Add 'sequence' zmq publisher to track all block (dis)connects, mempool deltas

Using the zmq notifications to avoid excessive mempool polling can be difficult
given the current notifications available. It announces all transactions
being added to mempool or included in blocks, but announces no evictions
and gives no indication if the transaction is in the mempool or a block.

Block notifications for zmq are also substandard, in that it only announces
block tips, while all block transactions are still announced.

This commit adds a unified stream which can be used to closely track mempool:

1) getrawmempool to fill out mempool knowledge
2) if txhash is announced, add or remove from set
based on add/remove flag
3) if blockhash is announced, get block txn list,
remove from those transactions local view of mempool
4) if we drop a sequence number, go to (1)

The mempool sequence number starts at the value 1, and
increments each time a transaction enters the mempool,
or is evicted from the mempool for any reason, including
block inclusion. The mempool sequence number is published
via ZMQ for any transaction-related notification.

These features allow for ZMQ/RPC consumer to track mempool
state in a more exacting way, without unnecesarily polling
getrawmempool. See interface_zmq.py::test_mempool_sync for
example usage.

19 files changed, 206 insertions, 41 deletions

diff --git a/src/init.cpp b/src/init.cpp
index 023aa9aba5..132acbfc81 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -488,19 +488,23 @@ void SetupServerArgs(NodeContext& node)
     argsman.AddArg("-zmqpubhashtx=<address>", "Enable publish hash transaction in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
     argsman.AddArg("-zmqpubrawblock=<address>", "Enable publish raw block in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
     argsman.AddArg("-zmqpubrawtx=<address>", "Enable publish raw transaction in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
+    argsman.AddArg("-zmqpubsequence=<address>", "Enable publish hash block and tx sequence in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
     argsman.AddArg("-zmqpubhashblockhwm=<n>", strprintf("Set publish hash block outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
     argsman.AddArg("-zmqpubhashtxhwm=<n>", strprintf("Set publish hash transaction outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
     argsman.AddArg("-zmqpubrawblockhwm=<n>", strprintf("Set publish raw block outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
     argsman.AddArg("-zmqpubrawtxhwm=<n>", strprintf("Set publish raw transaction outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
+    argsman.AddArg("-zmqpubsequencehwm=<n>", strprintf("Set publish hash sequence message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
 #else
     hidden_args.emplace_back("-zmqpubhashblock=<address>");
     hidden_args.emplace_back("-zmqpubhashtx=<address>");
     hidden_args.emplace_back("-zmqpubrawblock=<address>");
     hidden_args.emplace_back("-zmqpubrawtx=<address>");
+    hidden_args.emplace_back("-zmqpubsequence=<n>");
     hidden_args.emplace_back("-zmqpubhashblockhwm=<n>");
     hidden_args.emplace_back("-zmqpubhashtxhwm=<n>");
     hidden_args.emplace_back("-zmqpubrawblockhwm=<n>");
     hidden_args.emplace_back("-zmqpubrawtxhwm=<n>");
+    hidden_args.emplace_back("-zmqpubsequencehwm=<n>");
 #endif
 
