diff --git a/services/mediametrics/Android.bp b/services/mediametrics/Android.bp
index 5989181..0351d2d 100644
--- a/services/mediametrics/Android.bp
+++ b/services/mediametrics/Android.bp
@@ -148,7 +148,8 @@
         "statsd_mediaparser.cpp",
         "statsd_nuplayer.cpp",
         "statsd_recorder.cpp",
-        "StringUtils.cpp"
+        "StringUtils.cpp",
+        "ValidateId.cpp",
     ],
 
     proto: {
diff --git a/services/mediametrics/AudioAnalytics.cpp b/services/mediametrics/AudioAnalytics.cpp
index 11ec993..a0dcb55 100644
--- a/services/mediametrics/AudioAnalytics.cpp
+++ b/services/mediametrics/AudioAnalytics.cpp
@@ -28,6 +28,7 @@
 #include "AudioTypes.h"           // string to int conversions
 #include "MediaMetricsService.h"  // package info
 #include "StringUtils.h"
+#include "ValidateId.h"
 
 #define PROP_AUDIO_ANALYTICS_CLOUD_ENABLED "persist.audio.analytics.cloud.enabled"
 
@@ -562,7 +563,7 @@
         const auto flagsForStats = types::lookup<types::INPUT_FLAG, short_enum_type_t>(flags);
         const auto sourceForStats = types::lookup<types::SOURCE_TYPE, short_enum_type_t>(source);
         // Android S
-        const auto logSessionIdForStats = stringutils::sanitizeLogSessionId(logSessionId);
+        const auto logSessionIdForStats = ValidateId::get()->validateId(logSessionId);
 
         LOG(LOG_LEVEL) << "key:" << key
               << " id:" << id
@@ -717,7 +718,7 @@
                  types::lookup<types::TRACK_TRAITS, short_enum_type_t>(traits);
         const auto usageForStats = types::lookup<types::USAGE, short_enum_type_t>(usage);
         // Android S
-        const auto logSessionIdForStats = stringutils::sanitizeLogSessionId(logSessionId);
+        const auto logSessionIdForStats = ValidateId::get()->validateId(logSessionId);
 
