I am new to scalding world. My scalding job will have multiple stages, and I need to tune each stage individually.
I have found that we might be able to change the number of reducers by using withReducers. Also, I am able to set the split size for the input data by the job config. However, I didn't see there is any way to change the number of mappers for my sub-tasks on the fly.
Did I miss something? Does anyone know how to specify the number of mappers for my sub-tasks? Thanks.
Got some answers/ideas might be helpful for someone else who shared the same question.
It is much easier to control reducers compared to mappers.
Mappers are controlled by hadoop without a similar simple knob. You can set some config parameters to give hadoop an idea of how many map tasks to launch.
This stack overflow may be helpful:
Setting the number of map tasks and reduce tasks
One workaround I could think of is changing your major task to small ones, which you could individually tweak the size (# of mappers) of your input data.
Related
What I'm trying to do is to deposit into HDFS blocks of size of 128MB I've been trying several processors but can't get the good one or I haven't identify the correct property:
This is how prety much the flow looks like:
Right now I'm using PutParquet but this processor doesn't have a property to do that
The previous processor is a MergeContent and this is the configuration
and on the SplitAvro I have next configuration
Hope someone can help I'm really stuck trying to do this.
You shouldn't need the SplitAvro or ConvertAvroToJSON, if you use MergeRecord instead you can supply an AvroReader and JsonRecordSetWriter and it will do the conversion for you. If you know the approximate number of records that will fit in an HDFS block, you can set that as the Maximum Number of Entries and also the Max Group Size. Keep in mind those are soft limits though, so you might want to set it to something safer like 100MB.
When you tried with your flow from the description, what did you observe? Were the files still too big, or did it not seem to obey the min/max limits, etc.?
I have data I have to join at the record level. For example data about users is coming in from different source systems but there is not a common primary key or user identifier
Example Data
Source System 1:
{userid = 123, first_name="John", last_name="Smith", many other columns...}
Source System 2:
{userid = EFCBA-09DA0, fname="J.", lname="Smith", many other columns...}
There are about 100 rules I can use to compare one record to another
to see if customer in source system 1 is the same as source system 2.
Some rules may be able to infer record values and add data to a master record about a customer.
Because some rules may infer/add data to any particular record, the rules must be re-applied again when a record changes.
We have millions of records per day we'd have to unify
Apache Beam / Dataflow implementation
Apache beam DAG is by definition acyclic but I could just republish the data through pubsub to the same DAG to make it a cyclic algorithm.
I could create a PCollection of hashmaps that continuously do a self join against all other elements but this seems it's probably an inefficient method
Immutability of a PCollection is a problem if I want to be constantly modifying things as it goes through the rules. This sounds like it would be more efficient with Flink Gelly or Spark GraphX
Is there any way you may know in dataflow to process such a problem efficiently?
Other thoughts
Prolog: I tried running on subset of this data with a subset of the rules but swi-prolog did not seem scalable, and I could not figure out how I would continuously emit the results to other processes.
JDrools/Jess/Rete: Forward chaining would be perfect for the inference and efficient partial application, but this algorithm is more about applying many many rules to individual records, rather than inferring record information from possibly related records.
Graph database: Something like neo4j or datomic would be nice since joins are at the record level rather than row/column scans, but I don't know if it's possible in beam to do something similar
BigQuery or Spanner: Brute forcing these rules in SQL and doing full table scans per record is really slow. It would be much preferred to keep the graph of all records in memory and compute in-memory. We could also try to concat all columns and run multiple compare and update across all columns
Or maybe there's a more standard way to solving these class of problems.
It is hard to say what solution works best for you from what I can read so far. I would try to split the problem further and try to tackle different aspects separately.
From what I understand, the goal is to combine together the matching records that represent the same thing in different sources:
records come from a number of sources:
it is logically the same data but formatted differently;
there are rules to tell if the records represent the same entity:
collection of rules is static;
So, the logic probably roughly goes like:
read a record;
try to find existing matching records;
if matching record found:
update it with new data;
otherwise save the record for future matching;
repeat;
To me this looks very high level and there's probably no single 'correct' solution at this level of detail.
