diff --git a/src/txrequest.h b/src/txrequest.h
--- a/src/txrequest.h
+++ b/src/txrequest.h
@@ -117,9 +117,47 @@
  *   announcements, plus the number of announcements affected by an operation
  *   (amortized O(1) per announcement).
  */
+
+// Avoid littering this header file with implementation details.
+class Impl {
+    friend class TxRequestTracker;
+
+    // The base class is responsible for building the child implementation.
+    // This is a hack that allows for hiding the concrete implementation details
+    // from the callsite.
+    static std::unique_ptr<Impl> BuildImpl(bool deterministic);
+
+protected:
+    using clearExpiredFun = const std::function<void()> &;
+    using emplaceExpiredFun =
+        const std::function<void(const NodeId &, const uint256 &)> &;
+
+    virtual void ReceivedInv(NodeId peer, const uint256 &txid, bool preferred,
+                             std::chrono::microseconds reqtime) = 0;
+    virtual void DisconnectedPeer(NodeId peer) = 0;
+    virtual void ForgetTxId(const uint256 &txid) = 0;
+    virtual std::vector<uint256>
+    GetRequestable(NodeId peer, std::chrono::microseconds now,
+                   clearExpiredFun clearExpired,
+                   emplaceExpiredFun emplaceExpired) = 0;
+    virtual void RequestedTx(NodeId peer, const uint256 &txid,
+                             std::chrono::microseconds expiry) = 0;
+    virtual void ReceivedResponse(NodeId peer, const uint256 &txid) = 0;
+    virtual size_t CountInFlight(NodeId peer) const = 0;
+    virtual size_t CountCandidates(NodeId peer) const = 0;
+    virtual size_t Count(NodeId peer) const = 0;
+    virtual size_t Size() const = 0;
+    virtual uint64_t ComputePriority(const uint256 &txid, NodeId peer,
+                                     bool preferred) const = 0;
+    virtual void SanityCheck() const = 0;
+    virtual void
+    PostGetRequestableSanityCheck(std::chrono::microseconds now) const = 0;
+
+public:
+    virtual ~Impl() = default;
+};
+
 class TxRequestTracker {
-    // Avoid littering this header file with implementation details.
-    class Impl;
     const std::unique_ptr<Impl> m_impl;
 
 public:
diff --git a/src/txrequest.cpp b/src/txrequest.cpp
--- a/src/txrequest.cpp
+++ b/src/txrequest.cpp
@@ -351,14 +351,10 @@
     return ret;
 }
 
-using clearExpiredFun = const std::function<void()> &;
-using emplaceExpiredFun =
-    const std::function<void(const NodeId &, const uint256 &)> &;
-
 } // namespace
 
 /** Actual implementation for TxRequestTracker's data structure. */
-class TxRequestTracker::Impl {
+class ImplConcrete : public Impl {
     //! The current sequence number. Increases for every announcement. This is
     //! used to sort txid returned by GetRequestable in announcement order.
     SequenceNumber m_current_sequence{0};
@@ -624,7 +620,7 @@
     }
 
 public:
-    Impl(bool deterministic)
+    ImplConcrete(bool deterministic)
         : m_computer(deterministic),
           // Explicitly initialize m_index as we need to pass a reference to
           // m_computer to ByTxHashViewExtractor.
@@ -637,8 +633,8 @@
 
     // Disable copying and assigning (a default copy won't work due the stateful
     // ByTxIdViewExtractor).
-    Impl(const Impl &) = delete;
-    Impl &operator=(const Impl &) = delete;
+    ImplConcrete(const ImplConcrete &) = delete;
+    ImplConcrete &operator=(const ImplConcrete &) = delete;
 
     void DisconnectedPeer(NodeId peer) {
         auto &index = m_index.get<ByPeer>();
@@ -861,8 +857,12 @@
     }
 };
 
+std::unique_ptr<Impl> Impl::BuildImpl(bool deterministic) {
+    return std::unique_ptr<Impl>(new ImplConcrete(deterministic));
+}
+
 TxRequestTracker::TxRequestTracker(bool deterministic)
-    : m_impl{std::make_unique<TxRequestTracker::Impl>(deterministic)} {}
+    : m_impl{Impl::BuildImpl(deterministic)} {}
 
 TxRequestTracker::~TxRequestTracker() = default;
 
@@ -911,13 +911,13 @@
 std::vector<TxId> TxRequestTracker::GetRequestable(
     NodeId peer, std::chrono::microseconds now,
     std::vector<std::pair<NodeId, TxId>> *expired) {
-    clearExpiredFun clearExpired = [expired]() {
+    Impl::clearExpiredFun clearExpired = [expired]() {
         if (expired) {
             expired->clear();
         }
     };
-    emplaceExpiredFun emplaceExpired = [expired](const NodeId &nodeid,
-                                                 const uint256 &txid) {
+    Impl::emplaceExpiredFun emplaceExpired = [expired](const NodeId &nodeid,
+                                                       const uint256 &txid) {
         if (expired) {
             expired->emplace_back(nodeid, TxId(txid));
         }