1DBMAPREDUCE(1) User Contributed Perl Documentation DBMAPREDUCE(1)
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6 dbmapreduce - reduce all input rows with the same key
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9 dbmapreduce [-dMS] [-k KeyField] [-f CodeFile] [-C Filtercode] [--] [ReduceCommand [ReduceArguments...]]
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12 Group input data by KeyField, then apply a function (the "reducer") to
13 each group. The reduce function can be an external program given by
14 ReduceCommand and ReduceArguments, or an Perl subroutine given in
15 CodeFile or FilterCode.
16 If a "--" appears before reduce command, arguments after the -- passed
18 the the command.
19 Grouping (The Mapper)
21 By default the KeyField is the first field in the row. Unlike Hadoop
22 streaming, the -k KeyField option can explicitly name where the key is
23 in any column of each input row.
24 By default, we sort the data to make sure data is grouped by key. If
26 the input is already grouped, the "-S" option avoids this cost.
27 The Reducer
29 Reduce functions default to be shell commands. However, with "-C", one
30 can use arbitrary Perl code
31 (see the "-C" option below for details). the "-f" option is useful to
33 specify complex Perl code somewhere other than the command line.
34 Finally, as a special case, if there are no rows of input, the reducer
36 will be invoked once with the empty value (if it's an external reducer)
37 or with undef (if it's a subroutine). It is expected to generate the
38 output header, and it may generate no data rows itself, or a null data
39 row of its choosing.
40 Output
42 For non-multi-key-aware reducers, we add the KeyField use for each
43 Reduce is in the output stream. (If the reducer passes the key we
44 trust that it gives a correct value.) We also insure that the output
45 field separator is the same as the input field separator.
46 Adding the key and adjusting the output field separator is not possible
48 for non-multi-key-aware reducers.
49 Comparison to Related Work
51 This program thus implements Google-style map/reduce, but executed
52 sequentially.
53 For input, these systems include a map function and apply it to input
55 data to generate the key. We assume this key generation (the map
56 function) has occurred head of time.
57 We also allow the grouping key to be in any column. Hadoop Streaming
59 requires it to be in the first column.
60 By default, the reducer gets exactly (and only) one key. This
62 invariant is stronger than Google and Hadoop. They both pass multiple
63 keys to the reducer, insuring that each key is grouped together. With
64 the "-M" option, we also pass multiple multiple groups to the reducer.
65 Unlike those systems, with the "-S" option we do not require the groups
67 arrive in any particular order, just that they be grouped together.
68 (They guarantees they arrive in lexically sorted order). However, with
69 "-S" we create lexical ordering.
70 With "--prepend-key" we insure that the KeyField is in the output
72 stream; other systems do not enforce this.
73 Assumptions and requirements
75 By default, data can be provided in arbitrary order and the program
76 consumes O(number of unique tags) memory, and O(size of data) disk
77 space.
78 With the "-S" option, data must arrive group by tags (not necessarily
80 sorted), and the program consumes O(number of tags) memory and no disk
81 space. The program will check and abort if this precondition is not
82 met.
83 With two "-S"'s, program consumes O(1) memory, but doesn't verify that
85 the data-arrival precondition is met.
86 The field separators of the input and the output can now be different
88 (early versions of this tool prohibited such variation.) With
89 "--copy-fs" we copy the input field separator to the output, but only
90 for non-multi-key-aware reducers. (this used to be done
91 automatically). Alternatively, one can specify the output field
92 separator with "--fieldseparator", in which case the output had better
93 generate that format. An explicit "--fieldseparator" takes priority
94 over "--copy-fs".
95 Known bugs
97 As of 2013-09-21, we don't verify key order with options "-M -S".
98
100 -k or --key KeyField
101 Specify which column is the key for grouping (default: the first
102 column).
103 Note that dbmapreduce can only operate on one column as the key.
105 To group on the combination of multiple columns, one must merge
106 them, perhaps with dbcolmerge.
107 -S or --pre-sorted
109 Assume data is already grouped by tag. Provided twice, it removes
110 the validation of this assertion.
111 -M or --multiple-ok
113 Assume the ReduceCommand can handle multiple grouped keys, and the
114 ReduceCommand is responsible for outputting the with each output
115 row. (By default, a separate ReduceCommand is run for each key,
116 and dbmapreduce adds the key to each output row.)
117 -K or --pass-current-key
119 Pass the current key as an argument to the external, non-map-aware
120 ReduceCommand. This is only done optionally since some external
121 commands do not expect an extra argument. (Internal, non-map-aware
122 Perl reducers are always given the current key as an argument.)
123 --prepend-key
125 Add the current key into the reducer output for non-multi-key-aware
126 reducers only. Not done by default.
