hadoop - 如何从复杂的 pig 数据类型中提取简单的 pig 数据类型

Question

我正在尝试使用内置的 BuildBloom 和 Bloom UDF 在 PIG 中编写布隆过滤器构建器。调用 BuildBloom UDF 的语法是:

define bb BuildBloom('hash_type', 'vector_size', 'false_positive_rate');

其中向量大小和误报率参数作为字符数组传入。因为我不一定事先知道矢量大小，但在调用 BuildBloom UDF 之前它总是在脚本中可用，所以我想使用内置的 COUNT UDF而不是一些硬编码的值。像这样的东西:

records = LOAD '$input' using PigStorage();
records = FOREACH records GENERATE 
    (long)     $0 AS value_fld:long, 
    (chararray)$1 AS filter_fld:chararray;
records_fltr = FILTER records by (filter_fld=='$filter_value') AND (value_fld is not null);
records_grp = GROUP records_fltr all;
records_count = FOREACH records_grp GENERATE (chararray) COUNT(records_fltr.value_fld) AS count:chararray;
n = FOREACH records_count GENERATE flatten(count);
define bb BuildBloom('jenkins', n, '$false_positive_rate');

问题是，当我描述 n 时，我得到:n: {count: chararray}。不出所料，BuildBloom UDF 调用失败了，因为它得到了一个元组作为输入，而它期望的是一个简单的 chararray。我应该如何只提取 chararray(即从 COUNT 转换为 chararray 的整数返回)并将其分配给 n 以便在调用中使用到 BuildBloom(...)？

编辑:这是我尝试将 N::count 传递到 BuildBloom(...) UDF 时产生的错误。 describe N 产生:N {count: chararray}。违规行(第 40 行)显示为:define bb BuildBloom('jenkins', N::count, '$fpr');

ERROR 1200: <file buildBloomFilter.pig, line 40, column 32>  mismatched input 'N::count' expecting set null

org.apache.pig.impl.logicalLayer.FrontendException: ERROR 1000: Error during parsing. <file buildBloomFilter.pig, line 40, column 32>  mismatched input 'N::count' expecting set null
    at org.apache.pig.PigServer$Graph.parseQuery(PigServer.java:1607)
    at org.apache.pig.PigServer$Graph.registerQuery(PigServer.java:1546)
    at org.apache.pig.PigServer.registerQuery(PigServer.java:516)
    at org.apache.pig.tools.grunt.GruntParser.processPig(GruntParser.java:991)
    at org.apache.pig.tools.pigscript.parser.PigScriptParser.parse(PigScriptParser.java:412)
    at org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:194)
    at org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:170)
    at org.apache.pig.tools.grunt.Grunt.exec(Grunt.java:84)
    at org.apache.pig.Main.run(Main.java:604)
    at org.apache.pig.Main.main(Main.java:157)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.apache.hadoop.util.RunJar.main(RunJar.java:197)
Caused by: Failed to parse: <file buildBloomFilter.pig, line 40, column 32>  mismatched input 'N::count' expecting set null
    at org.apache.pig.parser.QueryParserDriver.parse(QueryParserDriver.java:235)
    at org.apache.pig.parser.QueryParserDriver.parse(QueryParserDriver.java:177)
    at org.apache.pig.PigServer$Graph.parseQuery(PigServer.java:1599)
    ... 14 more

Answer 1

如果您使用的是 grunt shell，那么执行此操作的明显方法是调用 DUMP n;，等待作业完成运行，然后将值复制到您的 define绽放... 调用。

我猜这不是一个非常令人满意的答案。您很可能希望在脚本中运行它。这是一个非常 hacky 的方法。您需要 3 个文件:

1. 'n_start.txt' 其中包含:

   n='
   

2. 'n_end.txt' 包含单个字符:

   '
   

3. 'bloom_build.pig' 包含:

   define bb BuildBloom('jenkins', '$n', '0.0001');
   

一旦你有了这些，你就可以运行这个脚本:

records = LOAD '$input' using PigStorage();
records = FOREACH records GENERATE 
    (long)     $0 AS value_fld:long, 
    (chararray)$1 AS filter_fld:chararray;
records_fltr = FILTER records by (filter_fld=='$filter_value') 
    AND (value_fld is not null);
records_grp = GROUP records_fltr all;
records_count = FOREACH records_grp GENERATE 
    (chararray) COUNT(records_fltr.value_fld) AS count:chararray;
n = FOREACH records_count GENERATE flatten(count);

--the new part
STORE records_count INTO 'n' USING PigStorgae(',');
--this will copy what you just stored into a local directory
fs -copyToLocal n n
--this will cat the two static files we created prior to running pig
--with the count we just generated.  it will pass it through tr which will
--strip out the newlines and then store it into a file called 'n.txt' which we
--will use as a parameter file
sh cat -s nstart.txt n/part-r-00000 nend.txt| tr -d '\n' > n.txt
--RUN makes pig call one script within another.  Be forewarned that if pig returns 
--a message with an error on a certain line, it is the line number of the expanded script
RUN -param_file n.txt bloom_bulid.pig;

在此之后，您可以像之前那样调用 bb。这很丑陋，可能更精通 unix 的人可以摆脱 n_start.txt 和 n_end.txt 文件。

另一个更简洁但更复杂的选择是编写一个新的 UDF(如 BuildBloom)扩展 BuildBloomBase.java 但具有空构造函数并且可以处理 exec() 方法中的所有内容。