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Deja View (/ops/dejaview) is a time-travel debugger for LangWatch’s event-sourcing system. It lets you search for any aggregate, inspect its full event history, and compute any projection’s state at any point in that history — all from a single interface.
Deja View showing event timeline and projection state

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When to Use Deja View

  • Debugging projection state — “Why does this trace show the wrong evaluation result?” Look at the events and compute the projection to see where state diverged.
  • Investigating processing failures — Find the aggregate, check what events were stored, and identify whether the issue is in the events or the projection.
  • Auditing — Review the complete history of any aggregate: every event that happened, in order.
  • Verifying a replay — After running a projection replay, check that the rebuilt state looks correct.

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Searching for Aggregates

The search bar at the top accepts:
  • Aggregate ID (required) — the primary key of the aggregate you’re looking for
  • Tenant ID (optional) — filter to a specific project/tenant
Results appear in a table showing matching aggregates. Click a row to load its event stream.
Deja View search interface

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Event Timeline

Once an aggregate is selected, the bottom of the screen shows a horizontal event timeline — a color-coded sequence of numbered event boxes.
  • Each box represents one event, numbered sequentially
  • Colors are assigned by event type, with a legend at the top
  • The current event is highlighted with a border
  • Click any event to jump to it, or use keyboard shortcuts to navigate

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Keyboard Shortcuts

KeyAction
h or ←Previous event
l or →Next event
eToggle event detail panel

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Event Payload

The center panel shows the current event’s payload. Toggle between:
  • Raw view — the complete event data structure
  • Diff view — shows what changed compared to the previous event
Press e or click the toggle to open a detailed JSON viewer in the right panel for deeply nested event payloads.

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Projection State

The left panel lists all available projections and reactors for the aggregate type. Select a projection to compute its state at the current event position. This is the core of the time-travel capability: you can step through events and watch how a projection’s state evolves with each event. For example:
  1. Select a projection (e.g., “TraceAnalytics”)
  2. Navigate to event #5 — see the projection state after events 1-5
  3. Step forward to event #6 — see how the state changed
  4. Compare with the current state to identify where things went wrong

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Deep Linking

Deja View encodes its full state in the URL fragment, making every view shareable: 
/ops/dejaview#query=trace_abc&tenant=project_xyz&event=5&proj=TraceAnalytics
Copy the URL from your browser to share the exact view — aggregate, event position, and selected projection — with a colleague. No additional setup required.

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Common Workflows

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”Why is this trace’s evaluation wrong?”

  1. Search for the trace’s aggregate ID
  2. Select the evaluation projection
  3. Step through events to find where the evaluation result was computed
  4. Check the event payload — is the input data correct?
  5. Check the projection state — does the fold logic produce the expected result?

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”Did the replay fix this aggregate?”

  1. After running a projection replay, search for the aggregate
  2. Select the replayed projection
  3. Navigate to the latest event
  4. Verify the projection state matches expectations

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”What happened to this aggregate at 2pm yesterday?”

  1. Search for the aggregate ID
  2. Use the event timeline to find events around that timestamp
  3. Step through them to see the sequence of state changes