diff --git a/plugin/federated/federated.proto b/plugin/federated/federated.proto
index d8ef5bd92f43..8450659fd180 100644
--- a/plugin/federated/federated.proto
+++ b/plugin/federated/federated.proto
@@ -7,6 +7,7 @@ package xgboost.federated;
 
 service Federated {
   rpc Allgather(AllgatherRequest) returns (AllgatherReply) {}
+  rpc AllgatherV(AllgatherVRequest) returns (AllgatherVReply) {}
   rpc Allreduce(AllreduceRequest) returns (AllreduceReply) {}
   rpc Broadcast(BroadcastRequest) returns (BroadcastReply) {}
 }
@@ -42,6 +43,17 @@ message AllgatherReply {
   bytes receive_buffer = 1;
 }
 
+message AllgatherVRequest {
+  // An incrementing counter that is unique to each round to operations.
+  uint64 sequence_number = 1;
+  int32 rank = 2;
+  bytes send_buffer = 3;
+}
+
+message AllgatherVReply {
+  bytes receive_buffer = 1;
+}
+
 message AllreduceRequest {
   // An incrementing counter that is unique to each round to operations.
   uint64 sequence_number = 1;
diff --git a/plugin/federated/federated_client.h b/plugin/federated/federated_client.h
index d104cb2319b8..ac1fbd57d9dd 100644
--- a/plugin/federated/federated_client.h
+++ b/plugin/federated/federated_client.h
@@ -44,11 +44,11 @@ class FederatedClient {
         }()},
         rank_{rank} {}
 
-  std::string Allgather(std::string const &send_buffer) {
+  std::string Allgather(std::string_view send_buffer) {
     AllgatherRequest request;
     request.set_sequence_number(sequence_number_++);
     request.set_rank(rank_);
-    request.set_send_buffer(send_buffer);
+    request.set_send_buffer(send_buffer.data(), send_buffer.size());
 
     AllgatherReply reply;
     grpc::ClientContext context;
@@ -63,6 +63,25 @@ class FederatedClient {
     }
   }
 
+  std::string AllgatherV(std::string_view send_buffer) {
+    AllgatherVRequest request;
+    request.set_sequence_number(sequence_number_++);
+    request.set_rank(rank_);
+    request.set_send_buffer(send_buffer.data(), send_buffer.size());
+
+    AllgatherVReply reply;
+    grpc::ClientContext context;
+    context.set_wait_for_ready(true);
+    grpc::Status status = stub_->AllgatherV(&context, request, &reply);
+
+    if (status.ok()) {
+      return reply.receive_buffer();
+    } else {
+      std::cout << status.error_code() << ": " << status.error_message() << '\n';
+      throw std::runtime_error("AllgatherV RPC failed");
+    }
+  }
+
   std::string Allreduce(std::string const &send_buffer, DataType data_type,
                         ReduceOperation reduce_operation) {
     AllreduceRequest request;
diff --git a/plugin/federated/federated_communicator.h b/plugin/federated/federated_communicator.h
index 996b433cb2ea..46c6b0fda672 100644
--- a/plugin/federated/federated_communicator.h
+++ b/plugin/federated/federated_communicator.h
@@ -125,14 +125,19 @@ class FederatedCommunicator : public Communicator {
   [[nodiscard]] bool IsFederated() const override { return true; }
 
   /**
-   * \brief Perform in-place allgather.
-   * \param send_receive_buffer Buffer for both sending and receiving data.
-   * \param size Number of bytes to be gathered.
+   * \brief Perform allgather.
+   * \param input Buffer for sending data.
+   */
+  std::string AllGather(std::string_view input) override {
+    return client_->Allgather(input);
+  }
+
+  /**
+   * \brief Perform variable-length allgather.
+   * \param input Buffer for sending data.
    */
-  void AllGather(void *send_receive_buffer, std::size_t size) override {
-    std::string const send_buffer(reinterpret_cast<char const *>(send_receive_buffer), size);
-    auto const received = client_->Allgather(send_buffer);
-    received.copy(reinterpret_cast<char *>(send_receive_buffer), size);
+  std::string AllGatherV(std::string_view input) override {
+    return client_->AllgatherV(input);
   }
 
   /**
diff --git a/plugin/federated/federated_server.cc b/plugin/federated/federated_server.cc
index ae42f6d28920..ad6cf6022e6d 100644
--- a/plugin/federated/federated_server.cc
+++ b/plugin/federated/federated_server.cc
@@ -19,6 +19,13 @@ grpc::Status FederatedService::Allgather(grpc::ServerContext*, AllgatherRequest
   return grpc::Status::OK;
 }
 
+grpc::Status FederatedService::AllgatherV(grpc::ServerContext*, AllgatherVRequest const* request,
+                                          AllgatherVReply* reply) {
+  handler_.AllgatherV(request->send_buffer().data(), request->send_buffer().size(),
+                      reply->mutable_receive_buffer(), request->sequence_number(), request->rank());
+  return grpc::Status::OK;
+}
+
 grpc::Status FederatedService::Allreduce(grpc::ServerContext*, AllreduceRequest const* request,
                                          AllreduceReply* reply) {
   handler_.Allreduce(request->send_buffer().data(), request->send_buffer().size(),
@@ -36,8 +43,8 @@ grpc::Status FederatedService::Broadcast(grpc::ServerContext*, BroadcastRequest
   return grpc::Status::OK;
 }
 
-void RunServer(int port, int world_size, char const* server_key_file, char const* server_cert_file,
-               char const* client_cert_file) {
+void RunServer(int port, std::size_t world_size, char const* server_key_file,
+               char const* server_cert_file, char const* client_cert_file) {
   std::string const server_address = "0.0.0.0:" + std::to_string(port);
   FederatedService service{world_size};
 
