diff --git a/TODO b/TODO
index cc7710a..3cb9593 100644
--- a/TODO
+++ b/TODO
@@ -7,14 +7,11 @@
 0.2 release
   ? why isn't the LMDB mapping CLOEXEC
   ? plugin for stream: make sure bortloff@github didn't make a mess of it
-  fix sync
-    * logging of bytes up/down
-    * up/both directions
-    * error handling and reporting
-    * way to close sync request
-    * limit on number of concurrent sync requests
-    * full-db scan limited by since/until
-    * `strfry sync` command always takes at least 1 second due to batching delay. figure out better way to flush
+
+sync
+  logging of bytes up/down
+  pre-calcuated tree of XOR
+    full-db scan limited by since/until
 
 features
   less verbose default logging
diff --git a/src/RelayNegentropy.cpp b/src/RelayNegentropy.cpp
index f35e0b0..4403630 100644
--- a/src/RelayNegentropy.cpp
+++ b/src/RelayNegentropy.cpp
@@ -1,4 +1,4 @@
-#include "Negentropy.h"
+#include <Negentropy.h>
 
 #include "RelayServer.h"
 #include "DBQuery.h"
diff --git a/src/WriterPipeline.h b/src/WriterPipeline.h
index bd10d0c..ed6ce48 100644
--- a/src/WriterPipeline.h
+++ b/src/WriterPipeline.h
@@ -24,6 +24,10 @@ struct WriterPipeline {
     std::thread validatorThread;
     std::thread writerThread;
 
+    std::condition_variable shutdownCv;
+    std::mutex shutdownMutex;
+    bool shutdown = false;
+
   public:
     WriterPipeline() {
         validatorThread = std::thread([&]() {
@@ -61,7 +65,12 @@ struct WriterPipeline {
             while (1) {
                 // Debounce
                 writerInbox.wait();
-                std::this_thread::sleep_for(std::chrono::milliseconds(1'000));
+
+                {
+                    std::unique_lock<std::mutex> lk(shutdownMutex);
+                    shutdownCv.wait_for(lk, std::chrono::milliseconds(1'000), [&]{return shutdown;}); 
+                }
+
                 auto newEvents = writerInbox.pop_all();
 
                 bool flush = false;
@@ -120,6 +129,12 @@ struct WriterPipeline {
     }
 
     void flush() {
+        {
+            std::lock_guard<std::mutex> lk(shutdownMutex);
+            shutdown = true;
+        }
+        shutdownCv.notify_all();
+
         inbox.push_move({ tao::json::null, EventSourceType::None, "" });
         flushInbox.wait();
     }
diff --git a/src/cmd_sync.cpp b/src/cmd_sync.cpp
index 32a673f..f4893a9 100644
--- a/src/cmd_sync.cpp
+++ b/src/cmd_sync.cpp
@@ -1,5 +1,6 @@
 #include <docopt.h>
 #include <tao/json.hpp>
+#include <Negentropy.h>
 
 #include "golpe.h"
 
@@ -18,7 +19,7 @@ R"(
 
     Options:
       --filter=<filter>  Nostr filter (either single filter object or array of filters)
-      --dir=<dir>        Direction: down, up, or both [default: down]
+      --dir=<dir>        Direction: both, down, up, none [default: both]
 )";
 
 
@@ -31,11 +32,163 @@ void cmd_sync(const std::vector<std::string> &subArgs) {
     std::string filterStr;
     if (args["--filter"]) filterStr = args["--filter"].asString();
     else filterStr = "{}";
+    std::string dir = args["--dir"] ? args["--dir"].asString() : "both";
+    if (dir != "both" && dir != "up" && dir != "down" && dir != "none") throw herr("invalid direction: ", dir, ". Should be one of both/up/down/none");
 
-    std::string dir = args["--dir"] ? args["--dir"].asString() : "down";
-    if (dir != "up" && dir != "down" && dir != "both") throw herr("invalid direction: ", dir, ". Should be one of up/down/both");
-    if (dir != "down") throw herr("only down currently supported"); // FIXME
+    const uint64_t idSize = 16;
+    const bool doUp = dir == "both" || dir == "up";
+    const bool doDown = dir == "both" || dir == "down";
 
