Mandatory compatibility rules when changing journal file readers, writers, fixtures, conformance tests, interoperability tests, or journalctl rewrites.
Project Journal Compatibility
Purpose
Keep all implementations aligned with the systemd journal file format, Netdata Rust reader/writer sources, and the project scope decisions.
Scope
Use this skill when:
importing or adapting Rust journal code;
implementing journal readers or writers in Rust or Go;
porting systemd tests or fixtures;
building shared interoperability tests;
implementing journalctl rewrites.
Do not use this skill for:
pure repository bootstrap work;
generic SOW maintenance unrelated to journal behavior.
Mandatory Knowledge
Baseline compatibility target is systemd/systemd tag v260.1.
The test scope is SDK conformance plus file-backed journalctl behavior.
Do not implement daemon-only journalctl commands such as daemon sync, flush, rotate, or relinquish-var operations.
journalctl already treats repeated matches for the same field as OR alternatives and different fields as AND.
The + separator is a systemd journalctl disjunction feature to replicate for file-backed journalctl behavior; it is not a new extension.
Each product language must provide two API layers: idiomatic SDK API plus a libsystemd-compatible reader facade. The facade is required unless a SOW records concrete evidence that it would require native bindings, violate the pure-language policy, or create an unsafe/unrepresentable API in that language.
Each product-language writer must expose two compatible append shapes: a systemd-compatible raw full-field KEY=value byte payload layer and a structured binary-safe {name, value} SDK layer. Structured is the SDK hot path for already-structured producers; raw full payloads are the systemd-compatible low-level layer.
Each product-language writer must expose the same three field-name policy layers: RAW, JOURNALD, and JOURNAL-APP. JOURNALD is the default and preserves trusted protected fields such as _HOSTNAME; JOURNAL-APP emulates untrusted application-facing journald restrictions and drops invalid caller fields; RAW allows every field name the DATA structure can represent directly, currently non-empty and no = in the field name.
The SDK must not perform producer-specific field-name remapping. Do not add or keep SDK behavior that emits project-specific mapping marker fields or project-specific mapped field prefixes. Consumers that need naming transformations must perform them before calling the SDK.
Core journal readers and writers are file-format implementations only. They must not execute external programs, discover host identity, read host identity sources, or enforce cooperating-writer locks by default. They must operate on explicit caller-provided paths, bytes, timestamps, machine IDs, boot IDs, seqnum IDs, and options.
Systemd/journald compatibility is a policy/API layer above the file-format core. It may require caller-provided machine and boot identity, but it must not silently probe host identity. Callers that want automatic identity discovery must explicitly call the optional identity helper and pass the result in.
Cooperating-writer locking is an optional helper/wrapper, independent from systemd compatibility. The journal format has a one-writer operational contract, but the lock protocol is not part of the systemd journal file format and must not be described as systemd compatibility. Core writer constructors must not expose lock-enable options; callers acquire and release the optional lock helper separately around writer use.
Host-observation mechanisms such as /proc, /host/proc, /etc/machine-id, platform registries, sysctl, system_profiler, ps, shell commands, and subprocess APIs are forbidden in core reader/writer runtime paths. They are allowed only in explicitly named optional helper code and tests for that helper.
Low-level writers must sort ENTRY DATA references by on-disk DATA object offset and remove duplicate DATA references by default, matching systemd. A trusted unique-payload option may skip only duplicate elimination when the caller guarantees no duplicate full payloads in one entry; it must not skip offset sorting unless a later SOW records measured evidence, compatibility validation, and a user decision for a non-byte-identity mode.
Jenkins lookup3 hashing must match systemd jenkins_hashlittle2(), including the empty payload value 0xdeadbeefdeadbeef.
The final writer target includes compression and Forward Secure Sealing, but implementation may be phased.
Live concurrency compatibility is mandatory for every writer and reader. A writer is not production-compatible unless stock journalctl --file and stock libsystemd readers can safely read the file while that writer is appending. A reader is not production-compatible unless it can safely read files while they are being appended by each repository writer and, where testable without violating repository-boundary rules, stock systemd writers.
The live concurrency contract is one writer plus multiple readers on the same journal file. Tests must cover online state, append publication windows, tail metadata changes, entry-array growth, reader follow/tail behavior, clean close verification, and interruption/reopen scenarios for the claimed feature slice.
Writers expose a cross-language live_publish_every_entries publication
cadence. The default 1 is the stock live-reader compatibility mode. 0
disables explicit SDK live publication, and N > 1 publishes after every
N appended entries for latency-tolerant consumers. Non-default modes must
not be described as stock journalctl --follow compatible unless their exact
mode is covered by live matrix evidence; they still must pass clean-close
verify and final reads.
The reusable live-concurrency harness is under tests/conformance/live/. Writer tests should use the configured monotonically increasing sequence field, default LIVE_SEQ, so stock readers prove complete ordered visibility.
Stock reader harness adapters may retry transient active-writer ENODATA open/read failures or partial snapshots only while the writer is active. After the writer exits, final ordered reads and journalctl --verify --file must pass.
