abort.
-                if (tc.ws_meta().seqno().nil())
+                if (tc.ws_meta().seqno().is_undefined())
                 {
                     ++group_seqno_;
                     ws_meta = wsrep::ws_meta(
@@ -198,6 +198,10 @@ namespace wsrep
             return wsrep::provider::success;
         }
 
+        enum wsrep::provider::status causal_read(int) const WSREP_OVERRIDE
+        {
+            return wsrep::provider::success;
+        }
         int sst_sent(const wsrep::gtid&, int) { return 0; }
         int sst_received(const wsrep::gtid&, int) { return 0; }
 
@@ -225,7 +229,7 @@ namespace wsrep
                  wsrep::seqno& victim_seqno)
         {
             bf_abort_map_.insert(std::make_pair(trx_id, bf_seqno));
-            if (bf_seqno.nil() == false)
+            if (bf_seqno.is_undefined() == false)
             {
                 group_seqno_ = bf_seqno.get();
             }