HTTP Subscription Link

httpSubscriptionLink is a terminating link that's uses Server-sent Events (SSE) for subscriptions.

SSE is a good option for real-time as it's a bit easier than setting up a WebSockets-server.

info

We have prefixed this as unstable_ as it's a new API, but you're safe to use it! Read more.

Setup

info

If your client's environment doesn't support EventSource, you need an EventSource polyfill. For React Native specific instructions please defer to the compatibility section.

To use httpSubscriptionLink, you need to use a splitLink to make it explicit that we want to use SSE for subscriptions.

client/index.ts
ts
import type { TRPCLink } from '@trpc/client';
import {
  httpBatchLink,
  loggerLink,
  splitLink,
  unstable_httpSubscriptionLink,
} from '@trpc/client';
const trpcClient = createTRPCClient<AppRouter>({
  /**
   * @see https://trpc.io/docs/v11/client/links
   */
  links: [
    // adds pretty logs to your console in development and logs errors in production
    loggerLink(),
    splitLink({
      // uses the httpSubscriptionLink for subscriptions
      condition: (op) => op.type === 'subscription',
      true: unstable_httpSubscriptionLink({
        url: `/api/trpc`,
      }),
      false: httpBatchLink({
        url: `/api/trpc`,
      }),
    }),
  ],
});

client/index.ts
ts
import type { TRPCLink } from '@trpc/client';
import {
  httpBatchLink,
  loggerLink,
  splitLink,
  unstable_httpSubscriptionLink,
} from '@trpc/client';
const trpcClient = createTRPCClient<AppRouter>({
  /**
   * @see https://trpc.io/docs/v11/client/links
   */
  links: [
    // adds pretty logs to your console in development and logs errors in production
    loggerLink(),
    splitLink({
      // uses the httpSubscriptionLink for subscriptions
      condition: (op) => op.type === 'subscription',
      true: unstable_httpSubscriptionLink({
        url: `/api/trpc`,
      }),
      false: httpBatchLink({
        url: `/api/trpc`,
      }),
    }),
  ],
});

tip

The document here outlines the specific details of using httpSubscriptionLink. For general usage of subscriptions, see our subscriptions guide.

Headers and authorization / authentication

Web apps

Same domain

If you're doing a web application, cookies are sent as part of the request as long as your client is on the same domain as the server.

Cross-domain

If the client and server are not on the same domain, you can use withCredentials: true (read more on MDN here).

Example:

tsx
// [...]
unstable_httpSubscriptionLink({
  url: 'https://example.com/api/trpc',
  eventSourceOptions() {
    return {
      withCredentials: true, // <---
    };
  },
});

tsx
// [...]
unstable_httpSubscriptionLink({
  url: 'https://example.com/api/trpc',
  eventSourceOptions() {
    return {
      withCredentials: true, // <---
    };
  },
});

Custom headers through ponyfill

Recommended for non-web environments

You can ponyfill EventSource and use the eventSourceOptions -callback to populate headers.

tsx
import {
  createTRPCClient,
  httpBatchLink,
  splitLink,
  unstable_httpSubscriptionLink,
} from '@trpc/client';
import { EventSourcePolyfill } from 'event-source-polyfill';
import type { AppRouter } from '../server/index.js';
// Initialize the tRPC client
const trpc = createTRPCClient<AppRouter>({
  links: [
    splitLink({
      condition: (op) => op.type === 'subscription',
      true: unstable_httpSubscriptionLink({
        url: 'http://localhost:3000',
        // ponyfill EventSource
        EventSource: EventSourcePolyfill,
        // options to pass to the EventSourcePolyfill constructor
        eventSourceOptions: async ({ op }) => {
          //                          ^ Includes the operation that's being executed
          // you can use this to generate a signature for the operation
          const signature = await getSignature(op);
          return {
            headers: {
              authorization: 'Bearer supersecret',
              'x-signature': signature,
            },
          };
        },
      }),
      false: httpBatchLink({
        url: 'http://localhost:3000',
      }),
    }),
  ],
});

tsx
import {
  createTRPCClient,
  httpBatchLink,
  splitLink,
  unstable_httpSubscriptionLink,
} from '@trpc/client';
import { EventSourcePolyfill } from 'event-source-polyfill';
import type { AppRouter } from '../server/index.js';
// Initialize the tRPC client
const trpc = createTRPCClient<AppRouter>({
  links: [
    splitLink({
      condition: (op) => op.type === 'subscription',
      true: unstable_httpSubscriptionLink({
        url: 'http://localhost:3000',
        // ponyfill EventSource
        EventSource: EventSourcePolyfill,
        // options to pass to the EventSourcePolyfill constructor
        eventSourceOptions: async ({ op }) => {
          //                          ^ Includes the operation that's being executed
          // you can use this to generate a signature for the operation
          const signature = await getSignature(op);
          return {
            headers: {
              authorization: 'Bearer supersecret',
              'x-signature': signature,
            },
          };
        },
      }),
      false: httpBatchLink({
        url: 'http://localhost:3000',
      }),
    }),
  ],
});