     argsman.AddArg("-checkblocks=<n>", strprintf("How many blocks to check at startup (default: %u, 0 = all)", DEFAULT_CHECKBLOCKS), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::DEBUG_TEST);
diff --git a/src/interfaces/chain.cpp b/src/interfaces/chain.cpp
index 13d885a20d..4c5ebe66fc 100644
--- a/src/interfaces/chain.cpp
+++ b/src/interfaces/chain.cpp
@@ -59,13 +59,13 @@ public:
     explicit NotificationsProxy(std::shared_ptr<Chain::Notifications> notifications)
         : m_notifications(std::move(notifications)) {}
     virtual ~NotificationsProxy() = default;
-    void TransactionAddedToMempool(const CTransactionRef& tx) override
+    void TransactionAddedToMempool(const CTransactionRef& tx, uint64_t mempool_sequence) override
     {
-        m_notifications->transactionAddedToMempool(tx);
+        m_notifications->transactionAddedToMempool(tx, mempool_sequence);
     }
-    void TransactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) override
+    void TransactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason, uint64_t mempool_sequence) override
     {
-        m_notifications->transactionRemovedFromMempool(tx, reason);
+        m_notifications->transactionRemovedFromMempool(tx, reason, mempool_sequence);
     }
     void BlockConnected(const std::shared_ptr<const CBlock>& block, const CBlockIndex* index) override
     {
@@ -405,7 +405,7 @@ public:
         if (!m_node.mempool) return;
         LOCK2(::cs_main, m_node.mempool->cs);
         for (const CTxMemPoolEntry& entry : m_node.mempool->mapTx) {
-            notifications.transactionAddedToMempool(entry.GetSharedTx());
+            notifications.transactionAddedToMempool(entry.GetSharedTx(), 0 /* mempool_sequence */);
         }
     }
     NodeContext& m_node;
diff --git a/src/interfaces/chain.h b/src/interfaces/chain.h
index 6e50ccb27a..85d09be0f3 100644
--- a/src/interfaces/chain.h
+++ b/src/interfaces/chain.h
@@ -242,8 +242,8 @@ public:
     {
     public:
         virtual ~Notifications() {}
-        virtual void transactionAddedToMempool(const CTransactionRef& tx) {}
-        virtual void transactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) {}
+        virtual void transactionAddedToMempool(const CTransactionRef& tx, uint64_t mempool_sequence) {}
+        virtual void transactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason, uint64_t mempool_sequence) {}
         virtual void blockConnected(const CBlock& block, int height) {}
         virtual void blockDisconnected(const CBlock& block, int height) {}
         virtual void updatedBlockTip() {}
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
index 9b464bee72..0bb7342db0 100644
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -500,9 +500,12 @@ static void entryToJSON(const CTxMemPool& pool, UniValue& info, const CTxMemPool
     info.pushKV("unbroadcast", pool.IsUnbroadcastTx(tx.GetHash()));
 }
 
-UniValue MempoolToJSON(const CTxMemPool& pool, bool verbose)
+UniValue MempoolToJSON(const CTxMemPool& pool, bool verbose, bool include_mempool_sequence)
 {
     if (verbose) {
+        if (include_mempool_sequence) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Verbose results cannot contain mempool sequence values.");
+        }
         LOCK(pool.cs);
         UniValue o(UniValue::VOBJ);
         for (const CTxMemPoolEntry& e : pool.mapTx) {
@@ -516,14 +519,25 @@ UniValue MempoolToJSON(const CTxMemPool& pool, bool verbose)
         }
         return o;
     } else {
+        uint64_t mempool_sequence;
         std::vector<uint256> vtxid;
-        pool.queryHashes(vtxid);
-
+        {
+            LOCK(pool.cs);
+            pool.queryHashes(vtxid);
+            mempool_sequence = pool.GetSequence();
+        }
         UniValue a(UniValue::VARR);
         for (const uint256& hash : vtxid)
             a.push_back(hash.ToString());
 
-        return a;
+        if (!include_mempool_sequence) {
+            return a;
+        } else {
+            UniValue o(UniValue::VOBJ);
+            o.pushKV("txids", a);
+            o.pushKV("mempool_sequence", mempool_sequence);
+            return o;
+        }
     }
 }
 
@@ -534,6 +548,7 @@ static RPCHelpMan getrawmempool()
                 "\nHint: use getmempoolentry to fetch a specific transaction from the mempool.\n",
                 {
                     {"verbose", RPCArg::Type::BOOL, /* default */ "false", "True for a json object, false for array of transaction ids"},
+                    {"mempool_sequence", RPCArg::Type::BOOL, /* default */ "false", "If verbose=false, returns a json object with transaction list and mempool sequence number attached."},
                 },
                 {
                     RPCResult{"for verbose = false",
@@ -546,6 +561,15 @@ static RPCHelpMan getrawmempool()
                         {
                             {RPCResult::Type::OBJ, "transactionid", "", MempoolEntryDescription()},
                         }},
+                    RPCResult{"for verbose = false and mempool_sequence = true",
+                        RPCResult::Type::OBJ, "", "",
+                        {
+                            {RPCResult::Type::ARR, "txids", "",
+                            {
+                                {RPCResult::Type::STR_HEX, "", "The transaction id"},
+                            }},
+                            {RPCResult::Type::NUM, "mempool_sequence", "The mempool sequence value."},
+                        }},
                 },
                 RPCExamples{
                     HelpExampleCli("getrawmempool", "true")
@@ -557,7 +581,12 @@ static RPCHelpMan getrawmempool()
     if (!request.params[0].isNull())
         fVerbose = request.params[0].get_bool();
 