I would probably try to approach this by first understanding it in more detail (maybe you already do), few thoughts:
what are the properties of the data?
are there patterns? E.g. when one system publishes something, do you expect something else from other systems?
what are the requirements in general?
latency, consistency, availability, etc;
how data is read from the sources?
can all the systems publish the records in batches in files, submit them into PubSub, does your solution need to poll them, etc?
can the data be read in parallel or is it a single stream?
then the main question of how can you efficiently match a record in general will probably look different under different assumptions and requirements as well. For example I would think about:
can you fit all data in memory;
are your rules dynamic. Do they change at all, what happens when they do;
can you split the data into categories that can be stored separately and matched efficiently, e.g. if you know you can try to match some things by id field, some other things by hash of something, etc;
do you need to match against all of historical/existing data?
can you have some quick elimination logic to not do expensive checks?
what is the output of the solution? What are the requirements for the output?
I want to understand MapReduce of Hbase from application point of view, Need some real use cases of it to better understand the efficient use case of writing these jobs.
If there is any link to document or examples that explains the real use cases, Please share.
I can give some example based on my use cases. If you already store your data in hbase, you can write a java program, which scans a table and do something, then write the output to hbase or somewhere else. OR you can use mapreduce to do the same. The difference is, mapreduce will run where the data is and network traffic is used only for result data. We have hourly jobs to calculate sum and average of kpis and input data is huge but output data is tiny for this task. If i did not use mapreduce, i need to move one hour of data over network which is 18gb. But mapreduce output is only 1mb and i can write it to hbase or file or somewhere else.
Also mapreduce gives you parallel task execution ability, which you can have in java but why :)
Keep in mind that YARN creates map tasks according to your hbase table's split count. So if you need more map task, split your table.
If you already store your data in hadoop hdfs, you are lucky, a mapreduce reading from hdfs is much faster than reading from hbase. Also you can still write mapreduce output to hbase, if you want.
Please look into the usecases given
1. here.
2. And a small reference here - 30.Joins
3. May be an end to end example here
In the end, it all depends on your understanding of each concept Map reduce, Hbase and use it as per your need in your project. The same task can be done with or without map reduce. Happy coding
I know this simple question, I need some help on this query from this community, When I create PartitionTable with ORC format, When I try to dump data from non partition table which is pointing to 2 GB File with 210 columns, I see Number of Mapper are 2 and reducer are 2 . is there a way to increase Mapper and reducer. My assumption is we cant set number of Mapper and reducer like MR 1.0, It is based on Settings like Yarn container size, Mapper minimum memory and maximum memory . can any one suggest me TEz Calculates mappers and reducers. What is best value to keep memory size setting, so that i dont come across : Java heap space, Java Out of Memory problem. My file size may grow upto 100GB. Please help me on this.
You can still set the number of mappers and reducers in Yarn. Have you tried that? If so, please get back here.
Yarn changes the underlying execution mechanism, but #mappers and #reducers is describing the Job requirements - not the way the job resources are allocated (which is how yarn and mrv1 differ).
Traditional Map/Reduce has a hard coded number of map and reduce "slot". As you say - Yarn uses containers - which are per-application. Yarn is thus more flexible. But the #mappers and #reducers are inputs of the job in both cases. And also in both cases the actual number of mappers and reducers may differ from the requested number. Typically the #reducers would either be
(a) precisely the number that was requested
(b) exactly ONE reducer - that is if the job required it such as in total ordering
For the memory settings, if you are using hive with tez, the following 2 settings will be of use to you:
1) hive.tez.container.size - this is the size of the Yarn Container that will be used ( value in MB ).
2) hive.tez.java.opts - this is for the java opts that will be used for each task. If container size is set to 1024 MB, set java opts to say something like "-Xmx800m" and not "-Xmx1024m". YARN kills processes that use more memory than specified container size and given that a java process's memory footprint usually can exceed the specified Xmx value, setting Xmx to be the same value as the container size usually leads to problems.
Following the instructions on this link, I implemented a wordcount program in c++ using single mapper and single reducer. Now I need to use two mappers and one reducer for the same problem.
Can someone help me please in this regard?
The number of mappers depends on the number of input splits created. The number of input splits depends on the size of the input, the size of a block, the number of input files (each input file creates at least one input split), whether the input files are splittable or not, etc. See also this post in SO.
You can set the number of reducers to as many as you wish. I guess in hadoop pipes you can do this by setting the -D mapred.reduce.tasks=... when running hadoop. See this post in SO.
If you want to quickly test how your program works with more than one mappers, you can simply put a new file in your input path. This will make hadoop create another input split and thus another map task.
PS: The link that you provide is not reachable.