“distinct” and “mapReduce”

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 }