JuliaFEM.jl/docs/src/assembler_choice.md

[Choosing an assembly strategy](@id assembler_choice)

JuliaFEM 0.x exposes more than one assembly path. This page matches the split under src/assemblers/ and the regression tests.

DOF-based COO (DOFBasedCOOAssembler)

• Use when: you want the modern, matrix-free-friendly pipeline: one cache drives both sparse triplets and apply_K! / Krylov operators.
• Strengths: walks global DOF rows with local_dof_layout decoding; the hot path is written for zero allocations after warmup (test/assemblers/test_dof_based_zero_alloc.jl).
• Tests: test/assemblers/test_dof_based_apply_K.jl and related test_dof_based_*.jl files.

Element-based COO / CSC

• Use when: you want the classical element-by-element scatter into COO or CSC as a reference implementation or for tooling that expects element loops.
• Strengths: straightforward comparison to textbook element assembly.
• Code: src/assemblers/element_based/; see src/assemblers/README.md.

Matrix-free operators

• Use when: you do not want to form K explicitly (large systems, matrix-free Newton–Krylov).
• Strengths: composable Dirichlet, MPC, loads, and preconditioners under src/assemblers/matrix_free/.
• Tests: test/assemblers/test_matrix_free_operator.jl and neighbors.
• Site guide: juliafem.github.io/docs/user-guide/matrix_free_cookbook.md.

Rule of thumb

Start with DOF-based COO for new physics and examples unless you have a specific reason to call the element-based scatter directly.