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From "Bernd Mathiske" <>
Subject Re: Review Request 30774: Fetcher Cache
Date Wed, 11 Mar 2015 10:41:33 GMT

> On March 6, 2015, 2:15 p.m., Timothy Chen wrote:
> > include/mesos/fetcher/fetcher.proto, line 58
> > <>
> >
> >     It's harder to make a optional field required, but it's much easier the other
way around.
> >     
> >     If we always want it to be required, I think we should make the sandbox a required
> Bernd Mathiske wrote:
>     There was some discussion about whether this field should be required or not. The
general idea here is that a task might be able to run without fetching anything into its sandbox.
In this case, the framework may get away without naming the sandbox. But since a task always
has one, we could also make it required. I am impartial in this choice, but I see that your
argument that required->optional is easier has pull.

I have heard good arguments both ways. Here is how I see it. 

For the recipient of a message, "optional" is the preferred choice. Then any legacy recipient's
code is always prepared for everything and robust wrt. changing to "required". Not the other
way around.

But for the sender, "required" is the better choice, making sender code more robust. If legacy
senders still provide the field when it has become optional, that's OK. Not the other way

So which side are we on in this case? As much as this is an internal protocol, we are on neither
side and we can change this in arbitrary ways. 

This is an external protocol if someone else than a Mesos slave uses mesos-fetcher. (Maybe
a special external containerizer.) Then we are providing the message recipient and we have
to be on that side. Therefore I am voting for "optional".

- Bernd

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On March 7, 2015, 7:21 a.m., Bernd Mathiske wrote:
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> -----------------------------------------------------------
> (Updated March 7, 2015, 7:21 a.m.)
> Review request for mesos, Adam B, Benjamin Hindman, Till Toenshoff, and Timothy Chen.
> Bugs: MESOS-2057, MESOS-2069, MESOS-2070, MESOS-2072, MESOS-2073, and MESOS-2074
> Repository: mesos
> Description
> -------
> Almost all of the functionality in epic MESOS-336. Downloaded files from CommandInfo::URIs
can now be cached in a cache directory designated by a slave flag. This only happens when
asked for by an extra flag in the URI and is thus backwards-compatible. The cache has a size
limit also given by a new slave flag. Cache-resident files are evicted as necessary to make
space for newly fetched ones. Concurrent attempts to cache the same URI leads to only one
download. The fetcher program remains external for safety reasons, but is now augmented with
more elaborate parameters packed into a JSON object to implement specific fetch actions for
all of the above. Additional testing includes fetching from (mock) HDFS and coverage of the
new features.
> Diffs
> -----
>   docs/ PRE-CREATION 
>   docs/ PRE-CREATION 
>   include/mesos/fetcher/fetcher.proto 311af9aebc6a85dadba9dbeffcf7036b70896bcc 
>   include/mesos/mesos.proto 9df972d750ce1e4a81d2e96cc508d6f83cad2fc8 
>   src/ d299f07d865080676ca8a550cf6005c6ab32839f 
>   src/hdfs/hdfs.hpp 968545d9af896f3e72e156484cc58135405cef6b 
>   src/launcher/fetcher.cpp 796526f59c25898ef6db2b828b0e2bb7b172ba25 
>   src/slave/constants.hpp fd1c1aba0aa62372ab399bee5709ce81b8e92cec 
>   src/slave/containerizer/docker.hpp b7bf54ac65d6c61622e485ac253513eaac2e4f88 
>   src/slave/containerizer/docker.cpp 5f4b4ce49a9523e4743e5c79da4050e6f9e29ed7 
>   src/slave/containerizer/fetcher.hpp 1db0eaf002c8d0eaf4e0391858e61e0912b35829 
>   src/slave/containerizer/fetcher.cpp 9e9e9d0eb6b0801d53dec3baea32a4cd4acdd5e2 
>   src/slave/containerizer/mesos/containerizer.hpp ae61a0fcd19f2ba808624312401f020121baf5d4

>   src/slave/containerizer/mesos/containerizer.cpp ec4626f903d44c0911093ff763ef16ad27c418a9

