Chainer is an open-source deep learning framework built around define-by-run automatic differentiation, where computation graphs are constructed dynamically during forward execution. This imperative approach allows networks to be built using standard Python control flow, with gradients computed automatically through reverse-mode differentiation on the dynamically recorded graph. The framework supports GPU acceleration through a NumPy-compatible array backend with CUDA and cuDNN support, and provides a pluggable device abstraction that lets users switch between CPU and GPU computation without code changes.
The framework distinguishes itself through flexible network construction that imposes no architectural constraints, allowing layers and functions to be connected in any directed graph structure including feed-forward, convolutional, recurrent, recursive, or per-batch dynamic architectures. Training is managed through an extensible loop that triggers user-defined hooks for logging, validation, and snapshotting at specified intervals. For large-scale workloads, Chainer supports distributed training across multiple nodes using collective communication for gradient synchronization. Model parameters and optimizer states can be serialized to a portable hierarchical binary format for checkpointing and training resumption.
Chainer provides a compatibility layer that wraps existing arrays in a standard variable interface, enabling incremental migration to faster backends without rewriting code. The framework also includes image data loading from disk for vision tasks, and supports deployment in containerized environments with GPU acceleration.