# -*- coding: utf-8 -*-
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#     http://www.apache.org/licenses/LICENSE-2.0
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"""Additional help about using gsutil for production tasks."""

from __future__ import absolute_import

from gslib.help_provider import HelpProvider
from gslib.utils.constants import RESUMABLE_THRESHOLD_MIB

_DETAILED_HELP_TEXT = ("""
<B>OVERVIEW</B>
  If you use gsutil in large production tasks (such as uploading or
  downloading many GiBs of data each night), there are a number of things
  you can do to help ensure success. Specifically, this section discusses
  how to script large production tasks around gsutil's resumable transfer
  mechanism.


<B>BACKGROUND ON RESUMABLE TRANSFERS</B>
  First, it's helpful to understand gsutil's resumable transfer mechanism,
  and how your script needs to be implemented around this mechanism to work
  reliably. gsutil uses resumable transfer support when you attempt to download
  a file of any size or to upload a file larger than a configurable threshold
  (by default, this threshold is %d MiB). If a transfer fails partway through
  (e.g., because of an intermittent network problem), gsutil uses a truncated
  randomized binary exponential backoff-and-retry strategy that by default will
  retry transfers up to 23 times over a 10 minute period of time (see
  "gsutil help retries" for details). If the transfer fails each of these
  attempts with no intervening progress, gsutil gives up on the transfer, but
  keeps a "tracker" file for it in a configurable location (the default
  location is ~/.gsutil/, in a file named by a combination of the SHA1 hash of
  the name of the bucket and object being transferred and the last 16
  characters of the file name). When transfers fail in this fashion, you can
  rerun gsutil at some later time (e.g., after the networking problem has been
  resolved), and the resumable transfer picks up where it left off.


<B>SCRIPTING DATA TRANSFER TASKS</B>
  To script large production data transfer tasks around this mechanism,
  you can implement a script that runs periodically, determines which file
  transfers have not yet succeeded, and runs gsutil to copy them. Below,
  we offer a number of suggestions about how this type of scripting should
  be implemented:

  1. When resumable transfers fail without any progress 23 times in a row
     over the course of up to 10 minutes, it probably won't work to simply
     retry the transfer immediately. A more successful strategy would be to
     have a cron job that runs every 30 minutes, determines which transfers
     need to be run, and runs them. If the network experiences intermittent
     problems, the script picks up where it left off and will eventually
     succeed (once the network problem has been resolved).

  2. If your business depends on timely data transfer, you should consider
     implementing some network monitoring. For example, you can implement
     a task that attempts a small download every few minutes and raises an
     alert if the attempt fails for several attempts in a row (or more or less
     frequently depending on your requirements), so that your IT staff can
     investigate problems promptly. As usual with monitoring implementations,
     you should experiment with the alerting thresholds, to avoid false
     positive alerts that cause your staff to begin ignoring the alerts.

  3. There are a variety of ways you can determine what files remain to be
     transferred. We recommend that you avoid attempting to get a complete
     listing of a bucket containing many objects (e.g., tens of thousands
     or more). One strategy is to structure your object names in a way that
     represents your transfer process, and use gsutil prefix wildcards to
     request partial bucket listings. For example, if your periodic process
     involves downloading the current day's objects, you could name objects
     using a year-month-day-object-ID format and then find today's objects by
     using a command like gsutil ls "gs://bucket/2011-09-27-*". Note that it
     is more efficient to have a non-wildcard prefix like this than to use
     something like gsutil ls "gs://bucket/*-2011-09-27". The latter command
     actually requests a complete bucket listing and then filters in gsutil,
     while the former asks Google Storage to return the subset of objects
     whose names start with everything up to the "*".

     For data uploads, another technique would be to move local files from a "to
     be processed" area to a "done" area as your script successfully copies
     files to the cloud. You can do this in parallel batches by using a command
     like:

       gsutil -m cp -r to_upload/subdir_$i gs://bucket/subdir_$i

     where i is a shell loop variable. Make sure to check the shell $status
     variable is 0 after each gsutil cp command, to detect if some of the copies
     failed, and rerun the affected copies.

     With this strategy, the file system keeps track of all remaining work to
     be done.

  4. If you have really large numbers of objects in a single bucket
     (say hundreds of thousands or more), you should consider tracking your
     objects in a database instead of using bucket listings to enumerate
     the objects. For example this database could track the state of your
     downloads, so you can determine what objects need to be downloaded by
     your periodic download script by querying the database locally instead
     of performing a bucket listing.

  5. Make sure you don't delete partially downloaded temporary files after a
     transfer fails: gsutil picks up where it left off (and performs a hash
     of the final downloaded content to ensure data integrity), so deleting
     partially transferred files will cause you to lose progress and make
     more wasteful use of your network.

  6. If you have a fast network connection, you can speed up the transfer of
     large numbers of files by using the gsutil -m (multi-threading /
     multi-processing) option. Be aware, however, that gsutil doesn't attempt to
     keep track of which files were downloaded successfully in cases where some
     files failed to download. For example, if you use multi-threaded transfers
     to download 100 files and 3 failed to download, it is up to your scripting
     process to determine which transfers didn't succeed, and retry them. A
     periodic check-and-run approach like outlined earlier would handle this
     case.

     If you use parallel transfers (gsutil -m) you might want to experiment with
     the number of threads being used (via the parallel_thread_count setting
     in the .boto config file). By default, gsutil uses 10 threads for Linux
     and 24 threads for other operating systems. Depending on your network
     speed, available memory, CPU load, and other conditions, this may or may
     not be optimal. Try experimenting with higher or lower numbers of threads
     to find the best number of threads for your environment.
""" % RESUMABLE_THRESHOLD_MIB)


class CommandOptions(HelpProvider):
  """Additional help about using gsutil for production tasks."""

  # Help specification. See help_provider.py for documentation.
  help_spec = HelpProvider.HelpSpec(
      help_name='prod',
      help_name_aliases=[
          'production', 'resumable', 'resumable upload', 'resumable transfer',
          'resumable download', 'scripts', 'scripting'],
      help_type='additional_help',
      help_one_line_summary='Scripting Production Transfers',
      help_text=_DETAILED_HELP_TEXT,
      subcommand_help_text={},
  )
