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Storage Meter Pressure Lab

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About This MicroSim

LRS-Lite keeps each student's record in an IndexedDB database under a self-imposed 10 MB budget. The design's storage meter shows how full the local record is and how safe it is, and it drives four pressure levels:

Level Condition Behavior
Normal below 60% Nothing beyond the meter
Offer 60% or more, or 7 days since the last backup, or unsynced events older than 24 hours A backup card appears
Reclaim 80% or more Drop mirrored segments from other devices first, then this device's covered segments
Protect 95% or more with unsynced data Force all sims into summary mode and fold the oldest unsynced statements into summaries (truncated_statements: N)

Summaries and evidence are never pruned, because they are the student's state. This device's segments can only be dropped once they are synced and covered by a verified checkpoint.

The lab hides the level until you predict it. Set the sliders, pick Normal, Offer, Reclaim or Protect, and the lab reveals the level, the rule that fired, and an "after" bar showing which slices shrink and in what order. Hover a slice or a legend entry to see whether it may be pruned. Advance 10 days grows the database at the design's heavy-student rate for the chosen emission mode (0.37 MB per semester in Summary mode, 3.6 MB in Full mode). Keep my record on this device simulates a persistent storage request; it protects against eviction but does not change the level.

The meter, the slices and the levels are a design that has not been built. The split of the total into slices and the growth rates are illustrative.

Learning objective: The learner will predict which pressure level a given storage state triggers and which data the library discards first.

Bloom's taxonomy level: Apply (verb: predict)

How to Use

  1. At the defaults (0.34 MB, backup 2 days ago, nothing unsynced), predict the level. Check the rule that fired.
  2. Press Advance 10 days once and predict again. Why does the level change although the database barely grew?
  3. Drag Database size to 8.5 MB and predict. Read the discard order in the "after" bar.
  4. Now drag it to 9.7 MB with Oldest unsynced at 0 h, predict, then set unsynced to 30 h and predict again. What does the "with unsynced data" condition add?
  5. Choose each persistence scenario and press Keep my record on this device. Does the level change?

Iframe Embed Code

You can add this MicroSim to any web page by adding this to your HTML:

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<iframe src="https://dmccreary.github.io/microsims/sims/storage-meter-pressure-lab/main.html"
        height="702px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Audience

Teachers, instructional designers, learning-technology developers and learning-analytics practitioners (college undergraduate and professional development).

Duration

15 minutes

Prerequisites

  • IndexedDB storage and the five object stores (Chapter 22)
  • The 10 MB budget and its slices (Chapter 22)
  • The difference between syncing and a verified backup (Chapter 23 preview)

Activities

  1. Predict five states (6 min): Work through the five How to Use steps, predicting before each reveal. Aim for five correct predictions in a row.
  2. Order the discards (4 min): Without the lab, write the order in which Reclaim and Protect free space and name the one slice that is never pruned. Check with the lab.
  3. Summary versus Full (3 min): From the defaults, count how many times you must press Advance 10 days in each mode to reach Offer by size alone (60%). Explain what this says about a summary-mode pilot.
  4. Discuss (2 min): Why does a granted persistence request not change the pressure level?

Assessment

  • Given a size, a backup age and an unsynced age, the learner names the level and the rule that decides it, including when a higher level overrides a lower one.
  • The learner states that mirrored segments go first, covered local segments second, and summaries never.
  • The learner explains that Protect needs both 95% fullness and unsynced data.

References

  1. IndexedDB API - MDN Web Docs. The browser database LRS-Lite uses for the local record.
  2. Storage quotas and eviction criteria - MDN Web Docs. How browsers limit and evict an origin's data.
  3. StorageManager: persist() method - MDN Web Docs. The persistent storage request behind "Keep my record on this device".
  4. p5.js reference: createSlider - p5.js. The control used for the three storage sliders.