diff --git a/byteps/common/compressor/strategy/onebit.h b/byteps/common/compressor/strategy/onebit.h
index 8b935d612..d917dd456 100644
--- a/byteps/common/compressor/strategy/onebit.h
+++ b/byteps/common/compressor/strategy/onebit.h
@@ -34,7 +34,6 @@ namespace compressor {
  * server: majority vote
  *    sign(\sum_i c_i)
  *
- * \note this is a deterministic algorithm.
  * \note 0 represents positive and 1 represents negative.
  */
 class OnebitCompressor : public BaseCompressor {
diff --git a/byteps/common/compressor/strategy/randomk.cc b/byteps/common/compressor/strategy/randomk.cc
index d7b147e16..63827fbdd 100644
--- a/byteps/common/compressor/strategy/randomk.cc
+++ b/byteps/common/compressor/strategy/randomk.cc
@@ -37,18 +37,123 @@ CompressorRegistry::Register
         });
 }
 
-RandomkCompressor::RandomkCompressor(int k) : _k(k){};
+RandomkCompressor::RandomkCompressor(int k) : _k(k) { _gen.seed(_rd()); };
 
 RandomkCompressor::~RandomkCompressor() = default;
+template <typename index_t, typename scalar_t>
+size_t RandomkCompressor::_Packing(index_t* dst, const scalar_t* src,
+                                   size_t len) {
+  static_assert(sizeof(index_t) == sizeof(scalar_t),
+                "index_t should be the same size as scalar_t");
+  BPS_CHECK_LE(this->_k, len / 2);
+  using pair_t = std::pair<index_t, scalar_t>;
+  std::uniform_int_distribution<> dis(0, len-1);
+  auto ptr = reinterpret_cast<pair_t*>(dst);
+
+  for (size_t i = 0; i < this->_k; ++i) {
+    auto index = dis(_gen);
+    ptr[i] = std::make_pair(index, src[index]);
+  }
+
+  return this->_k * sizeof(pair_t);
+}
+
+size_t RandomkCompressor::Packing(const void* src, size_t size, int dtype) {
+  switch (dtype) {
+    case BYTEPS_INT8:
+      return _Packing(reinterpret_cast<int8_t*>(_buf.get()),
+                      reinterpret_cast<const int8_t*>(src),
+                      size / sizeof(int8_t));
+    case BYTEPS_UINT8:
+      return _Packing(reinterpret_cast<uint8_t*>(_buf.get()),
+                      reinterpret_cast<const uint8_t*>(src),
+                      size / sizeof(uint8_t));
+    // case BYTEPS_FLOAT16:
+    //   return _Packing(reinterpret_cast<int8_t*>(_buf.get()),
+    //                   reinterpret_cast<const int8_t*>(src), size);
+    case BYTEPS_FLOAT32:
+      return _Packing(reinterpret_cast<int32_t*>(_buf.get()),
+                      reinterpret_cast<const float*>(src),
+                      size / sizeof(int32_t));
+    case BYTEPS_FLOAT64:
+      return _Packing(reinterpret_cast<int64_t*>(_buf.get()),
+                      reinterpret_cast<const double*>(src),
+                      size / sizeof(int64_t));
+    default:
+      BPS_CHECK(0) << "Unsupported data type: " << dtype;
+  }
+  return 0;
+}
 
 void RandomkCompressor::Compress(ByteBuf grad, int dtype, ByteBuf& compressed) {
-  // TODO
+  compressed.size = Packing(grad.data, grad.size, dtype);
+  compressed.data = _buf.get();
+}
+
+template <typename index_t, typename scalar_t>
+size_t RandomkCompressor::_Unpacking(scalar_t* dst, const index_t* src,
+                                     size_t len) {
+  static_assert(sizeof(index_t) == sizeof(scalar_t),
+                "index_t should be the same size as scalar_t");
+  using pair_t = std::pair<index_t, scalar_t>;
+  auto ptr = reinterpret_cast<const pair_t*>(src);
+
+  if ((void*)dst == (void*)src) {
+    auto buf = reinterpret_cast<pair_t*>(_buf.get());
+    std::copy(ptr, ptr + len, buf);
+    ptr = const_cast<const pair_t*>(buf);
+  }
+
+  // reset to zeros
+  std::fill(dst, dst + this->_src_len, 0);
+  for (auto i = 0; i < len; ++i) {
+    auto& pair = ptr[i];
+    dst[pair.first] = pair.second;
+  }
+}
+
+size_t RandomkCompressor::Unpacking(void* dst, const void* src, size_t size,
+                                    int dtype) {
+  switch (dtype) {
+    case BYTEPS_INT8:
+      return _Unpacking(reinterpret_cast<int8_t*>(dst),
+                        reinterpret_cast<const int8_t*>(src),
+                        size / sizeof(int8_t) / 2);
+    case BYTEPS_UINT8:
+      return _Unpacking(reinterpret_cast<uint8_t*>(dst),
+                        reinterpret_cast<const uint8_t*>(src),
+                        size / sizeof(uint8_t) / 2);
+    // case BYTEPS_FLOAT16:
+    //   return _Unpacking(reinterpret_cast<int8_t*>(_buf.get()),
+    //                   reinterpret_cast<const int8_t*>(src), size);
+    case BYTEPS_FLOAT32:
+      return _Unpacking(reinterpret_cast<float*>(dst),
+                        reinterpret_cast<const int32_t*>(src),
+                        size / sizeof(float) / 2);
+    case BYTEPS_FLOAT64:
+      return _Unpacking(reinterpret_cast<double*>(dst),
+                        reinterpret_cast<const int64_t*>(src),
+                        size / sizeof(double) / 2);
+    default:
+      BPS_CHECK(0) << "Unsupported data type: " << dtype;
+  }
+  return 0;
 }
 
