diff --git a/Framework/Core/include/Framework/MessageSet.h b/Framework/Core/include/Framework/MessageSet.h
index 281f9c42a0773..166934238d647 100644
--- a/Framework/Core/include/Framework/MessageSet.h
+++ b/Framework/Core/include/Framework/MessageSet.h
@@ -82,12 +82,6 @@ struct MessageSet {
     return *this;
   }
 
-  /// get number of in-flight O2 messages
-  [[nodiscard]] size_t size() const
-  {
-    return messages | count_parts{};
-  }
-
   /// get number of header-payload pairs
   [[nodiscard]] size_t getNumberOfPairs() const
   {
diff --git a/Framework/Core/src/DataRelayer.cxx b/Framework/Core/src/DataRelayer.cxx
index cece5b343659f..5b85a63bf6c95 100644
--- a/Framework/Core/src/DataRelayer.cxx
+++ b/Framework/Core/src/DataRelayer.cxx
@@ -184,11 +184,11 @@ DataRelayer::ActivityStats DataRelayer::processDanglingInputs(std::vector<Expira
       // We check that no data is already there for the given cell
       // it is enough to check the first element
       auto& part = mCache[ti * mDistinctRoutesIndex.size() + expirator.routeIndex.value];
-      if (part.size() > 0 && part.header(0) != nullptr) {
+      if (!part.messages.empty() && part.header(0) != nullptr) {
         headerPresent++;
         continue;
       }
-      if (part.size() > 0 && part.payload(0) != nullptr) {
+      if (!part.messages.empty() && part.payload(0) != nullptr) {
         payloadPresent++;
         continue;
       }
@@ -213,7 +213,7 @@ DataRelayer::ActivityStats DataRelayer::processDanglingInputs(std::vector<Expira
       auto partial = getPartialRecord(ti);
       // TODO: get the data ref from message model
       auto getter = [&partial](size_t idx, size_t part) {
-        if (partial[idx].size() > 0 && partial[idx].header(part).get()) {
+        if (!partial[idx].messages.empty() && partial[idx].header(part).get()) {
           auto header = partial[idx].header(part).get();
           auto payload = partial[idx].payload(part).get();
           return DataRef{nullptr,
@@ -224,7 +224,7 @@ DataRelayer::ActivityStats DataRelayer::processDanglingInputs(std::vector<Expira
         return DataRef{};
       };
       auto nPartsGetter = [&partial](size_t idx) {
-        return partial[idx].size();
+        return partial[idx].messages | count_parts{};
       };
       auto refCountGetter = [&partial](size_t idx) -> int {
         auto& header = static_cast<const fair::mq::shmem::Message&>(*partial[idx].header(0));
@@ -327,7 +327,7 @@ void DataRelayer::setOldestPossibleInput(TimesliceId proposed, ChannelIndex chan
     for (size_t mi = 0; mi < mInputs.size(); ++mi) {
       auto& input = mInputs[mi];
       auto& element = mCache[si * mInputs.size() + mi];
-      if (element.size() != 0) {
+      if (!element.messages.empty()) {
         if (input.lifetime != Lifetime::Condition && mCompletionPolicy.name != "internal-dpl-injected-dummy-sink") {
           didDrop = true;
           auto& state = mContext.get<DeviceState>();
@@ -353,7 +353,7 @@ void DataRelayer::setOldestPossibleInput(TimesliceId proposed, ChannelIndex chan
           continue;
         }
         auto& element = mCache[si * mInputs.size() + mi];
-        if (element.size() == 0) {
+        if (element.messages.empty()) {
           auto& state = mContext.get<DeviceState>();
           if (state.transitionHandling != TransitionHandlingState::NoTransition && DefaultsHelpers::onlineDeploymentMode()) {
             if (state.allowedProcessing == DeviceState::CalibrationOnly) {
@@ -411,11 +411,11 @@ void DataRelayer::pruneCache(TimesliceSlot slot, OnDropCallback onDrop)
         cachedStateMetrics[cacheId] = CacheEntryStatus::RUNNING;
         // TODO: in the original implementation of the cache, there have been only two messages per entry,
         // check if the 2 above corresponds to the number of messages.
-        if (cache[cacheId].size() > 0) {
+        if (!cache[cacheId].messages.empty()) {
           dropped[ai] = std::move(cache[cacheId]);
         }
       }
-      bool anyDropped = std::any_of(dropped.begin(), dropped.end(), [](auto& m) { return m.size(); });
+      bool anyDropped = std::any_of(dropped.begin(), dropped.end(), [](auto& m) { return !m.messages.empty(); });
       if (anyDropped) {
         O2_SIGNPOST_ID_GENERATE(aid, data_relayer);
         O2_SIGNPOST_EVENT_EMIT(data_relayer, aid, "pruneCache", "Dropping stuff from slot %zu with timeslice %zu", slot.index, oldestPossibleTimeslice.timeslice.value);
