command_sequence_with_retries.js

/**
 * A Promise-like interface for running a sequence of commands.
 *
 * If a network error occurs while running a command, then a reconnect is automatically
 * attempted and the sequence will resume by sending the same command again. Any other errors
 * that occur while running a command will cause the entire sequence of commands to abort.
 *
 * @param {Mongo} conn - a connection to the server
 */

function attemptReconnect(conn) {
    try {
        conn.adminCommand({ping: 1});
    } catch (e) {
        return false;
    }
    return true;
}

export class CommandSequenceWithRetries {
    constructor(conn) {
        this.conn = conn;
        this.steps = [];
    }

    then(phase, action) {
        this.steps.push({phase, action});
        return this;
    }

    execute() {
        let i = 0;
        let stepHadNetworkErrorAlready = false;

        while (i < this.steps.length) {
            try {
                // Treat no explicit return statement inside the action function as returning
                // {shouldStop: false} for syntactic convenience.
                const result = this.steps[i].action(this.conn);
                if (result !== undefined && result.shouldStop) {
                    return {
                        ok: 0,
                        msg: "giving up after " + this.steps[i].phase + ": " + result.reason
                    };
                }
            } catch (e) {
                if (!isNetworkError(e)) {
                    throw e;
                }

                // We retry running the action function a second time after a network error
                // because it is possible that the node is in the process of stepping down. We
                // won't be able to reconnect to the node until it has finished closing all of
                // its open connections.
                if (stepHadNetworkErrorAlready) {
                    return {
                        ok: 0,
                        msg: "giving up after " + this.steps[i].phase +
                            " because we encountered multiple network errors"
                    };
                }

                if (!attemptReconnect(this.conn)) {
                    return {
                        ok: 0,
                        msg: "giving up after " + this.steps[i].phase +
                            " because attempting to reconnect failed"
                    };
                }

                stepHadNetworkErrorAlready = true;
                continue;
            }

            ++i;
            stepHadNetworkErrorAlready = false;
        }

        return {ok: 1};
    }
}