Updating configuration on an active connection

httpSubscriptionLink leverages SSE through EventSource, ensuring that connections encountering errors like network failures or bad response codes are automatically retried. However, EventSource does not allow re-execution of the eventSourceOptions() or url() options to update its configuration, which is particularly important in scenarios where authentication has expired since the last connection.

To address this limitation, you can use a retryLink in conjunction with httpSubscriptionLink. This approach ensures that the connection is re-established with the latest configuration, including any updated authentication details.

caution

Please note that restarting the connection will result in the EventSource being recreated from scratch, which means any previously tracked events will be lost.

tsx
import {
  createTRPCClient,
  httpBatchLink,
  retryLink,
  splitLink,
  unstable_httpSubscriptionLink,
} from '@trpc/client';
import {
  EventSourcePolyfill,
  EventSourcePolyfillInit,
} from 'event-source-polyfill';
import type { AppRouter } from '../server/index.js';
// Initialize the tRPC client
const trpc = createTRPCClient<AppRouter>({
  links: [
    splitLink({
      condition: (op) => op.type === 'subscription',
      false: httpBatchLink({
        url: 'http://localhost:3000',
      }),
      true: [
        retryLink({
          retry: (opts) => {
            opts.op.type;
            //       ^? will always be 'subscription' since we're in a splitLink
            const code = opts.error.data?.code;
            if (!code) {
              // This shouldn't happen as our httpSubscriptionLink will automatically retry within when there's a non-parsable response
              console.error('No error code found, retrying', opts);
              return true;
            }
            if (code === 'UNAUTHORIZED' || code === 'FORBIDDEN') {
              console.log('Retrying due to 401/403 error');
              return true;
            }
            return false;
          },
        }),
        unstable_httpSubscriptionLink({
          url: async () => {
            // calculate the latest URL if needed...
            return getAuthenticatedUri();
          },
          // ponyfill EventSource
          EventSource: EventSourcePolyfill,
          eventSourceOptions: async () => {
            // ...or maybe renew an access token
            const token = await auth.getOrRenewToken();
            return {
              headers: {
                authorization: `Bearer ${token}`,
              },
            };
          },
        }),
      ],
    }),
  ],
});

tsx
import {
  createTRPCClient,
  httpBatchLink,
  retryLink,
  splitLink,
  unstable_httpSubscriptionLink,
} from '@trpc/client';
import {
  EventSourcePolyfill,
  EventSourcePolyfillInit,
} from 'event-source-polyfill';
import type { AppRouter } from '../server/index.js';
// Initialize the tRPC client
const trpc = createTRPCClient<AppRouter>({
  links: [
    splitLink({
      condition: (op) => op.type === 'subscription',
      false: httpBatchLink({
        url: 'http://localhost:3000',
      }),
      true: [
        retryLink({
          retry: (opts) => {
            opts.op.type;
            //       ^? will always be 'subscription' since we're in a splitLink
            const code = opts.error.data?.code;
            if (!code) {
              // This shouldn't happen as our httpSubscriptionLink will automatically retry within when there's a non-parsable response
              console.error('No error code found, retrying', opts);
              return true;
            }
            if (code === 'UNAUTHORIZED' || code === 'FORBIDDEN') {
              console.log('Retrying due to 401/403 error');
              return true;
            }
            return false;
          },
        }),
        unstable_httpSubscriptionLink({
          url: async () => {
            // calculate the latest URL if needed...
            return getAuthenticatedUri();
          },
          // ponyfill EventSource
          EventSource: EventSourcePolyfill,
          eventSourceOptions: async () => {
            // ...or maybe renew an access token
            const token = await auth.getOrRenewToken();
            return {
              headers: {
                authorization: `Bearer ${token}`,
              },
            };
          },
        }),
      ],
    }),
  ],
});

Connection params

In order to authenticate with EventSource, you can define connectionParams in httpSubscriptionLink. This will be sent as part of the URL, which is why other methods are preferred).