@@ -786,7 +786,7 @@ void DataRelayer::getReadyToProcess(std::vector<DataRelayer::RecordAction>& comp
     auto partial = getPartialRecord(li);
     // TODO: get the data ref from message model
     auto getter = [&partial](size_t idx, size_t part) {
-      if (partial[idx].size() > 0 && partial[idx].header(part).get()) {
+      if (!partial[idx].messages.empty() && partial[idx].header(part).get()) {
         auto header = partial[idx].header(part).get();
         auto payload = partial[idx].payload(part).get();
         return DataRef{nullptr,
@@ -797,7 +797,7 @@ void DataRelayer::getReadyToProcess(std::vector<DataRelayer::RecordAction>& comp
       return DataRef{};
     };
     auto nPartsGetter = [&partial](size_t idx) {
-      return partial[idx].size();
+      return partial[idx].messages | count_parts{};
     };
     auto refCountGetter = [&partial](size_t idx) -> int {
       auto& header = static_cast<const fair::mq::shmem::Message&>(*partial[idx].header(0));
@@ -897,7 +897,7 @@ std::vector<o2::framework::MessageSet> DataRelayer::consumeAllInputsForTimeslice
     cachedStateMetrics[cacheId] = CacheEntryStatus::RUNNING;
     // TODO: in the original implementation of the cache, there have been only two messages per entry,
     // check if the 2 above corresponds to the number of messages.
-    if (cache[cacheId].size() > 0) {
+    if (!cache[cacheId].messages.empty()) {
       messages[arg] = std::move(cache[cacheId]);
     }
     index.markAsInvalid(s);
@@ -951,7 +951,7 @@ std::vector<o2::framework::MessageSet> DataRelayer::consumeExistingInputsForTime
     cachedStateMetrics[cacheId] = CacheEntryStatus::RUNNING;
     // TODO: in the original implementation of the cache, there have been only two messages per entry,
     // check if the 2 above corresponds to the number of messages.
-    for (size_t pi = 0; pi < cache[cacheId].size(); pi++) {
+    for (size_t pi = 0; pi < (cache[cacheId].messages | count_parts{}); pi++) {
       auto& header = cache[cacheId].header(pi);
       auto&& newHeader = header->GetTransport()->CreateMessage();
       newHeader->Copy(*header);
diff --git a/Framework/Core/test/benchmark_DataRelayer.cxx b/Framework/Core/test/benchmark_DataRelayer.cxx
index 3c3d2294fdd7e..e983f3604cfab 100644
--- a/Framework/Core/test/benchmark_DataRelayer.cxx
+++ b/Framework/Core/test/benchmark_DataRelayer.cxx
@@ -96,7 +96,7 @@ static void BM_RelaySingleSlot(benchmark::State& state)
     assert(ready[0].op == CompletionPolicy::CompletionOp::Consume);
     auto result = relayer.consumeAllInputsForTimeslice(ready[0].slot);
     assert(result.size() == 1);
-    assert(result.at(0).size() == 1);
+    assert((result.at(0).messages | count_parts{}) == 1);
     inflightMessages = std::move(result[0].messages);
   }
 }
@@ -153,7 +153,7 @@ static void BM_RelayMultipleSlots(benchmark::State& state)
     assert(ready[0].op == CompletionPolicy::CompletionOp::Consume);
     auto result = relayer.consumeAllInputsForTimeslice(ready[0].slot);
     assert(result.size() == 1);
-    assert(result.at(0).size() == 1);
+    assert((result.at(0).messages | count_parts{}) == 1);
     inflightMessages = std::move(result[0].messages);
   }
 }
@@ -228,8 +228,8 @@ static void BM_RelayMultipleRoutes(benchmark::State& state)
     assert(ready[0].op == CompletionPolicy::CompletionOp::Consume);
     auto result = relayer.consumeAllInputsForTimeslice(ready[0].slot);
     assert(result.size() == 2);
-    assert(result.at(0).size() == 1);
-    assert(result.at(1).size() == 1);
+    assert((result.at(0).messages | count_parts{}) == 1);
+    assert((result.at(1).messages | count_parts{}) == 1);
     inflightMessages = std::move(result[0].messages);
     inflightMessages.emplace_back(std::move(result[1].messages[0]));
     inflightMessages.emplace_back(std::move(result[1].messages[1]));
diff --git a/Framework/Core/test/test_DataRelayer.cxx b/Framework/Core/test/test_DataRelayer.cxx
index 8957e361cb8a2..e5ca7c5d235e5 100644
--- a/Framework/Core/test/test_DataRelayer.cxx
+++ b/Framework/Core/test/test_DataRelayer.cxx
@@ -115,7 +115,7 @@ TEST_CASE("DataRelayer")
     auto result = relayer.consumeAllInputsForTimeslice(ready[0].slot);
     // one MessageSet with one PartRef with header and payload
     REQUIRE(result.size() == 1);
-    REQUIRE(result.at(0).size() == 1);
+    REQUIRE((result.at(0).messages | count_parts{}) == 1);
   }
 
