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Effective Bid

The effective bid is the real-time value of a contract's cache slot bid after accounting for time-based decay. It determines whether a contract can retain its cache slot or risks eviction.

How is the Effective Bid Calculated?

The effective bid decays over time according to the following formula:

effectiveBid = lastBid - (timeElapsed * decayRate)
  • lastBid: The most recent bid amount placed for the contract (in wei)
  • timeElapsed: Seconds since the last bid was placed
  • decayRate: The current decay rate (in wei per second)

If the decay amount exceeds the last bid, the effective bid is set to zero.

Time Decay Bonus for New Bids

When you place a new bid, the contract adds a time decay bonus to your bid. This means that for the same nominal bid value, a newer bid is always worth more than an older one, because it includes the current decay amount. This mechanism ensures that new bids are always competitive with decayed older bids.

Note: As a user, you only pay the value you specify. The extra "decay bonus" is added by the smart contract internally for the purpose of effective bid calculation. You do not pay the bonus amount out of pocket—the contract logic simply makes your bid more competitive by design.

  • Placing a bid: The actual bid value used for cache slot competition is:
bid = value + decay

where decay is the current decay amount at the time of bidding.

  • Eviction/decay: Over time, the effective value of all bids decays at the same rate, so only the relative age and size of the bid matter for eviction.

Reference:

Example function:

/// @dev Converts a value to a bid by adding the time decay term.
function _toBid(uint256 value) internal view returns (uint192 bid) {
    uint256 _bid = value + _calcDecay();
    if (_bid > type(uint192).max) {
        revert BidTooLarge(_bid);
    }
    return uint192(_bid);
}

Why Does It Matter?

  • The effective bid is used to determine a contract's position in the cache queue.
  • If your effective bid is outbid by another contract, your contract risks eviction from the cache.
  • Monitoring the effective bid is essential for understanding your place in the cache queue.

Example Calculation

Suppose:

  • lastBid = 1,000,000 wei
  • decayRate = 100 wei/sec
  • timeElapsed = 3,600 sec (1 hour)
decayAmount = 3,600 * 100 = 360,000 wei
effectiveBid = 1,000,000 - 360,000 = 640,000 wei

If the decay amount ever exceeds the last bid, the effective bid is set to 0.

See Also