cosmos-sdk/docs/core/baseapp.md

BaseApp

Pre-requisite Reading

• Anatomy of an SDK application
• Lifecycle of an SDK transaction

Synopsis

This document describes baseapp, the abstraction that implements most of the common functionalities of an SDK application.

• Introduction
• Type Definition
• Constructor
• States
• Routing
• Main ABCI Messages
  • CheckTx
  • DeliverTx
• RunTx(), AnteHandler and RunMsgs
  • RunTx()
  • AnteHandler
  • RunMsgs()
• Other ABCI Message
  • InitChain
  • BeginBlock
  • EndBlock
  • Commit
  • Info
  • Query

Introduction

baseapp is a base class that implements the core of an SDK application, namely:

• The Application-Blockchain Interface, for the state-machine to communicate with the underlying consensus engine (e.g. Tendermint).
• A Router, to route messages and queries to the appropriate module.
• Different states, as the state-machine can have different parallel states updated based on the ABCI message received.

The goal of baseapp is to provide the fundamental layer of an SDK application that developers can easily extend to build their own custom application. Usually, developers will create a custom type for their application, like so:

type app struct {
    *bam.BaseApp // reference to baseapp
    cdc *codec.Codec

    // list of application store keys

    // list of application keepers

    // module manager
}

Extending the application with baseapp gives the former access to all of baseapp's methods. This allows developers to compose their custom application with the modules they want, while not having to concern themselves with the hard work of implementing the ABCI, the routing and state management logic.

Type Definition

The baseapp type holds many important parameters for any Cosmos SDK based application. Let us go through the most important components.

Note: Not all parameters are described, only the most important ones. Refer to the type definition for the full list

First, the important parameters that are initialized during the initialization of the application:

• CommitMultiStore: This is the main store of the application, which holds the canonical state that is committed at the end of each block. This store is not cached, meaning it is not used to update the application's intermediate (un-committed) states. The CommitMultiStore is a multi-store, meaning a store of stores. Each module of the application uses one or multiple KVStores in the multi-store to persist their subset of the state.
• Database: The db is used by the CommitMultiStore to handle data storage.
• Router: The router facilitates the routing of messages to the appropriate module for it to be processed. Here message refers to the transaction components that need to be processed by the application in order to update the state, and not to ABCI messages which implement the interface between the application and the underlying consensus engine.
• Query Router: The query router facilitates the routing of queries to the appropriate module for it to be processed. These queries are not ABCI messages themselves, but they are relayed to the application from the underlying consensus engine via the ABCI message Query.
• TxDecoder: It is used to decode transaction []byte relayed by the underlying Tendermint engine.
• [BaseKey]: This key is used to access the main store in the CommitMultiStore. The main store is used to persist data related to the core of the application, like consensus parameters.
• AnteHandler: This handler is used to handle signature verification and fee payment when a transaction is received.
• initChainer, beginBlocker and endBlocker: These are the functions executed when the application receives the [InitChain], [BeginBlock] and [EndBlock] ABCI messages from the underlying Tendermint engine.

Then, parameters used to define volatile states (i.e. cached states):

• checkState: This state is updated during CheckTx, and reset on Commit.
• deliverState: This state is updated during DeliverTx, and reset on Commit.

Finally, a few more important parameters:

• voteInfos: This parameter carries the list of validators whose precommit is missing, either because they did not vote or because the proposer did not include their vote. This information is carried by the context and can be used by the application for various things like punishing absent validators.
• minGasPrices: This parameter defines the minimum gas prices accepted by the node. This is a local parameter, meaning each full-node can set a different minGasPrices. It is run by the anteHandler during CheckTx, mainly as a spam protection mechanism. The transaction enters the mempool only if the gas prices of the transaction is superior to one of the minimum gas price in minGasPrices (i.e. if minGasPrices == 1uatom, 1upho, the gas-price of the transaction must be superior to 1uatom OR 1upho).
• appVersion: Version of the application. It is set in the application's constructor function.

Constructor

NewBaseApp(name string, logger log.Logger, db dbm.DB, txDecoder sdk.TxDecoder, options ...func(*BaseApp),) is the constructor function for baseapp. It is called from the application's constructor function each time the full-node is started.

baseapp's constructor function is pretty straightforward. The only thing worth noting is the possibility to add additional options to baseapp by passing options functions to the constructor function, which will execute them in order. options are generally setter functions for important parameters, like SetPruning() to active pruning or SetMinGasPrices() to set the node's min-gas-prices.

A list of options examples can be found here. Naturally, developers can add additional options based on their application's needs.

States

baseapp handles various parallel states for different purposes. There is the main state, which is the canonical state of the application, and volatile states like checkState and deliverState, which are used to handle temporary states in-between updates of the main state made during Commit.

            Updated whenever an unconfirmed   Updated whenever a transaction         To serve user queries relayed
            transaction is received from the  is received from the underlying        from the underlying consensus
            underlying consensus engine via   consensus engine (as part of a block)  engine via the Query ABCI message
            CheckTx                           proposal via DeliverTx
                +----------------------+      +----------------------+       +----------------------+
                |   CheckState(t)(0)   |      |  DeliverState(t)(0)  |       |    QueryState(t)     |
                +----------------------+      |                      |       |                      |
CheckTx(tx1)               |                  |                      |       |                      |
                           v                  |                      |       |                      |
                +----------------------+      |                      |       |                      |
                |   CheckState(t)(1)   |      |                      |       |                      |
                +----------------------+      |                      |       |                      |
CheckTx(tx2)               |                  |                      |       |                      |
                           v                  |                      |       |                      |
                +----------------------+      |                      |       |                      |
                |   CheckState(t)(2)   |      |                      |       |                      |
                +----------------------+      |                      |       |                      |
CheckTx(tx3)               |                  |                      |       |                      |
                           v                  |                      |       |                      |
                +----------------------+      |                      |       |                      |
                |   CheckState(t)(3)   |      |                      |       |                      |
                +----------------------+      +----------------------+       |                      |
DeliverTx(tx1)             |                             |                   |                      |
                           v                             v                   |                      |
                +----------------------+      +----------------------+       |                      |
                |                      |      |  DeliverState(t)(1)  |       |                      |
                |                      |      +----------------------+       |                      |
DeliverTx(tx2)  |                      |                 |                   |                      |
                |                      |                 v                   |                      |
                |                      |      +----------------------+       |                      |
                |                      |      |  DeliverState(t)(2)  |       |                      |
                |                      |      +----------------------+       |                      |
DeliverTx(tx3)  |                      |                 |                   |                      |
                |                      |                 v                   |                      |
                |                      |      +----------------------+       |                      |
                |                      |      |  DeliverState(t)(3)  |       |                      |
                +----------------------+      +----------------------+       +----------------------+
Commit()                  |                              |                               |
                          v                              v                               v
                +----------------------+      +----------------------+       +----------------------+
                |  CheckState(t+1)(0)  |      | DeliverState(t+1)(0) |       |   QueryState(t+1)    |
                +----------------------+      |                      |       |                      |
                          .                              .                               .
                          .                              .                               .
                          .                              .                               .

Main State

The main state is the canonical state of the application. It is initialized on InitChain and updated on Commit at the end of each block.

+--------+                              +--------+
|        |                              |        |
|   S    +----------------------------> |   S'   |
|        |   For each T in B: apply(T)  |        |
+--------+                              +--------+

The main state is held by baseapp in a structure called the CommitMultiStore. This multi-store is used by developers to instantiate all the stores they need for each of their application's modules.

Volatile States

Volatile - or cached - states are used in between Commits to manage temporary states. They are reset to the latest version of the main state after it is committed. There are two main volatile states:

• checkState: This cached state is initialized during InitChain, updated during CheckTx when an unconfirmed transaction is received, and reset to the main state on Commit.
• deliverState: This cached state is initialized during BeginBlock, updated during DeliverTx when a transaction included in a block is processed, and reset to the main state on Commit.

Both checkState and deliverState are of type state, which includes:

• A CacheMultiStore, which is a cached version of the main CommitMultiStore. A new version of this store is committed at the end of each successful CheckTx()/DeliverTx() execution.
• A Context, which carries general information (like raw transaction size, block height, ...) that might be needed in order to process the transaction during CheckTx() and DeliverTx(). The context also holds a cache-wrapped version of the CacheMultiStore, so that the CacheMultiStore can maintain the correct version even if an internal step of CheckTx() or DeliverTx() fails.

Routing

When messages and queries are received by the application, they must be routed to the appropriate module in order to be processed. Routing is done via baseapp, which holds a router for messages, and a query router for queries.

Message Routing

Messages need to be routed after they are extracted from transactions, which are sent from the underlying Tendermint engine via the CheckTx and DeliverTx ABCI messages. To do so, baseapp holds a router which maps paths (string) to the appropriate module handler. Usually, the path is the name of the module.

The application's router is initialized with all the routes using the application's module manager, which itself is initialized with all the application's modules in the application's constructor.

Query Routing

Similar to messages, queries need to be routed to the appropriate module's querier. To do so, baseapp holds a query router, which maps paths (string) to the appropriate module querier. Usually, the path is the name of the module.

Just like the router, the query router is initialized with all the query routes using the application's module manager, which itself is initialized with all the application's modules in the application's constructor.

Main ABCI Messages

The Application-Blockchain Interface (ABCI) is a generic interface that connects a state-machine with a consensus engine to form a functional full-node. It can be wrapped in any language, and needs to be implemented by each application-specific blockchain built on top of an ABCI-compatible consensus engine like Tendermint.

The consensus engine handles two main tasks:

• The networking logic, which mainly consists in gossiping block parts, transactions and consensus votes.
• The consensus logic, which results in the deterministic ordering of transactions in the form of blocks.

It is not the role of the consensus engine to define the state or the validity of transactions. Generally, transactions are handled by the consensus engine in the form of []bytes, and relayed to the application via the ABCI to be decoded and processed. At keys moments in the networking and consensus processes (e.g. beginning of a block, commit of a block, reception of an unconfirmed transaction, ...), the consensus engine emits ABCI messages for the state-machine to act on.

Developers building on top of the Cosmos SDK need not implement the ABCI themselves, as all the ABCI messages are implemented as a set of baseapp's methods in the Cosmos SDK. Let us go through the main ABCI messages that baseapp handles: CheckTx and DeliverTx. Note that these ABCI messages are different from the messages contained in transactions, the purpose of which is to trigger state-transitions.

CheckTx

The CheckTx ABCI message is sent by the underlying consensus engine when a new unconfirmed (i.e. not yet included in a valid block) transaction is received by a full-node, and it is handled by the CheckTx(req abci.RequestCheckTx) method of baseapp (abbreviated to CheckTx() thereafter). The role of CheckTx() is to guard the full-node's mempool (where unconfirmed transactions are stored until they are included in a block) from spam transactions. Unconfirmed transactions are relayed to peers only if they pass CheckTx().

CheckTx() can perform both stateful and stateless checks, but developers should strive to make them lightweight. In the Cosmos SDK, after decoding transactions, CheckTx() is implemented to do the following checks:

1. Extract the messages from the transaction.
2. Perform stateless checks by calling ValidateBasic() on each of the messages. This is done first, as stateless checks are less computationally expensive than stateful checks. If ValidateBasic() fail, CheckTx returns before running stateful checks, which saves resources.
3. Perform non-module related stateful checks on the account. This step is mainly about checking that the message signatures are valid, that enough fees are provided and that the sending account has enough funds to pay for said fees. Note that no precise gas counting occurs here, as messages are not processed. Usually, the anteHandler will check that the gas provided with the transaction is superior to a minimum reference gas amount based on the raw transaction size, in order to avoid spam with transactions that provide 0 gas.
4. Ensure that a Route exists for each message, but do not actually process messages. Messages only need to be processed when the canonical state need to be updated, which happens during DeliverTx.

Steps 2. and 3. are performed by the anteHandler in the RunTx() function, which CheckTx() calls with the runTxModeCheck mode. During each step of CheckTx(), a special volatile state called checkState is updated. This state is used to keep track of the temporary changes triggered by the CheckTx() calls of each transaction without modifying the main canonical state . For example, when a transaction goes through CheckTx(), the transaction's fees are deducted from the sender's account in checkState. If a second transaction is received from the same account before the first is processed, and the account has consumed all its funds in checkState during the first transaction, the second transaction will fail CheckTx() and be rejected. In any case, the sender's account will not actually pay the fees until the transaction is actually included in a block, because checkState never gets committed to the main state. checkState is reset to the latest state of the main state each time a blocks gets committed.

CheckTx() returns a response to the underlying consensus engine of type abci.ResponseCheckTx. The response contains:

• Code (uint32): Response Code. 0 if successful.
• Data ([]byte): Result bytes, if any.
• Log (string): The output of the application's logger. May be non-deterministic.
• Info (string): Additional information. May be non-deterministic.
• GasWanted (int64): Amount of gas requested for transaction. It is provided by users when they generate the transaction.
• GasUsed (int64): Amount of gas consumed by transaction. During CheckTx, this value is computed by multiplying the standard cost of a transaction byte by the size of the raw transaction (click here for an example).
• Tags ([]cmn.KVPair): Key-Value tags for filtering and indexing transactions (eg. by account).
• Codespace (string): Namespace for the Code.

DeliverTx

When the underlying consensus engine receives a block proposal, each transaction in the block needs to be processed by the application. To that end, the underlying consensus engine sends a DeliverTx message to the application for each transaction in a sequential order. This ABCI message is handled by the DeliverTx() method of baseapp (abbreviated to DeliverTx() thereafter).

Before the first transaction of a given block is processed, a volatile state called deliverState is initialized during BeginBlock. This state is updated each time a transaction is processed via DeliverTx(), and committed to the main state when the block is committed, after what is is set to nil.

DeliverTx() performs the exact same steps as CheckTx(), with a little caveat at step 3 and the addition of a fifth step:

3. The anteHandler does not check that the transaction's gas-prices is sufficient. That is because the min-gas-prices value gas-prices is checked against is local to the node, and therefore what is enough for one full-node might not be for another. This means that the proposer can potentially include transactions for free, although they are not incentivised to do so, as they earn a bonus on the total fee of the block they propose.
4. For each message in the transaction, route to the appropriate module's handler. Additional stateful checks are performed, and the cache-wrapped multistore held in deliverState's context is updated by the module's keeper. If the handler returns successfully, the cache-wrapped multistore held in context is written to deliverState CacheMultiStore.

During step 5., each read/write to the store increases the value of GasConsumed. You can find the default cost of each operation here. At any point, if GasConsumed > GasWanted, the function returns with Code != 0 and DeliverTx() fails.

DeliverTx() returns a response to the underlying consensus engine of type abci.ResponseCheckTx. The response contains:

• Code (uint32): Response Code. 0 if successful.
• Data ([]byte): Result bytes, if any.
• Log (string): The output of the application's logger. May be non-deterministic.
• Info (string): Additional information. May be non-deterministic.
• GasWanted (int64): Amount of gas requested for transaction. It is provided by users when they generate the transaction.
• GasUsed (int64): Amount of gas consumed by transaction. During DeliverTx, this value is computed by multiplying the standard cost of a transaction byte by the size of the raw transaction (click here for an example), and by adding gas each time a read/write to the store occurs.
• Tags ([]cmn.KVPair): Key-Value tags for filtering and indexing transactions (eg. by account).
• Codespace (string): Namespace for the Code.

RunTx(), AnteHandler and RunMsgs()

RunTx()

RunTx() is called from CheckTx()/DeliverTx() to handle the transaction, with runTxModeCheck or runTxModeDeliver as parameter to differentiate between the two modes of execution. Note that when RunTx() receives a transaction, it has already been decoded.

The first thing RunTx() does upon being called is to retrieve the context's CacheMultiStore by calling the getContextForTx() function with the appropriate mode (either runTxModeCheck or runTxModeDeliver). This CacheMultiStore is a cached version of the main store instantiated during BeginBlock for DeliverTx and during the Commit of the previous block for CheckTx. After that, two defer func() are called for gas management. They are executed when RunTx() returns and make sure gas is actually consumed, and will throw errors, if any.

After that, RunTx() calls ValidateBasic() on each messagein the Tx, which runs prelimary stateless validity checks. If any message fails to pass ValidateBasic(), RunTx() returns with an error.

Then, the anteHandler of the application is run (if it exists). In preparation of this step, both the checkState/deliverState's context and context's CacheMultiStore are cached-wrapped using the cacheTxContext() function. This allows RunTx() not to commit the changes made to the state during the execution of anteHandler if it ends up failing. It also prevents the module implementing the anteHandler from writing to state, which is an important part of the object-capabilities of the Cosmos SDK.

Finally, the RunMsgs() function is called to process the messagess in the Tx. In preparation of this step, just like with the anteHandler, both the checkState/deliverState's context and context's CacheMultiStore are cached-wrapped using the cacheTxContext() function.

AnteHandler

The AnteHandler is a special handler that implements the anteHandler interface and is used to authenticate the transaction before the transaction's internal messages are processed.

The AnteHandler is theoretically optional, but still a very important component of public blockchain networks. It serves 3 primary purposes:

• Be a primary line of defense against spam and second line of defense (the first one being the mempool) against transaction replay with fees deduction and sequencechecking.
• Perform preliminary stateful validity checks like ensuring signatures are valid or that the sender has enough funds to pay for fees.
• Play a role in the incentivisation of stakeholders via the collection of transaction fees.

baseapp holds an anteHandler as parameter, which is initialized in the application's constructor. The most widely used anteHandler today is that of the auth module.

RunMsgs()

RunMsgs() is called from RunTx() with runTxModeCheck as parameter to check the existence of a route for each message contained in the transaction, and with runTxModeDeliver to actually process the messages.

First, it retrieves the message's route using the Msg.Route() method. Then, using the application's router and the route, it checks for the existence of a handler. At this point, if mode == runTxModeCheck, RunMsgs() returns. If instead mode == runTxModeDeliver, the handler function for the message is executed, before RunMsgs() returns.

Other ABCI Messages

InitChain

The InitChain ABCI message is sent from the underlying Tendermint engine when the chain is first started, and is handled by the InitChain(req abci.RequestInitChain) method of baseapp. It is mainly used to initialize parameters and state like:

• Consensus Parameters via setConsensusParams.
• checkState and deliverState via setCheckState and setDeliverState.
• The block gas meter, with infinite gas to process genesis transactions.

Finally, the InitChain(req abci.RequestInitChain) method of baseapp calls the initChainer() of the application in order to initialize the main state of the application from the genesis file and, if defined, call the InitGenesis function of each of the application's modules.

BeginBlock

The BeginBlock ABCI message is sent from the underlying Tendermint engine when a block proposal created by the correct proposer is received, before DeliverTx() is run for each transaction in the block. It allows developers to have logic be executed at the beginning of each block. In the Cosmos SDK, the BeginBlock(req abci.RequestBeginBlock) method does the following:

• Initialize deliverState with the latest header using the req abci.RequestBeginBlock passed as parameter via the setDeliverState function.
• Initialize the block gas meter with the maxGas limit. The gas consumed within the block cannot go above maxGas. This parameter is defined in the application's consensus parameters.
• Run the application's begingBlocker(), which mainly runs the BeginBlocker() method of each of the application's modules.
• Set the VoteInfos of the application, i.e. the list of validators whose precommit for the previous block was included by the proposer of the current block. This information is carried into the Context so that it can be used during DeliverTx and EndBlock.

EndBlock

The EndBlock ABCI message is sent from the underlying Tendermint engine after DeliverTx as been run for each transactioni n the block. It allows developers to have logic be executed at the end of each block. In the Cosmos SDK, the bulk EndBlock(req abci.RequestEndBlock) method is to run the application's endBlocker(), which mainly runs the EndBlocker() method of each of the application's modules.

Commit

The Commit ABCI message is sent from the underlying Tendermint engine after the full-node has received precommits from 2/3+ of validators (weighted by voting power). On the baseapp end, the Commit(res abci.ResponseCommit) function is implemented to commit all the valid state transitions that occured during BeginBlock(), DeliverTx() and EndBlock() and to reset state for the next block.

To commit state-transitions, the Commit function calls the Write() function on deliverState.ms, where deliverState.ms is a cached multistore of the main store app.cms. Then, the Commit function sets checkState to the latest header (obtained from deliverState.ctx.BlockHeader) and deliverState to nil.

Finally, Commit returns the hash of the commitment of app.cms back to the underlying consensus engine. This hash is used as a reference in the header of the next block.

Info

The Info ABCI message is a simple query from the underlying consensus engine, notably used to sync the latter with the application during a handshake that happens on startup. When called, the Info(res abci.ResponseInfo) function from baseapp will return the application's name, version and the hash of the last commit of app.cms.

Query

The Query ABCI message is used to serve queries received from the underlying consensus engine, including queries received via RPC like Tendermint RPC. It is the main entrypoint to build interfaces with the application. The application must respect a few rules when implementing the Query method, which are outlined here.

The baseapp implementation of the Query(req abci.RequestQuery) method is a simple dispatcher serving 4 main categories of queries:

• Application-related queries like querying the application's version, which are served via the handleQueryApp method.
• Direct queries to the multistore, which are served by the handlerQueryStore method. These direct queries are different from custom queries which go through app.queryRouter, and are mainly used by third-party service provider like block explorers.
• P2P queries, which are served via the handleQueryP2P method. These queries return either app.addrPeerFilter or app.ipPeerFilter that contain the list of peers filtered by address or IP respectively. These lists are first initialized via options in baseapp's constructor.
• Custom queries, which encompass most queries, are served via the handleQueryCustom method. The handleQueryCustom cache-wraps the multistore before using the queryRoute obtained from app.queryRouter to map the query to the appropriate module's querier.

Next

Learn more about stores.