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* Audit sink interface and implementations for pluggable log output.
*
* **Intelligence Perspective:** Pluggable sinks enable routing audit trails to
* centralised SIEM platforms, improving threat-detection fidelity.
*
* **Business Perspective:** Durable audit storage is a prerequisite for
* enterprise customers requiring demonstrable GDPR Article 30 compliance.
*
* **Marketing Perspective:** Configurable audit sinks differentiate against
* solutions that only log to stderr with no persistence option.
*
* ISMS Policy: AU-002 (Audit Logging and Monitoring), GDPR Articles 5, 17, 30
*
* @module utils/auditSink
* @since 0.9.0
*/
import { isAbsolute } from 'node:path';
import { appendFile, rename, stat } from 'node:fs/promises';
// ---------------------------------------------------------------------------
// Core data model
// ---------------------------------------------------------------------------
/**
* Represents a single audited operation and its contextual metadata.
*
* Designed for serialisation to append-only log sinks.
*/
export interface AuditLogEntry {
/** Timestamp of the event */
timestamp: Date;
/** Action performed (e.g. `'get_meps'`, `'tool_call'`) */
action: string;
/** Sanitised parameters used in the action */
params?: Record<string, unknown>;
/** Outcome metadata */
result?: {
count?: number;
success: boolean;
error?: string;
};
/** Wall-clock duration of the operation in milliseconds */
duration?: number;
/** User identifier (if authenticated) */
userId?: string;
/** Client identifier */
clientId?: string;
/** IP address (for security monitoring) */
ipAddress?: string;
}
// ---------------------------------------------------------------------------
// Query / access-control types
// ---------------------------------------------------------------------------
/**
* Filter for querying audit log entries.
*/
export interface AuditFilter {
/** Restrict to a specific action name */
action?: string;
/** Include only entries on or after this date */
since?: Date;
/** Include only entries on or before this date */
until?: Date;
/** Restrict to a specific user ID */
userId?: string;
}
/**
* Authorization token for privileged audit operations such as
* `getLogs()` and `eraseByUser()`.
*
* @security Must be kept secret; treat as a capability key.
*/
export type AuthToken = string;
// ---------------------------------------------------------------------------
// Sink interface
// ---------------------------------------------------------------------------
/**
* Pluggable audit sink interface.
*
* Implement this to create custom log destinations (files, SIEM, syslog …).
*
* @example
* ```typescript
* class MyCustomSink implements AuditSink {
* write(entry: AuditLogEntry): void {
* myExternalSystem.send(entry);
* }
* }
* ```
*/
export interface AuditSink {
/** Write a single audit entry to the sink */
write(entry: AuditLogEntry): void | Promise<void>;
/**
* Query entries matching a filter.
* Implemented by in-memory sinks; write-only sinks omit this.
*/
query?(filter: AuditFilter): AuditLogEntry[];
/**
* Clear all entries.
* Implemented by in-memory sinks; write-only sinks omit this.
* The `authorization` parameter may be provided by callers; validation is
* the responsibility of the calling context (typically {@link AuditLogger}).
*/
clear?(authorization?: AuthToken): void;
}
// ---------------------------------------------------------------------------
// AuditLogger constructor options
// ---------------------------------------------------------------------------
/**
* Constructor options for {@link AuditLogger}.
*/
export interface AuditLoggerOptions {
/**
* Extra write-only sinks (e.g. `FileAuditSink`, `StructuredJsonSink`).
* Replaces the default `StderrAuditSink` when provided.
*/
sinks?: AuditSink[];
/** Maximum age of log entries in milliseconds (data retention enforcement) */
retentionMs?: number;
/**
* Authorization token required to call `getLogs()`, `queryLogs()`,
* `clear()`, and `eraseByUser()`.
* When absent, those methods are unrestricted (suitable for testing).
*/
requiredAuthToken?: AuthToken;
/**
* Top-level parameter keys treated as PII and redacted to `'[REDACTED]'`.
* Defaults to {@link DEFAULT_SENSITIVE_KEYS}.
