Ray

Core
----
- Add `delete_creating_tasks` option for `internal.free()` 4588

Tune
----
- Add filter flag for Tune CLI. 4337
- Better handling of ``tune.function`` in global checkpoint. 4519
- Add compatibility to nevergrad 0.2.0+. 4529
- Add `--columns` flag for CLI. 4564
- Add checkpoint eraser. 4490
- Fix checkpointing for Gym types. 4619

RLlib
-----
- Report sampler performance metrics. 4427
- Ensure stats are consistently reported across all algos. 4445
- Cleanup ``TFPolicyGraph``. 4478
- Make batch timeout for remote workers tunable. 4435
- Fix inconsistent weight assignment operations in ``DQNPolicyGraph``. 4504
- Add support for LR schedule to DQN/APEX. 4473
- Add option for RNN state and value estimates to span episodes. 4429
- Create a combination of ``ExternalEnv`` and ``MultiAgentEnv``, called ``ExternalMutliAgentEnv``. 4200
- Support ``prev_state``/``prev_action`` in rollout and fix multiagent. 4565
- Support torch device and distributions. 4553

Java
----
- TestNG outputs more verbose error messages. 4507
- Implement ``GcsClient``. 4601
- Avoid unnecessary memory copy and addd a benchmark. 4611

Autoscaler
----------
- Add support for separate docker containers on head and worker nodes. 4537
- Add an aggressive autoscaling flag. 4285

Core
----
- Build system fully converted to Bazel. 4284, 4280, 4281
- Introduce a set data structure in the GCS. 4199
- Make all arguments to `_remote()` optional. 4305
- Improve object transfer latency by setting `TCP_NODELAY` on all TCP connections. 4318
- Add beginning of experimental serving module. 4095
- Remove Jupyter notebook based UI. 4301
- Add `ray timeline` command line command for dumping Chrome trace. 4239

Tune
----
- Add custom field for serializations. 4237
- Begin adding Tune CLI. 3983, 4321, 4322
- Add optimization to reuse actors. 4218
- Add warnings if the Tune event loop gets clogged. 4353
- Switch preferred API from `tune.run_experiments` to `tune.run`. 4234
- Make the logging from the function API consistent and predictable. 4011

RLlib
-----
- **Breaking:** Flip sign of entropy coefficient in A2C and Impala. 4374
- Add option to continue training even if some workers crash. 4376
- Add asynchronous remote workers. 4253
- Add callback accessor for raw observations. 4212

Java
----
- Improve single-process mode. 4245, 4265
- Package native dependencies into jar. 4367
- Initial support for calling Python functions from Java. 4166

Autoscaler
----------
- Restore error messages for setup errors. 4388

Known Issues
------------
- Object broadcasts on large clusters are inefficient. 2945

Breaking
---------
- Removed `redirect_output` and `redirect_worker_output` from `ray.init`, removed deprecated `_submit` method. 4025
- Move `TensorFlowVariables` to `ray.experimental.tf_utils`. 4145

Core
------
- Stream worker logging statements to driver by default. 3892
- Added experimental ray signaling mechanism, see the [documentation](https://ray.readthedocs.io/en/latest/signals.html). #3624
- Make Bazel the default build system. 3898
- Preliminary experimental streaming API for Python. 4126
- Added web dashboard for monitoring node resource usage. 4066
- Improved propagation of backend errors to user. 4039
- Many improvements for the Java frontend. 3687, 3978, 4014, 3943, 3839, 4038, 4039, 4063, 4100, 4179, 4178
- Support for dataclass serialization. 3964
- Implement actor checkpointing. 3839
- First steps toward cross-language invocations. 3675
- Better defaults for Redis memory usage. 4152

Tune
------
- **Breaking**: Introduce ability to turn off default logging. Deprecates custom_loggers. 4104
- Support custom resources. 2979
- Add initial parameter suggestions for HyperOpt. 3944
- Add scipy-optimize to Tune. 3924
- Add Nevergrad. 3985
- Add number of trials to the trial runner logger. 4068
- Support RESTful API for the webserver. 4080
- Local mode support. 4138
- Dynamic resources for trials. 3974

RLlib
-------
- Basic infrastructure for off-policy estimation. 3941
- Add simplex action space and Dirichlet action distribution. 4070
- Exploration with parameter space noise. 4048
- Custom supervised loss API. 4083
- Add torch policy gradient implementation. 3857

