How to sync backlink events safely

Backlink event sync needs a simple ordering rule: save the events you received before saving the cursor that moves past them. Keep separate cursors for separate feeds. A crash should cause a repeat that you can identify, rather than a silent jump over unread history.

This article describes the AgentLinkOps local sync contract reviewed on September 12, 2026. The code exists and has local acceptance; that does not establish hosted readiness for every account. Confirm access and the connected environment before scheduling a recurring sync job.

Treat a cursor as a position in one feed

A cursor tells a consumer where to continue reading a particular ordered result. Do not compare its value with timestamps or manufacture the next value yourself. Store what the service returned, beside the identity of the feed it belongs to.

AgentLinkOps separates source-link events from destination-health events. A source event concerns the referring page and observed link. A target event concerns the destination. The sequences are independent, so a position from one cannot stand in for a position from the other.

Consider a synthetic sync state: source events are current through their sequence 120, while target events are current through their sequence 18. Writing 120 into both positions could skip target history. Writing 18 into both could repeat source history. The same-looking number has meaning only within its own feed.

Save a page before advancing its checkpoint

A consumer should read a page, validate it, record its events and only then persist the returned continuation state. In this article, checkpoint means that durable local continuation state.

The following is explanatory pseudocode, not the AgentLinkOps API schema:

read page using this feed's saved cursor
validate page and identify events already stored
write new events durably
save the cursor returned with that page
continue only when the response says more pages exist

There is still a crash window after events are written but before the cursor is saved. That window should produce a replay. Use stable event identity to identify records already applied. A repeat is easier to reason about than an event your files never received.

The current CLI writes observations before saving updated cursors. Keep that order if you build another consumer. The repository ledger article explains why cursor state and observations have different responsibilities.

Handle expired history as a visible gap

A service may stop retaining events older than a certain point. AgentLinkOps can return CURSOR_EXPIRED with recovery details when a cursor falls behind that retained history. Its HTTP response uses 410; the HTTP semantics specification defines that status, while the service's error body defines the recovery procedure.

Do not fix the error by replacing the saved cursor with today's position and declaring success. That would hide the period your consumer missed. Record the gap, follow the recovery information and explain what current state can be rebuilt.

The source-event path in the reviewed CLI can request a watch snapshot before resuming. The target-event path has no equivalent snapshot adapter in that code. It records the history gap and resumes according to the response; it cannot reconstruct every missed target transition. This limit matters when someone asks whether a destination failed during an unattended interval.

A current snapshot can establish current state. It cannot restore an exact change timeline that the service no longer retains.

Keep reads and remote writes separate

The CLI has linktrail sync --pull-only for reading remote evidence into local files. It still needs the configured cloud endpoint, project and authorization token. “Pull only” means no expectation push; it does not mean no local file changes.

Use this mode when you want to inspect incoming evidence before sending ledger expectations upstream. Confirm the account and project before running the full sync command. Our API permissions article describes the difference between reading events and changing watches.

A sync report should name which feeds advanced, how many records arrived, whether repeats occurred and whether recovery found a gap. If one feed fails, preserve the other feed's valid position without inventing progress for the failed one.

Test failure cases with synthetic records

Before relying on an unattended consumer, exercise three cases: repeat the same page, interrupt after event persistence, and expire one feed while the other remains current. Inspect the resulting files directly. A test that only returns one successful page misses the behavior that protects history.

Keep these tests free of real publisher changes or customer data. They test how your consumer handles records, not whether a backlink is live. The MCP workflow guide connects saved evidence to research tasks, and the agent workflow hub provides the surrounding process.

Sources

1. HTTP Semantics · RFC Editor · 2022-06