         LOG(LOG_LEVEL) << "key:" << key
               << " id:" << id
diff --git a/services/mediametrics/LruSet.h b/services/mediametrics/LruSet.h
new file mode 100644
index 0000000..1f0ab60
--- /dev/null
+++ b/services/mediametrics/LruSet.h
@@ -0,0 +1,123 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <list>
+#include <sstream>
+#include <unordered_map>
+
+namespace android::mediametrics {
+
+/**
+ * LruSet keeps a set of the last "Size" elements added or accessed.
+ *
+ * (Lru stands for least-recently-used eviction policy).
+ *
+ * Runs in O(1) time for add, remove, and check.  Internally implemented
+ * with an unordered_map and a list.  In order to remove elements,
+ * a list iterator is stored in the unordered_map
+ * (noting that std::list::erase() contractually
+ * does not affect iterators other than the one erased).
+ */
+
+template <typename T>
+class LruSet {
+    const size_t mMaxSize;
+    std::list<T> mAccessOrder;                 // front is the most recent, back is the oldest.
+    // item T with its access order iterator.
+    std::unordered_map<T, typename std::list<T>::iterator> mMap;
+
+public:
+    /**
+     * Constructs a LruSet which checks whether the element was
+     * accessed or added recently.
+     *
+     * The parameter maxSize is used to cap growth of LruSet;
+     * eviction is based on least recently used LRU.
+     * If maxSize is zero, the LruSet contains no elements
+     * and check() always returns false.
+     *
+     * \param maxSize the maximum number of elements that are tracked.
+     */
+    explicit LruSet(size_t maxSize) : mMaxSize(maxSize) {}
+
+    /**
+     * Returns the number of entries in the LruSet.
+     *
+     * This is a number between 0 and maxSize.
+     */
+    size_t size() const {
+        return mMap.size();
+    }
+
+    /** Clears the container contents. */
+    void clear() {
+        mMap.clear();
+        mAccessOrder.clear();
+    }
+
+    /** Returns a string dump of the last n entries. */
+    std::string dump(size_t n) const {
+        std::stringstream ss;
+        auto it = mAccessOrder.cbegin();
+        for (size_t i = 0; i < n && it != mAccessOrder.cend(); ++i) {
+            ss << *it++ << "\n";
+        }
+        return ss.str();
+    }
+
+    /** Adds a new item to the set. */
+    void add(const T& t) {
+        if (mMaxSize == 0) return;
+        auto it = mMap.find(t);
+        if (it != mMap.end()) { // already exists.
+            mAccessOrder.erase(it->second);  // move-to-front on the chronologically ordered list.
+        } else if (mAccessOrder.size() >= mMaxSize) {
+            const T last = mAccessOrder.back();
+            mAccessOrder.pop_back();
+            mMap.erase(last);
+        }
+        mAccessOrder.push_front(t);
+        mMap[t] = mAccessOrder.begin();
+    }
+
+    /**
+     * Removes an item from the set.
+     *
+     * \param t item to be removed.
+     * \return false if the item doesn't exist.
+     */
+    bool remove(const T& t) {
+        auto it = mMap.find(t);
+        if (it == mMap.end()) return false;
+        mAccessOrder.erase(it->second);
+        mMap.erase(it);
+        return true;
+    }
+
+    /** Returns true if t is present (and moves the access order of t to the front). */
+    bool check(const T& t) { // not const, as it adjusts the least-recently-used order.
+        auto it = mMap.find(t);
+        if (it == mMap.end()) return false;
+        mAccessOrder.erase(it->second);
+        mAccessOrder.push_front(it->first);
+        it->second = mAccessOrder.begin();
+        return true;
+    }
+};
+
+} // namespace android::mediametrics
diff --git a/services/mediametrics/MediaMetricsService.cpp b/services/mediametrics/MediaMetricsService.cpp
index 1d64878..35e0ae4 100644
--- a/services/mediametrics/MediaMetricsService.cpp
+++ b/services/mediametrics/MediaMetricsService.cpp
@@ -19,6 +19,7 @@
 #include <utils/Log.h>
 
 #include "MediaMetricsService.h"
+#include "ValidateId.h"
 #include "iface_statsd.h"
 
 #include <pwd.h> //getpwuid
@@ -204,6 +205,15 @@
     // now attach either the item or its dup to a const shared pointer
     std::shared_ptr<const mediametrics::Item> sitem(release ? item : item->dup());
 
+    // register log session ids with singleton.
+    if (startsWith(item->getKey(), "metrics.manager")) {
+        std::string logSessionId;
+        if (item->get("logSessionId", &logSessionId)
+                && mediametrics::stringutils::isLogSessionId(logSessionId.c_str())) {
+            mediametrics::ValidateId::get()->registerId(logSessionId);
+        }
+    }
+
     (void)mAudioAnalytics.submit(sitem, isTrusted);
 
     (void)dump2Statsd(sitem, mStatsdLog);  // failure should be logged in function.
@@ -309,6 +319,9 @@
                 result << "-- some lines may be truncated --\n";
             }
 
