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"distinct" 和 "mapReduce"

本文档提供了一些实用、简单的示例来演示 distinct和mapReduce命令。

设置

首先，我们将写入一些代码来插入示例数据：

bool
insert_data (mongoc_collection_t *collection)
{
   mongoc_bulk_operation_t *bulk;
   enum N { ndocs = 4 };
   bson_t *docs[ndocs];
   bson_error_t error;
   int i = 0;
   bool ret;
   bulk = mongoc_collection_create_bulk_operation_with_opts (collection, NULL);
   docs[0] = BCON_NEW ("x", BCON_DOUBLE (1.0), "tags", "[", "dog", "cat", "]");
   docs[1] = BCON_NEW ("x", BCON_DOUBLE (2.0), "tags", "[", "cat", "]");
   docs[2] = BCON_NEW ("x", BCON_DOUBLE (2.0), "tags", "[", "mouse", "cat", "dog", "]");
   docs[3] = BCON_NEW ("x", BCON_DOUBLE (3.0), "tags", "[", "]");
   for (i = 0; i < ndocs; i++) {
      mongoc_bulk_operation_insert (bulk, docs[i]);
      bson_destroy (docs[i]);
      docs[i] = NULL;
   }
   ret = mongoc_bulk_operation_execute (bulk, NULL, &error);
   if (!ret) {
      fprintf (stderr, "Error inserting data: %s\n", error.message);
   }
   mongoc_bulk_operation_destroy (bulk);
   return ret;
}
/* A helper which we'll use a lot later on */
void
print_res (const bson_t *reply)
{
   char *str;
   BSON_ASSERT (reply);
   str = bson_as_canonical_extended_json (reply, NULL);
   printf ("%s\n", str);
   bson_free (str);
}

"distinct" 命令

以下是使用distinct命令获取大于1的x不同值的方法：

distinct.c

bool
distinct (mongoc_database_t *database)
{
   bson_t *command;
   bson_t reply;
   bson_error_t error;
   bool res;
   bson_iter_t iter;
   bson_iter_t array_iter;
   double val;
   command = BCON_NEW ("distinct",
                       BCON_UTF8 (COLLECTION_NAME),
                       "key",
                       BCON_UTF8 ("x"),
                       "query",
                       "{",
                       "x",
                       "{",
                       "$gt",
                       BCON_DOUBLE (1.0),
                       "}",
                       "}");
   res = mongoc_database_command_simple (database, command, NULL, &reply, &error);
   if (!res) {
      fprintf (stderr, "Error with distinct: %s\n", error.message);
      goto cleanup;
   }
   /* Do something with reply (in this case iterate through the values) */
   if (!(bson_iter_init_find (&iter, &reply, "values") && BSON_ITER_HOLDS_ARRAY (&iter) &&
         bson_iter_recurse (&iter, &array_iter))) {
      fprintf (stderr, "Couldn't extract \"values\" field from response\n");
      goto cleanup;
   }
   while (bson_iter_next (&array_iter)) {
      if (BSON_ITER_HOLDS_DOUBLE (&array_iter)) {
         val = bson_iter_double (&array_iter);
         printf ("Next double: %f\n", val);
      }
   }
cleanup:
   /* cleanup */
   bson_destroy (command);
   bson_destroy (&reply);
   return res;
}

"mapReduce" — 基本示例

使用 map reduce 框架的简单示例。它只是将每个“标签”出现的次数相加。

首先定义map和reduce函数：

constants.c

const char *const COLLECTION_NAME = "things";
/* Our map function just emits a single (key, 1) pair for each tag
   in the array: */
const char *const MAPPER = "function () {"
                           "this.tags.forEach(function(z) {"
                           "emit(z, 1);"
                           "});"
                           "}";
/* The reduce function sums over all of the emitted values for a
   given key: */
const char *const REDUCER = "function (key, values) {"
                            "var total = 0;"
                            "for (var i = 0; i < values.length; i++) {"
                            "total += values[i];"
                            "}"
                            "return total;"
                            "}";
/* Note We can't just return values.length as the reduce function
   might be called iteratively on the results of other reduce
   steps. */

