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+# Custom Protocol Buffer Codecs
+
+As of [v0.40](https://github.com/cosmos/cosmos-sdk/releases/tag/v0.40.0), the
+Cosmos SDK uses
+[protocol buffers](https://developers.google.com/protocol-buffers) as its
+standard serialization format for blockchain state and wire communication.
+CosmJS by default supports protocol buffer serialization for many of the
+standard queries and messages defined by the Cosmos SDK, as well as
+[CosmWasm](https://github.com/CosmWasm/wasmd). This document explains how you
+can make use of protocol buffer serialization for your own custom modules with
+CosmJS.
+
+## Prerequisites
+
+- You are working on a TypeScript project (plain JS is possible but not covered
+  by this document).
+- You have installed `@cosmjs/proto-signing`, `@cosmjs/stargate` and
+  `@cosmjs/tendermint-rpc` as dependencies.
+- You have installed `ts-proto` as a development dependency.
+- You have installed [`protoc`](https://github.com/protocolbuffers/protobuf).
+- This document assumes that the protocol buffer definitions which you need are
+  already available somewhere in `.proto` files.
+
+## Step 1: Acquire the definition files
+
+You will need these files locally. For example, we use
+[this script](https://github.com/cosmos/cosmjs/blob/v0.25.0-alpha.0/packages/stargate/scripts/get-proto.sh)
+to download the definition files from the Cosmos SDK repository.
+
+## Step 2: Generate codec files
+
+In CosmJS we use [ts-proto](https://github.com/stephenh/ts-proto) to generate
+codec files, and in this document we assume you will follow the same route. Here
+is an example usage:
+
+```sh
+protoc \
+  --plugin="./node_modules/.bin/protoc-gen-ts_proto" \
+  --ts_proto_out="./path/to/output/directory" \
+  --proto_path="./path/to/definitions" \
+  --ts_proto_opt="esModuleInterop=true,forceLong=long,useOptionals=true" \
+  "./path/to/definitions/file.proto" \
+  "./path/to/definitions/another.proto"
+```
+
+Note that the available `ts-proto` options are described
+[here](https://github.com/stephenh/ts-proto#supported-options). You can see the
+script we use for the `@cosmjs/stargate` package
+[here](https://github.com/cosmos/cosmjs/blob/v0.25.0-alpha.0/packages/stargate/scripts/define-proto.sh).
+
+## Step 3a: Instantiate a signing client using your custom message types
+
+This section assumes that your definition files included `MsgXxx` `message`
+definitions for use in submitting transactions to a Cosmos SDK blockchain. You
+can instantiate a signing client for Stargate which supports those message types
+using a custom registry. We expose a `Registry` class from
+`@cosmjs/proto-signing` for you to use, which maps type URLs to codec objects.
+For example:
+
+```ts
+import { DirectSecp256k1HdWallet, Registry } from "@cosmjs/proto-signing";
+import { defaultRegistryTypes, SigningStargateClient } from "@cosmjs/stargate";
+import { MsgXxx } from "./path/to/generated/codec";
+
+const myRegistry = new Registry([
+  ...defaultRegistryTypes,
+  ["/my.custom.MsgXxx", MsgXxx],
+]);
+const mnemonic =
+  "economy stock theory fatal elder harbor betray wasp final emotion task crumble siren bottom lizard educate guess current outdoor pair theory focus wife stone";
+
+// Inside an async function...
+const signer = await DirectSecp256k1HdWallet.fromMnemonic(
+  mnemonic,
+  undefined,
+  "myprefix",
+);
+const client = await SigningStargateClient.connectWithSigner(
+  "my.endpoint.com",
+  signer,
+  {
+    registry: myRegistry,
+  },
+);
+```
+
+Now when you want to sign and broadcast a transaction which contains a message
+of your custom type, the client will know how to serialize (and deserialize) it:
+
+```ts
+const myAddress = "wasm1pkptre7fdkl6gfrzlesjjvhxhlc3r4gm32kke3";
+const message = {
+  typeUrl: "/my.custom.MsgXxx",
+  value: {
+    foo: "bar",
+  },
+};
+const fee = {
+  amount: [
+    {
+      denom: "udenom",
+      amount: "120000",
+    },
+  ],
+  gas: "10000",
+};
+
+// Inside an async function...
+// This method uses the registry you provided
+const response = await client.signAndBroadcast(myAddress, [message], fee);
+```
+
+You can see a more complete example in Confio’s
+[`ts-relayer` repo](https://github.com/confio/ts-relayer/blob/1200f6f998edb2fed6c595aefd39b2e12de4972a/src/lib/ibcclient.ts).
+
+### Step 3b: Instantiate a query client using your custom query service
+
+This section assumes that your definition files included a `Query` `service`
+with `rpc` methods. `ts-proto` will generate a `QueryClientImpl` class which
+needs to be provided with an RPC client.
+
+Creating an RPC client with the functionality required by this generated class
+currently requires a few layers of abstraction. Here is how you can achieve it
+using CosmJS helpers:
+
+```ts
+import { createRpc, QueryClient } from "@cosmjs/stargate";
+import { Tendermint34Client } from "@cosmjs/tendermint-rpc";
+import { QueryClientImpl } from "./path/to/generated/codec";
+
+// Inside an async function...
+// The Tendermint client knows how to talk to the Tendermint RPC endpoint
+const tendermintClient = await Tendermint34Client.connect("my.endpoint.com");
+
+// The generic Stargate query client knows how to use the Tendermint client to submit unverified ABCI queries
+const queryClient = new QueryClient(tendermintClient);
+
+// This helper function wraps the generic Stargate query client for use by the specific generated query client
+const rpcClient = createRpc(queryClient);
+
+// Here we instantiate a specific query client which will have the custom methods defined in the .proto file
+const queryService = new QueryClientImpl(rpcClient);
+
+// Now you can use this service to submit queries
+const queryResult = await queryService.MyCustomQuery({
+  foo: "bar",
+});
+```
+
+Additionally, we provide a system for extending `@cosmjs/stargate`’s
+`QueryClient` with methods of your own design, wrapping those of the query
+service. For this you will need to define your own `setupXxxExtension` functions
+and pass them to the `QueryClient.withExtensions` static method like this:
+
+```ts
+// Define your extensions
+function setupXxxExtension(base: QueryClient) {
+  const rpcClient = createRpc(base);
+  const queryService = new QueryClientImpl(rpcClient);
+
+  return {
+    my: {
+      nested: {
+        customQuery: async (foo: string) =>
+          queryService.MyCustomQuery({ foo: foo }),
+      },
+    },
+  };
+}
+function setupYyyExtension(base: QueryClient) {
+  // ...
+}
+
+// Setup the query client
+const queryClient = QueryClient.withExtensions(
+  tendermintClient,
+  setupXxxExtension,
+  setupYyyExtension,
+  // You can add up to 8 extensions
+);
+
+// Inside an async function...
+// Now your query client has been extended
+const queryResult = await queryClient.my.nested.customQuery("bar");
+```
+
+You can see how CosmJS sets up the `bank` extension for its default query client
+[here](https://github.com/cosmos/cosmjs/blob/v0.25.0-alpha.0/packages/stargate/src/queries/bank.ts).