server/context.ts
ts
import type { CreateHTTPContextOptions } from '@trpc/server/adapters/standalone';
 
export const createContext = async (opts: CreateHTTPContextOptions) => {
  const token = opts.info.connectionParams?.token;
         
const token: string | undefined
 
  // [... authenticate]
 
  return {};
};
 
export type Context = Awaited<ReturnType<typeof createContext>>;

server/context.ts
ts
import type { CreateHTTPContextOptions } from '@trpc/server/adapters/standalone';
 
export const createContext = async (opts: CreateHTTPContextOptions) => {
  const token = opts.info.connectionParams?.token;
         
const token: string | undefined
 
  // [... authenticate]
 
  return {};
};
 
export type Context = Awaited<ReturnType<typeof createContext>>;

client/trpc.ts
ts
import {
  createTRPCClient,
  httpBatchLink,
  splitLink,
  unstable_httpSubscriptionLink,
} from '@trpc/client';
import type { AppRouter } from '../server/index.js';
// Initialize the tRPC client
const trpc = createTRPCClient<AppRouter>({
  links: [
    splitLink({
      condition: (op) => op.type === 'subscription',
      true: unstable_httpSubscriptionLink({
        url: 'http://localhost:3000',
        connectionParams: async () => {
          // Will be serialized as part of the URL
          return {
            token: 'supersecret',
          };
        },
      }),
      false: httpBatchLink({
        url: 'http://localhost:3000',
      }),
    }),
  ],
});

client/trpc.ts
ts
import {
  createTRPCClient,
  httpBatchLink,
  splitLink,
  unstable_httpSubscriptionLink,
} from '@trpc/client';
import type { AppRouter } from '../server/index.js';
// Initialize the tRPC client
const trpc = createTRPCClient<AppRouter>({
  links: [
    splitLink({
      condition: (op) => op.type === 'subscription',
      true: unstable_httpSubscriptionLink({
        url: 'http://localhost:3000',
        connectionParams: async () => {
          // Will be serialized as part of the URL
          return {
            token: 'supersecret',
          };
        },
      }),
      false: httpBatchLink({
        url: 'http://localhost:3000',
      }),
    }),
  ],
});

Timeout Configuration

The httpSubscriptionLink supports configuring a timeout for inactivity through the reconnectAfterInactivityMs option. If no messages (including ping messages) are received within the specified timeout period, the connection will be marked as "connecting" and automatically attempt to reconnect.

ts
import {
  createTRPCClient,
  httpBatchLink,
  splitLink,
  unstable_httpSubscriptionLink,
} from '@trpc/client';
const trpc = createTRPCClient<AppRouter>({
  links: [
    splitLink({
      condition: (op) => op.type === 'subscription',
      true: unstable_httpSubscriptionLink({
        url: '/api/trpc',
        // Reconnect if no messages received for 30 seconds
        reconnectAfterInactivityMs: 30_000,
      }),
      false: httpBatchLink({
        url: '/api/trpc',
      }),
    }),
  ],
});

ts
import {
  createTRPCClient,
  httpBatchLink,
  splitLink,
  unstable_httpSubscriptionLink,
} from '@trpc/client';
const trpc = createTRPCClient<AppRouter>({
  links: [
    splitLink({
      condition: (op) => op.type === 'subscription',
      true: unstable_httpSubscriptionLink({
        url: '/api/trpc',
        // Reconnect if no messages received for 30 seconds
        reconnectAfterInactivityMs: 30_000,
      }),
      false: httpBatchLink({
        url: '/api/trpc',
      }),
    }),
  ],
});

When a timeout occurs:

1. The connection state will change to "connecting"
2. The client will automatically attempt to reconnect
3. Any tracked event IDs will be preserved and sent with the new connection to resume from the last received message

This is useful for:

• Detecting stale connections that may have silently failed
• Automatically recovering from network interruptions
• Ensuring your subscriptions stay active even during periods of inactivity

tip

The timeout includes ping messages sent by the server. If you're experiencing timeouts during normal operation, you may want to:

1. Increase the reconnectAfterInactivityMs value
2. Adjust your server's ping interval to be more frequent

Server Ping Configuration

The server can be configured to send periodic ping messages to keep the connection alive and prevent timeout disconnections. This is particularly useful when combined with the reconnectAfterInactivityMs option on the client.

