Backend.ai

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* NEW: Add option to specify the amount of shared memory via `resource_opts` parameter
in the kernelc reation config API (lablup/backend.ai52)
* UPDATE: Enhance vfolder download APIs to support ranged HTTP requests for partial downloads and
fix the browser-side fetch() API content decoding error due to the default behavior of
aiohttp.web.FileResponse implementation.
* Alembic migrations are now distributed as a part of the source and wheel package.
Set `script_location = ai.backend.manager.models:alembic` in your alembic.ini configuration file.
* Various bug fixes for GQL APIs and statistics.
* Update dependencies including aiohttp 3.6, wheel, twine, setuptools, and typing-extensions.

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* NEW: Add support for specifying shared memory for containers (lablup/backend.ai52, 140)
* Internally applied static type checks to avoid potential bugs due to human mistakes. (138)
Also refactored the codebase to split the manager-agent communication part and the kernel interaction
part (which is now replacible!) for extensible development.
* Update dependencies including aiohttp 3.6, twine, setuptools, etc.

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* NEW: Add shared-memory stats
* CHANGE: watcher commands are now executed with "sudo".

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* FIX: regression of batch-mode execution (file uploads to kernels) due to refactoring

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* FIX: Apply a keepalive messaging at the 10-sec interval for agent-container RPC connection to avoid
kernel-enforced NAT connection tracker timeout (126, lablup/backend.ai46)

This allow execution of very long computation (more than 5 days) without interruption as long as
the idle timeout configuration allows.
* FIX: When reading plugin configurations, merge scaling-group and global configurations correctly.
* FIX: No longer change the fstab if mount operations fail. Also delete the unmounted folder
if it is empty after unmount was successful.

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* NEW: Add support for running CentOS-based kernel images by adding CentOS 7.6-based builds for
libbaihook and su-exec binaries.
* NEW: watcher: Add support for fstab/mount/unmount management APIs for superadmins (134)
* Improve stability of cancellation during shutdown via refactoring and let uvloop work more consistently
with vanilla asyncio. (133)
- Now the agent daemon handles SIGINT and SIGTERM much more gracefully.
- Upgrade aiotools to v0.8.2+
- Rewrite kernel's `list_files` RPC call to work safer and faster (124).