Autoscaler and Cluster Setup
--------------------------------------
- Add docker run option (e.g. to support nvidia-docker). 3921

Modin
-----
- Upgrade Modin to 0.3.1, see the [release notes](https://github.com/modin-project/modin/releases/tag/v0.3.1). #4058

Known Issues
------------
- Object broadcasts on large clusters are inefficient. 2945
- IMPALA is broken 4329

Core
------
- Initial work on porting the build system to Bazel. 3918, 3806, 3867, 3842
- Allow starting Ray processes inside valgrind, gdb, tmux. 3824, 3847
- Stability improvements and bug fixes. 3861, 3962, 3958, 3855, 3736, 3822, 3821, 3925
- Convert Python C extensions to Cython. 3541
- `ray start` can now be used to start Java workers. 3838, 3852
- Enable LZ4 compression in `pyarrow` build. 3931
- Update Redis to version 5.0.3. 3886
- Use one memory-mapped file for Plasma store. 3871,

Tune
------
- Support for BayesOpt. 3864
- Support for SigOpt. 3844
- Support executing infinite recovery retries for a trial. 3901
- Support `export_formats` option to export policy graphs. 3868
- Cluster and logging improvements. 3906

RLlib
-------
- Support for Asynchronous Proximal Policy Optimization (APPO). 3779
- Support for MARWIL. 3635
- Support for evaluation option in DQN. 3835
- Bug fixes. 3865, 3810, 3938
- Annotations for API stability. 3808

Autoscaler and Cluster Setup
--------------------------------------
- Faster cluster launch and update. 3720
- Bug fixes. 3916, 3860, 3937, 3782, 3969
- Kubernetes configuration improvements. 3875, 3909

Modin
-----
- Update Modin to 0.3.0. 3936

Breaking Changes
- The `timeout` argument of `ray.wait` now uses seconds instead of milliseconds. 3706

Core
- Limit default redis max memory to 10GB. 3630
- Define a `Node` class to manage Ray processes. 3733
- Garbage collection of actor dummy objects. 3593
- Split profile table among many keys in the GCS. 3676
- Automatically try to figure out the memory limit in a docker container. 3605
- Improve multi-threading support. 3672
- Push a warning to all users when large number of workers have been started. 3645
- Refactor code `ray.ObjectID` code. 3674

Tune
- Change log handling for Tune. 3661
- Tune now supports resuming from cluster failure. 3309, 3725, 3657, 3681
- Support Configuration Merging for Suggestion Algorithms. 3584
- Support nested PBT mutations. 3455

RLlib
- Add starcraft multiagent env as example. 3542
- Allow development without needing to compile Ray. 3623
- Documentation for I/O API and multi-agent improvements. 3650
- Export policy model checkpoint. 3637
- Refactor PyTorch custom model support. 3634

Autoscaler
- Add an initial_workers option. 3530
- Add kill and get IP commands to CLI for testing. 3731
- GCP allow manual network configuration. 3748

Known Issues:
- Object broadcasts on large clusters are inefficient. 2945

Core
- Added experimental option to limit Redis memory usage. 3499
- Added option for restarting failed actors. 3332
- Fixed Plasma TensorFlow operator memory leak. 3448
- Fixed compatibility issue with TensorFlow and PyTorch. 3574
- Miscellaneous code refactoring and cleanup. 3563 3564 3461 3511
- Documentation. 3427 3535 3138
- Several stability improvements. 3592 3597

RLlib
- Multi-GPU support for Multi-agent PPO. 3479
- Unclipped actions are sent to learner. 3496
- `rllib rollout` now also preprocesses observations. 3512
- Basic Offline Data API added. 3473
- Improvements to metrics reporting in DQN. 3491
- AsyncSampler no longer auto-concats. 3556
- QMIX Implementation (Experimental). 3548
- IMPALA performance improvements. 3402
- Better error messages. 3444
- PPO performance improvements. 3552

Autoscaler
- Bug fixes, botocore handling fix. 3454 3503 3447

Ray Tune
- Lambdas now require `tune.function` wrapper. 3457
- Custom loggers, sync functions, and trial names are now supported. 3465
- Improvements to fault tolerance. 3414
- Variant Generator docs clarification. 3583
- `trial_resources` now renamed to `resources_per_trial`. 3580

Modin
- Modin 0.2.5 is now bundled with Ray
- `import modin` after `import ray`