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From "Michel Tourn (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-288) RFC: Efficient file caching
Date Tue, 20 Jun 2006 00:12:30 GMT
    [ http://issues.apache.org/jira/browse/HADOOP-288?page=comments#action_12416824 ] 

Michel Tourn commented on HADOOP-288:
-------------------------------------

Hadoop proposal: file caching
updated description with more details.
----------

Efficient file caching 
(on Hadoop Task nodes, for benefit of MapReduce Tasks)

Overview
========
This document describes a mechanism for caching files or archives on taskTracker nodes' local
file system.
Exporting and extracting large archives from HDFS to local filesystem is expensive.
And is often required by the applications.
Currently this would happen at the beginning of every Task of a MapReduce Job.

The purpose of the Hadoop file cache is to minimize this overhead by
preserving and reusing the extracted data

During a MapRed job there are two kinds of data being uploaded to a Hadoop cluster:
  Java code and Out-of-band data.

Java code may include libraries so this can easily get large. (megabytes)

Out-of-band data is any data used by the job, in addition to the map input or the reduce input.
For example a large dictionary of words. This can also get large (gigabytes)


There are two main kinds of cacheable files:
1. The MapReduce job jar. 
   This contains Java code and possibly out-of-band data.
2. Additional archives
   This contains out-of-band data.

The proposed solution suggests that
Cacheable files:
are stored in HDFS, and specified in the JobConf of a MapReduce job.
A special case is the job jar file, which will get cached by default.

Supported formats for cacheable files are jar, tar and gzip, 
Additional formats could be added at a future time.
Regular files are also supported


Workflow:
========
For out-of-band data, the user must first explicitly upload archives to HDFS.
This can be done using any HDFS client.
It is typical for out-of-band data to be reused across Jobs and users.

The user specifies the out-of-band data using:
JobConf.addCachedArchive() or JobConf.addCachedFile()

The user specifies the job jar as today:
JobConf.setJar("f.jar") which implicitly has the effect of:
JobConf.addCachedArchive("f.jar"). 

When a Job starts, the JobTracker does the following for each cached archive.
Compute a strong hash of the archive and store the hash in the HDFS.
To avoid reading and scanning the archive, the strong hash is based
on the existing HDFS block-CRC codes rather than on the actual content.

When a Task starts, the TaskTracker does the following for each cached archive.
Retrieve the strong hash from HDFS, compare with the hash of the local copy.
If the local hash does not exist or is different, then
  retrieve the archive, unarchive it, update the local hash.
If the archive is the job jar, then
  copy or hard-link the archive contents to the Task working directory.
Then start the TaskRunner as usual.

Once the Task is running, the user code may access the cached archive contents.
This usually happens at initialization time.
If the JobConf added the cached archive: /hdfsdir/path/f.jar
Then the task can expect to access the archive content at:
$HADOOP_CACHE/hdfsdir/path/fdir/ffile 
or maybe:
$HADOOP_CACHE/hdfsdir/path/f_jar/fdir/ffile
The second option guarantees that multiple archives 
in the same directory will not clobber each other.
The translation of f.jar to f_jar is a convention to ease the distinction of file names and
directory names.


Note that in the above, the HDFS paths are mirrored on the local filesystem.
The intent is to provide namespace protection.
[i.e. the contents of hdfs1/archive.jar and hdfs2/archive.jar should not collide in the cache]
The intent is not to make cache paths interchangeable with HDFS paths. 


The variable HADOOP_CACHE is made available to the task as
a JobConf property that is dynamically set by the TaskRunner code.

Cache size control:
------------------
We cannot let the cache grow unbounded.

The cache is always up-to-date at the start of a job.
So the configurable parameter should not be the age of the cached data 
but the total size of the cache. 
The cache size is a static TaskTracker configuration parameter.

LRU (least recently used) policy:
On each Task tracker, the cache manager will measure the total size of the cache
and expire the oldest cached items. 
When a cached item is requested again in a different job, it goes back to the top.

The cached archive contents are required for the MapReduce task to function.
So when the promised cache contents cannot be provided, 
the cache manager will force a job failure 

Before new files are added to the cache, we do this size test.
If the cache size limit WOULD require to expire files...
1. .. expire files for completed jobs then everything is fine: delete them.
2. .. expire files for jobs that are already running, then the NEW job fails.
3. .. expire files for the new job then the new job fails.