运行 mapReduce 命令。使用通用命令助手（例如mongoc_database_command_simple ）。不要将读取命令助手（例如mongoc_database_read_command_with_opts），因为它们被视为可重试的读取操作。如果启用了可重试读取，这些操作将在出现可重试错误时重试一次，从而给 mapReduce 带来不良行为。

map-reduce-basic.c

bool
map_reduce_basic (mongoc_database_t *database)
{
   bson_t reply;
   bool res = false;
   bson_error_t error;
   mongoc_cursor_t *cursor = NULL;
   bool query_done = false;
   const char *out_collection_name = "outCollection";
   mongoc_collection_t *out_collection = NULL;
   /* Empty find query */
   bson_t find_query = BSON_INITIALIZER;
   /* Construct the mapReduce command */
   /* Other arguments can also be specified here, like "query" or
      "limit" and so on */
   bson_t *const command = BCON_NEW ("mapReduce",
                                     BCON_UTF8 (COLLECTION_NAME),
                                     "map",
                                     BCON_CODE (MAPPER),
                                     "reduce",
                                     BCON_CODE (REDUCER),
                                     "out",
                                     BCON_UTF8 (out_collection_name));
   res = mongoc_database_command_simple (database, command, NULL, &reply, &error);
   if (!res) {
      fprintf (stderr, "MapReduce failed: %s\n", error.message);
      goto cleanup;
   }
   /* Do something with the reply (it doesn't contain the mapReduce results) */
   print_res (&reply);
   /* Now we'll query outCollection to see what the results are */
   out_collection = mongoc_database_get_collection (database, out_collection_name);
   cursor = mongoc_collection_find_with_opts (out_collection, &find_query, NULL, NULL);
   query_done = true;
   /* Do something with the results */
   const bson_t *doc = NULL;
   while (mongoc_cursor_next (cursor, &doc)) {
      print_res (doc);
   }
   if (mongoc_cursor_error (cursor, &error)) {
      fprintf (stderr, "ERROR: %s\n", error.message);
      res = false;
      goto cleanup;
   }
cleanup:
   /* cleanup */
   if (query_done) {
      mongoc_cursor_destroy (cursor);
      mongoc_collection_destroy (out_collection);
   }
   bson_destroy (&reply);
   bson_destroy (command);
   return res;
}

"mapReduce" — 更复杂的示例

为此，必须运行副本集。

在此示例从节点(secondary node from replica set)，我们联系副本集的从节点并执行“内联”map reduce，因此会立即返回结果：

map-reduce-advanced.c

bool
map_reduce_advanced (mongoc_database_t *database)
{
   bson_t *command;
   bson_error_t error;
   bool res = true;
   mongoc_cursor_t *cursor;
   mongoc_read_prefs_t *read_pref;
   const bson_t *doc;
   /* Construct the mapReduce command */
   /* Other arguments can also be specified here, like "query" or "limit"
      and so on */
   /* Read the results inline from a secondary replica */
   command = BCON_NEW ("mapReduce",
                       BCON_UTF8 (COLLECTION_NAME),
                       "map",
                       BCON_CODE (MAPPER),
                       "reduce",
                       BCON_CODE (REDUCER),
                       "out",
                       "{",
                       "inline",
                       "1",
                       "}");
   read_pref = mongoc_read_prefs_new (MONGOC_READ_SECONDARY);
   cursor = mongoc_database_command (database, MONGOC_QUERY_NONE, 0, 0, 0, command, NULL, read_pref);
   /* Do something with the results */
   while (mongoc_cursor_next (cursor, &doc)) {
      print_res (doc);
   }
   if (mongoc_cursor_error (cursor, &error)) {
      fprintf (stderr, "ERROR: %s\n", error.message);
      res = false;
   }
   mongoc_cursor_destroy (cursor);
   mongoc_read_prefs_destroy (read_pref);
   bson_destroy (command);
   return res;
}