   //
@@ -165,7 +165,7 @@ TEST_CASE("DataRelayer")
     auto result = relayer.consumeAllInputsForTimeslice(ready[0].slot);
     // one MessageSet with one PartRef with header and payload
     REQUIRE(result.size() == 1);
-    REQUIRE(result.at(0).size() == 1);
+    REQUIRE((result.at(0).messages | count_parts{}) == 1);
   }
 
   // This test a more complicated set of inputs, and verifies that data is
@@ -245,8 +245,8 @@ TEST_CASE("DataRelayer")
     auto result = relayer.consumeAllInputsForTimeslice(ready[0].slot);
     // two MessageSets, each with one PartRef
     REQUIRE(result.size() == 2);
-    REQUIRE(result.at(0).size() == 1);
-    REQUIRE(result.at(1).size() == 1);
+    REQUIRE((result.at(0).messages | count_parts{}) == 1);
+    REQUIRE((result.at(1).messages | count_parts{}) == 1);
   }
 
   // This test a more complicated set of inputs, and verifies that data is
@@ -733,7 +733,7 @@ TEST_CASE("DataRelayer")
     // we have one input route and thus one message set containing pairs for all
     // payloads
     REQUIRE(messageSet.size() == 1);
-    REQUIRE(messageSet[0].size() == nSplitParts);
+    REQUIRE((messageSet[0].messages | count_parts{}) == nSplitParts);
     REQUIRE(messageSet[0].getNumberOfPayloads(0) == 1);
   }
 
@@ -796,7 +796,7 @@ TEST_CASE("DataRelayer")
     // we have one input route
     REQUIRE(messageSet.size() == 1);
     // one message set containing number of added sequences of messages
-    REQUIRE(messageSet[0].size() == sequenceSize.size());
+    REQUIRE((messageSet[0].messages | count_parts{}) == sequenceSize.size());
     size_t counter = 0;
     for (auto seqid = 0; seqid < sequenceSize.size(); ++seqid) {
       REQUIRE(messageSet[0].getNumberOfPayloads(seqid) == sequenceSize[seqid]);
diff --git a/Framework/Core/test/test_ForwardInputs.cxx b/Framework/Core/test/test_ForwardInputs.cxx
index 7081d600080b1..e3031b7e72a69 100644
--- a/Framework/Core/test/test_ForwardInputs.cxx
+++ b/Framework/Core/test/test_ForwardInputs.cxx
@@ -92,7 +92,7 @@ TEST_CASE("ForwardInputsSingleMessageSingleRoute")
   auto channelAlloc = o2::pmr::getTransportAllocator(transport.get());
   auto header = o2::pmr::getMessage(o2::header::Stack{channelAlloc, dh, dph});
   messageSet.add(PartRef{std::move(header), std::move(payload)});
-  REQUIRE(messageSet.size() == 1);
+  REQUIRE((messageSet.messages | count_parts{}) == 1);
   currentSetOfInputs.emplace_back(std::move(messageSet));
 