Note that a file (archive) may belong to multiple jobs.

In normal use the cache size is expected to be significantly larger 
than the files requested by a single job. 
So the failure modes due to cache overflow should rarely occur.

THE END.


> RFC: Efficient file caching
> ---------------------------
>
>          Key: HADOOP-288
>          URL: http://issues.apache.org/jira/browse/HADOOP-288
>      Project: Hadoop
>         Type: Bug

>     Reporter: Michel Tourn
>     Assignee: Michel Tourn

>
> RFC: Efficient file caching 
> (on Hadoop Task nodes, for benefit of MapReduce Tasks)
> ------------------------------------------------------
> We will start implementing this soon. Please provide feedback and improvements to this
plan.
> The header "Options:" indicates places where simple choices must be made.
> Problem:
> -------
> o MapReduce tasks require access to additional out-of-band data ("dictionaries")
> This out-of-band data is:
> o in addition to the map/reduce inputs.
> o large (1GB+)
> o broadcast (same data is required on all the Task nodes)
> o changes "infrequently", in particular:
> oo it is always constant for all the Tasks in a Job. 
> oo it is often constant for a month at a time 
> oo it may be shared across team members
> o sometimes used by pure-Java MapReduce programs
> o sometimes used by non-Java MapReduce programs (using Hadoop-Streaming)
> o (future) used by programs that use HDFS and Task-trackers but not MapReduce.
> Existing Solutions to the problem:
> ---------------------------------
> These solutions are not good enough. The present proposal is to do Sol 1 with caching.
> Sol 1: Pure Hadoop: package the out-of-band data in the MapReduce Job jar file.
> Sol 2: Non  Hadoop: for each task node run rsync from single source for data.
> Sol 3: Non  Hadoop: use BitTorrent, etc.
> Sol.1 is correct but slow for many reasons:
>  The Job submitter must recreate a large jar(tar) file for every Job.
>   (The jar contains both changing programs and stable dictionaries)
>  The large Jar file must be propagated from the client to HDFS with 
>  a large replication factor. 
>  At the beginning of every Task, the Task tracker gets the job jar from HDFS 
>  and unjars it in the working directory. This can dominate task execution time.
>  
> Sol.2 has nice properties but also some problems.
>  It does not scale well with large clusters (many concurrent rsync read requests i.e.
single-source broadcast)
>  It assumes that Hadoop users can upload data using rsync to the cluster nodes. As a
policy, this is not allowed.
>  It requires rsync.
>  
> Sol.3 alleviates the rsync scalability problems but 
>       It is a dependency on an external system. 
>       We want something simpler and more tightly integrated with Hadoop.
>       
> Staging (uploading) out-of-band data:
> ------------------------------------
> The out-of-band data will often originate on the local filesystem of a user machine 
>  (i.e. a MapReduce job submitter)
> Nevertheless it makes sense to use HDFS to store the original out-of-band data because:
> o HDFS has (wide) replication. This enables scalable broadcast later.
> o HDFS is an available channel to move data from clients to all task machines.
> o HDFS is convenient as a shared location among Hadoop team members.
> Accessing (downloading) out-of-band data:
> ----------------------------------------
> The non-Java MapReduce programs do not have or want[1] APIs for HDFS.
> Instead these programs just want to access out-of-band data as 
>  local files at predefined paths.
> ([1] Existing programs should be reusable with no changes. 
>  This is often possible bec. communication is over stdin/stdout.)
> Job's jar file as a special case:
> --------------------------------
> One use case is to allow users to make the job jar itself cachable.
> This is only useful in cases where NOTHING changes when a job is resubmitted
>  (no MapRed code changes and no changes in shipped data)
> This situation might occur with an 'extractor' job (gets data from an external source:
like Nutch crawler)
> Currently the Hadoop mapred-jar mechanism works in this way:
>  the job jar data is unjarred in the "working directory" of the Task 
>  the jar contains both MapRed java code (added to classpath)
> Cache synchronization:
> ---------------------
> The efficient implementation of the out-of-band data distribution
> is mostly a cache synchronization problem.
> A list of the various aspects where choices must be made follows.
> Cache key:
> ---------
> How do you test that the cached copy is out-of-date?
> Options: 
> 1. the archive/file timestamp 
> 2. the MD5 of the archive/file content
> Comparing source and destination Timestamps is problematic bec. it assumes synchronized
clocks.
> Also there is no last-modif metadata in HDFS (for good reasons, like scalability of metadata
ops)
> Timestamps stored with the source ('last-propagate-time') do 
>  not require synchronized clocks, only locally monotonic time. 
> (and the worse which can happen at daylight-savings switch is a missed update or an extra-update)
> The cache code could store a copy of the local timestamp 
> in the same way that it caches the value of the content hash along with the source data.
>  
> Cachable unit:
> -------------
> Options: individual files or archives or both.
> Note:
> At the API level, directories will be processed recursively 
> (and the local FS directories will parallel HDFS directories)
> So bulk operations are always possible using directories.
> The question here is whether to handle archives as an additional bulk mechanism.
> Archives are special because:
> o unarchiving occurs transparently as part of the cache sync
> o The cache key is computed on the archive and preserved although 
>   the archive itself is not preserved.
> Supported archive format will be: tar (maybe tgz or compressed jar)
> Archive detection test: by filename extension ".tar" or ".jar"
> Suppose we don't handle archives as special files:
> Pros:
>  o less code, no discussion about which archive formats are supported
>  o fine for large dictionary files. And when files are not large, user may as well
>    put them in the Job jar as usual.
>  o user code could always check and unarchive specific cached files
>    (as a side-effect of MapRed task initialization)
> Cons:
>  o handling small files may be inefficient 
>   (multiple HDFS operations, multiple hash computation, 
>    one 'metadata' hash file along with each small file)
>  o It will not be possible to handle the Job's jar file as a special case of caching