High-level directory writers must apply configured retention once when an active writer is opened or created. Existing-active reopen and eager open enforce during construction; lazy archived-only construction remains side-effect-free until the first append opens the active file, then retention runs before the first entry is written. Active/current files must remain protected and normal retention deletion lifecycle events must be reused.
For deterministic regular uncompressed writer output, the layout target is byte-for-byte identity with the systemd v260.1 reference ingester for the accepted corpus. Writers must match systemd object order, alignment, initial allocation envelope, v260 header fields, entry-array growth, tail metadata, and hash-chain header behavior for that slice.
Deterministic byte-identity validation must cover systemd final-state variants: online/plain close, offline close, and archived close.
Header readers must use the on-disk header_size when validating object and hash-table locations. Do not compare historical file offsets against the current in-memory v260 JournalHeader struct size, and do not expose bytes beyond the on-disk header as newer header fields.
Readers must not require HEADER_INCOMPATIBLE_KEYED_HASH for opening existing historical files. If that flag is absent, readers must use the unkeyed Jenkins lookup3 hash path and still support applicable compression flags such as LZ4. Writers may remain conservative when appending and reject unkeyed historical files unless a SOW explicitly proves safe mutation of that historical format.
Writer append-open rejection for historical unkeyed files must be controlled and must occur before entry mutation. Do not leave assertion panics or partial state changes as the user-facing failure mode.
High-level directory writers own reliable active replacement. If a low-level
append-open rejects an existing active file as unsupported, the directory
writer must follow systemd's reliable-open shape: use readable header
metadata to continue sequence identity where possible, move the old active
file out of the normal .journal set with a collision-safe *.journal~
disposed name, and create a fresh active file. Do not archive unsupported
active files into normal readable history.
Journal-native API performance is part of compatibility. If the format provides a hash table, FIELD/DATA chain, DATA entry array, ENTRY array, offset, or mmap-backed path for an operation, implementations must use that path instead of scanning and expanding all entries unless a SOW records measured evidence and an explicit accepted reason.
Field-name enumeration should use FIELD hash-table traversal on valid indexed files. A compatibility fallback to entry scanning is acceptable only when a historical or damaged FIELD table cannot be traversed safely, and that fallback must be documented in the active SOW.
Unfiltered unique value enumeration must match systemd's algorithmic shape: find the FIELD object and walk its DATA chain, then de-duplicate across files. It must not scan every entry or expand unrelated fields.
Changes to journal-engine index semantics, FileIndexKey, serialized index
contents, cache schema, or consumer-visible cache partitioning must explicitly
decide whether CACHE_VERSION changes. If bumped, record the one-time
reindex/invalidation impact in the active SOW, product scope, and release
notes. If not bumped, record why old cache entries remain semantically safe.
Avoid unnecessary DATA decompression, repeated FIELD=value parsing, allocation, sorting, hashing, or syscalls in reader/writer hot paths. Treat these as regressions unless tests and benchmark evidence prove they are required.
Explorer column catalogs must be built from FIELD indexes. The Rust
ExplorerQuery::debug_collect_column_fields_by_row_traversal switch is a
debug-only discrepancy tool; production reports, benchmarks, or compatibility
claims with that switch enabled are invalid and must be treated as an
explorer defect.
Smoke tests are not sufficient evidence for production compatibility. SOW validation must record exact stock systemd version, commands/helpers, stress duration, entry counts, reader counts, and failure criteria.
Common compression-library dependencies are allowed after dependency review. Journal parsing/writing must not depend on systemd/libjournal; CGO and linking to system journal libraries remain disallowed unless the user explicitly changes those separate constraints.
Every external-agent prompt must include the canonical repository-boundary block verbatim from AGENTS.md or .agents/skills/project-agent-orchestration/SKILL.md.
Compatibility probing must not use the workstation's live journal. Do not run systemd-cat, logger, live journalctl, or write under /var/log/journal or /run/log/journal; stock-reader validation must use journalctl --file or repository-local --directory fixtures only.
Best Practices
Treat systemd source and tests as the compatibility authority when documentation and implementation disagree.
Keep shared tests language-neutral and run them against both product SDKs.
Add shared live-concurrency tests before accepting a writer or reader as production-compatible.
Prove cross-language interoperability with files written by each implementation and read by every implementation.
Prove stock-reader interoperability while repository writers are actively appending, not only after close.
For live writer/reader compatibility changes, run
tests/interoperability/run_live_matrix.py and require the feature matrix to
cover regular, zstd/xz/lz4 DATA compression, compact, compact plus DATA
compression, and sealed/FSS files. The matrix must include stock
journalctl --file, stock libsystemd, all repository readers, final
journalctl --verify --file, and --verify-key for sealed files.
For compressed-DATA writer changes, run tests/interoperability/run_compression_matrix.py
and require the structural oracle to validate object order, offsets, flags,
counters, hash-chain consistency, tail metadata, references, and compression
flags plus stock journalctl, stock libsystemd, and all repository reader
checks.