>   src/slave/flags.hpp 56b25caf3901b38bdecb50310e8bcae0b114efa8 
>   src/slave/slave.cpp a06d68032f26ccb3f786b6ea7c3a6c3c52449bd2 
>   src/tests/docker_containerizer_tests.cpp 06cd3d89ecbaaac17ae6970604b21fbe29f6e887 
>   src/tests/fetcher_cache_tests.cpp PRE-CREATION 
>   src/tests/fetcher_tests.cpp 4549e6a631e2c17cec3766efaa556593eeac9a1e 
>   src/tests/mesos.hpp e91e5e484eea4587ac8f2eb9cefeab4acc9f4615 
>   src/tests/mesos.cpp c8f43d21b214e75eaac2870cbdf4f03fd18707d1 
> Diff:
> Testing
> -------
> make check
> --- longer Description: ---
> -Replaces all other reviews for the fetcher cache except those related to stout: 30006,
30033, 30034, 30036, 30037, 30039, 30124, 30173, 30614, 30616, 30618, 30621, 30626. See descriptions
of those. In dependency order:
> 30033: Removes the fetcher env tests since these won't be needed any more when the fetcher
uses JSON in a single env var as a parameter. They never tested anything that won't be covered
by other tests anyway.
> 30034: Makes the code structure of all fetcher tests the same. Instead of calling the
run method of the fetcher directly, calling through fetch(). Also removes all uses of I/O
redirection, which is not really needed for debugging, and thus the next patch can refactor
fetch() and run(). (The latter comes in two varieties, which complicates matters without much
> 30036: Extends the CommandInfo::URI protobuf with a boolean "caching" field that will
later cause fetcher cache actions. Also introduces the notion of a cache directory to the
fetcher info protobuf. And then propagates these additions throughout the rest of the code
base where applicable. This includes passing the slave ID all the way down to the place where
the cache dir name is constructed.
> 30037: Extends the fetcher info protobuf with "actions" (fetch directly bypassing the
cache, fetch through the cache, retrieve from the cache). Switches the basis for dealing with
uris to "items", which contain the uri, the action, and potentially a cache file name. Refactors
fetch() and run(), so there is only one of each. Introduces about half of the actual cache
logic, including a hashmap of cache file objects for bookkeeping and basic operations on it.

> 30039: Enables fetcher cache actions in the mesos fetcher program.
> 30006: Enables concurrent downloading into the fetcher cache. Reuse of download results
in the cache when multiple fetcher runs occur concurrently. 
> 30614: This is to ensure that all this refactoring of fetcher code has not broken HDFS
fetching. Adds a test that exercises the C++ code paths in Mesos and mesos-fetcher related
to fetching from HDFS. Uses a mock HDFS client written in bash that acts just like a real
"hadoop" command if used in the right limited way.
> 30124: Inserted fetcher cache zap upon slave startup, recovery and shutdown. This implements
recovery in an acceptable, yet most simple way.
> 30173: Created fetcher cache tests. Adds a new test source file containing a test fixture
and tests to find out if the fetcher cache works with a variety of settings.
> 30616: Adds hdfs::du() which calls "hadoop fs -du -h" and returns a string that contains
the file size for the URI passed as argument. This is needed to determine the size of a file
on HDFS before downloading it to the fetcher cache (to ensure there is enough space).
> 30621: Refactored URI type separation in mesos-fetcher. Moved the URI type separation
code (distinguishes http, hdfs, local copying, etc.) from mesos-fetcher to the fetcher process/actor,
since it is going to be reused by download size queries when we introduce fetcher cache management.
Also factored out URI validation, which will be used the same way by mesos-fetcher and the
fetcher process/actor.
> 30626: Fetcher cache eviction. This happens when the cache does not have enough space
to accomodate upcoming downloads to the cache. Necessary provisions included here:
> - mesos-fetcher does not run until evictions have been successful
> - Cache space is reserved while (async) waiting for eviction to succeed. If it fails,
the reservation gets undone.
> - Reservations can be partly from available space, partly from evictions. All math included
> - To find out how much space is needed, downloading has a prelude in which we query the
download size from the URI. This works for all URI types that mesos-fetcher currently supports,
including http and hdfs.
> - Size-determination requests are now synchronized, too. Only one per URI in play happens.
> - There is cleanup code for all kinds of error situations. At the very end of the fetch
attempt, each list is processed for undoing things like space reservations and eviction disabling.
> - Eviction gets disabled for URIs that are currently in use, i.e. the related cache files
are. We use reference counting for this, since there may be concurrent fetch attempts using
the same cache files.
> Thanks,
> Bernd Mathiske

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