@@ -59,7 +66,7 @@ void RunServer(int port, int world_size, char const* server_key_file, char const
   server->Wait();
 }
 
-void RunInsecureServer(int port, int world_size) {
+void RunInsecureServer(int port, std::size_t world_size) {
   std::string const server_address = "0.0.0.0:" + std::to_string(port);
   FederatedService service{world_size};
 
diff --git a/plugin/federated/federated_server.h b/plugin/federated/federated_server.h
index 7738248ea729..711ef55880b3 100644
--- a/plugin/federated/federated_server.h
+++ b/plugin/federated/federated_server.h
@@ -12,11 +12,14 @@ namespace federated {
 
 class FederatedService final : public Federated::Service {
  public:
-  explicit FederatedService(int const world_size) : handler_{world_size} {}
+  explicit FederatedService(std::size_t const world_size) : handler_{world_size} {}
 
   grpc::Status Allgather(grpc::ServerContext* context, AllgatherRequest const* request,
                          AllgatherReply* reply) override;
 
+  grpc::Status AllgatherV(grpc::ServerContext* context, AllgatherVRequest const* request,
+                          AllgatherVReply* reply) override;
+
   grpc::Status Allreduce(grpc::ServerContext* context, AllreduceRequest const* request,
                          AllreduceReply* reply) override;
 
@@ -27,10 +30,10 @@ class FederatedService final : public Federated::Service {
   xgboost::collective::InMemoryHandler handler_;
 };
 
-void RunServer(int port, int world_size, char const* server_key_file, char const* server_cert_file,
-               char const* client_cert_file);
+void RunServer(int port, std::size_t world_size, char const* server_key_file,
+               char const* server_cert_file, char const* client_cert_file);
 
-void RunInsecureServer(int port, int world_size);
+void RunInsecureServer(int port, std::size_t world_size);
 
 }  // namespace federated
 }  // namespace xgboost
diff --git a/src/c_api/c_api.cc b/src/c_api/c_api.cc
index 858047af84ca..4fb6d90ff432 100644
--- a/src/c_api/c_api.cc
+++ b/src/c_api/c_api.cc
@@ -1724,7 +1724,7 @@ XGB_DLL int XGCommunicatorAllreduce(void *send_receive_buffer, size_t count, int
 }
 
 #if defined(XGBOOST_USE_FEDERATED)
-XGB_DLL int XGBRunFederatedServer(int port, int world_size, char const *server_key_path,
+XGB_DLL int XGBRunFederatedServer(int port, std::size_t world_size, char const *server_key_path,
                                   char const *server_cert_path, char const *client_cert_path) {
   API_BEGIN();
   federated::RunServer(port, world_size, server_key_path, server_cert_path, client_cert_path);
@@ -1732,7 +1732,7 @@ XGB_DLL int XGBRunFederatedServer(int port, int world_size, char const *server_k
 }
 
 // Run a server without SSL for local testing.
-XGB_DLL int XGBRunInsecureFederatedServer(int port, int world_size) {
+XGB_DLL int XGBRunInsecureFederatedServer(int port, std::size_t world_size) {
   API_BEGIN();
   federated::RunInsecureServer(port, world_size);
   API_END();
diff --git a/src/collective/communicator-inl.h b/src/collective/communicator-inl.h
index 59cc4cc45736..c58a9f3bcf83 100644
--- a/src/collective/communicator-inl.h
+++ b/src/collective/communicator-inl.h
@@ -57,9 +57,7 @@ namespace collective {
  *   - federated_client_key: Client key file path. Only needed for the SSL mode.
  *   - federated_client_cert: Client certificate file path. Only needed for the SSL mode.
  */
-inline void Init(Json const& config) {
-  Communicator::Init(config);
-}
+inline void Init(Json const &config) { Communicator::Init(config); }
 
 /*!
  * \brief Finalize the collective communicator.
@@ -141,17 +139,89 @@ inline void Broadcast(std::string *sendrecv_data, int root) {
   }
 }
 
