# Query Lifecycle
This document describes the lifecycle of a query in a SDK application, from the user interface to application stores and back. The query will be referred to as
# Pre-requisite Readings
# Query Creation
A query is a request for information made by end-users of applications through an interface and processed by a full-node. Users can query information about the network, the application itself, and application state directly from the application's stores or modules. Note that queries are different from transactions (view the lifecycle here), particularly in that they do not require consensus to be processed (as they do not trigger state-transitions); they can be fully handled by one full-node.
For the purpose of explaining the query lifecycle, let's say
Query is requesting a list of delegations made by a certain delegator address in the application called
app. As to be expected, the
staking module handles this query. But first, there are a few ways
Query can be created by users.
The main interface for an application is the command-line interface. Users connect to a full-node and run the CLI directly from their machines - the CLI interacts directly with the full-node. To create
Query from their terminal, users type the following command:
This query command was defined by the
staking module developer and added to the list of subcommands by the application developer when creating the CLI. The code for this particular command is the following:
Note that the general format is as follows:
To provide values such as
--node (the full-node the CLI connects to), the user can use the
config command to set themn or provide them as flags.
The CLI understands a specific set of commands, defined in a hierarchical structure by the application developer: from the root command (
appcli), the type of command (
query), the module that contains the command (
staking), and command itself (
delegations). Thus, the CLI knows exactly which module handles this command and directly passes the call there.
To provide values such as
--node (the full-node the CLI connects to) that are required by
baseReq, the user must configure their local REST server with the values or provide them in the request body.
The router automatically routes the
Query HTTP request to the staking module
Since this function is defined within the module and thus has no inherent knowledge of the application
Query belongs to, it takes in the application
Context as parameters.
To summarize, when users interact with the interfaces, they create a CLI command or HTTP request.
Query now exists in one of these two forms, but needs to be transformed into an object understood by a full-node.
# Query Preparation
The interactions from the users' perspective are a bit different, but the underlying functions are almost identical because they are implementations of the same command defined by the module developer. This step of processing happens within the CLI or REST server and heavily involves a
The first thing that is created in the execution of a CLI command is a
Context, while the REST Server directly provides a
Context for the REST Request handler. A Context is an immutable object that stores all the data needed to process a request on the user side. In particular, a
Context stores the following:
- Codec: The encoder/decoder used by the application, used to marshal the parameters and query before making the Tendermint RPC request and unmarshal the returned response into a JSON object.
- Account Decoder: The account decoder from the
authmodule, which translates
bytes into accounts.
- RPC Client: The Tendermint RPC Client, or node, to which the request will be relayed to.
- Keybase: A Key Manager used to sign transactions and handle other operations with keys.
- Output Writer: A Writer used to output the response.
- Configurations: The flags configured by the user for this command, including
--height, specifying the height of the blockchain to query and
--indent, which indicates to add an indent to the JSON response.
Context also contains various functions such as
Query() which retrieves the RPC Client and makes an ABCI call to relay a query to a full-node.
Context's primary role is to store data used during interactions with the end-user and provide methods to interact with this data - it is used before and after the query is processed by the full-node. Specifically, in handling
Context is utilized to encode the query parameters, retrieve the full-node, and write the output. Prior to being relayed to a full-node, the query needs to be encoded into a
byte form, as full-nodes are application-agnostic and do not understand specific types. The full-node (RPC Client) itself is retrieved using the
Context, which knows which node the user CLI is connected to. The query is relayed to this full-node to be processed. Finally, the
Context contains a
Writer to write output when the response is returned. These steps are further described in later sections.
# Arguments and Route Creation
At this point in the lifecycle, the user has created a CLI command or HTTP Request with all of the data they wish to include in their
Context exists to assist in the rest of the
Query's journey. Now, the next step is to parse the command or request, extract the arguments, create a
queryRoute, and encode everything. These steps all happen on the user side within the interface they are interacting with.
In this case,
Query contains an address
delegatorAddress as its only argument. However, the request can only contain
bytes, as it will be relayed to a consensus engine (e.g. Tendermint Core) of a full-node that has no inherent knowledge of the application types. Thus, the
Context is used to marshal the address.
Here is what the code looks like for the CLI command:
Here is what the code looks like for the HTTP Request:
# Query Route Creation
Important to note is that there will never be a "query" object created for
Query; the SDK actually takes a simpler approach. Instead of an object, all the full-node needs to process a query is its
route which specifies exactly which module to route the query to and the name of this query type. The
route will be passed to the application
baseapp, then module, then querier, and each will understand the
route and pass it to the appropriate next step.
baseapp will understand this query to be a
custom query in the module
staking, and the
staking module querier supports the type
QueryDelegatorDelegations. Thus, the route will be
Here is what the code looks like:
Query exists as a set of encoded arguments and a route to a specific module and its query type. It is ready to be relayed to a full-node.
# ABCI Query Call
Context has a
Query() function used to retrieve the pre-configured node and relay a query to it; the function takes the query
route and arguments as parameters. It first retrieves the RPC Client (called the node) configured by the user to relay this query to, and creates the
ABCIQueryOptions (parameters formatted for the ABCI call). The node is then used to make the ABCI call,
Here is what the code looks like:
With a call to
Query is received by a full-node which will then process the request. Note that, while the RPC is made to the consensus engine (e.g. Tendermint Core) of a full-node, queries are not part of consensus and will not be broadcasted to the rest of the network, as they do not require anything the network needs to agree upon.
Read more about ABCI Clients and Tendermint RPC in the Tendermint documentation here.
# Application Query Handling
When a query is received by the full-node after it has been relayed from the underlying consensus engine, it is now being handled within an environment that understands application-specific types and has a copy of the state.
baseapp implements the ABCI
Query() function and handles four different types of queries:
queryRoute is parsed such that the first string must be one of the four options, then the rest of the path is parsed within the subroutines handling each type of query. The first three types (
p2p) are purely application-level and thus directly handled by
baseapp or the stores, but the
custom query type requires
baseapp to route the query to a module's querier.
Query is a custom query type from the
baseapp first parses the path, then uses the
QueryRouter to retrieve the corresponding querier, and routes the query to the module. The querier is responsible for recognizing this query, retrieving the appropriate values from the application's stores, and returning a response. Read more about queriers here.
Once a result is received from the querier,
baseapp begins the process of returning a response to the user.
Query() is an ABCI function,
baseapp returns the response as an
abci.ResponseQuery type. The
Query() routine receives the response and.
# CLI Response
codec is used to unmarshal the response to a JSON and the
Context prints the output to the command line, applying any configurations such as
# REST Response
The REST server uses the
Context to format the response properly, then uses the HTTP package to write the appropriate response or error.