*/
sensitiveKeys?: readonly string[];
}
// ---------------------------------------------------------------------------
// Parameter sanitisation
// ---------------------------------------------------------------------------
/**
* Default set of top-level parameter keys treated as personally identifiable
* information (PII) and redacted before storage.
*/
export const DEFAULT_SENSITIVE_KEYS: readonly string[] = [
'name',
'email',
'fullName',
'address',
'firstName',
'lastName',
'phone',
];
/**
* Returns a copy of `params` with sensitive values replaced by `'[REDACTED]'`.
*
* Only **top-level** keys are inspected. Nested objects are passed through
* unchanged, so callers should sanitise nested params separately if needed.
*
* @param params - Original parameter map
* @param sensitiveKeys - Keys to redact (defaults to {@link DEFAULT_SENSITIVE_KEYS})
* @returns Sanitised copy of `params`
*
* @security This function does NOT recurse into nested objects.
* Callers are responsible for sanitising nested parameter structures.
* @since 0.9.0
*/
export function sanitizeParams(
params: Record<string, unknown>,
sensitiveKeys: readonly string[] = DEFAULT_SENSITIVE_KEYS
): Record<string, unknown> {
return Object.fromEntries(
Object.entries(params).map(([key, value]): [string, unknown] => [
key,
sensitiveKeys.includes(key) ? '[REDACTED]' : value,
])
);
}
// ---------------------------------------------------------------------------
// Data retention
// ---------------------------------------------------------------------------
/**
* Enforces a configurable data-retention window by filtering out expired entries.
*
* GDPR Article 5(1)(e) — Storage limitation principle.
*
* @example
* ```typescript
* const policy = new RetentionPolicy(30 * 24 * 60 * 60 * 1000); // 30 days
* const fresh = policy.enforce(auditLogger.getLogs());
* ```
* @since 0.9.0
*/
export class RetentionPolicy {
constructor(private readonly maxAgeMs: number) {}
/**
* Returns only entries whose timestamp is within the retention window.
*/
enforce(entries: AuditLogEntry[]): AuditLogEntry[] {
const cutoff = Date.now() - this.maxAgeMs;
return entries.filter((e): boolean => e.timestamp.getTime() >= cutoff);
}
/**
* Returns `true` if the given entry has exceeded the retention period.
*/
isExpired(entry: AuditLogEntry): boolean {
return Date.now() - entry.timestamp.getTime() > this.maxAgeMs;
}
}
// ---------------------------------------------------------------------------
// MemoryAuditSink
// ---------------------------------------------------------------------------
/**
* In-memory audit sink.
*
* Buffers entries in a private array and supports querying and per-user
* erasure (GDPR Article 17 — Right to Erasure).
*
* @since 0.9.0
*/
export class MemoryAuditSink implements AuditSink {
private entries: AuditLogEntry[] = [];
/** Appends the entry to the internal buffer. */
write(entry: AuditLogEntry): void {
this.entries.push(entry);
}
/** Returns entries matching the supplied filter. */
query(filter: AuditFilter): AuditLogEntry[] {
return this.entries.filter((e): boolean => this.matchesFilter(e, filter));
}
/**
* Clears the internal buffer.
* The `authorization` param is accepted but unused — access control is
* enforced by `AuditLogger`.
*/
clear(_authorization?: AuthToken): void {
this.entries = [];
}
/**
* Removes all entries associated with `userId`.
*
* GDPR Article 17 — Right to Erasure.
*/
eraseByUser(userId: string): void {
this.entries = this.entries.filter((e): boolean => e.userId !== userId);
}
private matchesFilter(entry: AuditLogEntry, filter: AuditFilter): boolean {
if (filter.action !== undefined && entry.action !== filter.action) {
return false;
}
if (filter.since !== undefined && entry.timestamp < filter.since) {
return false;
}
if (filter.until !== undefined && entry.timestamp > filter.until) {
return false;
}
if (filter.userId !== undefined && entry.userId !== filter.userId) {
return false;
}
return true;
}
}
// ---------------------------------------------------------------------------
// StderrAuditSink
// ---------------------------------------------------------------------------
/**
* Writes structured JSON audit lines to `stderr`.