+/**
+ * @brief Gathers a single value all processes and distributes the result to all processes.
+ *
+ * @param input The single value.
+ */
+template <typename T>
+inline std::vector<T> Allgather(T const &input) {
+  std::string_view str_input{reinterpret_cast<char const *>(&input), sizeof(T)};
+  auto const output = Communicator::Get()->AllGather(str_input);
+  CHECK_EQ(output.size() % sizeof(T), 0);
+  std::vector<T> result(output.size() / sizeof(T));
+  std::memcpy(reinterpret_cast<void *>(result.data()), output.data(), output.size());
+  return result;
+}
+
 /**
  * @brief Gathers data from all processes and distributes it to all processes.
  *
- * This assumes all ranks have the same size, and input data has been sliced into the
- * corresponding position.
+ * This assumes all ranks have the same size.
  *
- * @param send_receive_buffer Buffer storing the data.
- * @param size                Size of the data in bytes.
+ * @param input Buffer storing the data.
  */
-inline void Allgather(void *send_receive_buffer, std::size_t size) {
-  Communicator::Get()->AllGather(send_receive_buffer, size);
+template <typename T>
+inline std::vector<T> Allgather(std::vector<T> const &input) {
+  if (input.empty()) {
+    return input;
+  }
+  std::string_view str_input{reinterpret_cast<char const *>(input.data()),
+                             input.size() * sizeof(T)};
+  auto const output = Communicator::Get()->AllGather(str_input);
+  CHECK_EQ(output.size() % sizeof(T), 0);
+  std::vector<T> result(output.size() / sizeof(T));
+  std::memcpy(reinterpret_cast<void *>(result.data()), output.data(), output.size());
+  return result;
+}
+
+/**
+ * @brief Gathers variable-length data from all processes and distributes it to all processes.
+ * @param input Buffer storing the data.
+ */
+template <typename T>
+inline std::vector<T> AllgatherV(std::vector<T> const &input) {
+  std::string_view str_input{reinterpret_cast<char const *>(input.data()),
+                             input.size() * sizeof(T)};
+  auto const output = Communicator::Get()->AllGatherV(str_input);
+  CHECK_EQ(output.size() % sizeof(T), 0);
+  std::vector<T> result(output.size() / sizeof(T));
+  if (!output.empty()) {
+    std::memcpy(reinterpret_cast<void *>(result.data()), output.data(), output.size());
+  }
+  return result;
+}
+
+/**
+ * @brief Gathers variable-length strings from all processes and distributes them to all processes.
+ * @param input Variable-length list of variable-length strings.
+ */
+inline std::vector<std::string> AllgatherStrings(std::vector<std::string> const &input) {
+  std::size_t total_size{0};
+  for (auto const &s : input) {
+    total_size += s.length() + 1;  // +1 for null-terminators
+  }
+  std::string flat_string;
+  flat_string.reserve(total_size);
+  for (auto const &s : input) {
+    flat_string.append(s);
+    flat_string.push_back('\0');  // Append a null-terminator after each string
+  }
+
+  auto const output = Communicator::Get()->AllGatherV(flat_string);
+
+  std::vector<std::string> result;
+  std::size_t start_index = 0;
+  // Iterate through the output, find each null-terminated substring.
+  for (std::size_t i = 0; i < output.size(); i++) {
+    if (output[i] == '\0') {
+      // Construct a std::string from the char* substring
+      result.emplace_back(&output[start_index]);
+      // Move to the next substring
+      start_index = i + 1;
+    }
+  }
+  return result;
 }
 
 /*!
@@ -226,7 +296,7 @@ inline void Allreduce(double *send_receive_buffer, size_t count) {
 }
 
 template <typename T>
-struct AllgatherVResult {
+struct SpecialAllgatherVResult {
   std::vector<std::size_t> offsets;
   std::vector<std::size_t> sizes;
   std::vector<T> result;
@@ -241,14 +311,10 @@ struct AllgatherVResult {
  * @param sizes  Sizes of each input.
  */
 template <typename T>
-inline AllgatherVResult<T> AllgatherV(std::vector<T> const &inputs,
-                                      std::vector<std::size_t> const &sizes) {
-  auto num_inputs = sizes.size();
-
+inline SpecialAllgatherVResult<T> SpecialAllgatherV(std::vector<T> const &inputs,
+                                                    std::vector<std::size_t> const &sizes) {
   // Gather the sizes across all workers.
-  std::vector<std::size_t> all_sizes(num_inputs * GetWorldSize());
-  std::copy_n(sizes.cbegin(), sizes.size(), all_sizes.begin() + num_inputs * GetRank());
-  collective::Allgather(all_sizes.data(), all_sizes.size() * sizeof(std::size_t));
+  auto const all_sizes = Allgather(sizes);
 
   // Calculate input offsets (std::exclusive_scan).
   std::vector<std::size_t> offsets(all_sizes.size());
@@ -257,11 +323,7 @@ inline AllgatherVResult<T> AllgatherV(std::vector<T> const &inputs,
   }
 
   // Gather all the inputs.
-  auto total_input_size = offsets.back() + all_sizes.back();
-  std::vector<T> all_inputs(total_input_size);
-  std::copy_n(inputs.cbegin(), inputs.size(), all_inputs.begin() + offsets[num_inputs * GetRank()]);
-  // We cannot use allgather here, since each worker might have a different size.
-  Allreduce<Operation::kMax>(all_inputs.data(), all_inputs.size());
+  auto const all_inputs = AllgatherV(inputs);
 
   return {offsets, all_sizes, all_inputs};
 }
diff --git a/src/collective/communicator.h b/src/collective/communicator.h
index def9615135df..feb446355b5d 100644
--- a/src/collective/communicator.h
+++ b/src/collective/communicator.h
@@ -125,13 +125,17 @@ class Communicator {
   /**
    * @brief Gathers data from all processes and distributes it to all processes.
    *
-   * This assumes all ranks have the same size, and input data has been sliced into the
-   * corresponding position.
+   * This assumes all ranks have the same size.
    *
-   * @param send_receive_buffer Buffer storing the data.
-   * @param size                Size of the data in bytes.
+   * @param input Buffer storing the data.
+   */
+  virtual std::string AllGather(std::string_view input) = 0;
+
+  /**
+   * @brief Gathers variable-length data from all processes and distributes it to all processes.
+   * @param input Buffer storing the data.
    */
-  virtual void AllGather(void *send_receive_buffer, std::size_t size) = 0;
+  virtual std::string AllGatherV(std::string_view input) = 0;
 