-    return MempoolToJSON(EnsureMemPool(request.context), fVerbose);
+    bool include_mempool_sequence = false;
+    if (!request.params[1].isNull()) {
+        include_mempool_sequence = request.params[1].get_bool();
+    }
+
+    return MempoolToJSON(EnsureMemPool(request.context), fVerbose, include_mempool_sequence);
 },
     };
 }
@@ -2451,7 +2480,7 @@ static const CRPCCommand commands[] =
     { "blockchain",         "getmempooldescendants",  &getmempooldescendants,  {"txid","verbose"} },
     { "blockchain",         "getmempoolentry",        &getmempoolentry,        {"txid"} },
     { "blockchain",         "getmempoolinfo",         &getmempoolinfo,         {} },
-    { "blockchain",         "getrawmempool",          &getrawmempool,          {"verbose"} },
+    { "blockchain",         "getrawmempool",          &getrawmempool,          {"verbose", "mempool_sequence"} },
     { "blockchain",         "gettxout",               &gettxout,               {"txid","n","include_mempool"} },
     { "blockchain",         "gettxoutsetinfo",        &gettxoutsetinfo,        {"hash_type"} },
     { "blockchain",         "pruneblockchain",        &pruneblockchain,        {"height"} },
diff --git a/src/rpc/blockchain.h b/src/rpc/blockchain.h
index 5c9a43b13e..5b362bf211 100644
--- a/src/rpc/blockchain.h
+++ b/src/rpc/blockchain.h
@@ -43,7 +43,7 @@ UniValue blockToJSON(const CBlock& block, const CBlockIndex* tip, const CBlockIn
 UniValue MempoolInfoToJSON(const CTxMemPool& pool);
 
 /** Mempool to JSON */
-UniValue MempoolToJSON(const CTxMemPool& pool, bool verbose = false);
+UniValue MempoolToJSON(const CTxMemPool& pool, bool verbose = false, bool include_mempool_sequence = false);
 
 /** Block header to JSON */
 UniValue blockheaderToJSON(const CBlockIndex* tip, const CBlockIndex* blockindex) LOCKS_EXCLUDED(cs_main);
diff --git a/src/rpc/client.cpp b/src/rpc/client.cpp
index 6ef3294132..3c432464f2 100644
--- a/src/rpc/client.cpp
+++ b/src/rpc/client.cpp
@@ -142,6 +142,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "pruneblockchain", 0, "height" },
     { "keypoolrefill", 0, "newsize" },
     { "getrawmempool", 0, "verbose" },
+    { "getrawmempool", 1, "mempool_sequence" },
     { "estimatesmartfee", 0, "conf_target" },
     { "estimaterawfee", 0, "conf_target" },
     { "estimaterawfee", 1, "threshold" },
diff --git a/src/txmempool.cpp b/src/txmempool.cpp
index 6a525c97db..0c2b731967 100644
--- a/src/txmempool.cpp
+++ b/src/txmempool.cpp
@@ -409,12 +409,16 @@ void CTxMemPool::addUnchecked(const CTxMemPoolEntry &entry, setEntries &setAnces
 
 void CTxMemPool::removeUnchecked(txiter it, MemPoolRemovalReason reason)
 {
+    // We increment mempool sequence value no matter removal reason
+    // even if not directly reported below.
+    uint64_t mempool_sequence = GetAndIncrementSequence();
+
     if (reason != MemPoolRemovalReason::BLOCK) {
         // Notify clients that a transaction has been removed from the mempool
         // for any reason except being included in a block. Clients interested
         // in transactions included in blocks can subscribe to the BlockConnected
         // notification.
-        GetMainSignals().TransactionRemovedFromMempool(it->GetSharedTx(), reason);
+        GetMainSignals().TransactionRemovedFromMempool(it->GetSharedTx(), reason, mempool_sequence);
     }
 
     const uint256 hash = it->GetTx().GetHash();
diff --git a/src/txmempool.h b/src/txmempool.h
index 664fb5986a..f513f14af6 100644
--- a/src/txmempool.h
+++ b/src/txmempool.h
@@ -501,6 +501,11 @@ private:
     mutable uint64_t m_epoch;
     mutable bool m_has_epoch_guard;
 