运行示例

运行示例代码的方法如下

Basic-aggregation.c

/*
 * Copyright 2009-present MongoDB, Inc.
 *
 * 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.
 */
#include <mongoc/mongoc.h>
#include <stdio.h>
#include "constants.c"
#include "../doc-common-insert.c"
#include "distinct.c"
#include "map-reduce-basic.c"
#include "map-reduce-advanced.c"
int
main (int argc, char *argv[])
{
   mongoc_database_t *database = NULL;
   mongoc_client_t *client = NULL;
   mongoc_collection_t *collection = NULL;
   mongoc_uri_t *uri = NULL;
   bson_error_t error;
   char *host_and_port = NULL;
   int exit_code = EXIT_FAILURE;
   if (argc != 2) {
      fprintf (stderr, "usage: %s CONNECTION-STRING\n", argv[0]);
      fprintf (stderr, "the connection string can be of the following forms:\n");
      fprintf (stderr, "localhost\t\t\t\tlocal machine\n");
      fprintf (stderr, "localhost:27018\t\t\t\tlocal machine on port 27018\n");
      fprintf (stderr,
               "mongodb://user:pass@localhost:27017\t"
               "local machine on port 27017, and authenticate with username "
               "user and password pass\n");
      return exit_code;
   }
   mongoc_init ();
   if (strncmp (argv[1], "mongodb://", 10) == 0) {
      host_and_port = bson_strdup (argv[1]);
   } else {
      host_and_port = bson_strdup_printf ("mongodb://%s", argv[1]);
   }
   uri = mongoc_uri_new_with_error (host_and_port, &error);
   if (!uri) {
      fprintf (stderr,
               "failed to parse URI: %s\n"
               "error message:       %s\n",
               host_and_port,
               error.message);
      goto cleanup;
   }
   client = mongoc_client_new_from_uri (uri);
   if (!client) {
      goto cleanup;
   }
   mongoc_client_set_error_api (client, 2);
   database = mongoc_client_get_database (client, "test");
   collection = mongoc_database_get_collection (database, COLLECTION_NAME);
   printf ("Inserting data\n");
   if (!insert_data (collection)) {
      goto cleanup;
   }
   printf ("distinct\n");
   if (!distinct (database)) {
      goto cleanup;
   }
   printf ("map reduce\n");
   if (!map_reduce_basic (database)) {
      goto cleanup;
   }
   printf ("more complicated map reduce\n");
   if (!map_reduce_advanced (database)) {
      goto cleanup;
   }
   exit_code = EXIT_SUCCESS;
cleanup:
   if (collection) {
      mongoc_collection_destroy (collection);
   }
   if (database) {
      mongoc_database_destroy (database);
   }
   if (client) {
      mongoc_client_destroy (client);
   }
   if (uri) {
      mongoc_uri_destroy (uri);
   }
   if (host_and_port) {
      bson_free (host_and_port);
   }
   mongoc_cleanup ();
   return exit_code;
}

如果您想使用从节点尝试高级从节点(secondary node from replica set)示例，请启动一个副本集（可以在此处找到有关如何执行此操作的说明）。

否则，仅启动一个 MongoDB 实例：

$ mongod

现在编译并运行示例程序：

$ cd examples/basic_aggregation/
$ gcc -Wall -o agg-example basic-aggregation.c $(pkg-config --cflags --libs libmongoc-1.0)
$ ./agg-example localhost
Inserting data
distinct
Next double: 2.000000
Next double: 3.000000
map reduce
{ "result" : "outCollection", "timeMillis" : 155, "counts" : { "input" : 84, "emit" : 126, "reduce" : 3, "output" : 3 }, "ok" : 1 }
{ "_id" : "cat", "value" : 63 }
{ "_id" : "dog", "value" : 42 }
{ "_id" : "mouse", "value" : 21 }
more complicated map reduce
{ "results" : [ { "_id" : "cat", "value" : 63 }, { "_id" : "dog", "value" : 42 }, { "_id" : "mouse", "value" : 21 } ], "timeMillis" : 14, "counts" : { "input" : 84, "emit" : 126, "reduce" : 3, "output" : 3 }, "ok" : 1 }

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