server/trpc.ts
ts
import { initTRPC } from '@trpc/server';
export const t = initTRPC.create({
  sse: {
    // Maximum duration of a single SSE connection in milliseconds
    // maxDurationMs: 60_00,
    ping: {
      // Enable periodic ping messages to keep connection alive
      enabled: true,
      // Send ping message every 200ms
      intervalMs: 200,
    },
  },
});

server/trpc.ts
ts
import { initTRPC } from '@trpc/server';
export const t = initTRPC.create({
  sse: {
    // Maximum duration of a single SSE connection in milliseconds
    // maxDurationMs: 60_00,
    ping: {
      // Enable periodic ping messages to keep connection alive
      enabled: true,
      // Send ping message every 200ms
      intervalMs: 200,
    },
  },
});

When configuring both client timeout and server ping, ensure that:

1. The server's experimental_streamingPingInterval is less than the client's reconnectAfterInactivityMs
2. Allow some buffer time for network latency

For example:

• Server: experimental_streamingPingInterval: 15_000 (15 seconds)
• Client: reconnectAfterInactivityMs: 30_000 (30 seconds)

This gives a 15-second buffer for network delays or missed pings before triggering a reconnection.

Compatibility (React Native)

The httpSubscriptionLink makes use of the EventSource API, Streams API, and AsyncIterators, these are not natively supported by React Native and will have to be ponyfilled.

To ponyfill EventSource we recommend to use a polyfill that utilizes the networking library exposed by React Native, over using a polyfill that using the XMLHttpRequest API. Libraries that polyfill EventSource using XMLHttpRequest fail to reconnect after the app has been in the background. Consider using the rn-eventsource-reborn package.

The Streams API can be ponyfilled using the web-streams-polyfill package.

AsyncIterators can be polyfilled using the @azure/core-asynciterator-polyfill package.

Installation

Install the required polyfills:

npm

npm install rn-eventsource-reborn web-streams-polyfill @azure/core-asynciterator-polyfill

yarn

yarn add rn-eventsource-reborn web-streams-polyfill @azure/core-asynciterator-polyfill

pnpm

pnpm add rn-eventsource-reborn web-streams-polyfill @azure/core-asynciterator-polyfill

bun

bun add rn-eventsource-reborn web-streams-polyfill @azure/core-asynciterator-polyfill

Add the polyfills to your project before the link is used (e.g. where you add your TRPCReact.Provider):

utils/api.tsx
ts
import '@azure/core-asynciterator-polyfill';
import { RNEventSource } from 'rn-eventsource-reborn';
import { ReadableStream, TransformStream } from 'web-streams-polyfill';
globalThis.ReadableStream = globalThis.ReadableStream || ReadableStream;
globalThis.TransformStream = globalThis.TransformStream || TransformStream;

utils/api.tsx
ts
import '@azure/core-asynciterator-polyfill';
import { RNEventSource } from 'rn-eventsource-reborn';
import { ReadableStream, TransformStream } from 'web-streams-polyfill';
globalThis.ReadableStream = globalThis.ReadableStream || ReadableStream;
globalThis.TransformStream = globalThis.TransformStream || TransformStream;

Once the ponyfills are added, you can continue setting up the httpSubscriptionLink as described in the setup section.

httpSubscriptionLink Options

ts
type HTTPSubscriptionLinkOptions<
  TRoot extends AnyClientTypes,
  TEventSource extends EventSourceLike.AnyConstructor = typeof EventSource,
> = {
  /**
   * EventSource ponyfill
   */
  EventSource?: TEventSource;
  /**
   * EventSource options or a callback that returns them
   */
  eventSourceOptions?:
    | EventSourceLike.InitDictOf<TEventSource>
    | ((opts: {
        op: Operation;
      }) =>
        | EventSourceLike.InitDictOf<TEventSource>
        | Promise<EventSourceLike.InitDictOf<TEventSource>>);
  /**
   * Timeout after inactivity in milliseconds.
   * If no messages (including pings) are received within this period,
   * the connection will be marked as "connecting" and attempt to reconnect.
   */
  reconnectAfterInactivityMs?: number;
};

ts
type HTTPSubscriptionLinkOptions<
  TRoot extends AnyClientTypes,
  TEventSource extends EventSourceLike.AnyConstructor = typeof EventSource,
> = {
  /**
   * EventSource ponyfill
   */
  EventSource?: TEventSource;
  /**
   * EventSource options or a callback that returns them
   */
  eventSourceOptions?:
    | EventSourceLike.InitDictOf<TEventSource>
    | ((opts: {
        op: Operation;
      }) =>
        | EventSourceLike.InitDictOf<TEventSource>
        | Promise<EventSourceLike.InitDictOf<TEventSource>>);
  /**
   * Timeout after inactivity in milliseconds.
   * If no messages (including pings) are received within this period,
   * the connection will be marked as "connecting" and attempt to reconnect.
   */
  reconnectAfterInactivityMs?: number;
};