+    // In-memory counter for external mempool tracking purposes.
+    // This number is incremented once every time a transaction
+    // is added or removed from the mempool for any reason.
+    mutable uint64_t m_sequence_number{1};
+
     void trackPackageRemoved(const CFeeRate& rate) EXCLUSIVE_LOCKS_REQUIRED(cs);
 
     bool m_is_loaded GUARDED_BY(cs){false};
@@ -776,6 +781,15 @@ public:
         return m_unbroadcast_txids.count(txid) != 0;
     }
 
+    /** Guards this internal counter for external reporting */
+    uint64_t GetAndIncrementSequence() const EXCLUSIVE_LOCKS_REQUIRED(cs) {
+        return m_sequence_number++;
+    }
+
+    uint64_t GetSequence() const EXCLUSIVE_LOCKS_REQUIRED(cs) {
+        return m_sequence_number;
+    }
+
 private:
     /** UpdateForDescendants is used by UpdateTransactionsFromBlock to update
      *  the descendants for a single transaction that has been added to the
diff --git a/src/validation.cpp b/src/validation.cpp
index a96913e3a0..7020b59cb8 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -1058,7 +1058,7 @@ bool MemPoolAccept::AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs
 
     if (!Finalize(args, workspace)) return false;
 
-    GetMainSignals().TransactionAddedToMempool(ptx);
+    GetMainSignals().TransactionAddedToMempool(ptx, m_pool.GetAndIncrementSequence());
 
     return true;
 }
diff --git a/src/validationinterface.cpp b/src/validationinterface.cpp
index 3dfbcc581c..1e07ff23ae 100644
--- a/src/validationinterface.cpp
+++ b/src/validationinterface.cpp
@@ -199,18 +199,18 @@ void CMainSignals::UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockInd
                           fInitialDownload);
 }
 
-void CMainSignals::TransactionAddedToMempool(const CTransactionRef& tx) {
-    auto event = [tx, this] {
-        m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.TransactionAddedToMempool(tx); });
+void CMainSignals::TransactionAddedToMempool(const CTransactionRef& tx, uint64_t mempool_sequence) {
+    auto event = [tx, mempool_sequence, this] {
+        m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.TransactionAddedToMempool(tx, mempool_sequence); });
     };
     ENQUEUE_AND_LOG_EVENT(event, "%s: txid=%s wtxid=%s", __func__,
                           tx->GetHash().ToString(),
                           tx->GetWitnessHash().ToString());
 }
 
-void CMainSignals::TransactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) {
-    auto event = [tx, reason, this] {
-        m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.TransactionRemovedFromMempool(tx, reason); });
+void CMainSignals::TransactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason, uint64_t mempool_sequence) {
+    auto event = [tx, reason, mempool_sequence, this] {
+        m_internals->Iterate([&](CValidationInterface& callbacks) { callbacks.TransactionRemovedFromMempool(tx, reason, mempool_sequence); });
     };
     ENQUEUE_AND_LOG_EVENT(event, "%s: txid=%s wtxid=%s", __func__,
                           tx->GetHash().ToString(),
diff --git a/src/validationinterface.h b/src/validationinterface.h
index e96f2883fc..7c3ce00fbc 100644
--- a/src/validationinterface.h
+++ b/src/validationinterface.h
@@ -97,7 +97,8 @@ protected:
      *
      * Called on a background thread.
      */
-    virtual void TransactionAddedToMempool(const CTransactionRef& tx) {}
+    virtual void TransactionAddedToMempool(const CTransactionRef& tx, uint64_t mempool_sequence) {}
+
     /**
      * Notifies listeners of a transaction leaving mempool.
      *
@@ -130,7 +131,7 @@ protected:
      *
      * Called on a background thread.
      */
-    virtual void TransactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) {}
+    virtual void TransactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason, uint64_t mempool_sequence) {}
     /**
      * Notifies listeners of a block being connected.
      * Provides a vector of transactions evicted from the mempool as a result.
@@ -197,8 +198,8 @@ public:
 