+            result << "LogSessionId:\n"
+                   << mediametrics::ValidateId::get()->dump();
+
             // Dump the statsd atoms we sent out.
             result << "Statsd atoms:\n"
                    << mStatsdLog->dumpToString("  " /* prefix */,
diff --git a/services/mediametrics/ValidateId.cpp b/services/mediametrics/ValidateId.cpp
new file mode 100644
index 0000000..0cc8593
--- /dev/null
+++ b/services/mediametrics/ValidateId.cpp
@@ -0,0 +1,65 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+//#define LOG_NDEBUG 0
+#define LOG_TAG "MediaMetricsService"  // not ValidateId
+#include <utils/Log.h>
+
+#include "ValidateId.h"
+
+namespace android::mediametrics {
+
+std::string ValidateId::dump() const
+{
+    std::stringstream ss;
+    ss << "Entries:" << mIdSet.size() << "  InvalidIds:" << mInvalidIds << "\n";
+    ss << mIdSet.dump(10);
+    return ss.str();
+}
+
+void ValidateId::registerId(const std::string& id)
+{
+    if (id.empty()) return;
+    if (!mediametrics::stringutils::isLogSessionId(id.c_str())) {
+        ALOGW("%s: rejecting malformed id %s", __func__, id.c_str());
+        return;
+    }
+    ALOGV("%s: registering %s", __func__, id.c_str());
+    mIdSet.add(id);
+}
+
+const std::string& ValidateId::validateId(const std::string& id)
+{
+    static const std::string empty{};
+    if (id.empty()) return empty;
+
+    // reject because the id is malformed
+    if (!mediametrics::stringutils::isLogSessionId(id.c_str())) {
+        ALOGW("%s: rejecting malformed id %s", __func__, id.c_str());
+        ++mInvalidIds;
+        return empty;
+    }
+
+    // reject because the id is unregistered
+    if (!mIdSet.check(id)) {
+        ALOGW("%s: rejecting unregistered id %s", __func__, id.c_str());
+        ++mInvalidIds;
+        return empty;
+    }
+    return id;
+}
+
+} // namespace android::mediametrics
diff --git a/services/mediametrics/ValidateId.h b/services/mediametrics/ValidateId.h
new file mode 100644
index 0000000..166b39a
--- /dev/null
+++ b/services/mediametrics/ValidateId.h
@@ -0,0 +1,80 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include "LruSet.h"
+#include "StringUtils.h"
+#include "Wrap.h"
+
+namespace android::mediametrics {
+
+/*
+ * ValidateId is used to check whether the log session id is properly formed
+ * and has been registered (i.e. from the Java MediaMetricsManagerService).
+ *
+ * The default memory window to track registered ids is set to SINGLETON_LRU_SET_SIZE.
+ *
+ * This class is not thread-safe, but the singleton returned by get() uses LockWrap<>
+ * to ensure thread-safety.
+ */
+class ValidateId {
+    mediametrics::LruSet<std::string> mIdSet;
+    size_t mInvalidIds = 0;  // count invalid ids encountered.
+public:
+    /** Creates a ValidateId object with size memory window. */
+    explicit ValidateId(size_t size) : mIdSet{size} {}
+
+    /** Returns a string dump of recent contents and stats. */
+    std::string dump() const;
+
+    /**
+     * Registers the id string.
+     *
+     * If id string is malformed (not 16 Base64Url chars), it is ignored.
+     * Once registered, calling validateId() will return id (instead of the empty string).
+     * ValidateId may "forget" the id after not encountering it within the past N ids,
+     * where N is the size set in the constructor.
+     *
+     * param id string (from MediaMetricsManagerService).
+     */
+    void registerId(const std::string& id);
+
+    /**
+     * Returns the empty string if id string is malformed (not 16 Base64Url chars)
+     * or if id string has not been seen (in the recent size ids);
+     * otherwise it returns the same id parameter.
+     *
+     * \param id string (to be sent to statsd).
+     */
+    const std::string& validateId(const std::string& id);
+
+    /** Singleton set size */
+    static inline constexpr size_t SINGLETON_LRU_SET_SIZE = 2000;
+
+    using LockedValidateId = mediametrics::LockWrap<ValidateId>;
+    /**
+     * Returns a singleton locked ValidateId object that is thread-safe using LockWrap<>.
+     *
+     * The Singleton ValidateId object is created with size LRU_SET_SIZE (during first call).
+     */
+    static inline LockedValidateId& get() {
+        static LockedValidateId privateSet{SINGLETON_LRU_SET_SIZE};
+        return privateSet;
+    }
+};
+
+} // namespace android::mediametrics
diff --git a/services/mediametrics/statsd_audiorecord.cpp b/services/mediametrics/statsd_audiorecord.cpp
index 41efcaa..c53b6f3 100644
--- a/services/mediametrics/statsd_audiorecord.cpp
+++ b/services/mediametrics/statsd_audiorecord.cpp
@@ -32,7 +32,7 @@
 #include <statslog.h>
 
