Cold Start Strategies

New candidates enter the system with zero impressions. Promovolve uses three distinct strategies depending on the state of the candidate pool.

Strategy 1: Full Cold Start

Condition: All candidates in the slot have 0 impressions.

Algorithm: Use categoryScore from the auction phase as a prior, with noise:

sampledCTR = categoryScore + random(-0.1, +0.1)
score = sampledCTR × log(1 + CPM)

The categoryScore = classifierConfidence × rankerWeight provides a signal from the TaxonomyRankerEntity. The ±0.1 noise ensures different candidates are selected across requests even when they have identical category scores.

Strategy 2: Warmup Phase

Condition: All candidates have fewer than 10 impressions (WarmupImpressions = 10).

Algorithm: Round-robin — always select the candidate with the fewest impressions:

select = argmin(candidate.impressions)

No Thompson Sampling runs during warmup. This guarantees every candidate gets at least 10 impressions before exploitation begins.

Why 10? At 10 impressions with a typical 2-5% CTR, the expected number of clicks is 0-1. The Beta distribution Beta(1, 10) or Beta(2, 9) has sufficient shape to distinguish different CTRs but is still wide enough for continued exploration after warmup ends.

Strategy 3: Partial Cold Start

Condition: Some candidates have data (≥ 10 impressions) and some are new (0 impressions).

Algorithm: Epsilon-greedy with ExplorationRate = 0.30:

if random() < 0.30:
    select randomly from cold candidates (impressions == 0)
else:
    run Thompson Sampling on all candidates

The 30% rate is aggressive by design — new candidates need data quickly. Once they accumulate impressions, Thompson Sampling’s Beta posterior handles exploration naturally.

Note: When Thompson Sampling runs in the else branch, it runs on all candidates including cold ones. Cold candidates use categoryScore + random(-0.15, +0.15) as their sampled CTR, so they still have a chance to win through the normal scoring mechanism.

Strategy Selection Flow

Are all candidates at 0 impressions?
  └── Yes → Full Cold Start (categoryScore ± 0.1 noise)
  └── No  → Are all candidates under 10 impressions?
              └── Yes → Warmup (round-robin by fewest impressions)
              └── No  → Are some candidates at 0 impressions?
                          └── Yes → Partial Cold Start (30% epsilon-greedy)
                          └── No  → Standard Thompson Sampling

Key Constants