# Pre-requisites Readings
# Context Definition
Context is a custom data structure that contains Go's stdlib
context (opens new window) as its base, and has many additional types within its definition that are specific to the Cosmos SDK. he
Context is integral to transaction processing in that it allows modules to easily access their respective store in the
multistore and retrieve transactional context such as the block header and gas meter.
- Context: The base type is a Go Context (opens new window), which is explained further in the Go Context Package section below.
- Multistore: Every application's
CommitMultiStorewhich is provided when a
Contextis created. Calling the
TransientStore()methods allows modules to fetch their respective
KVStoreusing their unique
- ABCI Header: The header (opens new window) is an ABCI type. It carries important information about the state of the blockchain, such as block height and proposer of the current block.
- Chain ID: The unique identification number of the blockchain a block pertains to.
- Transaction Bytes: The
byterepresentation of a transaction being processed using the context. Every transaction is processed by various parts of the SDK and consensus engine (e.g. Tendermint) throughout its lifecycle, some of which to not have any understanding of transaction types. Thus, transactions are marshaled into the generic
bytetype using some kind of encoding format such as Amino.
- Logger: A
loggerfrom the Tendermint libraries. Learn more about logs here (opens new window). Modules call this method to create their own unique module-specific logger.
- VoteInfo: A list of the ABCI type
VoteInfo(opens new window), which includes the name of a validator and a boolean indicating whether they have signed the block.
- Gas Meters: Specifically, a
gasMeterfor the transaction currently being processed using the context and a
blockGasMeterfor the entire block it belongs to. Users specify how much in fees they wish to pay for the execution of their transaction; these gas meters keep track of how much gas has been used in the transaction or block so far. If the gas meter runs out, execution halts.
- CheckTx Mode: A boolean value indicating whether a transaction should be processed in
- Min Gas Price: The minimum gas price a node is willing to take in order to include a transaction in its block. This price is a local value configured by each node individually, and should therefore not be used in any functions used in sequences leading to state-transitions.
- Consensus Params: The ABCI type Consensus Parameters (opens new window), which specify certain limits for the blockchain, such as maximum gas for a block.
- Event Manager: The event manager allows any caller with access to a
Events. Modules may define module specific
Eventsby defining various
Attributesor use the common definitions found in
types/. Clients can subscribe or query for these
Eventsare collected throughout
EndBlockand are returned to Tendermint for indexing. For example:
# Go Context Package
Context is defined in the Golang Context Package (opens new window). A
is an immutable data structure that carries request-scoped data across APIs and processes. Contexts
are also designed to enable concurrency and to be used in goroutines.
Contexts are intended to be immutable; they should never be edited. Instead, the convention is
to create a child context from its parent using a
With function. For example:
The Golang Context Package (opens new window) documentation instructs developers to
explicitly pass a context
ctx as the first argument of a process.
# Cache Wrapping
Context contains a
MultiStore, which allows for cache-wrapping functionality: a
KVStore is is wrapped with an ephemeral cache. Processes are free to write changes to
CacheMultiStore, then write the changes back to the original state or disregard them if something
goes wrong. The pattern of usage for a Context is as follows:
- A process receives a Context
ctxfrom its parent process, which provides information needed to perform the process.
ctx.msis cache wrapped, i.e. a cached copy of the multistore is made so that the process can make changes to the state as it executes, without changing the original
ctx.ms. This is useful to protect the underlying multistore in case the changes need to be reverted at some point in the execution.
- The process may read and write from
ctxas it is executing. It may call a subprocess and pass
ctxto it as needed.
- When a subprocess returns, it checks if the result is a success or failure. If a failure, nothing
needs to be done - the cache wrapped
ctxis simply discarded. If successful, the changes made to the cache-wrapped
MultiStorecan be committed to the original
Here is the process:
- Prior to calling
runMsgson the message(s) in the transaction, it uses
app.cacheTxContext()to cache-wrap the context and multistore.
- The cache-wrapped context,
runMsgCtx, is used in
runMsgsto return a result.
- If the process is running in
checkTxMode, there is no need to write the changes - the result is returned immediately.
- If the process is running in
deliverTxModeand the result indicates a successful run over all the messages, the cached multistore is written back to the original.
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