 #include "MediaMetricsService.h"
-#include "StringUtils.h"
+#include "ValidateId.h"
 #include "frameworks/proto_logging/stats/message/mediametrics_message.pb.h"
 #include "iface_statsd.h"
 
@@ -143,8 +143,7 @@
     // log_session_id (string)
     std::string logSessionId;
     (void)item->getString("android.media.audiorecord.logSessionId", &logSessionId);
-    const auto log_session_id =
-            mediametrics::stringutils::sanitizeLogSessionId(logSessionId);
+    const auto log_session_id = mediametrics::ValidateId::get()->validateId(logSessionId);
 
     android::util::BytesField bf_serialized( serialized.c_str(), serialized.size());
     int result = android::util::stats_write(android::util::MEDIAMETRICS_AUDIORECORD_REPORTED,
diff --git a/services/mediametrics/statsd_audiotrack.cpp b/services/mediametrics/statsd_audiotrack.cpp
index 59627ae..707effd 100644
--- a/services/mediametrics/statsd_audiotrack.cpp
+++ b/services/mediametrics/statsd_audiotrack.cpp
@@ -32,7 +32,7 @@
 #include <statslog.h>
 
 #include "MediaMetricsService.h"
-#include "StringUtils.h"
+#include "ValidateId.h"
 #include "frameworks/proto_logging/stats/message/mediametrics_message.pb.h"
 #include "iface_statsd.h"
 
@@ -137,8 +137,7 @@
     // log_session_id (string)
     std::string logSessionId;
     (void)item->getString("android.media.audiotrack.logSessionId", &logSessionId);
-    const auto log_session_id =
-            mediametrics::stringutils::sanitizeLogSessionId(logSessionId);
+    const auto log_session_id = mediametrics::ValidateId::get()->validateId(logSessionId);
 
     android::util::BytesField bf_serialized( serialized.c_str(), serialized.size());
     int result = android::util::stats_write(android::util::MEDIAMETRICS_AUDIOTRACK_REPORTED,
diff --git a/services/mediametrics/statsd_codec.cpp b/services/mediametrics/statsd_codec.cpp
index 46cbdc8..8581437 100644
--- a/services/mediametrics/statsd_codec.cpp
+++ b/services/mediametrics/statsd_codec.cpp
@@ -34,7 +34,7 @@
 
 #include "cleaner.h"
 #include "MediaMetricsService.h"
-#include "StringUtils.h"
+#include "ValidateId.h"
 #include "frameworks/proto_logging/stats/message/mediametrics_message.pb.h"
 #include "iface_statsd.h"
 
@@ -228,7 +228,7 @@
 
     std::string sessionId;
     if (item->getString("android.media.mediacodec.log-session-id", &sessionId)) {
-        sessionId = mediametrics::stringutils::sanitizeLogSessionId(sessionId);
+        sessionId = mediametrics::ValidateId::get()->validateId(sessionId);
         metrics_proto.set_log_session_id(sessionId);
     }
     AStatsEvent_writeString(event, codec.c_str());
diff --git a/services/mediametrics/statsd_extractor.cpp b/services/mediametrics/statsd_extractor.cpp
index bcf2e0a..a8bfeaa 100644
--- a/services/mediametrics/statsd_extractor.cpp
+++ b/services/mediametrics/statsd_extractor.cpp
@@ -32,7 +32,7 @@
 #include <statslog.h>
 
 #include "MediaMetricsService.h"
-#include "StringUtils.h"
+#include "ValidateId.h"
 #include "frameworks/proto_logging/stats/message/mediametrics_message.pb.h"
 #include "iface_statsd.h"
 