127 --copy-fs or --copy-fieldseparator
129 Change the field separator of a non-multi-key-aware reducers to
130 match the input's field separator. Not done by default.
131 --parallelism=N or -j N
133 Allow up to N reducers to run in parallel. Default is the number
134 of CPUs in the machine.
135 -F or --fs or --fieldseparator S
137 Specify the field (column) separator as "S". See dbfilealter for
138 valid field separators.
139 -C FILTER-CODE or --filter-code=FILTER-CODE
141 Provide FILTER-CODE, Perl code that generates and returns a
142 Fsdb::Filter object that implements the reduce function. The
143 provided code should be an anonymous sub that creates a Fsdb Filter
144 that implements the reduce object.
145 The reduce object will then be called with --input and --output
147 parameters that hook it into a the reduce with queues.
148 One sample fragment that works is just:
150 dbcolstats(qw(--nolog duration))
152 So this command:
154 cat DATA/stats.fsdb | \
156 dbmapreduce -k experiment -C 'dbcolstats(qw(--nolog duration))'
157 is the same as the example
159 cat DATA/stats.fsdb | \
161 dbmapreduce -k experiment -- dbcolstats duration
162 except that with "-C" there is no forking and so things run faster.
164 If "dbmapreduce" is invoked from within Perl, then one can use a
166 code SUB as well:
167 dbmapreduce(-k => 'experiment', -C => sub {
168 dbcolstats(qw(--nolong duration)) });
169 The reduce object must consume all input as a Fsdb stream, and
171 close the output Fsdb stream. (If this assumption is not met the
172 map/reduce will be aborted.)
173 For non-map-reduce-aware filters, when the filter-generator code
175 runs, $_[0] will be the current key.
176 -f CODE-FILE or --code-file=CODE-FILE
178 Includes CODE-FILE in the program. This option is useful for more
179 complicated perl reducer functions.
180 Thus, if reducer.pl has the code.
182 sub make_reducer {
184 my($current_key) = @_;
185 dbcolstats(qw(--nolog duration));
186 }
187 Then the command
189 cat DATA/stats.fsdb | \
191 dbmapreduce -k experiment -f reducer.pl -C make_reducer
192 does the same thing as the example.
194 -w or --warnings
196 Enable warnings in user supplied code. Warnings are issued if an
197 external reducer fails to consume all input. (Default to include
198 warnings.)
199 -T TmpDir
201 where to put tmp files. Also uses environment variable TMPDIR, if
202 -T is not specified. Default is /tmp.
203 This module also supports the standard fsdb options:
205 -d Enable debugging output.
207 -i or --input InputSource
209 Read from InputSource, typically a file name, or "-" for standard
210 input, or (if in Perl) a IO::Handle, Fsdb::IO or Fsdb::BoundedQueue
211 objects.
212 -o or --output OutputDestination
214 Write to OutputDestination, typically a file name, or "-" for
215 standard output, or (if in Perl) a IO::Handle, Fsdb::IO or
216 Fsdb::BoundedQueue objects.
217 --autorun or --noautorun
219 By default, programs process automatically, but Fsdb::Filter
220 objects in Perl do not run until you invoke the run() method. The
221 "--(no)autorun" option controls that behavior within Perl.
222 --header H
224 Use H as the full Fsdb header, rather than reading a header from
225 then input.
226 --help
228 Show help.
229 --man
231 Show full manual.
232
234 Input:
235 #fsdb experiment duration
236 ufs_mab_sys 37.2
237 ufs_mab_sys 37.3
238 ufs_rcp_real 264.5
239 ufs_rcp_real 277.9
240 Command:
242 cat DATA/stats.fsdb | \
243 dbmapreduce --prepend-key -k experiment -- dbcolstats duration
244 Output:
246 #fsdb experiment mean stddev pct_rsd conf_range conf_low conf_high conf_pct sum sum_squared min max n
247 ufs_mab_sys 37.25 0.070711 0.18983 0.6353 36.615 37.885 0.95 74.5 2775.1 37.2 37.3 2
248 ufs_rcp_real 271.2 9.4752 3.4938 85.13 186.07 356.33 0.95 542.4 1.4719e+05 264.5 277.9 2
249 # | dbmapreduce -k experiment dbstats duration
250
252 Fsdb. dbmultistats dbrowsplituniq
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255 Copyright (C) 1991-2018 by John Heidemann <johnh@isi.edu>
256 This program is distributed under terms of the GNU general public
258 license, version 2. See the file COPYING with the distribution for
259 details.
260perl v5.38.0 2023-07-20 DBMAPREDUCE(1)