diff --git a/include/wsrep/client_state.hpp b/include/wsrep/client_state.hpp
index 0299cf9..f38768f 100644
--- a/include/wsrep/client_state.hpp
+++ b/include/wsrep/client_state.hpp
@@ -788,8 +788,10 @@ namespace wsrep
          * Begin non-blocking operation phase two. The keys argument
          * passed to this call must contain the same keys which were
          * passed to begin_nbo_phase_one().
+         *
+         * @param keys Key array.
          */
-        int begin_nbo_phase_two();
+        int begin_nbo_phase_two(const wsrep::key_array& keys);
 
         /**
          * End non-blocking operation phase two. This call will
@@ -998,6 +1000,7 @@ namespace wsrep
             wsrep::unique_lock<wsrep::mutex>& lock);
         void update_last_written_gtid(const wsrep::gtid&);
         void debug_log_state(const char*) const;
+        void debug_log_keys(const wsrep::key_array& keys) const;
         void state(wsrep::unique_lock<wsrep::mutex>& lock, enum state state);
         void mode(wsrep::unique_lock<wsrep::mutex>& lock, enum mode mode);
 
diff --git a/include/wsrep/key.hpp b/include/wsrep/key.hpp
index fb0e009..5296976 100644
--- a/include/wsrep/key.hpp
+++ b/include/wsrep/key.hpp
@@ -27,6 +27,10 @@
 
 namespace wsrep
 {
+    /** @class key
+     *
+     * Certification key type.
+     */
     class key
     {
     public:
@@ -44,6 +48,14 @@ namespace wsrep
             , key_parts_len_()
         { }
 
+        /**
+         * Append key part to key.
+         *
+         * @param ptr Pointer to key part data. The caller is supposed to take
+         *            care that the pointer remains valid over the lifetime
+         *            if the key object.
+         * @param len Length of the key part data.
+         */
         void append_key_part(const void* ptr, size_t len)
         {
             if (key_parts_len_ == 3)
diff --git a/include/wsrep/transaction.hpp b/include/wsrep/transaction.hpp
index fa7b82c..bc81288 100644
--- a/include/wsrep/transaction.hpp
+++ b/include/wsrep/transaction.hpp
@@ -251,7 +251,7 @@ namespace wsrep
         void clear_fragments();
         void cleanup();
         void debug_log_state(const char*) const;
-        void debug_log_key_append(const wsrep::key& key);
+        void debug_log_key_append(const wsrep::key& key) const;
 
         wsrep::server_service& server_service_;
         wsrep::client_service& client_service_;
diff --git a/src/client_state.cpp b/src/client_state.cpp
index e21eaac..f59b41c 100644
--- a/src/client_state.cpp
+++ b/src/client_state.cpp
@@ -512,6 +512,7 @@ int wsrep::client_state::begin_nbo_phase_one(
     std::chrono::time_point<wsrep::clock> wait_until)
 {
     debug_log_state("begin_nbo_phase_one: enter");
+    debug_log_keys(keys);
     wsrep::unique_lock<wsrep::mutex> lock(mutex_);
     assert(state_ == s_exec);
     assert(mode_ == m_local);
@@ -583,9 +584,10 @@ int wsrep::client_state::enter_nbo_mode(const wsrep::ws_meta& ws_meta)
     return 0;
 }
 
-int wsrep::client_state::begin_nbo_phase_two()
+int wsrep::client_state::begin_nbo_phase_two(const wsrep::key_array& keys)
 {
     debug_log_state("begin_nbo_phase_two: enter");
+    debug_log_keys(keys);
     assert(state_ == s_exec);
     assert(mode_ == m_nbo);
     assert(toi_mode_ == m_undefined);
@@ -597,7 +599,7 @@ int wsrep::client_state::begin_nbo_phase_two()
     // Output stored in nbo_meta_ is copied to toi_meta_ for
     // phase two end.
     enum wsrep::provider::status status(
-        provider().enter_toi(id_, wsrep::key_array(),
+        provider().enter_toi(id_, keys,
                              wsrep::const_buffer(), nbo_meta_,
                              wsrep::provider::flag::commit));
     int ret;
@@ -725,6 +727,18 @@ void wsrep::client_state::debug_log_state(const char* context) const
                     << ",nbo: " << nbo_meta_.seqno() << ")");
 }
 
+void wsrep::client_state::debug_log_keys(const wsrep::key_array& keys) const
+{
+    for (const auto& k : keys)
+    {
+        WSREP_LOG_DEBUG(debug_log_level(),
+                        wsrep::log::debug_level_client_state,
+                        "TOI keys: "
+                        << " id: " << id_
+                        << "key: " << k);
+    }
+}
+
 void wsrep::client_state::state(
     wsrep::unique_lock<wsrep::mutex>& lock WSREP_UNUSED,
     enum wsrep::client_state::state state)
diff --git a/src/transaction.cpp b/src/transaction.cpp
index 4b8835f..2922ccc 100644
--- a/src/transaction.cpp
+++ b/src/transaction.cpp
@@ -1790,7 +1790,7 @@ void wsrep::transaction::debug_log_state(
         << "");
 }
 
-void wsrep::transaction::debug_log_key_append(const wsrep::key& key)
+void wsrep::transaction::debug_log_key_append(const wsrep::key& key) const
 {
     WSREP_LOG_DEBUG(client_state_.debug_log_level(),
                     wsrep::log::debug_level_transaction,