@@ -86,7 +86,7 @@
 
     std::string log_session_id;
     if (item->getString("android.media.mediaextractor.logSessionId", &log_session_id)) {
-        log_session_id = mediametrics::stringutils::sanitizeLogSessionId(log_session_id);
+        log_session_id = mediametrics::ValidateId::get()->validateId(log_session_id);
         metrics_proto.set_log_session_id(log_session_id);
     }
 
diff --git a/services/mediametrics/statsd_mediaparser.cpp b/services/mediametrics/statsd_mediaparser.cpp
index 921b320..67ca874b 100644
--- a/services/mediametrics/statsd_mediaparser.cpp
+++ b/services/mediametrics/statsd_mediaparser.cpp
@@ -31,7 +31,7 @@
 #include <statslog.h>
 
 #include "MediaMetricsService.h"
-#include "StringUtils.h"
+#include "ValidateId.h"
 #include "frameworks/proto_logging/stats/enums/stats/mediametrics/mediametrics.pb.h"
 #include "iface_statsd.h"
 
@@ -81,7 +81,7 @@
 
     std::string logSessionId;
     item->getString("android.media.mediaparser.logSessionId", &logSessionId);
-    logSessionId = mediametrics::stringutils::sanitizeLogSessionId(logSessionId);
+    logSessionId = mediametrics::ValidateId::get()->validateId(logSessionId);
 
     int result = android::util::stats_write(android::util::MEDIAMETRICS_MEDIAPARSER_REPORTED,
                                timestamp_nanos,
diff --git a/services/mediametrics/statsd_recorder.cpp b/services/mediametrics/statsd_recorder.cpp
index b29ad73..5f54a68 100644
--- a/services/mediametrics/statsd_recorder.cpp
+++ b/services/mediametrics/statsd_recorder.cpp
@@ -32,7 +32,7 @@
 #include <statslog.h>
 
 #include "MediaMetricsService.h"
-#include "StringUtils.h"
+#include "ValidateId.h"
 #include "frameworks/proto_logging/stats/message/mediametrics_message.pb.h"
 #include "iface_statsd.h"
 
@@ -59,7 +59,7 @@
     // string kRecorderLogSessionId = "android.media.mediarecorder.log-session-id";
     std::string log_session_id;
     if (item->getString("android.media.mediarecorder.log-session-id", &log_session_id)) {
-        log_session_id = mediametrics::stringutils::sanitizeLogSessionId(log_session_id);
+        log_session_id = mediametrics::ValidateId::get()->validateId(log_session_id);
         metrics_proto.set_log_session_id(log_session_id);
     }
     // string kRecorderAudioMime = "android.media.mediarecorder.audio.mime";
diff --git a/services/mediametrics/tests/mediametrics_tests.cpp b/services/mediametrics/tests/mediametrics_tests.cpp
index 2336d6f..69ec947 100644
--- a/services/mediametrics/tests/mediametrics_tests.cpp
+++ b/services/mediametrics/tests/mediametrics_tests.cpp
@@ -28,6 +28,7 @@
 
 #include "AudioTypes.h"
 #include "StringUtils.h"
+#include "ValidateId.h"
 
 using namespace android;
 