*
* MCP-compatible: `stdout` is reserved for the MCP protocol wire format.
*
* @since 0.9.0
*/
export class StderrAuditSink implements AuditSink {
write(entry: AuditLogEntry): void {
console.error('[AUDIT]', JSON.stringify(entry));
}
}
// ---------------------------------------------------------------------------
// FileAuditSink
// ---------------------------------------------------------------------------
/**
* Constructor options for {@link FileAuditSink}.
*/
export interface FileAuditSinkOptions {
/** Absolute path to the NDJSON log file */
filePath: string;
/** Maximum file size in bytes before log rotation (default: 10 MiB) */
maxSizeBytes?: number;
}
/**
* Appends audit entries as newline-delimited JSON (NDJSON) to a file.
*
* **Log rotation:** when the file reaches `maxSizeBytes`, it is renamed to
* `<filePath>.<timestamp>.bak` before the new entry is written.
*
* @security The log file should be stored on a volume with restricted write
* permissions; only the server process account should have write access.
* @since 0.9.0
*/
export class FileAuditSink implements AuditSink {
private readonly filePath: string;
private readonly maxSizeBytes: number;
/**
* Serialises concurrent write calls so that stat + rename + appendFile
* sequences never interleave across parallel `log()` invocations.
*/
private writeQueue: Promise<void> = Promise.resolve();
constructor(options: FileAuditSinkOptions) {
if (!isAbsolute(options.filePath)) {
throw new Error(
`FileAuditSink: filePath must be an absolute path; received "${options.filePath}"`
);
}
this.filePath = options.filePath;
this.maxSizeBytes = options.maxSizeBytes ?? 10 * 1024 * 1024;
}
write(entry: AuditLogEntry): Promise<void> {
// Chain each write onto the tail of the previous one so concurrent callers
// are serialised — preventing interleaved stat/rename/append sequences.
// Swap `writeQueue` to a promise that always resolves (never rejects) so
// the next enqueued write is not blocked after a prior write failure.
const next = this.writeQueue.then(async (): Promise<void> => {
await this.rotateIfNeeded();
await appendFile(this.filePath, `${JSON.stringify(entry)}\n`, 'utf8');
});
// Always keep `writeQueue` as a resolved promise for the next caller;
// propagate the actual error only through the returned `next` promise.
this.writeQueue = next.then(
(): void => { /* success — queue stays resolved */ },
(): void => { /* failure — reset queue to resolved so next write proceeds */ }
);
return next;
}
private async rotateIfNeeded(): Promise<void> {
try {
const stats = await stat(this.filePath);
if (stats.size >= this.maxSizeBytes) {
await rename(
this.filePath,
`${this.filePath}.${String(Date.now())}.bak`
);
}
} catch (error: unknown) {
const err = error as NodeJS.ErrnoException;
if (err.code === 'ENOENT') {
// File does not exist yet — no rotation needed.
return;
}
// Surface unexpected errors (permissions, EBUSY, disk errors, etc.)
// so rotation failures are observable rather than silently swallowed.
console.error('[FileAuditSink] Failed to rotate audit log file:', err);
throw err;
}
}
}
// ---------------------------------------------------------------------------
// StructuredJsonSink
// ---------------------------------------------------------------------------
/**
* Serialises each audit entry to JSON and passes it to a writer callback.
*
* Suitable for forwarding to structured log aggregators such as
* AWS CloudWatch, Elasticsearch, or Splunk.
*
* @example
* ```typescript
* const sink = new StructuredJsonSink((json) => cloudwatch.putLogEvent(json));
* const logger = new AuditLogger({ sinks: [sink] });
* ```
* @since 0.9.0
*/
export class StructuredJsonSink implements AuditSink {
private readonly writer: (json: string) => void;
constructor(writer?: (json: string) => void) {
this.writer = writer ?? ((json: string): void => {
console.error('[AUDIT]', json);
});
}
write(entry: AuditLogEntry): void {
this.writer(JSON.stringify(entry));
}
}
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