   auto result = o2::framework::DataProcessingHelpers::routeForwardedMessageSet(proxy, currentSetOfInputs, copyByDefault, consume);
@@ -143,7 +143,7 @@ TEST_CASE("ForwardInputsSingleMessageSingleRouteNoConsume")
   auto channelAlloc = o2::pmr::getTransportAllocator(transport.get());
   auto header = o2::pmr::getMessage(o2::header::Stack{channelAlloc, dh, dph});
   messageSet.add(PartRef{std::move(header), std::move(payload)});
-  REQUIRE(messageSet.size() == 1);
+  REQUIRE((messageSet.messages | count_parts{}) == 1);
   currentSetOfInputs.emplace_back(std::move(messageSet));
 
   auto result = o2::framework::DataProcessingHelpers::routeForwardedMessageSet(proxy, currentSetOfInputs, copyByDefault, true);
@@ -198,7 +198,7 @@ TEST_CASE("ForwardInputsSingleMessageSingleRouteAtEOS")
   auto header = o2::pmr::getMessage(o2::header::Stack{channelAlloc, dh, dph, sih});
   REQUIRE(o2::header::get<SourceInfoHeader*>(header->GetData()));
   messageSet.add(PartRef{std::move(header), std::move(payload)});
-  REQUIRE(messageSet.size() == 1);
+  REQUIRE((messageSet.messages | count_parts{}) == 1);
   currentSetOfInputs.emplace_back(std::move(messageSet));
 
   auto result = o2::framework::DataProcessingHelpers::routeForwardedMessageSet(proxy, currentSetOfInputs, copyByDefault, consume);
@@ -256,7 +256,7 @@ TEST_CASE("ForwardInputsSingleMessageSingleRouteWithOldestPossible")
   auto header = o2::pmr::getMessage(o2::header::Stack{channelAlloc, dh, dph, dih});
   REQUIRE(o2::header::get<DomainInfoHeader*>(header->GetData()));
   messageSet.add(PartRef{std::move(header), std::move(payload)});
-  REQUIRE(messageSet.size() == 1);
+  REQUIRE((messageSet.messages | count_parts{}) == 1);
   currentSetOfInputs.emplace_back(std::move(messageSet));
 
   auto result = o2::framework::DataProcessingHelpers::routeForwardedMessageSet(proxy, currentSetOfInputs, copyByDefault, consume);
@@ -321,7 +321,7 @@ TEST_CASE("ForwardInputsSingleMessageMultipleRoutes")
   auto channelAlloc = o2::pmr::getTransportAllocator(transport.get());
   auto header = o2::pmr::getMessage(o2::header::Stack{channelAlloc, dh, dph});
   messageSet.add(PartRef{std::move(header), std::move(payload)});
-  REQUIRE(messageSet.size() == 1);
+  REQUIRE((messageSet.messages | count_parts{}) == 1);
   currentSetOfInputs.emplace_back(std::move(messageSet));
 
   auto result = o2::framework::DataProcessingHelpers::routeForwardedMessageSet(proxy, currentSetOfInputs, copyByDefault, consume);
@@ -384,7 +384,7 @@ TEST_CASE("ForwardInputsSingleMessageMultipleRoutesExternals")
   auto channelAlloc = o2::pmr::getTransportAllocator(transport.get());
   auto header = o2::pmr::getMessage(o2::header::Stack{channelAlloc, dh, dph});
   messageSet.add(PartRef{std::move(header), std::move(payload)});
-  REQUIRE(messageSet.size() == 1);
+  REQUIRE((messageSet.messages | count_parts{}) == 1);
   currentSetOfInputs.emplace_back(std::move(messageSet));
 
