diff --git a/src/fdb5/remote/client/ClientConnection.cc b/src/fdb5/remote/client/ClientConnection.cc
index 4ae663050..28601e226 100644
--- a/src/fdb5/remote/client/ClientConnection.cc
+++ b/src/fdb5/remote/client/ClientConnection.cc
@@ -23,6 +23,7 @@
 
 #include <unistd.h>
 
+#include <algorithm>
 #include <cstddef>
 #include <cstdint>
 #include <exception>
@@ -54,15 +55,19 @@ class DataWriteRequest {
 
 public:
 
-    DataWriteRequest() : client_(nullptr), msg_(Message::None), id_(0), data_(Buffer(0)) {}
+    DataWriteRequest() = default;
 
     DataWriteRequest(Client* client, Message msg, uint32_t id, Buffer&& data) :
         client_(client), msg_(msg), id_(id), data_(std::move(data)) {}
 
-    Client* client_;
-    Message msg_;
-    uint32_t id_;
-    Buffer data_;
+    /// @param barrier: no payload, signals when all queued writes before this barrier have been processed
+    explicit DataWriteRequest(std::shared_ptr<std::promise<void>> barrier) : barrier_{std::move(barrier)} {}
+
+    Client* client_{nullptr};
+    Message msg_{Message::None};
+    uint32_t id_{0};
+    Buffer data_{Buffer(0)};
+    std::shared_ptr<std::promise<void>> barrier_;
 };
 
 //----------------------------------------------------------------------------------------------------------------------
@@ -113,8 +118,11 @@ bool ClientConnection::remove(uint32_t clientID) {
 }
 
 ClientConnection::~ClientConnection() {
-    if (dataWriteQueue_) {
-        dataWriteQueue_->close();
+    {
+        std::lock_guard lock(dataWriteMutex_);
+        if (dataWriteQueue_) {
+            dataWriteQueue_->close();
+        }
     }
 
     disconnect();
@@ -282,23 +290,67 @@ void ClientConnection::dataWriteThreadLoop() {
     Timer timer;
     DataWriteRequest element;
 
-    try {
+    auto* queue = dataWriteQueue_.get();
+    ASSERT(queue);
 
-        ASSERT(dataWriteQueue_);
-        while (dataWriteQueue_->pop(element) != -1) {
+    try {
 
+        while (queue->pop(element) != -1) {
+            if (element.barrier_) {
+                std::lock_guard lock(dataWriteMutex_);
+                element.barrier_->set_value();
+                dataBarriers_.erase(std::remove(dataBarriers_.begin(), dataBarriers_.end(), element.barrier_),
+                                    dataBarriers_.end());
+                continue;
+            }
             dataWrite(element);
         }
 
+        std::lock_guard lock(dataWriteMutex_);
         dataWriteQueue_.reset();
     }
     catch (...) {
-        dataWriteQueue_->interrupt(std::current_exception());
-        throw;
+        // we must not rethrow in thread (that would std::terminate)
+        // propagate and unblock any waiting flushDataWrites()
+        queue->interrupt(std::current_exception());
+        failDataBarriers(std::current_exception());
+    }
+}
+
+void ClientConnection::flushDataWrites() {
+    std::future<void> written;
+    {
+        std::lock_guard lock(dataWriteMutex_);
+        if (!dataWriteQueue_) {
+            return;
+        }
+        auto barrier = std::make_shared<std::promise<void>>();
+        written = barrier->get_future();
+        dataBarriers_.push_back(barrier);
+        try {
+            dataWriteQueue_->emplace(barrier);
+        }
+        catch (...) {
+            LOG_DEBUG_LIB(LibFdb5) << "flushDataWrites - failed to enqueue barrier!" << std::endl;
+            dataBarriers_.pop_back();
+            throw;
+        }
     }
+    // block until the writer reaches the barrier (or the connection fails)
+    written.get();
+}
 
-    // We are inside an async, so don't need to worry about exceptions escaping.
-    // They will be released when flush() is called.
+void ClientConnection::failDataBarriers(const std::exception_ptr& eptr) {
+    std::lock_guard lock(dataWriteMutex_);
+    for (auto& barrier : dataBarriers_) {
+        try {
+            barrier->set_exception(eptr);
+        }
+        catch (...) {
+            Log::warning() << "failDataBarriers - barrier already satisfied." << std::endl;
+        }
+    }
+    dataBarriers_.clear();
 }
 
 void ClientConnection::writeControlStartupMessage(const Configuration& config) {
diff --git a/src/fdb5/remote/client/ClientConnection.h b/src/fdb5/remote/client/ClientConnection.h
index 7eaad1fe8..83a2500a8 100644
--- a/src/fdb5/remote/client/ClientConnection.h
+++ b/src/fdb5/remote/client/ClientConnection.h
@@ -29,6 +29,7 @@
 #include <mutex>
 #include <string>
 #include <thread>
+#include <vector>
 
 namespace eckit {
 
@@ -56,6 +57,9 @@ class ClientConnection : protected Connection {
 
     void dataWrite(Client& client, Message msg, uint32_t requestID, PayloadList payloads = {});
 
+    // Blocks until the data-writer thread has processed all data writes queued before this call.
+    void flushDataWrites();
+
     void add(Client& client);
     bool remove(uint32_t clientID);
 
@@ -93,6 +97,9 @@ class ClientConnection : protected Connection {
     // do not hang forever once the connection is known to be dead.
     void failPendingRequests(const std::exception_ptr& eptr);
 
+    // fail any waiting flush data barriers
+    void failDataBarriers(const std::exception_ptr& eptr);
+
     void dataWriteThreadLoop();
     void closeConnection();
 
@@ -135,6 +142,8 @@ class ClientConnection : protected Connection {
     std::thread dataWriteThread_;
 
     uint64_t agreedFeatures_{0};
+
+    std::vector<std::shared_ptr<std::promise<void>>> dataBarriers_;
 };
 
 //----------------------------------------------------------------------------------------------------------------------
diff --git a/src/fdb5/remote/client/RemoteCatalogue.cc b/src/fdb5/remote/client/RemoteCatalogue.cc
index 7674e901f..8731ceee5 100644
--- a/src/fdb5/remote/client/RemoteCatalogue.cc
+++ b/src/fdb5/remote/client/RemoteCatalogue.cc
@@ -149,6 +149,10 @@ void RemoteCatalogue::flush(size_t archivedFields) {
 
         LOG_DEBUG_LIB(LibFdb5) << " RemoteCatalogue::flush - flushing " << numLocations_ << " fields" << std::endl;
 
+        // Ensure the (field-location) "Blob"s arrive before the blocking "Flush"!
+        // (may deadlock server on a single connection)
+        connection_->flushDataWrites();
+
         // The flush call is blocking
         controlWriteCheckResponse(Message::Flush, generateRequestID(), false, sendBuf, s.position());