@@ -1127,3 +1128,100 @@
    validId2[3] = '!';
    ASSERT_EQ("", mediametrics::stringutils::sanitizeLogSessionId(validId2));
 }
+
+TEST(mediametrics_tests, LruSet) {
+    constexpr size_t LRU_SET_SIZE = 2;
+    mediametrics::LruSet<std::string> lruSet(LRU_SET_SIZE);
+
+    // test adding a couple strings.
+    lruSet.add("abc");
+    ASSERT_EQ(1u, lruSet.size());
+    ASSERT_TRUE(lruSet.check("abc"));
+    lruSet.add("def");
+    ASSERT_EQ(2u, lruSet.size());
+
+    // now adding the third string causes eviction of the oldest.
+    lruSet.add("ghi");
+    ASSERT_FALSE(lruSet.check("abc"));
+    ASSERT_TRUE(lruSet.check("ghi"));
+    ASSERT_TRUE(lruSet.check("def"));  // "def" is most recent.
+    ASSERT_EQ(2u, lruSet.size());      // "abc" is correctly discarded.
+
+    // adding another string will evict the oldest.
+    lruSet.add("foo");
+    ASSERT_FALSE(lruSet.check("ghi")); // note: "ghi" discarded when "foo" added.
+    ASSERT_TRUE(lruSet.check("foo"));
+    ASSERT_TRUE(lruSet.check("def"));
+
+    // manual removing of a string works, too.
+    ASSERT_TRUE(lruSet.remove("def"));
+    ASSERT_FALSE(lruSet.check("def")); // we manually removed "def".
+    ASSERT_TRUE(lruSet.check("foo"));  // "foo" is still there.
+    ASSERT_EQ(1u, lruSet.size());
+
+    // you can't remove a string that has not been added.
+    ASSERT_FALSE(lruSet.remove("bar")); // Note: "bar" doesn't exist, so remove returns false.
+    ASSERT_EQ(1u, lruSet.size());
+
+    lruSet.add("foo");   // adding "foo" (which already exists) doesn't change size.
+    ASSERT_EQ(1u, lruSet.size());
+    lruSet.add("bar");   // add "bar"
+    ASSERT_EQ(2u, lruSet.size());
+    lruSet.add("glorp"); // add "glorp" evicts "foo".
+    ASSERT_EQ(2u, lruSet.size());
+    ASSERT_TRUE(lruSet.check("bar"));
+    ASSERT_TRUE(lruSet.check("glorp"));
+    ASSERT_FALSE(lruSet.check("foo"));
+}
+
+TEST(mediametrics_tests, LruSet0) {
+    constexpr size_t LRU_SET_SIZE = 0;
+    mediametrics::LruSet<std::string> lruSet(LRU_SET_SIZE);
+
+    lruSet.add("a");
+    ASSERT_EQ(0u, lruSet.size());
+    ASSERT_FALSE(lruSet.check("a"));
+    ASSERT_FALSE(lruSet.remove("a")); // never added.
+    ASSERT_EQ(0u, lruSet.size());
+}
+
+// Returns a 16 Base64Url string representing the decimal representation of value
+// (with leading 0s) e.g. 0000000000000000, 0000000000000001, 0000000000000002, ...
+static std::string generateId(size_t value)
+{
+    char id[16 + 1]; // to be filled with 16 Base64Url chars (and zero termination)
+    char *sptr = id + 16; // start at the end.
+    *sptr-- = 0; // zero terminate.
+    // output the digits from least significant to most significant.
+    while (value) {
+        *sptr-- = value % 10;
+        value /= 10;
+    }
+    // add leading 0's
+    while (sptr > id) {
+        *sptr-- = '0';
+    }
+    return std::string(id);
+}
+
+TEST(mediametrics_tests, ValidateId) {
+    constexpr size_t LRU_SET_SIZE = 3;
+    constexpr size_t IDS = 10;
+    static_assert(IDS > LRU_SET_SIZE);  // IDS must be greater than LRU_SET_SIZE.
+    mediametrics::ValidateId validateId(LRU_SET_SIZE);
+
+
+    // register IDs as integer strings counting from 0.
+    for (size_t i = 0; i < IDS; ++i) {
+        validateId.registerId(generateId(i));
+    }
+
+    // only the last LRU_SET_SIZE exist.
+    for (size_t i = 0; i < IDS - LRU_SET_SIZE; ++i) {
+        ASSERT_EQ("", validateId.validateId(generateId(i)));
+    }
+    for (size_t i = IDS - LRU_SET_SIZE; i < IDS; ++i) {
+        const std::string id = generateId(i);
+        ASSERT_EQ(id, validateId.validateId(id));
+    }
+}