   /**
    * @brief Combines values from all processes and distributes the result back to all processes.
diff --git a/src/collective/device_communicator_adapter.cuh b/src/collective/device_communicator_adapter.cuh
index d10b10486693..7d3e836a0ec9 100644
--- a/src/collective/device_communicator_adapter.cuh
+++ b/src/collective/device_communicator_adapter.cuh
@@ -40,12 +40,10 @@ class DeviceCommunicatorAdapter : public DeviceCommunicator {
     }
 
     dh::safe_cuda(cudaSetDevice(device_ordinal_));
-    host_buffer_.resize(send_size * world_size_);
-    dh::safe_cuda(cudaMemcpy(host_buffer_.data() + rank_ * send_size, send_buffer, send_size,
-                             cudaMemcpyDefault));
-    Allgather(host_buffer_.data(), host_buffer_.size());
-    dh::safe_cuda(
-        cudaMemcpy(receive_buffer, host_buffer_.data(), host_buffer_.size(), cudaMemcpyDefault));
+    host_buffer_.resize(send_size);
+    dh::safe_cuda(cudaMemcpy(host_buffer_.data(), send_buffer, send_size, cudaMemcpyDefault));
+    auto const output = Allgather(host_buffer_);
+    dh::safe_cuda(cudaMemcpy(receive_buffer, output.data(), output.size(), cudaMemcpyDefault));
   }
 
   void AllGatherV(void const *send_buffer, size_t length_bytes, std::vector<std::size_t> *segments,
diff --git a/src/collective/in_memory_communicator.h b/src/collective/in_memory_communicator.h
index f41029af1dea..c712d32a8006 100644
--- a/src/collective/in_memory_communicator.h
+++ b/src/collective/in_memory_communicator.h
@@ -60,11 +60,16 @@ class InMemoryCommunicator : public Communicator {
   bool IsDistributed() const override { return true; }
   bool IsFederated() const override { return false; }
 
-  void AllGather(void* in_out, std::size_t size) override {
+  std::string AllGather(std::string_view input) override {
     std::string output;
-    handler_.Allgather(static_cast<const char*>(in_out), size, &output, sequence_number_++,
-                       GetRank());
-    output.copy(static_cast<char*>(in_out), size);
+    handler_.Allgather(input.data(), input.size(), &output, sequence_number_++, GetRank());
+    return output;
+  }
+
+  std::string AllGatherV(std::string_view input) override {
+    std::string output;
+    handler_.AllgatherV(input.data(), input.size(), &output, sequence_number_++, GetRank());
+    return output;
   }
 
   void AllReduce(void* in_out, std::size_t size, DataType data_type, Operation operation) override {
diff --git a/src/collective/in_memory_handler.cc b/src/collective/in_memory_handler.cc
index a45fe3e7dd78..944e5077b068 100644
--- a/src/collective/in_memory_handler.cc
+++ b/src/collective/in_memory_handler.cc
@@ -16,23 +16,49 @@ class AllgatherFunctor {
  public:
   std::string const name{"Allgather"};
 
-  AllgatherFunctor(int world_size, int rank) : world_size_{world_size}, rank_{rank} {}
+  AllgatherFunctor(std::size_t world_size, std::size_t rank)
+      : world_size_{world_size}, rank_{rank} {}
 
   void operator()(char const* input, std::size_t bytes, std::string* buffer) const {
     if (buffer->empty()) {
-      // Copy the input if this is the first request.
-      buffer->assign(input, bytes);
-    } else {
-      // Splice the input into the common buffer.
-      auto const per_rank = bytes / world_size_;
-      auto const index = rank_ * per_rank;
-      buffer->replace(index, per_rank, input + index, per_rank);
+      // Resize the buffer if this is the first request.
+      buffer->resize(bytes * world_size_);
+    }
+
+    // Splice the input into the common buffer.
+    buffer->replace(rank_ * bytes, bytes, input, bytes);
+  }
+
+ private:
+  std::size_t world_size_;
+  std::size_t rank_;
+};
+
+/**
+ * @brief Functor for variable-length allgather.
+ */
+class AllgatherVFunctor {
+ public:
+  std::string const name{"AllgatherV"};
+
+  AllgatherVFunctor(std::size_t world_size, std::size_t rank,
+                    std::map<std::size_t, std::string_view>* data)
+      : world_size_{world_size}, rank_{rank}, data_{data} {}
+
+  void operator()(char const* input, std::size_t bytes, std::string* buffer) const {
+    data_->emplace(rank_, std::string_view{input, bytes});
+    if (data_->size() == world_size_) {
+      for (auto const& kv : *data_) {
+        buffer->append(kv.second);
+      }
+      data_->clear();
     }
   }
 
  private:
-  int world_size_;
-  int rank_;
+  std::size_t world_size_;
+  std::size_t rank_;
+  std::map<std::size_t, std::string_view>* data_;
 };
 
 /**
@@ -154,7 +180,7 @@ class BroadcastFunctor {
  public:
   std::string const name{"Broadcast"};
 
-  BroadcastFunctor(int rank, int root) : rank_{rank}, root_{root} {}
+  BroadcastFunctor(std::size_t rank, std::size_t root) : rank_{rank}, root_{root} {}
 
   void operator()(char const* input, std::size_t bytes, std::string* buffer) const {
     if (rank_ == root_) {
@@ -164,11 +190,11 @@ class BroadcastFunctor {
   }
 
  private:
-  int rank_;
-  int root_;
+  std::size_t rank_;
+  std::size_t root_;
 };
 
-void InMemoryHandler::Init(int world_size, int) {
+void InMemoryHandler::Init(std::size_t world_size, std::size_t) {
   CHECK(world_size_ < world_size) << "In memory handler already initialized.";
 
   std::unique_lock<std::mutex> lock(mutex_);
@@ -178,7 +204,7 @@ void InMemoryHandler::Init(int world_size, int) {
   cv_.notify_all();
 }
 