 
     void UpdatedBlockTip(const CBlockIndex *, const CBlockIndex *, bool fInitialDownload);
-    void TransactionAddedToMempool(const CTransactionRef&);
-    void TransactionRemovedFromMempool(const CTransactionRef&, MemPoolRemovalReason);
+    void TransactionAddedToMempool(const CTransactionRef&, uint64_t mempool_sequence);
+    void TransactionRemovedFromMempool(const CTransactionRef&, MemPoolRemovalReason, uint64_t mempool_sequence);
     void BlockConnected(const std::shared_ptr<const CBlock> &, const CBlockIndex *pindex);
     void BlockDisconnected(const std::shared_ptr<const CBlock> &, const CBlockIndex* pindex);
     void ChainStateFlushed(const CBlockLocator &);
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 73e11a5b52..66857dbb39 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -1177,7 +1177,7 @@ void CWallet::SyncTransaction(const CTransactionRef& ptx, CWalletTx::Confirmatio
     MarkInputsDirty(ptx);
 }
 
-void CWallet::transactionAddedToMempool(const CTransactionRef& tx) {
+void CWallet::transactionAddedToMempool(const CTransactionRef& tx, uint64_t mempool_sequence) {
     LOCK(cs_wallet);
     SyncTransaction(tx, {CWalletTx::Status::UNCONFIRMED, /* block height */ 0, /* block hash */ {}, /* index */ 0});
 
@@ -1187,7 +1187,7 @@ void CWallet::transactionAddedToMempool(const CTransactionRef& tx) {
     }
 }
 
-void CWallet::transactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) {
+void CWallet::transactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason, uint64_t mempool_sequence) {
     LOCK(cs_wallet);
     auto it = mapWallet.find(tx->GetHash());
     if (it != mapWallet.end()) {
@@ -1234,7 +1234,7 @@ void CWallet::blockConnected(const CBlock& block, int height)
     m_last_block_processed = block_hash;
     for (size_t index = 0; index < block.vtx.size(); index++) {
         SyncTransaction(block.vtx[index], {CWalletTx::Status::CONFIRMED, height, block_hash, (int)index});
-        transactionRemovedFromMempool(block.vtx[index], MemPoolRemovalReason::BLOCK);
+        transactionRemovedFromMempool(block.vtx[index], MemPoolRemovalReason::BLOCK, 0 /* mempool_sequence */);
     }
 }
 
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index c54480612a..f15712dd0e 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -900,7 +900,7 @@ public:
 
     CWalletTx* AddToWallet(CTransactionRef tx, const CWalletTx::Confirmation& confirm, const UpdateWalletTxFn& update_wtx=nullptr, bool fFlushOnClose=true);
     bool LoadToWallet(const uint256& hash, const UpdateWalletTxFn& fill_wtx) EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
-    void transactionAddedToMempool(const CTransactionRef& tx) override;
+    void transactionAddedToMempool(const CTransactionRef& tx, uint64_t mempool_sequence) override;
     void blockConnected(const CBlock& block, int height) override;
     void blockDisconnected(const CBlock& block, int height) override;
     void updatedBlockTip() override;
@@ -922,7 +922,7 @@ public:
         uint256 last_failed_block;
     };
     ScanResult ScanForWalletTransactions(const uint256& start_block, int start_height, Optional<int> max_height, const WalletRescanReserver& reserver, bool fUpdate);
-    void transactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason) override;
+    void transactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason, uint64_t mempool_sequence) override;
     void ReacceptWalletTransactions() EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
     void ResendWalletTransactions();
     struct Balance {
diff --git a/src/zmq/zmqabstractnotifier.cpp b/src/zmq/zmqabstractnotifier.cpp
index 0d0428f3c0..3938f6fd2c 100644
--- a/src/zmq/zmqabstractnotifier.cpp
+++ b/src/zmq/zmqabstractnotifier.cpp
@@ -22,3 +22,23 @@ bool CZMQAbstractNotifier::NotifyTransaction(const CTransaction &/*transaction*/
 {
     return true;
 }
+
+bool CZMQAbstractNotifier::NotifyBlockConnect(const CBlockIndex * /*CBlockIndex*/)
+{
+    return true;
+}
+
+bool CZMQAbstractNotifier::NotifyBlockDisconnect(const CBlockIndex * /*CBlockIndex*/)
+{
+    return true;
+}
+
+bool CZMQAbstractNotifier::NotifyTransactionAcceptance(const CTransaction &/*transaction*/, uint64_t mempool_sequence)
+{
+    return true;
+}
+
+bool CZMQAbstractNotifier::NotifyTransactionRemoval(const CTransaction &/*transaction*/, uint64_t mempool_sequence)
+{
+    return true;
+}
diff --git a/src/zmq/zmqabstractnotifier.h b/src/zmq/zmqabstractnotifier.h
index 34d7e5ef03..dddba8d6b6 100644
--- a/src/zmq/zmqabstractnotifier.h
+++ b/src/zmq/zmqabstractnotifier.h
@@ -44,7 +44,17 @@ public:
     virtual bool Initialize(void *pcontext) = 0;
     virtual void Shutdown() = 0;
 