   auto result = o2::framework::DataProcessingHelpers::routeForwardedMessageSet(proxy, currentSetOfInputs, copyByDefault, consume);
@@ -455,12 +455,12 @@ TEST_CASE("ForwardInputsMultiMessageMultipleRoutes")
   auto header1 = o2::pmr::getMessage(o2::header::Stack{channelAlloc, dh1, dph});
   MessageSet messageSet1;
   messageSet1.add(PartRef{std::move(header1), std::move(payload1)});
-  REQUIRE(messageSet1.size() == 1);
+  REQUIRE((messageSet1.messages | count_parts{}) == 1);
 
   auto header2 = o2::pmr::getMessage(o2::header::Stack{channelAlloc, dh2, dph});
   MessageSet messageSet2;
   messageSet2.add(PartRef{std::move(header2), std::move(payload2)});
-  REQUIRE(messageSet2.size() == 1);
+  REQUIRE((messageSet2.messages | count_parts{}) == 1);
   currentSetOfInputs.emplace_back(std::move(messageSet1));
   currentSetOfInputs.emplace_back(std::move(messageSet2));
   REQUIRE(currentSetOfInputs.size() == 2);
@@ -525,7 +525,7 @@ TEST_CASE("ForwardInputsSingleMessageMultipleRoutesOnlyOneMatches")
   auto channelAlloc = o2::pmr::getTransportAllocator(transport.get());
   auto header = o2::pmr::getMessage(o2::header::Stack{channelAlloc, dh, dph});
   messageSet.add(PartRef{std::move(header), std::move(payload)});
-  REQUIRE(messageSet.size() == 1);
+  REQUIRE((messageSet.messages | count_parts{}) == 1);
   currentSetOfInputs.emplace_back(std::move(messageSet));
 
   auto result = o2::framework::DataProcessingHelpers::routeForwardedMessageSet(proxy, currentSetOfInputs, copyByDefault, consume);
@@ -607,7 +607,7 @@ TEST_CASE("ForwardInputsSplitPayload")
   PartRef part{std::move(header2), transport->CreateMessage()};
   messageSet.add(std::move(part));
 
-  REQUIRE(messageSet.size() == 2);
+  REQUIRE((messageSet.messages | count_parts{}) == 2);
   currentSetOfInputs.emplace_back(std::move(messageSet));
 
   auto result = o2::framework::DataProcessingHelpers::routeForwardedMessageSet(proxy, currentSetOfInputs, copyByDefault, consume);
@@ -727,7 +727,7 @@ TEST_CASE("ForwardInputEOSSingleRoute")
   auto channelAlloc = o2::pmr::getTransportAllocator(transport.get());
   auto header = o2::pmr::getMessage(o2::header::Stack{channelAlloc, sih});
   messageSet.add(PartRef{std::move(header), std::move(payload)});
-  REQUIRE(messageSet.size() == 1);
+  REQUIRE((messageSet.messages | count_parts{}) == 1);
   currentSetOfInputs.emplace_back(std::move(messageSet));
 
   auto result = o2::framework::DataProcessingHelpers::routeForwardedMessageSet(proxy, currentSetOfInputs, copyByDefault, consume);
@@ -772,7 +772,7 @@ TEST_CASE("ForwardInputOldestPossibleSingleRoute")
   auto channelAlloc = o2::pmr::getTransportAllocator(transport.get());
   auto header = o2::pmr::getMessage(o2::header::Stack{channelAlloc, dih});
   messageSet.add(PartRef{std::move(header), std::move(payload)});
-  REQUIRE(messageSet.size() == 1);
+  REQUIRE((messageSet.messages | count_parts{}) == 1);
   currentSetOfInputs.emplace_back(std::move(messageSet));
 
   auto result = o2::framework::DataProcessingHelpers::routeForwardedMessageSet(proxy, currentSetOfInputs, copyByDefault, consume);