-void InMemoryHandler::Shutdown(uint64_t sequence_number, int) {
+void InMemoryHandler::Shutdown(uint64_t sequence_number, std::size_t) {
   CHECK(world_size_ > 0) << "In memory handler already shutdown.";
 
   std::unique_lock<std::mutex> lock(mutex_);
@@ -194,24 +220,30 @@ void InMemoryHandler::Shutdown(uint64_t sequence_number, int) {
 }
 
 void InMemoryHandler::Allgather(char const* input, std::size_t bytes, std::string* output,
-                                std::size_t sequence_number, int rank) {
+                                std::size_t sequence_number, std::size_t rank) {
   Handle(input, bytes, output, sequence_number, rank, AllgatherFunctor{world_size_, rank});
 }
 
+void InMemoryHandler::AllgatherV(char const* input, std::size_t bytes, std::string* output,
+                                 std::size_t sequence_number, std::size_t rank) {
+  Handle(input, bytes, output, sequence_number, rank, AllgatherVFunctor{world_size_, rank, &aux_});
+}
+
 void InMemoryHandler::Allreduce(char const* input, std::size_t bytes, std::string* output,
-                                std::size_t sequence_number, int rank, DataType data_type,
+                                std::size_t sequence_number, std::size_t rank, DataType data_type,
                                 Operation op) {
   Handle(input, bytes, output, sequence_number, rank, AllreduceFunctor{data_type, op});
 }
 
 void InMemoryHandler::Broadcast(char const* input, std::size_t bytes, std::string* output,
-                                std::size_t sequence_number, int rank, int root) {
+                                std::size_t sequence_number, std::size_t rank, std::size_t root) {
   Handle(input, bytes, output, sequence_number, rank, BroadcastFunctor{rank, root});
 }
 
 template <class HandlerFunctor>
 void InMemoryHandler::Handle(char const* input, std::size_t bytes, std::string* output,
-                             std::size_t sequence_number, int rank, HandlerFunctor const& functor) {
+                             std::size_t sequence_number, std::size_t rank,
+                             HandlerFunctor const& functor) {
   // Pass through if there is only 1 client.
   if (world_size_ == 1) {
     if (input != output->data()) {
diff --git a/src/collective/in_memory_handler.h b/src/collective/in_memory_handler.h
index 4182c7b3ddb2..f9ac520079fd 100644
--- a/src/collective/in_memory_handler.h
+++ b/src/collective/in_memory_handler.h
@@ -3,6 +3,7 @@
  */
 #pragma once
 #include <condition_variable>
+#include <map>
 #include <string>
 
 #include "communicator.h"
@@ -31,7 +32,7 @@ class InMemoryHandler {
    *
    * This is used when the handler only needs to be initialized once with a known world size.
    */
-  explicit InMemoryHandler(int worldSize) : world_size_{worldSize} {}
+  explicit InMemoryHandler(std::size_t worldSize) : world_size_{worldSize} {}
 
   /**
    * @brief Initialize the handler with the world size and rank.
@@ -41,7 +42,7 @@ class InMemoryHandler {
    * This is used when multiple objects/threads are accessing the same handler and need to
    * initialize it collectively.
    */
-  void Init(int world_size, int rank);
+  void Init(std::size_t world_size, std::size_t rank);
 
   /**
    * @brief Shut down the handler.
@@ -51,7 +52,7 @@ class InMemoryHandler {
    * This is used when multiple objects/threads are accessing the same handler and need to
    * shut it down collectively.
    */
-  void Shutdown(uint64_t sequence_number, int rank);
+  void Shutdown(uint64_t sequence_number, std::size_t rank);
 
   /**
    * @brief Perform allgather.
@@ -62,7 +63,18 @@ class InMemoryHandler {
    * @param rank Index of the worker.
    */
   void Allgather(char const* input, std::size_t bytes, std::string* output,
-                 std::size_t sequence_number, int rank);
+                 std::size_t sequence_number, std::size_t rank);
+
+  /**
+   * @brief Perform variable-length allgather.
+   * @param input The input buffer.
+   * @param bytes Number of bytes in the input buffer.
+   * @param output The output buffer.
+   * @param sequence_number Call sequence number.
+   * @param rank Index of the worker.
+   */
+  void AllgatherV(char const* input, std::size_t bytes, std::string* output,
+                  std::size_t sequence_number, std::size_t rank);
 
   /**
    * @brief Perform allreduce.
@@ -75,7 +87,7 @@ class InMemoryHandler {
    * @param op The reduce operation.
    */
   void Allreduce(char const* input, std::size_t bytes, std::string* output,
-                 std::size_t sequence_number, int rank, DataType data_type, Operation op);
+                 std::size_t sequence_number, std::size_t rank, DataType data_type, Operation op);
 
   /**
    * @brief Perform broadcast.
@@ -87,7 +99,7 @@ class InMemoryHandler {
    * @param root Index of the worker to broadcast from.
    */
   void Broadcast(char const* input, std::size_t bytes, std::string* output,
-                 std::size_t sequence_number, int rank, int root);
+                 std::size_t sequence_number, std::size_t rank, std::size_t root);
 
  private:
   /**
@@ -102,15 +114,16 @@ class InMemoryHandler {
    */
   template <class HandlerFunctor>
   void Handle(char const* input, std::size_t size, std::string* output, std::size_t sequence_number,
-              int rank, HandlerFunctor const& functor);
+              std::size_t rank, HandlerFunctor const& functor);
 
-  int world_size_{};                    /// Number of workers.
-  int received_{};                      /// Number of calls received with the current sequence.
-  int sent_{};                          /// Number of calls completed with the current sequence.
-  std::string buffer_{};                /// A shared common buffer.
-  uint64_t sequence_number_{};          /// Call sequence number.
-  mutable std::mutex mutex_;            /// Lock.
-  mutable std::condition_variable cv_;  /// Conditional variable to wait on.
+  std::size_t world_size_{};  /// Number of workers.
+  std::size_t received_{};    /// Number of calls received with the current sequence.
+  std::size_t sent_{};        /// Number of calls completed with the current sequence.
+  std::string buffer_{};      /// A shared common buffer.
+  std::map<std::size_t, std::string_view> aux_{};  /// A shared auxiliary map.
+  uint64_t sequence_number_{};                     /// Call sequence number.
+  mutable std::mutex mutex_;                       /// Lock.
+  mutable std::condition_variable cv_;             /// Conditional variable to wait on.
 };
 