+    // Notifies of ConnectTip result, i.e., new active tip only
     virtual bool NotifyBlock(const CBlockIndex *pindex);
+    // Notifies of every block connection
+    virtual bool NotifyBlockConnect(const CBlockIndex *pindex);
+    // Notifies of every block disconnection
+    virtual bool NotifyBlockDisconnect(const CBlockIndex *pindex);
+    // Notifies of every mempool acceptance
+    virtual bool NotifyTransactionAcceptance(const CTransaction &transaction, uint64_t mempool_sequence);
+    // Notifies of every mempool removal, except inclusion in blocks
+    virtual bool NotifyTransactionRemoval(const CTransaction &transaction, uint64_t mempool_sequence);
+    // Notifies of transactions added to mempool or appearing in blocks
     virtual bool NotifyTransaction(const CTransaction &transaction);
 
 protected:
diff --git a/src/zmq/zmqnotificationinterface.cpp b/src/zmq/zmqnotificationinterface.cpp
index a22772baed..a7e9a34269 100644
--- a/src/zmq/zmqnotificationinterface.cpp
+++ b/src/zmq/zmqnotificationinterface.cpp
@@ -36,6 +36,7 @@ CZMQNotificationInterface* CZMQNotificationInterface::Create()
     factories["pubhashtx"] = CZMQAbstractNotifier::Create<CZMQPublishHashTransactionNotifier>;
     factories["pubrawblock"] = CZMQAbstractNotifier::Create<CZMQPublishRawBlockNotifier>;
     factories["pubrawtx"] = CZMQAbstractNotifier::Create<CZMQPublishRawTransactionNotifier>;
+    factories["pubsequence"] = CZMQAbstractNotifier::Create<CZMQPublishSequenceNotifier>;
 
     std::list<std::unique_ptr<CZMQAbstractNotifier>> notifiers;
     for (const auto& entry : factories)
@@ -140,31 +141,53 @@ void CZMQNotificationInterface::UpdatedBlockTip(const CBlockIndex *pindexNew, co
     });
 }
 
-void CZMQNotificationInterface::TransactionAddedToMempool(const CTransactionRef& ptx)
+void CZMQNotificationInterface::TransactionAddedToMempool(const CTransactionRef& ptx, uint64_t mempool_sequence)
 {
-    // Used by BlockConnected and BlockDisconnected as well, because they're
-    // all the same external callback.
     const CTransaction& tx = *ptx;
 
