MONGOC_DISTINCT_MAPREDUCE(3)

libmongoc

MONGOC_DISTINCT_MAPREDUCE(3)

This document provides some practical, simple, examples to demonstrate the distinct and mapReduce commands.

SETUP¶

First we'll write some code to insert sample data:

doc-common-insert.c

/* Don't try to compile this file on its own. It's meant to be #included


   by example code */
/* Insert some sample data */
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 COMMAND¶

This is how to use the distinct command to get the distinct values of x which are greater than 1:

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 - BASIC EXAMPLE¶

A simple example using the map reduce framework. It simply adds up the number of occurrences of each "tag".

First define the map and reduce functions:

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. */

Run the mapReduce command. Use the generic command helpers (e.g. mongoc_database_command_simple()). Do not the read command helpers (e.g. mongoc_database_read_command_with_opts()) because they are considered retryable read operations. If retryable reads are enabled, those operations will retry once on a retryable error, giving undesirable behavior for mapReduce.

map-reduce-basic.c

bool
map_reduce_basic (mongoc_database_t *database)
{


   bson_t reply;


   bson_t *command;


   bool res;


   bson_error_t error;


   mongoc_cursor_t *cursor;


   const bson_t *doc;


   bool query_done = false;


   const char *out_collection_name = "outCollection";


   mongoc_collection_t *out_collection;


   /* 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 */


   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 */


   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 - MORE COMPLICATED EXAMPLE¶

You must have replica set running for this.

In this example we contact a secondary in the replica set and do an "inline" map reduce, so the results are returned immediately:

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;
}

RUNNING THE EXAMPLES¶

Here's how to run the example code

basic-aggregation.c

/*


 * Copyright 2016 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;
}

If you want to try the advanced map reduce example with a secondary, start a replica set (instructions for how to do this can be found here).

Otherwise, just start an instance of MongoDB:

$ mongod

Now compile and run the example program:

$ 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 }

AUTHOR¶

MongoDB, Inc

COPYRIGHT¶

2017-present, MongoDB, Inc

February 25, 2024

1.26.0

Source file:	mongoc_distinct_mapreduce.3.en.gz (from libmongoc-doc )
Source last updated:	2024-02-25T08:22:45Z
Converted to HTML:	2024-10-21T18:04:02Z