feat: support describe group (#7)

Author: BewareMyPower

README.md

@@ -125,7 +125,21 @@ topic count: 2
 ```bash
 $ snctl-cpp groups list
 There are 1 group
-[0] my-group Stable
+[0] sub Stable
+```
+
+### Describe a specific consumer group
+
+```bash
+$ snctl-cpp groups describe sub
+Group ID: sub
+Assignor: range
+State: Stable
+Type: 2
+There are 2 members:
+| index | client id | consumer id | host | assignments |
+| 0 | consumer-sub-1 | consumer-sub-1-b97d2b45-86cf-4352-8e82-9ebdfd6fbff6 | /127.0.0.1:54214 | [test-0, test-1] |
+| 1 | consumer-sub-2 | consumer-sub-2-63b7c688-3007-4650-91eb-404284dfd837 | /127.0.0.1:54213 | [test-2, test-3] |
 ```
 
 The format of each line is `[index] <group-id> <state>`.

include/snctl-cpp/groups.h

@@ -15,6 +15,7 @@
  */
 #pragma once
 
+#include "snctl-cpp/groups/describe_group.h"
 #include "snctl-cpp/groups/list_groups.h"
 #include "snctl-cpp/subcommand.h"
 
@@ -25,19 +26,29 @@ class Groups : public SubCommand {
 public:
   explicit Groups(argparse::ArgumentParser &parent) : SubCommand("groups") {
     list_command_.add_description("List all consumer groups");
+    describe_command_.add_description("Describe a specific consumer group")
+        .add_argument("group")
+        .help("The group id")
+        .required();
 
     add_child(list_command_);
+    add_child(describe_command_);
+
     attach_parent(parent);
   }
 
   void run(rd_kafka_t *rk, rd_kafka_queue_t *rkqu) {
     if (is_subcommand_used(list_command_)) {
       list_groups(rk, rkqu);
+    } else if (is_subcommand_used(describe_command_)) {
+      auto group = describe_command_.get("group");
+      describe_group(rk, rkqu, group);
     } else {
       fail();
     }
   }
 
 private:
   argparse::ArgumentParser list_command_{"list"};
+  argparse::ArgumentParser describe_command_{"describe"};
 };

include/snctl-cpp/groups/describe_group.h

@@ -0,0 +1,117 @@
+/**
+ * Copyright 2025 Yunze Xu
+ *
+ * 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 "snctl-cpp/rk_event_wrapper.h"
+#include <cassert>
+#include <cstddef>
+#include <iostream>
+#include <librdkafka/rdkafka.h>
+#include <ostream>
+#include <string>
+
+static std::ostream &operator<<(std::ostream &os, const rd_kafka_Node_t *node) {
+  os << rd_kafka_Node_host(node) << ":" << rd_kafka_Node_port(node);
+  if (const auto *rack = rd_kafka_Node_rack(node); rack != nullptr) {
+    os << " (rack: " << rack << ")";
+  }
+  return os;
+}
+
+static std::ostream &operator<<(std::ostream &os,
+                                const rd_kafka_topic_partition_t &partition) {
+  os << partition.topic << "-" << partition.partition;
+  return os;
+}
+
+inline void describe_group(rd_kafka_t *rk, rd_kafka_queue_t *rkqu,
+                           const std::string &group) {
+  const char *groups[1] = {group.c_str()};
+  rd_kafka_DescribeConsumerGroups(rk, groups, 1, nullptr, rkqu);
+
+  try {
+    auto event = RdKafkaEvent::poll(rkqu);
+    const auto *result =
+        rd_kafka_event_DescribeConsumerGroups_result(event.handle());
+    assert(result != nullptr);
+
+    size_t group_count;
+    const auto *groups =
+        rd_kafka_DescribeConsumerGroups_result_groups(result, &group_count);
+    assert(groups);
+    if (group_count != 1) {
+      std::cerr << "Expected exactly one group, but got " << group_count
+                << std::endl;
+      return;
+    }
+
+    const auto *group = groups[0];
+    const auto *group_id = rd_kafka_ConsumerGroupDescription_group_id(group);
+    const auto *error = rd_kafka_ConsumerGroupDescription_error(group);
+    if (error != nullptr) {
+      std::cerr << "Error describing group '" << group_id
+                << "': " << rd_kafka_error_string(error) << std::endl;
+      return;
+    }
+
+    const auto *assignor =
+        rd_kafka_ConsumerGroupDescription_partition_assignor(group);
+    const auto state = rd_kafka_ConsumerGroupDescription_state(group);
+    const auto *state_name = rd_kafka_consumer_group_state_name(state);
+    const auto *node = rd_kafka_ConsumerGroupDescription_coordinator(group);
+    const auto type = rd_kafka_ConsumerGroupDescription_type(group);
+
+    std::cout << "Group ID: " << group_id << std::endl;
+    std::cout << "Assignor: " << assignor << std::endl;
+    std::cout << "State: " << state_name << std::endl;
+    std::cout << "Type: " << type << std::endl;
+    std::cout << "Coordinator: " << node << std::endl;
+    const auto member_count =
+        rd_kafka_ConsumerGroupDescription_member_count(group);
+
+    if (member_count > 0) {
+      std::cout << "There are " << member_count << " members:" << std::endl;
+      std::cout << "| index | client id | consumer id | host | assignments |"
+                << std::endl;
+    } else {
+      std::cout << "No members" << std::endl;
+    }
+    for (size_t i = 0; i < member_count; i++) {
+      const auto *member = rd_kafka_ConsumerGroupDescription_member(group, i);
+      assert(member != nullptr);
+      const auto *assignment = rd_kafka_MemberDescription_assignment(member);
+      assert(assignment != nullptr);
+      std::cout << "| " << i << " | "
+                << rd_kafka_MemberDescription_client_id(member) << " | "
+                << rd_kafka_MemberDescription_consumer_id(member) << " | "
+                << rd_kafka_MemberDescription_host(member);
+      const auto *partitions = rd_kafka_MemberAssignment_partitions(assignment);
+      assert(partitions != nullptr);
+      std::cout << " | [";
+      for (int j = 0; j < partitions->cnt; j++) {
+        if (j > 0) {
+          std::cout << ", ";
+        }
+        std::cout << partitions->elems[j];
+      }
+      std::cout << "] |" << std::endl;
+    }
+
+  } catch (const std::runtime_error &e) {
+    std::cerr << "Failed to describe group '" << group << "': " << e.what()
+              << std::endl;
+  }
+}