-    TryForEachAndRemoveFailed(notifiers, [&tx](CZMQAbstractNotifier* notifier) {
-        return notifier->NotifyTransaction(tx);
+    TryForEachAndRemoveFailed(notifiers, [&tx, mempool_sequence](CZMQAbstractNotifier* notifier) {
+        return notifier->NotifyTransaction(tx) && notifier->NotifyTransactionAcceptance(tx, mempool_sequence);
+    });
+}
+
+void CZMQNotificationInterface::TransactionRemovedFromMempool(const CTransactionRef& ptx, MemPoolRemovalReason reason, uint64_t mempool_sequence)
+{
+    // Called for all non-block inclusion reasons
+    const CTransaction& tx = *ptx;
+
+    TryForEachAndRemoveFailed(notifiers, [&tx, mempool_sequence](CZMQAbstractNotifier* notifier) {
+        return notifier->NotifyTransactionRemoval(tx, mempool_sequence);
     });
 }
 
 void CZMQNotificationInterface::BlockConnected(const std::shared_ptr<const CBlock>& pblock, const CBlockIndex* pindexConnected)
 {
     for (const CTransactionRef& ptx : pblock->vtx) {
-        // Do a normal notify for each transaction added in the block
-        TransactionAddedToMempool(ptx);
+        const CTransaction& tx = *ptx;
+        TryForEachAndRemoveFailed(notifiers, [&tx](CZMQAbstractNotifier* notifier) {
+            return notifier->NotifyTransaction(tx);
+        });
     }
+
+    // Next we notify BlockConnect listeners for *all* blocks
+    TryForEachAndRemoveFailed(notifiers, [pindexConnected](CZMQAbstractNotifier* notifier) {
+        return notifier->NotifyBlockConnect(pindexConnected);
+    });
 }
 
 void CZMQNotificationInterface::BlockDisconnected(const std::shared_ptr<const CBlock>& pblock, const CBlockIndex* pindexDisconnected)
 {
     for (const CTransactionRef& ptx : pblock->vtx) {
-        // Do a normal notify for each transaction removed in block disconnection
-        TransactionAddedToMempool(ptx);
+        const CTransaction& tx = *ptx;
+        TryForEachAndRemoveFailed(notifiers, [&tx](CZMQAbstractNotifier* notifier) {
+            return notifier->NotifyTransaction(tx);
+        });
     }
+
+    // Next we notify BlockDisconnect listeners for *all* blocks
+    TryForEachAndRemoveFailed(notifiers, [pindexDisconnected](CZMQAbstractNotifier* notifier) {
+        return notifier->NotifyBlockDisconnect(pindexDisconnected);
+    });
 }
 
 CZMQNotificationInterface* g_zmq_notification_interface = nullptr;
diff --git a/src/zmq/zmqnotificationinterface.h b/src/zmq/zmqnotificationinterface.h
index 0686960ed4..788a383517 100644
--- a/src/zmq/zmqnotificationinterface.h
+++ b/src/zmq/zmqnotificationinterface.h
@@ -26,7 +26,8 @@ protected:
     void Shutdown();
 
     // CValidationInterface
-    void TransactionAddedToMempool(const CTransactionRef& tx) override;
+    void TransactionAddedToMempool(const CTransactionRef& tx, uint64_t mempool_sequence) override;
+    void TransactionRemovedFromMempool(const CTransactionRef& tx, MemPoolRemovalReason reason, uint64_t mempool_sequence) override;
     void BlockConnected(const std::shared_ptr<const CBlock>& pblock, const CBlockIndex* pindexConnected) override;
     void BlockDisconnected(const std::shared_ptr<const CBlock>& pblock, const CBlockIndex* pindexDisconnected) override;
     void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) override;
diff --git a/src/zmq/zmqpublishnotifier.cpp b/src/zmq/zmqpublishnotifier.cpp
index d4d21b05ba..a0e7a0a600 100644
--- a/src/zmq/zmqpublishnotifier.cpp
+++ b/src/zmq/zmqpublishnotifier.cpp
@@ -26,6 +26,7 @@ static const char *MSG_HASHBLOCK = "hashblock";
 static const char *MSG_HASHTX    = "hashtx";
 static const char *MSG_RAWBLOCK  = "rawblock";
 static const char *MSG_RAWTX     = "rawtx";
+static const char *MSG_SEQUENCE  = "sequence";
 
