# Query Lifecycle
This document describes the lifecycle of a query in a Cosmos SDK application, from the user interface to application stores and back.
# 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
MyQuery is requesting a list of delegations made by a certain delegator address in the application called
simapp. As to be expected, the
staking module handles this query. But first, there are a few ways
MyQuery 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
MyQuery 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.
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
app.toml config file to set them 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 (
simd), the type of command (
Myquery), 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.
Another interface through which users can make queries is gRPC (opens new window) requests to a gRPC server. The endpoints are defined as Protocol Buffers (opens new window) service methods inside
.proto files, written in Protobuf's own language-agnostic interface definition language (IDL). The Protobuf ecosystem developed tools for code-generation from
*.proto files into various languages. These tools allow to build gRPC clients easily.
One such tool is grpcurl (opens new window), and a gRPC request for
MyQuery using this client looks like:
Another interface through which users can make queries is through HTTP Requests to a REST server. The REST server is fully auto-generated from Protobuf services, using gRPC-gateway (opens new window).
An example HTTP request for
MyQuery looks like:
# How Queries are Handled by the CLI
The examples above show how an external user can interact with a node by querying its state. To understand more in details the exact lifecycle of a query, let's dig into how the CLI prepares the query, and how the node handles it. 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, gRPC or REST server and heavily involves a
The first thing that is created in the execution of a CLI command is a
client.Context is an object that stores all the data needed to process a request on the user side. In particular, a
client.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. The default codec used by the CLI is Protobuf.
- 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.
- Keyring: A Key Manager used to sign transactions and handle other operations with keys.
- Output Writer: A Writer (opens new window) 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.
client.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.
client.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
client.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
client.Context, which knows which node the user CLI is connected to. The query is relayed to this full-node to be processed. Finally, the
client.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 with all of the data they wish to include in their query. A
client.Context exists to assist in the rest of the
MyQuery's journey. Now, the next step is to parse the command or request, extract the arguments, and encode everything. These steps all happen on the user side within the interface they are interacting with.
In our case (querying an address's delegations),
MyQuery 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
client.Context is used to marshal the address.
Here is what the code looks like for the CLI command:
# gRPC Query Client Creation
The Cosmos SDK leverages code generated from Protobuf services to make queries. The
MyQuery service generates a
queryClient, which the CLI will use to make queries. Here is the relevant code:
Under the hood, the
client.Context has a
Query() function used to retrieve the pre-configured node and relay a query to it; the function takes the query fully-qualified service method name as path (in our case:
/cosmos.staking.v1beta1.Query/Delegations), 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
MyQuery 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 (opens new window).
# 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 gRPC queries. The query route is parsed, and it matches the fully-qualified service method name of an existing service method (most likely in one of the modules), then
baseapp will relay the request to the relevant module.
Apart from gRPC routes,
baseapp also handles four different types of queries:
custom. 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 legacy queriers. To learn more about these queries, please refer to this guide.
MyQuery has a Protobuf fully-qualified service method name from the
staking module (recall
baseapp first parses the path, then uses its own internal
GRPCQueryRouter to retrieve the corresponding gRPC handler, and routes the query to the module. The gRPC handler is responsible for recognizing this query, retrieving the appropriate values from the application's stores, and returning a response. Read more about query services 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 (opens new window) type. The
Query() routine receives the response and.
# CLI Response
codec is used to unmarshal the response to a JSON and the
client.Context prints the output to the command line, applying any configurations such as the output type (text, JSON or YAML).
And that's a wrap! The result of the query is outputted to the console by the CLI.
Read more about accounts.