+#ifndef BYTEPS_BUILDING_SERVER
+// worker version decompressor
+void RandomkCompressor::Decompress(ByteBuf compressed, int dtype,
+                                   ByteBuf& decompressed) {
+  BPS_CHECK(decompressed.data);
+  Unpacking(decompressed.data, compressed.data, compressed.size, dtype);
+}
+#else
 void RandomkCompressor::Decompress(ByteBuf compressed, int dtype,
                                    ByteBuf& decompressed) {
-  // TODO
+  if (decompressed.data == nullptr) decompressed.data = _buf.get();
+  Unpacking(decompressed.data, compressed.data, compressed.size, dtype);
 }
+#endif
 }  // namespace compressor
 }  // namespace common
 }  // namespace byteps
\ No newline at end of file
diff --git a/byteps/common/compressor/strategy/randomk.h b/byteps/common/compressor/strategy/randomk.h
index 3ef1fc317..1e464663a 100644
--- a/byteps/common/compressor/strategy/randomk.h
+++ b/byteps/common/compressor/strategy/randomk.h
@@ -16,6 +16,8 @@
 #ifndef BYTEPS_COMPRESS_STRAT_RANDOMK_H
 #define BYTEPS_COMPRESS_STRAT_RANDOMK_H
 
+#include <random>
+
 #include "../base_compressor.h"
 
 namespace byteps {
@@ -23,20 +25,57 @@ namespace common {
 namespace compressor {
 
 /*!
- * \brief TODO
+ * \brief RandomK Compressor
+ *
+ * paper: Sparsified SGD with Memory
+ * https://arxiv.org/pdf/1809.07599.pdf
+ *
+ * randomly sending k entries of the stochastic gradient
  */
 class RandomkCompressor : public BaseCompressor {
  public:
   explicit RandomkCompressor(int k);
   virtual ~RandomkCompressor();
 
+  /*!
+   * \brief Compress function
+   *
+   * randomly select k entries and corresponding indices
+   *
+   * \param grad gradient tensor
+   * \param dtype data type
+   * \param compressed compressed tensor
+   */
   void Compress(ByteBuf grad, int dtype, ByteBuf& compressed) override;
 
+  /*!
+   * \brief Decompress function
+   *
+   * fill a zero tensor with topk entries and corresponding indices
+   *
+   * \param compressed compressed tensor
+   * \param dtype data type
+   * \param decompressed decompressed tensor
+   */
   void Decompress(ByteBuf compressed, int dtype,
                   ByteBuf& decompressed) override;
 
+ private:
+  size_t Packing(const void* src, size_t size, int dtype);
+
+  template <typename index_t, typename scalar_t>
+  size_t _Packing(index_t* dst, const scalar_t* src, size_t len);
+
+  size_t Unpacking(void* dst, const void* src, size_t size, int dtype);
+
+  template <typename index_t, typename scalar_t>
+  size_t _Unpacking(scalar_t* dst, const index_t* src, size_t len);
+
  private:
   int _k;
+  int _src_len;
+  std::random_device _rd;
+  std::mt19937 _gen;
 };
 }  // namespace compressor
 }  // namespace common
diff --git a/byteps/common/compressor/strategy/topk.h b/byteps/common/compressor/strategy/topk.h
index 8200e69d5..3c6d06956 100644
--- a/byteps/common/compressor/strategy/topk.h
+++ b/byteps/common/compressor/strategy/topk.h
@@ -30,7 +30,6 @@ namespace compressor {
  * 
  * sending the most significant entries of the stochastic gradient
  * 
- * \note this is a deterministic algorithm
  */
 class TopkCompressor : public BaseCompressor {
  public: