# 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 be included in a block during message selection, as long as they are profitable. Miners should configure their own worker addresses so that they include their own messages when they produce a new block. Default is empty. - `SizeLimitHigh` -- this is the maximum number of pending messages before triggering a prune in the message pool. Note that messages from priority addresses are never pruned. Defafult is 30000. - `SizeLimitLow` -- this is the number of pending messages that should be kept after a prune. Default is 20000. - `ReplaceByFeeRatio` -- this is the gas fee ratio for replacing messages in the mpool. Whenever a message is replaced, the `GasPremium` must be increased by this ratio. Default is 1.25. - `PruneCooldown` -- this is the period of time to wait before triggering a new prune. Default is 1min. - `GasLimitOverestimation` -- this is a parameter that controls the gas limit overestimation for new messages. Default is 1.25. ## 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`.