 }  // namespace collective
diff --git a/src/collective/noop_communicator.h b/src/collective/noop_communicator.h
index 28a0a1cada4d..2d88fd8024d2 100644
--- a/src/collective/noop_communicator.h
+++ b/src/collective/noop_communicator.h
@@ -17,10 +17,11 @@ class NoOpCommunicator : public Communicator {
   NoOpCommunicator() : Communicator(1, 0) {}
   bool IsDistributed() const override { return false; }
   bool IsFederated() const override { return false; }
-  void AllGather(void *, std::size_t) override {}
+  std::string AllGather(std::string_view) override { return {}; }
+  std::string AllGatherV(std::string_view) override { return {}; }
   void AllReduce(void *, std::size_t, DataType, Operation) override {}
   void Broadcast(void *, std::size_t, int) override {}
-  std::string GetProcessorName() override { return ""; }
+  std::string GetProcessorName() override { return {}; }
   void Print(const std::string &message) override { LOG(CONSOLE) << message; }
 
  protected:
diff --git a/src/collective/rabit_communicator.h b/src/collective/rabit_communicator.h
index 9b79624a2718..59a4bbbd889a 100644
--- a/src/collective/rabit_communicator.h
+++ b/src/collective/rabit_communicator.h
@@ -7,6 +7,7 @@
 #include <string>
 #include <vector>
 
+#include "communicator-inl.h"
 #include "communicator.h"
 #include "xgboost/json.h"
 
@@ -55,10 +56,27 @@ class RabitCommunicator : public Communicator {
 
   bool IsFederated() const override { return false; }
 
-  void AllGather(void *send_receive_buffer, std::size_t size) override {
-    auto const per_rank = size / GetWorldSize();
+  std::string AllGather(std::string_view input) override {
+    auto const per_rank = input.size();
+    auto const total_size = per_rank * GetWorldSize();
     auto const index = per_rank * GetRank();
-    rabit::Allgather(static_cast<char *>(send_receive_buffer), size, index, per_rank, per_rank);
+    std::string result(total_size, '\0');
+    rabit::Allgather(result.data(), total_size, index, per_rank, per_rank);
+    return result;
+  }
+
+  std::string AllGatherV(std::string_view input) override {
+    auto const size_node_slice = input.size();
+    auto const all_sizes = collective::Allgather(size_node_slice);
+    auto const total_size = std::accumulate(all_sizes.cbegin(), all_sizes.cend(), 0ul);
+    auto const begin_index =
+        std::accumulate(all_sizes.cbegin(), all_sizes.cbegin() + GetRank(), 0ul);
+    auto const size_prev_slice = GetRank() == 0 ? 0 : all_sizes[GetRank() - 1];
+
+    std::string result(total_size, '\0');
+    result.replace(begin_index, size_node_slice, input);
+    rabit::Allgather(result.data(), total_size, begin_index, size_node_slice, size_prev_slice);
+    return result;
   }
 
   void AllReduce(void *send_receive_buffer, std::size_t count, DataType data_type,
diff --git a/src/data/simple_dmatrix.cc b/src/data/simple_dmatrix.cc
index 1989f68a9444..48e764986966 100644
--- a/src/data/simple_dmatrix.cc
+++ b/src/data/simple_dmatrix.cc
@@ -76,10 +76,8 @@ DMatrix* SimpleDMatrix::SliceCol(int num_slices, int slice_id) {
 
 void SimpleDMatrix::ReindexFeatures(Context const* ctx) {
   if (info_.IsColumnSplit()) {
-    std::vector<uint64_t> buffer(collective::GetWorldSize());
-    buffer[collective::GetRank()] = info_.num_col_;
-    collective::Allgather(buffer.data(), buffer.size() * sizeof(uint64_t));
-    auto offset = std::accumulate(buffer.cbegin(), buffer.cbegin() + collective::GetRank(), 0ul);
+    auto const cols = collective::Allgather(info_.num_col_);
+    auto const offset = std::accumulate(cols.cbegin(), cols.cbegin() + collective::GetRank(), 0ul);
     if (offset == 0) {
       return;
     }
diff --git a/src/tree/hist/evaluate_splits.h b/src/tree/hist/evaluate_splits.h
index b4612e24c552..680c50398b42 100644
--- a/src/tree/hist/evaluate_splits.h
+++ b/src/tree/hist/evaluate_splits.h
@@ -292,20 +292,19 @@ class HistEvaluator {
    */
   std::vector<CPUExpandEntry> Allgather(std::vector<CPUExpandEntry> const &entries) {
     auto const world = collective::GetWorldSize();
-    auto const rank = collective::GetRank();
     auto const num_entries = entries.size();
 
     // First, gather all the primitive fields.
-    std::vector<CPUExpandEntry> all_entries(num_entries * world);
+    std::vector<CPUExpandEntry> local_entries(num_entries);
     std::vector<uint32_t> cat_bits;
     std::vector<std::size_t> cat_bits_sizes;
     for (std::size_t i = 0; i < num_entries; i++) {
-      all_entries[num_entries * rank + i].CopyAndCollect(entries[i], &cat_bits, &cat_bits_sizes);
+      local_entries[i].CopyAndCollect(entries[i], &cat_bits, &cat_bits_sizes);
     }
-    collective::Allgather(all_entries.data(), all_entries.size() * sizeof(CPUExpandEntry));
+    auto all_entries = collective::Allgather(local_entries);
 
