The BlockPolicyEstimator is used for estimating the feerate needed for a transaction to be included in a block within a certain number of blocks.
Declared in <policy/fees/block_policy_estimator.h>
class CBlockPolicyEstimator;At a high level the algorithm works by grouping transactions into buckets based on having similar feerates and then tracking how long it takes transactions in the various buckets to be mined. It operates under the assumption that in general transactions of higher feerate will be included in blocks before transactions of lower feerate. So for example if you wanted to know what feerate you should put on a transaction to be included in a block within the next 5 blocks, you would start by looking at the bucket with the highest feerate transactions and verifying that a sufficiently high percentage of them were confirmed within 5 blocks and then you would look at the next highest feerate bucket, and so on, stopping at the last bucket to pass the test. The average feerate of transactions in this bucket will give you an indication of the lowest feerate you can put on a transaction and still have a sufficiently high chance of being confirmed within your desired 5 blocks.
Here is a brief description of the implementation: When a transaction enters the mempool, we track the height of the block chain at entry. All further calculations are conducted only on this set of "seen" transactions. Whenever a block comes in, we count the number of transactions in each bucket and the total amount of feerate paid in each bucket. Then we calculate how many blocks Y it took each transaction to be mined. We convert from a number of blocks to a number of periods Y' each encompassing "scale" blocks. This is tracked in 3 different data sets each up to a maximum number of periods. Within each data set we have an array of counters in each feerate bucket and we increment all the counters from Y' up to max periods representing that a tx was successfully confirmed in less than or equal to that many periods. We want to save a history of this information, so at any time we have a counter of the total number of transactions that happened in a given feerate bucket and the total number that were confirmed in each of the periods or less for any bucket. We save this history by keeping an exponentially decaying moving average of each one of these stats. This is done for a different decay in each of the 3 data sets to keep relevant data from different time horizons. Furthermore we also keep track of the number unmined (in mempool or left mempool without being included in a block) transactions in each bucket and for how many blocks they have been outstanding and use both of these numbers to increase the number of transactions we've seen in that feerate bucket when calculating an estimate for any number of confirmations below the number of blocks they've been outstanding.
We want to be able to estimate feerates that are needed on tx's to be included in a certain number of blocks. Every time a block is added to the best chain, this class records stats on the transactions included in that block
| Name | Description |
|---|---|
CBlockPolicyEstimator [constructor] | Create new BlockPolicyEstimator and initialize stats tracking classes with default values. |
~CBlockPolicyEstimator [destructor] [virtual] | Destroy the estimator and release its tracking stats. |
EstimateFeeRate | Estimate the feerate needed to confirm within target blocks; wraps estimateSmartFee into a FeeRateEstimation. |
Flush | Drop still unconfirmed transactions and record current estimations, if the fee estimation file is present. |
FlushFeeEstimates | Record current fee estimations. |
FlushUnconfirmed | Empty mempool transactions on shutdown to record failure to confirm for txs still in mempool |
GetFeeEstimatorFileAge | Calculates the age of the file, since last modified. |
HighestTargetTracked [virtual] | Calculation of highest target that estimates are tracked for. |
MaximumTarget | Return the highest confirmation target for which an estimate can be provided. |
Read | Read estimation data from a file. |
Write | Write estimation data to a file. |
estimateFee | DEPRECATED. Return a feerate estimate. |
estimateRawFee | Return a specific fee estimate calculation with a given success threshold and time horizon, and optionally return detailed data about calculation. |
estimateSmartFee [virtual] | Estimate feerate needed to get be included in a block within confTarget blocks. If no answer can be given at confTarget, return an estimate at the closest target where one can be given. 'conservative' estimates are valid over longer time horizons also. |
processBlock | Process all the transactions that have been included in a block. |
processTransaction | Process a transaction accepted to the mempool. |
removeTx | Remove a transaction from the mempool tracking stats for non BLOCK removal reasons. |