 
-    throw herr("sync is temporarily not implemented");
+    tao::json::value filter = tao::json::from_string(filterStr);
+
+
+    Negentropy ne(idSize);
+
+    {
+        DBQuery query(filter);
+        Decompressor decomp;
+
+        auto txn = env.txn_ro();
+
+        uint64_t numEvents = 0;
+
+        while (1) {
+            bool complete = query.process(txn, [&](const auto &sub, uint64_t levId, std::string_view eventPayload){
+                auto ev = lookupEventByLevId(txn, levId);
+                ne.addItem(ev.flat_nested()->created_at(), sv(ev.flat_nested()->id()).substr(0, ne.idSize));
+
+                numEvents++;
+            });
+
+            if (complete) break;
+        }
+
+        LI << "Filter matches " << numEvents << " events";
+    }
+
+    ne.seal();
+
+
+
+    WriterPipeline writer;
+    WSConnection ws(url);
+    ws.reconnect = false;
+
+    ws.onConnect = [&]{
+        auto neMsg = to_hex(ne.initiate());
+        ws.send(tao::json::to_string(tao::json::value::array({
+            "NEG-OPEN",
+            "N",
+            filter,
+            idSize,
+            neMsg,
+        })));
+    };
+
+
+    const uint64_t highWaterUp = 100, lowWaterUp = 50;
+    const uint64_t batchSizeDown = 50;
+    uint64_t inFlightUp = 0, inFlightDown = 0;
+    std::vector<std::string> have, need;
+    bool syncDone = false;
+    uint64_t totalHaves = 0, totalNeeds = 0;
+    Decompressor decomp;
+
+    ws.onMessage = [&](auto msgStr, uWS::OpCode opCode, size_t compressedSize){
+        try {
+            tao::json::value msg = tao::json::from_string(msgStr);
+
+            if (msg.at(0) == "NEG-MSG") {
+                uint64_t origHaves = have.size(), origNeeds = need.size();
+
+                auto neMsg = ne.reconcile(from_hex(msg.at(2).get_string()), have, need);
+
+                totalHaves += have.size() - origHaves;
+                totalNeeds += need.size() - origNeeds;
+
+                if (!doUp) have.clear();
+                if (!doDown) need.clear();
+
+                if (neMsg.size() == 0) {
+                    syncDone = true;
+                    LI << "Set reconcile complete. Have " << totalHaves << " need " << totalNeeds;
+                } else {
+                    ws.send(tao::json::to_string(tao::json::value::array({
+                        "NEG-MSG",
+                        "N",
+                        to_hex(neMsg),
+                    })));
+                }
+            } else if (msg.at(0) == "OK") {
+                inFlightUp--;
+
+                if (!msg.at(2).get_boolean()) {
+                    LW << "Unable to upload event " << msg.at(1).get_string() << ": " << msg.at(3).get_string();
+                }
+            } else if (msg.at(0) == "EVENT") {
+                writer.inbox.push_move({ std::move(msg.at(2)), EventSourceType::Sync, url });
+            } else if (msg.at(0) == "EOSE") {
+                inFlightDown = 0;
+            }
+        } catch (std::exception &e) {
+            LE << "Error processing websocket message: " << e.what();
+            LW << "MSG: " << msgStr;
+        }
+
+        if (doUp && have.size() > 0 && inFlightUp < lowWaterUp) {
+            auto txn = env.txn_ro();
+
+            uint64_t numSent = 0;
+
+            while (have.size() > 0 && inFlightUp < highWaterUp) {
+                auto id = std::move(have.back());
+                have.pop_back();
+
+                auto ev = lookupEventById(txn, id);
+                if (!ev) {
+                    LW << "Couldn't upload event because not found (deleted?)";
+                    continue;
+                }
+
+                std::string sendEventMsg = "[\"EVENT\",";
+                sendEventMsg += getEventJson(txn, decomp, ev->primaryKeyId);
+                sendEventMsg += "]";
+                ws.send(sendEventMsg);
+
+                numSent++;
+                inFlightUp++;
+            }
+
+            if (numSent > 0) LI << "UP: " << numSent << " events (" << have.size() << " remaining)";
+        }
+
+        if (doDown && need.size() > 0 && inFlightDown == 0) {
+            tao::json::value ids = tao::json::empty_array;
+
+            while (need.size() > 0 && ids.get_array().size() < batchSizeDown) {
+                ids.emplace_back(to_hex(need.back()));
+                need.pop_back();
+            }
+
+            LI << "DOWN: " << ids.get_array().size() << " events (" << need.size() << " remaining)";
+
+            ws.send(tao::json::to_string(tao::json::value::array({
+                "REQ",
+                "R",
+                tao::json::value({
+                    { "ids", std::move(ids) }
+                }),
+            })));
+
+            inFlightDown = 1;
+        }
+
+        if (syncDone && have.size() == 0 && need.size() == 0 && inFlightUp == 0 && inFlightDown == 0) {
+            if (doDown) writer.flush();
+            ::exit(0);
+        }
+    };
+
+    ws.run();
 }