     // Gather all the cat_bits.
-    auto gathered = collective::AllgatherV(cat_bits, cat_bits_sizes);
+    auto gathered = collective::SpecialAllgatherV(cat_bits, cat_bits_sizes);
 
     common::ParallelFor(num_entries * world, ctx_->Threads(), [&] (auto i) {
       // Copy the cat_bits back into all expand entries.
@@ -579,28 +578,24 @@ class HistMultiEvaluator {
    */
   std::vector<MultiExpandEntry> Allgather(std::vector<MultiExpandEntry> const &entries) {
     auto const world = collective::GetWorldSize();
-    auto const rank = collective::GetRank();
     auto const num_entries = entries.size();
 
     // First, gather all the primitive fields.
-    std::vector<MultiExpandEntry> all_entries(num_entries * world);
+    std::vector<MultiExpandEntry> local_entries(num_entries);
     std::vector<uint32_t> cat_bits;
     std::vector<std::size_t> cat_bits_sizes;
     std::vector<GradientPairPrecise> gradients;
     for (std::size_t i = 0; i < num_entries; i++) {
-      all_entries[num_entries * rank + i].CopyAndCollect(entries[i], &cat_bits, &cat_bits_sizes,
-                                                         &gradients);
+      local_entries[i].CopyAndCollect(entries[i], &cat_bits, &cat_bits_sizes, &gradients);
     }
-    collective::Allgather(all_entries.data(), all_entries.size() * sizeof(MultiExpandEntry));
+    auto all_entries = collective::Allgather(local_entries);
 
     // Gather all the cat_bits.
-    auto gathered_cat_bits = collective::AllgatherV(cat_bits, cat_bits_sizes);
+    auto gathered_cat_bits = collective::SpecialAllgatherV(cat_bits, cat_bits_sizes);
 
     // Gather all the gradients.
     auto const num_gradients = gradients.size();
-    std::vector<GradientPairPrecise> all_gradients(num_gradients * world);
-    std::copy_n(gradients.cbegin(), num_gradients, all_gradients.begin() + num_gradients * rank);
-    collective::Allgather(all_gradients.data(), all_gradients.size() * sizeof(GradientPairPrecise));
+    auto const all_gradients = collective::Allgather(gradients);
 
     auto const total_entries = num_entries * world;
     auto const gradients_per_entry = num_gradients / num_entries;
diff --git a/tests/cpp/collective/test_in_memory_communicator.cc b/tests/cpp/collective/test_in_memory_communicator.cc
index f36e30e3391d..69c427a4e642 100644
--- a/tests/cpp/collective/test_in_memory_communicator.cc
+++ b/tests/cpp/collective/test_in_memory_communicator.cc
@@ -29,6 +29,11 @@ class InMemoryCommunicatorTest : public ::testing::Test {
     VerifyAllgather(comm, rank);
   }
 
+  static void AllgatherV(int rank) {
+    InMemoryCommunicator comm{kWorldSize, rank};
+    VerifyAllgatherV(comm, rank);
+  }
+
   static void AllreduceMax(int rank) {
     InMemoryCommunicator comm{kWorldSize, rank};
     VerifyAllreduceMax(comm, rank);
@@ -80,14 +85,19 @@ class InMemoryCommunicatorTest : public ::testing::Test {
 
  protected:
   static void VerifyAllgather(InMemoryCommunicator &comm, int rank) {
-    char buffer[kWorldSize] = {'a', 'b', 'c'};
-    buffer[rank] = '0' + rank;
-    comm.AllGather(buffer, kWorldSize);
+    std::string input{static_cast<char>('0' + rank)};
+    auto output = comm.AllGather(input);
     for (auto i = 0; i < kWorldSize; i++) {
-      EXPECT_EQ(buffer[i], '0' + i);
+      EXPECT_EQ(output[i], static_cast<char>('0' + i));
     }
   }
 
+  static void VerifyAllgatherV(InMemoryCommunicator &comm, int rank) {
+    std::vector<std::string_view> inputs{"a", "bb", "ccc"};
+    auto output = comm.AllGatherV(inputs[rank]);
+    EXPECT_EQ(output, "abbccc");
+  }
+
   static void VerifyAllreduceMax(InMemoryCommunicator &comm, int rank) {
     int buffer[] = {1 + rank, 2 + rank, 3 + rank, 4 + rank, 5 + rank};
     comm.AllReduce(buffer, sizeof(buffer) / sizeof(buffer[0]), DataType::kInt32, Operation::kMax);
@@ -205,6 +215,8 @@ TEST(InMemoryCommunicatorSimpleTest, IsDistributed) {
 
 TEST_F(InMemoryCommunicatorTest, Allgather) { Verify(&Allgather); }
 
+TEST_F(InMemoryCommunicatorTest, AllgatherV) { Verify(&AllgatherV); }
+
 TEST_F(InMemoryCommunicatorTest, AllreduceMax) { Verify(&AllreduceMax); }
 
