Stability Fees
Overview of stability fee parameters, timing, frequency, amounts, management, and updates.

Parameters Governing the Stability Fee

inflationPeriod how long to wait between rounds of applying inflation
inflationRate the multiplier by which the inflation factor is adjusted per inflationPeriod

Timing, Frequency, and Amount of Fee

The inflationRate is the multiplier by which the inflationFactor is increased per inflationPeriod. It is initially set to 1 which leaves it to governance to enable the stability fee later on.
Both, the inflationRate as well as the inflationPeriod, are specified for a given stable token and subject to changes based on governance decisions.

Stability Fee Levied on Balance

Each account’s stable token balance is stored as ‘units’, and inflationFactor describes the units/value ratio. The Celo Dollar value of an account can therefore be computed as follows.
Account cUSD Value = Account cUSD Units / inflationFactor
When a transaction occurs, a modifier checks if the stability fee needs updating and, if so, the inflationFactor is updated.

Updates to the Inflation Factor

To apply periodic inflation, the inflation factor must be updated at regular intervals. Every time an event triggering an inflationFactor update(eg a transfer) occurs, the updateInflationFactor modifier is called (pseudocode below), which does the following:
  1. 1.
    Decide if on or more inflationPeriod have passed since the last time inflationFactor was updated
  2. 2.
    If so, find out how many have passed
  3. 3.
    Compute the new inflationFactor and update the last updated time:
inflationFactor = inflationFactor * inflationRate ^ # inflationPeriods since last update

Changes to Inflation Factor

Desired inflation rates may vary over time. When a new rate needs to be set, a governance proposal is required to update the inflation rate. If successful, the above function is called, which ensures inflationFactor is up to date, then updates the inflationRate and inflationPeriod parameters.

Inflation Factor Update Schedule

The updateInflationFactor modifier is called by the following functions:
  • setInflationParameters
  • approve
  • mint
  • transferWithComment
  • burn
  • transferFrom
  • transfer
  • debitFrom