1 files changed, 65 insertions, 0 deletions
diff --git a/packages/subproviders/src/subproviders/redundant_subprovider.ts b/packages/subproviders/src/subproviders/redundant_subprovider.ts
new file mode 100644
index 000000000..37c8bba5a
--- /dev/null
+++ b/packages/subproviders/src/subproviders/redundant_subprovider.ts
@@ -0,0 +1,65 @@
+import { JSONRPCRequestPayload } from '@0xproject/types';
+import { promisify } from '@0xproject/utils';
+import * as _ from 'lodash';
+
+import { Callback } from '../types';
+
+import { Subprovider } from './subprovider';
+
+/**
+ * This class implements the [web3-provider-engine](https://github.com/MetaMask/provider-engine) subprovider interface.
+ * It attempts to handle each JSON RPC request by sequentially attempting to receive a valid response from one of a
+ * set of JSON RPC endpoints.
+ */
+export class RedundantSubprovider extends Subprovider {
+    private _subproviders: Subprovider[];
+    private static async _firstSuccessAsync(
+        subproviders: Subprovider[],
+        payload: JSONRPCRequestPayload,
+        next: Callback,
+    ): Promise<any> {
+        let lastErr: Error | undefined;
+        for (const subprovider of subproviders) {
+            try {
+                const data = await promisify(subprovider.handleRequest.bind(subprovider))(payload, next);
+                return data;
+            } catch (err) {
+                lastErr = err;
+                continue;
+            }
+        }
+        if (!_.isUndefined(lastErr)) {
+            throw lastErr;
+        }
+    }
+    /**
+     * Instantiates a new RedundantSubprovider
+     * @param endpoints JSON RPC endpoints to attempt. Attempts are made in the order of the endpoints.
+     */
+    constructor(subproviders: Subprovider[]) {
+        super();
+        this._subproviders = subproviders;
+    }
+    /**
+     * This method conforms to the web3-provider-engine interface.
+     * It is called internally by the ProviderEngine when it is this subproviders
+     * turn to handle a JSON RPC request.
+     * @param payload JSON RPC payload
+     * @param next Callback to call if this subprovider decides not to handle the request
+     * @param end Callback to call if subprovider handled the request and wants to pass back the request.
+     */
+    // tslint:disable-next-line:async-suffix
+    public async handleRequest(
+        payload: JSONRPCRequestPayload,
+        next: Callback,
+        end: (err: Error | null, data?: any) => void,
+    ): Promise<void> {
+        const subprovidersCopy = this._subproviders.slice();
+        try {
+            const data = await RedundantSubprovider._firstSuccessAsync(subprovidersCopy, payload, next);
+            end(null, data);
+        } catch (err) {
+            end(err);
+        }
+    }
+}