 TEST_F(InMemoryCommunicatorTest, AllreduceMin) { Verify(&AllreduceMin); }
diff --git a/tests/cpp/plugin/helpers.h b/tests/cpp/plugin/helpers.h
index 20b4afc3026b..b756adefd9b8 100644
--- a/tests/cpp/plugin/helpers.h
+++ b/tests/cpp/plugin/helpers.h
@@ -23,7 +23,7 @@ class ServerForTest {
   std::unique_ptr<grpc::Server> server_;
 
  public:
-  explicit ServerForTest(std::int32_t world_size) {
+  explicit ServerForTest(std::size_t world_size) {
     server_thread_.reset(new std::thread([this, world_size] {
       grpc::ServerBuilder builder;
       xgboost::federated::FederatedService service{world_size};
diff --git a/tests/cpp/plugin/test_federated_communicator.cc b/tests/cpp/plugin/test_federated_communicator.cc
index 8b0e1039adff..68b112f1c7b1 100644
--- a/tests/cpp/plugin/test_federated_communicator.cc
+++ b/tests/cpp/plugin/test_federated_communicator.cc
@@ -19,6 +19,11 @@ class FederatedCommunicatorTest : public BaseFederatedTest {
     CheckAllgather(comm, rank);
   }
 
+  static void VerifyAllgatherV(int rank, const std::string &server_address) {
+    FederatedCommunicator comm{kWorldSize, rank, server_address};
+    CheckAllgatherV(comm, rank);
+  }
+
   static void VerifyAllreduce(int rank, const std::string &server_address) {
     FederatedCommunicator comm{kWorldSize, rank, server_address};
     CheckAllreduce(comm);
@@ -31,14 +36,19 @@ class FederatedCommunicatorTest : public BaseFederatedTest {
 
  protected:
   static void CheckAllgather(FederatedCommunicator &comm, int rank) {
-    int buffer[kWorldSize] = {0, 0};
-    buffer[rank] = rank;
-    comm.AllGather(buffer, sizeof(buffer));
+    std::string input{static_cast<char>('0' + rank)};
+    auto output = comm.AllGather(input);
     for (auto i = 0; i < kWorldSize; i++) {
-      EXPECT_EQ(buffer[i], i);
+      EXPECT_EQ(output[i], static_cast<char>('0' + i));
     }
   }
 
+  static void CheckAllgatherV(FederatedCommunicator &comm, int rank) {
+    std::vector<std::string_view> inputs{"Federated", " Learning!!!"};
+    auto output = comm.AllGatherV(inputs[rank]);
+    EXPECT_EQ(output, "Federated Learning!!!");
+  }
+
   static void CheckAllreduce(FederatedCommunicator &comm) {
     int buffer[] = {1, 2, 3, 4, 5};
     comm.AllReduce(buffer, sizeof(buffer) / sizeof(buffer[0]), DataType::kInt32, Operation::kSum);
@@ -119,6 +129,16 @@ TEST_F(FederatedCommunicatorTest, Allgather) {
   }
 }
 
+TEST_F(FederatedCommunicatorTest, AllgatherV) {
+  std::vector<std::thread> threads;
+  for (auto rank = 0; rank < kWorldSize; rank++) {
+    threads.emplace_back(&FederatedCommunicatorTest::VerifyAllgatherV, rank, server_->Address());
+  }
+  for (auto &thread : threads) {
+    thread.join();
+  }
+}
+
 TEST_F(FederatedCommunicatorTest, Allreduce) {
   std::vector<std::thread> threads;
   for (auto rank = 0; rank < kWorldSize; rank++) {
diff --git a/tests/cpp/plugin/test_federated_server.cc b/tests/cpp/plugin/test_federated_server.cc
index 633d64df10f8..c40e58fa388f 100644
--- a/tests/cpp/plugin/test_federated_server.cc
+++ b/tests/cpp/plugin/test_federated_server.cc
@@ -18,6 +18,11 @@ class FederatedServerTest : public BaseFederatedTest {
     CheckAllgather(client, rank);
   }
 
+  static void VerifyAllgatherV(int rank, const std::string& server_address) {
+    federated::FederatedClient client{server_address, rank};
+    CheckAllgatherV(client, rank);
+  }
+
   static void VerifyAllreduce(int rank, const std::string& server_address) {
     federated::FederatedClient client{server_address, rank};
     CheckAllreduce(client);
@@ -39,8 +44,7 @@ class FederatedServerTest : public BaseFederatedTest {
 
  protected:
   static void CheckAllgather(federated::FederatedClient& client, int rank) {
-    int data[kWorldSize] = {0, 0};
-    data[rank] = rank;
+    int data[] = {rank};
     std::string send_buffer(reinterpret_cast<char const*>(data), sizeof(data));
     auto reply = client.Allgather(send_buffer);
     auto const* result = reinterpret_cast<int const*>(reply.data());
@@ -49,6 +53,12 @@ class FederatedServerTest : public BaseFederatedTest {
     }
   }
 
+  static void CheckAllgatherV(federated::FederatedClient& client, int rank) {
+    std::vector<std::string_view> inputs{"Hello,", " World!"};
+    auto reply = client.AllgatherV(inputs[rank]);
+    EXPECT_EQ(reply, "Hello, World!");
+  }
+
   static void CheckAllreduce(federated::FederatedClient& client) {
     int data[] = {1, 2, 3, 4, 5};
     std::string send_buffer(reinterpret_cast<char const*>(data), sizeof(data));
@@ -80,6 +90,16 @@ TEST_F(FederatedServerTest, Allgather) {
   }
 }
 
+TEST_F(FederatedServerTest, AllgatherV) {
+  std::vector<std::thread> threads;
+  for (auto rank = 0; rank < kWorldSize; rank++) {
+    threads.emplace_back(&FederatedServerTest::VerifyAllgatherV, rank, server_->Address());
+  }
+  for (auto& thread : threads) {
+    thread.join();
+  }
+}
+
 TEST_F(FederatedServerTest, Allreduce) {
   std::vector<std::thread> threads;
   for (auto rank = 0; rank < kWorldSize; rank++) {