 // Internal function to send multipart message
 static int zmq_send_multipart(void *sock, const void* data, size_t size, ...)
@@ -225,3 +226,51 @@ bool CZMQPublishRawTransactionNotifier::NotifyTransaction(const CTransaction &tr
     ss << transaction;
     return SendZmqMessage(MSG_RAWTX, &(*ss.begin()), ss.size());
 }
+
+
+// TODO: Dedup this code to take label char, log string
+bool CZMQPublishSequenceNotifier::NotifyBlockConnect(const CBlockIndex *pindex)
+{
+    uint256 hash = pindex->GetBlockHash();
+    LogPrint(BCLog::ZMQ, "zmq: Publish sequence block connect %s\n", hash.GetHex());
+    char data[sizeof(uint256)+1];
+    for (unsigned int i = 0; i < sizeof(uint256); i++)
+        data[sizeof(uint256) - 1 - i] = hash.begin()[i];
+    data[sizeof(data) - 1] = 'C'; // Block (C)onnect
+    return SendZmqMessage(MSG_SEQUENCE, data, sizeof(data));
+}
+
+bool CZMQPublishSequenceNotifier::NotifyBlockDisconnect(const CBlockIndex *pindex)
+{
+    uint256 hash = pindex->GetBlockHash();
+    LogPrint(BCLog::ZMQ, "zmq: Publish sequence block disconnect %s\n", hash.GetHex());
+    char data[sizeof(uint256)+1];
+    for (unsigned int i = 0; i < sizeof(uint256); i++)
+        data[sizeof(uint256) - 1 - i] = hash.begin()[i];
+    data[sizeof(data) - 1] = 'D'; // Block (D)isconnect
+    return SendZmqMessage(MSG_SEQUENCE, data, sizeof(data));
+}
+
+bool CZMQPublishSequenceNotifier::NotifyTransactionAcceptance(const CTransaction &transaction, uint64_t mempool_sequence)
+{
+    uint256 hash = transaction.GetHash();
+    LogPrint(BCLog::ZMQ, "zmq: Publish hashtx mempool acceptance %s\n", hash.GetHex());
+    unsigned char data[sizeof(uint256)+sizeof(mempool_sequence)+1];
+    for (unsigned int i = 0; i < sizeof(uint256); i++)
+        data[sizeof(uint256) - 1 - i] = hash.begin()[i];
+    data[sizeof(uint256)] = 'A'; // Mempool (A)cceptance
+    WriteLE64(data+sizeof(uint256)+1, mempool_sequence);
+    return SendZmqMessage(MSG_SEQUENCE, data, sizeof(data));
+}
+
+bool CZMQPublishSequenceNotifier::NotifyTransactionRemoval(const CTransaction &transaction, uint64_t mempool_sequence)
+{
+    uint256 hash = transaction.GetHash();
+    LogPrint(BCLog::ZMQ, "zmq: Publish hashtx mempool removal %s\n", hash.GetHex());
+    unsigned char data[sizeof(uint256)+sizeof(mempool_sequence)+1];
+    for (unsigned int i = 0; i < sizeof(uint256); i++)
+        data[sizeof(uint256) - 1 - i] = hash.begin()[i];
+    data[sizeof(uint256)] = 'R'; // Mempool (R)emoval
+    WriteLE64(data+sizeof(uint256)+1, mempool_sequence);
+    return SendZmqMessage(MSG_SEQUENCE, data, sizeof(data));
+}
diff --git a/src/zmq/zmqpublishnotifier.h b/src/zmq/zmqpublishnotifier.h
index eb9ae881be..f13ed6f537 100644
--- a/src/zmq/zmqpublishnotifier.h
+++ b/src/zmq/zmqpublishnotifier.h
@@ -52,4 +52,13 @@ public:
     bool NotifyTransaction(const CTransaction &transaction) override;
 };
 
+class CZMQPublishSequenceNotifier : public CZMQAbstractPublishNotifier
+{
+public:
+    bool NotifyBlockConnect(const CBlockIndex *pindex) override;
+    bool NotifyBlockDisconnect(const CBlockIndex *pindex) override;
+    bool NotifyTransactionAcceptance(const CTransaction &transaction, uint64_t mempool_sequence) override;
+    bool NotifyTransactionRemoval(const CTransaction &transaction, uint64_t mempool_sequence) override;
+};
+
 #endif // BITCOIN_ZMQ_ZMQPUBLISHNOTIFIER_H