Designing a Durable, Single-Host Append-Only Log with Dyalog Component Files

Durable Log Requirement: Single-Host, Ordered APL Array Storage

The core problem is maintaining a durable, append-only log of APL arrays across multiple concurrent Dyalog sessions on one host, ensuring strict ordering and preventing write overwrites.

Minimal Design for Concurrency Control

The smallest suitable design utilizes a single native component file, initialized via ⎕FCREATE. Concurrency is managed by session-level file ties.

• Writers must acquire an exclusive tie, typically using ⎕FHOLD, before performing a read-modify-write cycle and appending with ⎕FWRITE.
• Readers acquire a shared tie and sequentially use ⎕FREAD to validate the log order.
• Ties must be released (⎕FRELEASE) when the session concludes.

Trust and Data Boundaries

Crucially, the component file's security relies solely on the operating system's file permissions for the running Dyalog account. Dyalog component files do not offer built-in encryption or authentication. Furthermore, data integrity regarding text encoding is dependent on whether the Dyalog Classic or Unicode edition is used; do not mix editions without explicit character data validation.

Operational Checks and Safety

Robust operation requires strict resource management. Always confirm a tie is valid before attempting ⎕FREAD or ⎕FWRITE. To prevent resource leaks, wrap every ⎕FHOLD with a corresponding ⎕FRELEASE within a trap or finally block.

Example Check: To verify tie management, wrap the write operation in a trap block. If an error occurs between ⎕FHOLD and ⎕FRELEASE, the trap ensures ⎕FRELEASE executes, preventing a permanent lock on the file.

\n# In Dyalog session:\n⎕FHOLD \ntry\n    # Perform read-modify-write and ⎕FWRITE\n    ...\nfinally\n    ⎕FRELEASE \n

Verification: After running this structure, attempt to acquire a tie from a second session; it should block or fail as dictated by the exclusive tie mode, confirming the hold is active.

Failure Modes and Design Triggers

A crash during an ⎕FWRITE can leave an incomplete component. Without an exclusive hold, concurrent read-modify-write operations will result in lost updates. The current design is constrained by the single-host, single-file boundary.

A redesign is necessary if requirements shift to: adding encryption at rest, scaling beyond one host, requiring ACID guarantees across multiple files, or needing ad-hoc querying capabilities. In those cases, external solutions (OS encryption, Dyalog client/server architecture, or a dedicated database) are required, not modifications to the single component file.

Limitations

Do not assume tie numbers are stable across restarts; they are session-local. Furthermore, do not treat this single file as a disaster recovery mechanism; regular external backups are mandatory.