The Message Pool



The Message Pool (mpool) is the component of lotus that handles pending messages for inclusion in the chain. Messages are added to the mpool either directly for locally published messages or through pubsub propagation. Whenever a miner is ready to create a block for a tipset, it invokes the mpool selection algorithm which selects an appropriate set of messages such that it optimizes miner reward and chain capacity.



API



The full node API defines the following methods for interacting with the mpool:

    MpoolPending(context.Context, types.TipSetKey) ([]*types.SignedMessage, error)
    MpoolSelect(context.Context, types.TipSetKey, float64) ([]*types.SignedMessage, error)
    MpoolPush(context.Context, *types.SignedMessage) (cid.Cid, error)
    MpoolPushMessage(ctx context.Context, msg *types.Message, spec *MessageSendSpec) (*types.SignedMessage, error)
    MpoolGetNonce(context.Context, address.Address) (uint64, error)
    MpoolSub(context.Context) (<-chan MpoolUpdate, error)
    MpoolGetConfig(context.Context) (*types.MpoolConfig, error)
    MpoolSetConfig(context.Context, *types.MpoolConfig) error
    MpoolClear(context.Context, local bool) error



MpoolPending



Returns the list of messages that are pending for a tipset.



MpoolSelect



Selects and returns a list of pending messages for inclusion in the next block.



MpoolPush



Pushes a signed message to the mpool; returns the CID of the message.



MpoolPushMessage



Atomically assigns a nonce, signs, and pushes a message to the mpool.



The MaxFee field of the spec argument is only used when GasFeeCap/GasPremium fields aren't specified in the message. When maxFee is set to 0, MpoolPushMessage will guess appropriate fee based on current chain conditions.



MpoolGetNonce



Returns the next nonce for the specified sender. Note that this method may not be atomic. Use MpoolPushMessage instead.



MpoolSub



Returns a channel to receive notifications about updates to the message pool. Note that the context must be cancelled when the caller is done with the subscription.



MpoolGetConfig



Returns (a copy of) the current mpool configuration.



MpoolSetConfig



Sets the mpool configuration to (a copy of) the supplied configuration object.



MpoolClear



Clears pending messages from the mpool; if local is true then local messages are also cleared and removed from the datastore.



This should be used with extreme care and only in the case of errors during head changes that would leave the mpool in an inconsistent state.



Command Line Interfae



The lotus command line interface defines an mpool command which allows a user to interact with the mpool.



The following commands are supported:

lotus mpool pending [--local]
lotus mpool sub
lotus mpool stat [--local]
lotus mpool replace [--gas-feecap <feecap>] [--gas-premium <premium>] [--gas-limit <limit>] [from] [nonce]
lotus mpool find [--from <address>] [--to <address>] [--method <int>]
lotus mpool config [<configuration>]
lotus mpool clear [--local]



lotus mpool pending



Prints the pending messages in the mpool, as returned by the MpoolPending API call. If --local is specified, it only prints pending messages for addresses in the local wallet.



lotus mpool sub



Subscribes to mpool changes using the MpoolSub API call and prints the stream of mpool updates.



lotus mpool stat



Prints various mpool statistics. If --local is specified then only prints statistics for addresses in the local wallet.



lotus mpool replace



Replaces a message in the mpool.



lotus mpool find



Searches for messages in the mpool.



lotus mpool config



Gets or sets the current mpool configuration.



lotus mpool clear



Unconditionally clears pending messages from the mpool. If the --local flag is passed, then local messages are also cleared; otherwise local messages are retained.



Warning: this command should only be used in the case of head change errors leaving the mpool in an state.



Configuration



The mpool a few parameters that can be configured by the user, either through the API or the command line interface.



The config struct is defined as follows:

type MpoolConfig struct {
 PriorityAddrs          []address.Address
 SizeLimitHigh          int
 SizeLimitLow           int
 ReplaceByFeeRatio      float64
 PruneCooldown          time.Duration
 GasLimitOverestimation float64
}



The meaning of these fields is as follows:

  • PriorityAddrs -- these are the addresses of actors whose pending messages should always
  • SizeLimitHigh -- this is the maximum number of pending messages before triggering a
  • SizeLimitLow -- this is the number of pending messages that should be kept after a prune.
  • ReplaceByFeeRatio -- this is the gas fee ratio for replacing messages in the mpool.
  • PruneCooldown -- this is the period of time to wait before triggering a new prune.
  • GasLimitOverestimation -- this is a parameter that controls the gas limit overestimation for new messages.





Message Selection



A few words regarding message selection are pertinent. In message selection, a miner selects a set of messages from the pending messages for inclusion to the next block in order to maximize its reward. The problem however is NP-hard (it's an instance of knapsack packing), further confounded by the fact that miners don't communicate their tickets to each other. So at best we can approximate the optimal selection in a reasonable amount of time.



The mpool employs a sophisticated algorithm for selecting messages for inclusion, given the ticket quality of a miner. The ticket quality reflects the probability of execution order for a block in the tipset. Given the ticket quality the algorithm computes the probability of each block, and picks dependent chains of messages such that the reward is maximized, while also optimizing the capacity of the chain. If the ticket quality is sufficiently high, then a greedy selection algorithm is used instead that simply picks dependent chains of maximum reward. Note that pending message chains from priority addresses are always selected, regardless of their profitability.



For algorithm details, please prefer to the implementation in chain/messagepool/selection.go.