> Cache isolation: 
> ---------------
> In some cases it may be a problem if the cached HDFS files are updated while a Job is
in progress:
> The file may become unavailable for a short period of time and some tasks fail.
> The file may change (atomically) and different tasks use a different version.
> This isolation problem is not addressed in this proposal.
> Standard solutions to the isolation problem are:
> o Assume that Jobs and interfering cache updates won't occur concurrently.
> o Put a version number in the HDFS file paths and refer to a hard-coded version in the
Job code.
> o Before running the MapRed job, run a non-distributed application that tests
>   what is the latest available version of the out-of-band data. 
>   Then make this version available to the MapRed job.
>   Two ways to do this. 
>   o either set a job property just-in-time:
>     addCachePathPair("/mydata/v1234/", "localcache/mydata_latest"); 
>     (see Job Configuration for meaning of this)
>   o or publish the decision as an HDFS file containing the version.
>     then rely on user code to read the version, and manually populate the cache:
>     Cache.syncCache("/hdfs/path/fileordir", "relative/local/fileordir");
>     (see MapReduce API for meaning of this)
> Cache synchronization stages:
> ----------------------------
> There are two stages: Client-to-HDFS and HDFS-to-TaskTracker
> o Client-to-HDFS stage.
> Options: A simple option is to not do anything here, i.e. rely on the user.
> This is a reasonable option given previous remarks on the role of HDFS:
>  HDFS is a staging/publishing area and a natural shared location.
> In particular this means that the system need not track 
> where the client files come from.
> o HDFS-to-TaskTracker:
> Client-to-HDFS synchronization (if done at all) should happen before this.
> Then HDFS-to-TaskTracker synchronization must happen right before 
> the data is needed on a node.
> MapReduce cache API:
> -------------------
> Options:
> 1. No change in MapReduce framework code:
> require the user to put this logic in map() (or reduce) function:
>  in MyMapper constructor (or in map() on first record) user is asked to add:
>  
>     Cache.syncCache("/hdfs/path/fileordir", "relative/local/fileordir");
>     Cache.syncCache("..."); //etc.
>   
> -----
> 2. Put this logic in MapReduce framework and use Job properties to
>    communicate the list of pairs (hdfs path; local path)
>  
> Directories are processed recursively.
> If archives are treated specially then they are unarchived on destination.
>  
> MapReduce Job Configuration:
> ---------------------------
> Options:
> with No change in MapReduce framework code (see above)
>  no special Job configuration: 
>    it is up to the MapRed writer to configure and run the cache operations.
> ---
> with Logic in MapReduce framework (see above)
>  some simple Job configuration
> JobConf.addCachePathPair(String, String)
> JobConf.addCachePathPair("/hdfs/path/fileordir", "relative/local/fileordir");

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