For compact writer changes, run tests/interoperability/run_compact_matrix.py
and require the structural oracle to validate compact object layout,
32-bit compact offset constraints, optional compression flags, stock
journalctl, stock libsystemd, and all repository reader checks.
For deterministic writer layout changes, run
tests/interoperability/run_byte_identity.py --final-state all and require
all_equal: true for the accepted uncompressed corpus unless the active SOW
records exact byte deltas and a user decision changes the pass condition.
The accepted corpus must keep deliberate DATA hash-bucket collisions, and the
byte-identity runner must fail if data_hash_chain_depth does not match the
systemd reference value for every language and final state.
For writer lock changes, run tests/interoperability/run_lock_matrix.py
and require all SDK writer pairs to reject a second active writer before the
contender publishes a ready file, plus stale-lock cleanup after crashed
writers.
This matrix applies to optional lock helpers/wrappers, not to the core writer
constructors after SOW-0071.
For directory reader or file-backed journalctl --directory changes, run
tests/interoperability/run_directory_matrix.py and require stock
journalctl plus Rust and Go rewrites to agree on traversal,
ordering, filtering, boot listing, corrupt-file skipping, empty directories,
and repository directory discovery.
Bad Practices
Do not implement daemon/service behavior just to satisfy journalctl daemon-control options.
Do not rely on one language's tests as sufficient for all languages.
Do not claim compatibility from closed-file journalctl --verify alone.
Do not treat live-reader smoke tests as sufficient for production compatibility.
Do not load or link system journal libraries at runtime in any SDK implementation to pass tests.
Do not silently skip corrupted fixture behavior; record expected errors and recovery behavior.
Workflow Checklist
Confirm the active SOW names the exact compatibility surface being changed.
Identify relevant systemd tests, fixtures, and Netdata Rust source paths.
Add or update shared tests before accepting implementation as complete.
Add or update live stock-reader and cross-language concurrency tests before accepting writer/reader compatibility.
Run the same conformance suite across every affected language.
Run interoperability tests across every writer/reader pair affected by the SOW.
Run live one-writer/multiple-reader tests for every affected writer and reader.
Record benchmark or profiling evidence when the SOW includes performance claims.
Validation Checklist
Before claiming production-grade compatibility:
Shared conformance tests pass for every targeted language.
Cross-language writer/reader matrix passes for every targeted file variant.
journalctl behavior is tested against file-backed fixtures.
Stock journalctl --file reads each targeted writer's files while the writer is appending.
Stock libsystemd reader APIs read each targeted writer's files while the writer is appending.
Each targeted reader reads live files produced by each targeted writer.
Reader follow/tail behavior is compared with stock journalctl file-backed behavior.
Daemon-only journalctl commands are not implemented and have documented behavior.
Dependency audit confirms no CGO or system journal library linkage in the SDK runtime path unless a SOW records an explicit user policy change.
Evidence
AGENTS.md: project goals and scope decisions.
.agents/sow/specs/product-scope.md: product scope and compatibility contracts.
.agents/sow/done/SOW-0001-20260523-project-bootstrap-and-orchestration.md: initial decisions and evidence ledger.
Update Rules
Update this skill when:
compatibility baseline changes;
a new journal file feature becomes in scope;
reviewer findings expose a missed compatibility or validation requirement;
a phase adds a durable implementation workflow that future agents must repeat.
.journal.zst
For mixed directory feature changes, run
tests/interoperability/run_mixed_directory_matrix.py and require stock
journalctl plus Rust and Go rewrites to agree on mixed
regular/compact files, uncompressed and zstd/xz/lz4 DATA-compressed files,
sealed/unsealed reads, directory verification with and without keys, and the
repository whole-file .journal.zst directory extension.
For file-backed journalctl query or follow changes, run
tests/interoperability/run_journalctl_query_matrix.py and require stock
journalctl plus Rust and Go rewrites to agree on
--since/--until realtime ranges, --boot descriptors, and live
--follow reads from actively appended file and directory inputs, including
no-tail, default-tail, and boot-plus-since cases.
For journalctl parser or option-surface changes, run
tests/parser-parity/check_v260_manifest.py and
tests/parser-parity/run_parser_parity.py. Parser parity must cover option
names, short aliases, output modes, parser-required conflicts, recognized
options interspersed after show-action match arguments, stock short-option
attached values and clusters such as -n2, -ball, and -rn2, and
optional-argument edge cases where bare and explicit-empty forms differ, such
as --boot versus --boot=, bare --lines versus --lines=, and bare
--case-sensitive versus --case-sensitive=.
For verifier changes, run tests/interoperability/run_verify_matrix.py and
require stock journalctl --verify --file plus Rust and Go
verification paths to agree on positive regular, zstd/xz/lz4 DATA-compressed,
compact, compact plus DATA-compressed, and sealed files, and on negative
object type, object size, payload hash, hash-chain, entry-array,
header-counter, seqnum, monotonic, and TAG/FSS corruption classes.
Separate reader support for existing historical files from writer feature milestones.
Record excluded upstream tests with a reason and extract file-level behavior where practical.
